Prof. John S. Tatlock 
 
SELECT WORKS 
 
 OF 
 
 JOHN T Y N D A L L 
 
 n 
 
 FORMS OF WATER. 
 LESSONS IN ELECTRICITY, 
 SIX LECTURES ON LIGHT. 
 
 NEW YORK; 
 
 JOHN B. ALDEN, PUBLISHER, 
 
 1886, 
 
Tff 
 
FORMS OF WATER. 
 
 BY 
 
 JOHN TYNDALL. 
 
AUCTION. 
 
 \ v Clou.!?, Hair,s. and Rivers S t 
 
 :{. Too Waves of Li?.ht SO 
 
 i. The Wavej cf He it which produce the 
 VapLi- of our Atmosphere and ineit 
 
 our Glaciers 80 
 
 5. Experiments to prove the foregoing 
 
 statements 88 
 
 6 Oceanic 1 UstiUation tf9 
 
 7. Tropical Rains 90 
 
 8. Mountain Condensers 91 
 
 9. Architecture of Snow 92 
 
 10. Atomic Poles , 92 
 
 1 1. Architecture of Lake Ice 94 
 
 li'i. The Source of the Arveiron. Ice Pin- 
 nacles, Towers, and Chasms of the 
 Glacier des Bois. Passage to the 
 Montanvert 91 
 
 13. The Mer de Glace and its Sources. 
 
 Our First Climb to the Cleft Station.. . 93 
 
 1 1. Ice-cascade and Snows of the Col du 
 
 Geant 90 
 
 15. Questioning tue Glaciers 97 
 
 1(3. Branches and Medial Moraines of the 
 Mer de Glace from, the Cleft Station. . . 7 
 
 1". The Talefre and the Jardin. Work 
 among the Crevasses 98 
 
 1.1 First Questions regarding Glacier Mo- 
 tion. Drifting of Bodies Buried in 
 Crevassa 93 
 
 19. Tho Motion of Glaciers. Measurements 
 by Hugi and AgaseLf. Drifting of 
 Huts on the Ico 100 
 
 i.0. Precise Measurements of Agassiz and 
 Forbes. Motion of a Glacier proved 
 to resemble the Motion of a River 100 
 
 1\. The Theodolite and its Use. Our own 
 
 Measurements 101 
 
 22. Motion of the Mer de Glace 101 
 
 23. Unequal Motiou of the- two feides of 
 the Mer de Glace .103 
 
 24. Suggestion of a new Likene-ss of Gla- 
 cier Motiou to Biver Motion. Con- 
 jecture tested ... 104 
 
 25. New Law of Glacier Motion 106 
 
 26. Motion of Ax>a of Mer de Glace 106 
 
 27. Motion of Tributary Q-laxjiens IQn 
 
 2*. Motion ot Top r.nd Bottom of Glacier. .1^5 
 29. Lateral Compression of a Gi-iccer 106 
 
 SECTJ.ON. TAGS. 
 
 u'). Longitudinal Compression, of a Glacier IOC 
 ol. Sliding and Flowing. Hard Ice and 
 
 Soft l^e 107 
 
 3>. Winter ou the Mer d 3 Glace 107 
 
 33. Winter Motion of the Mer do Glace . . . . 108 
 84. Motion of th3 Grindelwaid and Aletsch 
 
 Glacier Kh 
 
 35. Motion of Morteratsch Glacier 109 
 
 36. Birth of a Crevasse: .Reflections lv>9 
 
 37. Icicles 110 
 
 38. The Bergscnrund 110 
 
 ;-9. TransvfcToO Crevasses Ill 
 
 40. Marginal Crevasses Ill 
 
 41. Longitudinal Crevasses 112 
 
 42. Crevasses in relation, to Curvature of 
 Glacier 112 
 
 43. Mor nine-ridge>, Glacier Tablc-s, and 
 Sand Conej . , 113 
 
 4 4 The Glacier Mills or Moulins 114 
 
 4') Tne Changes of Volume of Water l.y 
 
 Heat and Cold 115 
 
 43. Consequences flowing from the fore- 
 gning f roperties of Water. Correction 
 
 of Errors " 116 
 
 47- The Molecular Mechanism of Water- 
 Congelation 11 f> 
 
 4S. The Dirt Bands of the Mer de Glace. . .117 
 
 al) Sea-ice and Icebergs 119 
 
 50. The JEggischhorn, the MurgelLa See 
 
 and its icebergs , 119 
 
 01. The Bel Alp 121 
 
 r>2. The Kitfelherg and Gorner Glacier.. .. 121 
 
 ;~>3. Ancient Glaciers of Switzerland 122 
 
 54. Erratic Block* 123 
 
 f>5. Ancient Glaciers of Engkmd, Ireland, 
 
 Scotland, acd Wales 123 
 
 56. The Glacier Epoch 1 24 
 
 57. Glacial Theories 125 
 
 58. Dilation and Sliding Theories 125 
 
 59. Plastic Theory 12 " 
 
 00. Viscous Theory 1 -(> 
 
 61. Regelation Theory 127 
 
 02. Cause of Ke^eiatkm 12* 
 
 63. Faraday's View of Regelation ... 129 
 
 64. The Uluc Veins of Glaciers ISO 
 
 05 Relation of Mructur^ io Pressure 132 
 
 0<5. Slate Cleavage and Glacier Lamination 133 
 07. Conclusion ,, ....13 
 
 ivi300S91 
 
THE FORMS OF WATER 
 
 IN 
 
 CLOUDS AND RIVERS, ICE AND GLACIERS, 
 
 BY 
 
 JOHN TYNDALL, LL.D., F.R.S., 
 
 PROFESSOR OF NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION, LONDON. 
 
 WITH NINETEEN ILLUSTRATIONS DRAWN UNDER THE DIRECTION 
 
 OF THE AUTHOR. 
 
 PREFACE TO THE FOURTH EDITION. 
 
 AT a meeting of the Managers of the Royal 
 Institution held on December 12th, 1825, 
 " the Committee appointed to consider what 
 lectures should be delivered in the Institu- 
 tion in the next session," reported " that they 
 had consulted Mr. Furaday on the subject of 
 engaging him to take a part in the juvenile 
 lectures proposed to be given during the 
 Christmas and Easter recesses, and they 
 found his avocations were such that it 
 Would be exceedingly inconvenient for him 
 to engage in such lectures." 
 
 At a general monthly meeting of the mem- 
 bers of the Royal Institution, held on Do- 
 cember 4th, 1826, the Managers reported 
 " that they had engaged Mr. \Vallis to deliver 
 a course of lectures on Astronomy, adapted 
 to a juvenile auditory, during the Christmas 
 vacation." 
 
 In a report dated April 16th, 1827, the 
 Board of Visitors express " their satisfaction 
 at finding that the plan of juvenile courses 
 of lectures has been resorted to. They feel 
 sure that the influence of the Institution can- 
 not be extended too far, and the system of 
 nstructing the younger portion of the com- 
 munity is one of the most effective means 
 Which the Institution possesses foi the diffu- 
 :2on of science." 
 
 Faraday's holding aloof was but tempo- 
 
 IS", for at Christmas. 1827, we iiud him 
 
 giving a " Course of Six Elementary Lectures 
 on Chemistr}', adapted tc a Juvenile Audi- 
 tory."* 
 
 The Easter lectures were soon abandoned ; 
 but from the date here referred to to the pres- 
 ent time the Christmas lectures have been a 
 marked feature of the Royal Institution. 
 
 In 1871 it fell to my lot'to give one of these 
 courses. I had been frequently invited to 
 write on Glaciers in encyclopedias, journals, 
 and magazines, but had always declined to 
 do so. I had also abstained from making 
 them the subject of a course of "lectures, 
 wishing to take no advantage of my position 
 here, and indeed to avoid writing a line or 
 uttering a sentence on the subject for whicb 
 1 could not be held personally responsible. 
 In view of the discussions which the subject 
 had provoked, I thought this the fairest 
 course. 
 
 But, in 1871, the time (I imagined) had 
 come, when, without risk of offence, I might 
 tell our young people something about the 
 labors of those who had unravelled for their 
 instruction the various problems of the ice- 
 world. My lamented friend and ever-helpful 
 counsellor, Dr. Bence Jones, thought the 
 subject a good one, and accordingly it was 
 chosen. Strong in my sympathy with youth, 
 and remembering the damage done by defec- 
 tive exposition to my own young mind, I 
 sought, to the best of my abiiity, to confer 
 upon these lectures clearness, thoroughness, 
 
84 
 
 THE FORMS OF WATER 
 
 and life. 
 
 Wishing, moreover, to render them of per- 
 manent value, I wrote out copious Notes ol 
 the course, and had them distributed among 
 the boys and girls. In preparing these Notes 
 I aimed at nothing less than presenting to my 
 youthful audience, in a. concentrated but 
 perfectly digestible form, every essential 
 point embraced in the literature of the gia- 
 ciers, and some things in addition, which, 
 derived as they were from my own recent 
 researches, no book previously published on 
 the subject contained. 
 
 E'.H my theory of education agrees with 
 th*5 v>f JSmeraoii. according to which instruc- 
 tion is only half the battle, what he calls 
 provocation being the other half. By Ihis he 
 means that power of the teacher, through 
 the force of his character and the vitality of 
 his thought, to bring out all the latent 
 strength of his pupil, and to invest with in- 
 terest even the driest matters of detail. In 
 the present instance I was determined to 
 shirk nothing essential, however dry ; and, 
 to keep my mind alive to the requirements 
 of my pupil, I proposed a series of ideal 
 rambles, in which he should be always at my 
 side. Oddly enough, though 1 was here 
 dealing with what might be called the ab- 
 stract idea of a boy, I realized his presence 
 so fully as to entertain for him, before our 
 excursions ended, an affection consciously 
 warm and real. 
 
 The Notes here referred to were at first 
 intended for the use of my audience alone. 
 At the urgent request of a friend I slightly 
 expanded them, and converted them into the 
 little book here presented to the reader. 
 
 The amount of attention bestowed upon 
 the volume induces me to give this brief 
 history of Us origin. 
 
 A German critic, whom I have no reason 
 to regard as specially favorable to me or it, 
 makes the following remark on the style of 
 the book : " This passion [for the mountains] 
 tempts him to reveal more of his Alpine 
 wanderings than is necessary for his demon- 
 strations. Thu reader, however, will not 
 find this a disagreeable interruption of the 
 couise of thought; for the book thereby 
 gains wonderfully in vividness." This, I 
 would say, was the express aim of the breaks 
 referred to. I desired to keep my companion 
 fresh, as well as instructed, and these inter- 
 ruptions were so many breathing-places 
 where the intellectual tension was purposely 
 relaxed and the mind of tlie pupil braced to 
 fresh action. 
 
 Of other criticisms, flattering and other- 
 wise, I forbear to speak. A>5 regards some 
 ftf them, indeed, it would be a reproach to 
 that manliness which I have sought to en- 
 courage in my pupil to return blow for blow. 
 If the reader be acquainted with them, this 
 will let him know how I regard them ; and 
 if he be not acquainted with them, I wouM 
 recommend him to ignore them, and to form 
 his own judgment of this book. No fair- 
 minded person who reads it will dream that 
 I, in writing it, had thought of acting other- 
 
 wise than justly and generously toward K:? 
 predecessors, the last of whom, 1o the grief 
 of all who knew him, has recently passed 
 away. JOHN TYNDALL. 
 
 APBIL, 1874. 
 
 1. CLOUDS, RAINS, AND RIVERS. 
 
 1. EVERY occurrence in Nature is preceded 
 by otner occurrences which arc its causes, 
 and succeeded by others which arc its effects. 
 The human mind is not satisfied with observ- 
 ing and studying any natural occurrence, 
 alone, but takes pleasure in connecting every 
 natural fact with what has gone before if. 
 and with what is to come after it. 
 _ 2. Thus, when we enter upon the study of 
 rivers and glaciers, our interest will bo great- 
 ly augmented by taking into account not 
 only their actual appearances, but also their 
 causes and effects. 
 
 o. Let us trace a river to its source. Be- 
 ginning where it empties itself into the sea, 
 and following it backward, we find it from 
 time to tinu joined by tributaries which 
 swell its waters. The river of course be- 
 comes smaller as these tributaries are passed. 
 It shrinks first to a brook, then to a stream ; 
 this again divides itself into a number of 
 smaller streamlets, ending in mere threads of 
 water. These constitute the source of tlu 
 river, and are usually found among hills. j 
 
 4. Thus the Severn lias its source in th.>' 
 Welsh Mountains ; the Thames in the Cots- 
 wold Hills ; the Danube in tin? hills of tlie 
 Black Forest ; the Rhine and the Rhone in 
 the Alps ; the Ganges in the Himalaya 
 Mountains ; the Euphrates near Mount Ara 
 rat ; the Garonne in t.'ie Pyrenees ; the- Elbe 
 in the Giant Mountains of I5oheni.':i ; tho 
 Missouri in the Rocky Mountains, and the 
 Amazon in the Andes of Peru. 
 
 5. But it is quite plain that we have not 
 yet reached the real beginning of the liters. 
 Whence do the earliest streams derive their 
 water? A brief residence among tlie moun- 
 tains would prove to you that they are fed 
 by rains. In dry weather you would find 
 the streams feeble, sometime? indeed quitu 
 dried up. In wet weather yc'i would see 
 them foaming torrents. la general these 
 streams lose themselves as littlo threads of 
 water upon the hill-sides ; but. srmcliuies you 
 may trace a river to a definite spring. Tho 
 river Albula in Switzerland, for instance, 
 rushes at its origin in considerable volumo 
 from a mountain-side. But you very soon 
 assure yourself that such springs are also fed 
 by rain, which has percolated through the 
 rocks or soil, and which, through some ori- 
 fice that it has found or formed, comes to the 
 light of day. 
 
 0. But we cannot end here. Whence 
 comes the rain which forms the mountain 
 streams? Observation enables you to an- 
 swer the question. Rain does not'come from 
 a clear sky. It comes from clouds. But 
 w T hat are clouds ? Is there nothing you are 
 acquainted with which they resemble ? You 
 discover at once a likeness between them and 
 the condensed steam of a locomotive. 'At 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 85 
 
 every puff of the engine a cloud is projected 
 into the air. Watch the cloud sharply : you 
 notice that it first forms at a little distance 
 from the top of the funnel. Give close at- 
 tention and you will sometimes see a per- 
 fectly clear space between the funnel and the 
 cloud. Through that clear space the thing 
 which makes the cloud must pass. What, 
 then, is this thing which at one moment is 
 transparent and invisible, and at the next 
 moment visible as a dense opaque cloud ? 
 
 7. It is the steam or xapor of water from 
 the boiler. Within the boiler this steam is 
 transparent and invisible ; but to keep it in 
 this invisible state a heat would be required 
 as great as that within the boiler. When the 
 vapor mingles with the cold air above the hot 
 funnel it ceases to be vapor. Every bit of 
 steam shrinks, when chilled, to a much more 
 minute particle of water. The liquid parti- 
 cles thus produced form a kind of water-dust 
 of exceeding fineness, which floats in the air, 
 and is called a cloud. 
 
 8. Watch the cloud-banner from the fun- 
 nel of a running locomotive ; you see it 
 growing gradually less dense. It finally 
 nuilts away altogether, and it' you continue 
 your observations you will not fail to notice 
 that the speed of its disappearance depends 
 upon the character of the day. In humid 
 weather the cloud hangs long and lazily in 
 the air ; in dry weather it is rapidly licked 
 up. What has become of it? It luis been 
 reconverted into true invisible vapor. 
 
 9. The drier the air, and the Iwtter the air, 
 the greater is the amount of cloud which 
 can be thus dissolved in it. When the cloud 
 first forms, its quantity is far greater than the 
 air is able to maintain in an ^invisible stale. 
 But as the cloud mixes gradually with a 
 larger mass of air it is more an :1 more dis- 
 solved, and finally passes altogether from the 
 condition of a finely-divided liquid into that 
 of transparent vapor or gas. 
 
 10. Make the lid of a kettle air-tight, and 
 permit the steam t^> issue from the pipe ; a 
 cloud is precipitated in all respects similar to 
 that issuing from the funnel of the locomo- 
 tive. 
 
 11. Permit the steam as it issues from the 
 pipe to pass through the flame of a spirit- 
 lamp, the cloud is instantly dissolved by the 
 heat, and is not again precipitated. With a 
 special boiler and a special nozzle the exper- 
 iment may be made more striking, but not 
 more instructive, than with the kettle. 
 
 12. Look to your bedroom windows when 
 the weather is very cold outside ; they some- 
 times stream with water derived from the 
 condensation of the aqueous vapor from your 
 own lungs. The windows of" railway car- 
 riages in winter show this condensation in a 
 striking manner. Tour cold water into a dry 
 drinking-glass on a summer's day : the out- 
 fci'iu surface of the glass becomes instantly 
 uunmccl by the precipitation of moisture. 
 On a warm day you notice no vapor in front 
 of your mouth, but on a cold day yi-u form 
 there a little cloud derived from ihe conden- 
 sation of the aqueous vapor from the hir^r. 
 
 13. You may notice in a ball-room that as 
 long as the door and windows are kept, 
 closed, and the room remains hot, the air re- 
 mains clear ; but when the doors or windows 
 are opened a dimness is visible, caused by 
 the precipitation to fog of the aqueous vapor 
 of the ball-room. If the weather be intensely 
 cold the entrance of fresh air may even cause 
 now to fall. This has been observed in Rus- 
 sian ball-rooms ; and also in the subterranean 
 stables at Erzeroom, when the doors are 
 opened and the cold morning air is permitted 
 to enter. 
 
 14. Even on the driest day this vapor is 
 never absent from our atmosphere. The 
 vapor diffused through the air of this room 
 may be congealed to hoar frost in your pres- 
 ence This is done by filling a vessel with, a 
 mixture of pounded ice and salt, which is 
 colder than the ice itself, and which, there- 
 fore, condenses and freezes the aqueous 
 vapor. The surface of the vessel is finally 
 coated with a frozen fur, so thick that it may 
 he scraped away and formed into a snow- 
 ball. 
 
 15. To produce the cloud, in the case of 
 the locomotive and the kettle, heat is neces- 
 sary. By heating the wakr we first convert 
 it into steam. &n<l then, by chilling the steam 
 we convert it into cloud. Is there any file in 
 nature which produces the clouds of our at- 
 mosphere V There; is : the fire of the sun. 
 
 10. Thus, by tracing backward, without 
 any break in ihe chain of occurrences, our 
 river from its end t its real beginnings, we 
 come at length to the sun. 
 
 IT. There are, however, rivers which have 
 sources somewhat different from those just 
 mentioned. They do not begin by driblets 
 on a hill-side, nor can they be traced to a 
 spring. Go, for example, to the mouth of 
 the river Rhone, and trace it backward to 
 Lyons, where it turns to the cast. Bending 
 round by Chambery, you come at length to 
 the Lake of Geneva, from which the liver 
 rushes, and which } r ou might be disposed to 
 regard as the source of the Rhone. But go 
 to the head of ihe lake, and you find that 
 the Rhone there enteis it, that the lake is 
 in fact a kind of expansion of the liver. 
 Follow this upward ; you find it joined by 
 smaller rivers from the mountains right and 
 left. Pass these, and push your journey 
 higher still. You come at length to a lingo 
 mass of ice the end of a glacier which fills 
 the Rhone valley, and from the boiuuii of tiie 
 glacier the river rushes In the glacier of 
 the Rhone you thus find the souice of the 
 river Rhone. 
 
 18. But again we have not reached the real 
 beginning of the river. You soon convince 
 yourself that this earliest water of the Rhone 
 is produced by the melting of the ice. You 
 get upon the glacier and walk upward along 
 it. After a time the ice disappears and you 
 come upon snow. If you are a competent 
 mountaineer you may go to the very top of 
 this great snow-field, and if you cross the top 
 
THE FORMS OF WATER 
 
 and descend at the other side you finally ault 
 the snow, and get upon another glacier called 
 the Trift, from the end of which rushes a 
 river smaller than the Rhone. 
 
 19. You soon learn that the mountain 
 snow feeds the glacier. By some means or 
 other the snow is converted into ice. But 
 whence comes the snow ? Like the rain, it 
 comes from the clouds, which, as before, can 
 be traced to vapor raised by the sun. With- 
 out solar lire AVC could have no atmospheric 
 vapor, without vapor no clouds, without 
 clouds no snow, and without snow no gla- 
 ciers. Curious then as the conclusion maybe, 
 the cold ice of the Alps has its origin in the 
 heat of the sun. 
 
 3. THE WAVES OF LIGHT. 
 
 20. But what is the sun ? We knov/ its 
 size and its weight. We also know that it 
 is a globe of fire far hotter than any fire 
 upon earth. But we now enter upon another 
 inquiry. We have to learn definitely what is 
 the meaning of solar light and solar'heat ; in 
 what way they make themselves known to 
 our senses ; by what means they get from 
 the sun to the earth, and how, when there, 
 they produce the clouds of our atmosphere, 
 and thus originate our rivers and our glaciers. 
 
 21. If in a dark room you close your eyes 
 and press the eyelid with your finger-nail, a, 
 circle of light will be seen opposite to the 
 point pressed, while a sharp blow upon the 
 eye produces the impression of a flash of 
 light. There is a nerve specially devoted to 
 the purposes of vision which comes from the 
 brain to tho back of the eye, and there di- 
 vides into line filaments, which are woven 
 together to a kind of screen called the retina. 
 The retina can be excited in various ways so J 
 ns to produce the consciousness of light ; it 
 m?,y, as we have seen, be excited by the rude^ 
 mechanical action of a blow imparted to the 
 eye. 
 
 22. There is no spontaneous creation of 
 light by the healthy eye. To excite vision 
 tho relLia must be affected by something , 
 coming from without. What is that some- 
 thing? In some way or other, luminous 
 bo:lies have the power of affecting the retina 
 but hoio ? 
 
 23. It was long supposed that from such 
 bodies issued, with inconceivable rapidity, au 
 inconceivably fine matter, which flew 
 through spaca, passed through the pores sup- 
 posed to exist in the humors of the eye, 
 reached the retina behind, and by their shock 
 .-gainst the retina, aroused the sensation of 
 light. 
 
 2i. This theory, which was supported by 
 the greatest men, among others by Sir Isaac 
 Newton, was found competent to explain a 
 great number of the phenomena of light, but 
 it was not found competent to explain all the 
 phenomena. As the skill and knowledge of 
 experimenters increased, large classes of facts 
 were revealed which could only be explained 
 by assuming that light was produced, not by 
 a fine matter Hying through space and hitting 
 the retina, but by the shock of minute waxes 
 
 against the retina. .. 
 
 25. Dip your finger into a basin of water, 
 and cause it to quiver rapidly to and fro. 
 From the point of disturbance issue small 
 ripples which are carried forward by the 
 water, and which finally strike the basin. 
 Here, in the vibrating tinger, you have a 
 source of agitation ; in the water you have a 
 vehicle through which the finger's motion is 
 transmitted, and you have finally the side of 
 the basin which receives the shock of the 
 little waves. 
 
 26. In like manner, according to the warn 
 theory of light, you have a source of agitation 
 in the vibrating atoms, or smallest particles, 
 of the luminous body ; you have a vehicle of 
 transmission in a substance which is sup- 
 posed to fill all space, and to be diffused 
 through the humors of the eye ; and finally, 
 you have the retina, which receives the suc- 
 cessive shocks of the waves. These shocks 
 are supposed to produce the sensation of light. 
 
 27. We are here dealing, for the most part, 
 with suppositions and assumptions merely. 
 We have never seen the atoms of a luminous 
 body, nor their motions. We have never 
 seen the medium which transmits their 
 motions, nor the waves of that medium. 
 How, then, do \vc come to assume their ex- 
 istence ? 
 
 ; 28. Before such an idea could have taken 
 any real root in the human mind, it must have 
 been well disciplined and prepared by obser- 
 vations and calculations of ordinary wave- 
 motion. It was necessary to know how both 
 water- waves and sound-waves arc formed 
 and propagated. It was above all thing.-.' 
 necessary to know how waves, passing 
 through the same medium, act upon each 
 other. Thus disciplined, the mind was pre- 
 pared to detect any resemblance presenting 
 itself between the action of light and that of 
 waves. Great classes of optical phenomena 
 accordingly appeared which could be ac- 
 counted for in the most complete and satis- 
 factory manner by assuming them to be pro- 
 duced by waves, and which could not bo 
 otherwise accounted for. It is because of its 
 competence to explain all tho phenomena of 
 light that the wave theory now receives uni- 
 versal acceptance on tho part of scientific 
 men. 
 
 Let me use an illustration. We infer from 
 the flint implements recently found in such 
 profusion all over England and in other 
 countries, that they were produced by men, 
 and also that the Pyramids of Egypt were 
 built by men, because, as far as our expe- 
 rience goes, nothing but men could form such 
 implements or build such Pyramids. In like 
 manner, we infer from the phenomena of 
 light the agency of waves, because, as far as 
 our experience goes, no other agency could 
 produce the phenomena. 
 
 4. THE WAVES OF HEAT WHICH PRODUCE 
 THE VAPOR OF OUR ATMOSPHERE AND 
 MELT OUR GLACIERS. 
 29. Thus, in a general way, I have given 
 
 you the conception and the grounds of tho 
 
IN CLOUDS ASTD RIVERS, ICE AND GLACIERS. 
 
 87 
 
 Fia. 1. CLOUD-BANNER OF THK AIGUILLE DU BRU (par. 64 and 227X- 
 
 conception, which regards light a~s the pro- 
 duct of wave-motion ; but we must go far- 
 ther than ihis, and follow the conception into 
 some of its details. We have all seen the 
 waves of water, and we know they are of 
 different sizes different in length and differ- 
 ent in height. When, therefore, you are told 
 that the atoms of the sun, and of almost all 
 other luminous bodies, vibrate at diffcrci:: 
 rates, and produce waves of different sizes 
 your experience of water-waves will enable 
 you to form a tolerably clear notion of what 
 is meant. 
 
 30. As observed above, we have never seen 
 the light- waves, but we judge of their pres- 
 ence, their position, and their magnitude, by 
 their effects. Their lengths have been thus 
 determined, and found to vary from about 
 to ^ooouth of an inch. 
 
 31. But besides those which produce light 
 the sun sends forth incessantly a multitude 
 of waves which produce no" light. Ths 
 largest waves which the sun sen'is forth r.rc 
 of this non-luminous character, though they 
 possess the highest heating power. 
 
 32. A common sunbeam contains waves of 
 all kinds, but it is possible to ftift or filter the 
 beam so as to intercept all its light, and to 
 allow its obscure heat to pass unimpeded. 
 For substances have been discovered which, 
 while intensely opaque to the light- waves, 
 are almost perfectly transparent to^he others. 
 On the other hand, it is possible, by the 
 choice of proper substances, to intercept in a 
 great degree the pure heat-waves, and to 
 allow the pure light-waves free transmission. 
 This last separation is, however ; not so per- 
 fect as the h'rst. 
 
THE FORMS <jF WATER 
 
 33. We shall learn presently how to detach 
 the one class of waves from the other class, 
 and to prove that v/aves competent to light a 
 fire, fuse metal, or burn the hand like a hot 
 solid, may exist in a perfectly dark place. 
 
 34. Supposing, then, that we withdraw, in 
 the first instance, the large heat-waves, and 
 allow the light- waves alone to pass. These 
 nviy be concentrated by suitable lenses and 
 sent into water without sensibly warming it. 
 Let the light -waves now be withdrawn, and 
 tiie larger heat-waves concentrated in the 
 same manner , they may be caused to boil 
 the water almost instantaneously. 
 
 35. This is the point to which I wished to 
 ead you. and which without due preparation 
 could not l>e understood. You now per- 
 ceive the important, part played by these 
 large darkness-waves, if I may use the term, 
 in the work of evaporation. When they 
 plunge into seas, lakes, and rivers, they are 
 intercepted close to the surface, and they 
 heat the water at the surface, thus causing 
 it to evaporate ; the light-waves at the samd 
 time entering to great depths without sensibly 
 heating the water through which they pass. 
 Not only, therefore, is it the sun's fire which 
 produces evaporation, but a particular con- 
 stituent of that lire, the existence of which 
 you probably w r ere not aware of. 
 
 30. Further it is these self-same lightless 
 waves which, falling upon the g'.aei .TS of 
 the Alps, melt the ice and prot'-.ico all Hie 
 rivers flowing from the glaciers ; f;>i* I shall 
 prove to you presently that the light-waves, 
 even when concentrated to the uttermost, are 
 unable to melt the most delicate hoar-lro^t ; 
 much less would they be able to produce tlu 
 copious liquefaction observed upon the glo- 
 ciers. 
 
 37. These large lightless waves of the sun, 
 as well as the heat-waves issuing 1 _>m non- 
 luminous hot bodies, are frequently called 
 obscure or invisible heat. 
 
 We have here an example of the nv.nner 
 in which phenomena, apparently remote, are 
 connected together in this wonderful system 
 of things that we call Nature. You cannot 
 study a snow-Hake profoundly wiihout being- 
 led back by it step by step to the constitution 
 of the sun. It is thus throughout Nature. 
 All its parts are interdependent, and tho 
 study of any one part completely would really 
 involve the study of all 
 
 g 5. EXPERIMENTS TO PROVE VIIE FORE- 
 GOING STATEMENTS. 
 
 38 Heat issuing from any source not visi- 
 bly red cannot be concentrated so as to pro- 
 duce the intense effects just referred to. To 
 produce these it is necessary to employ the 
 obscure heat of a body raised to the highest 
 possible state of incandescence. The sun is 
 such a body, and its dark heat is therefore 
 suitable for experiments of this nature. 
 
 39. But in the atmosphere of London, and 
 for experiments such as ours, the heat-waves 
 emitted by coke raised to intense whiteness 
 by a current of electricity are much more 
 manageable than the sun's waves. The elec 
 
 trie light has also the advautage that rts dark 
 radiation embraces a larger proportion of the 
 total radiation than the dark heat of tiie sun. 
 In fact, the force 01 energy, if 1 may use the 
 term, of the daik Avaves of the electric light 
 is fully seven times that of its lightwaves. 
 The electric light, therefore, shall be em- 
 ployed in our experimental demons! rat ions. 
 
 40. From Ibis source a powerful beam u 
 sent throuirh the room, revealing its track by 
 the motes floating in the air of the room ; for 
 were the mo'es entirely absent the beam 
 would be unseen. It falls unon a concave 
 mirror (a glass one silvered behind will an- 
 swer) and is gathered up by the mirror into 
 a cone of reflected rays ; the luminous apex 
 of the cone, which is the/ocw of the mirror, 
 being about fifteen inches distant from its 
 reflecting SMI face. Let us uiaik the* focus 
 accurately by a pointer. 
 
 41. And now let us place in the path of 
 the beam a substance perfectly opaque tr 
 light. This substance is iodine dissolved m 
 a liquid called bisulphide of carbon. Tho 
 light at the focus instantly vanishes when 
 the dark solution is introduced. But the so 
 iution is intensely transparent to the dark 
 waves, and a focus of such waves remains m 
 the air of the room after the light has been 
 abolished. You may feel the heat of these 
 waves with your hand ; you may let them 
 fall upon a thermometer, and thus prove 
 their presence ; or, best of all, you may 
 cause them to produce a current of electric- 
 ity, which Reflects a large magnetic needle. 
 The magni' .do of the deflection is a measure 
 of the htMVi 
 
 42. Cm n \jeel now is, by the use of a 
 more power, til lamp, and a better mirror (one 
 silvered in front and with a shorter focal dis- 
 tance), to intensify the action here rendered 
 so sensible. As before, the focus is rendered 
 strikingly visible by the intense illumination 
 of the dust particles. We will first filter the 
 beam so as to intercept its dark waves, and 
 then permit the purely luminous waves to 
 sxert their utmost power on a small bundle 
 of gun-cotton placed at the focus. 
 
 4;j. No effect whatever b produced. The 
 gun-cotton might remain there for a week 
 without ignition Let us now permit the 
 uunitered beam t:> act upon the cotton. It 
 is instantly dissipated in an explosive flash. 
 This experiment proves that the light-waves 
 are incompetent to explode the cotton, while 
 th3 waves of the full beam are compel ent to 
 do so ; hence we may conclude that the dark 
 waves are the real airents in- the explosion 1 . 
 
 44. But this conclusion would be only 
 probable ; for it might be urged that the 
 mixture of the dark waves an.l the light waves 
 is necessary to produce the result. Let us then, 
 by means of our opaque solution, isolate our 
 dark waves and converge them on the cotton. 
 It explodes as before. 
 
 45. Hence it is the dark waves, and they 
 only, that are concerned in the ignition of 
 the cotton. 
 
 46. At the same dark focus sheets 01 plati- 
 num are raised to vivid redness ; zinc is 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 80 
 
 burned ur> , paper instantly blazes , magne- 
 sium wire is ignited ; charcoal witiim a ra- 
 ceivt- r coutainiTig oxygen is set binning : a 
 diamond similarly placed is caused to glow 
 like a star, being afterward gradually dissi- 
 pated. And all this while the air at the fo- 
 "iis remains as cool as in any other part of 
 the room. 
 
 47. To obtain the light-waves we employ 
 a clear solution of alum in water ; to obtain 
 the dark waves we employ tho solution of 
 iodine above referred to. But as before 
 stated (32), the aluui. is not so perfect a tiller 
 as the iodine ; / : or it uausmits a portion of 
 the obscure her i. 
 
 48. Though the light-waves here prove 
 their incompetence to ignite gun-cotton, they 
 arc able to burn up black paper ; or, indeed, 
 to explode the cotton when it is blackened. 
 The white cotton does not absorb the light, 
 and without absorption we have no heating. 
 The blackened cotton absorbs, is heated, and 
 explodes. 
 
 49. Instead of a solution of alum, we will 
 employ for our next experiment a cell of pure 
 water, through which the light passes with- 
 out sensible absorption. At the focus is 
 placed a test-tube also containing water, thtt 
 full force of the light being concentrated 
 upon it. The water is not sensibly warmed 
 by the conoentiated waves. We now re- 
 move the cell of water ; no change is visible 
 in the beam, but the water contained in the 
 test-tube now boils. 
 
 50. The light-waves being thus proved in- 
 effectual, and the full beam effectual, we may 
 infer that it is the dark waves that do the woi k 
 of heating. But we clinch our inference by 
 employing our opaque iodine filter. Placing 
 it on the path of the beam, the light is en- 
 tirely stopped, but the water boils exactly as 
 it did when the full beam fell upon it. 
 
 51. The truth of the statement made in 
 paragraph 34 is thus demonstrated. 
 
 52. And now with regard to the im llinrj of 
 ice. On the surface of a flask contain:^ a 
 freezing mixture we obtain n thick fur of hoar- 
 frost (Par. 14). Sending the beam through 
 a water-cell, its luminous waveo c,n; concen- 
 ( rated upon the surface of too E.a*"^ Not a 
 spicula of the frost is disso'l^ti. ''* e now 
 remove the water-cjll, and in ,'i moment a 
 patch of the frozen fur as le.rge as half-a-crown 
 is melted. Hence, inasmuch as the full beam 
 produces this effect, and the luminous part 
 of the beam does not produce it, we fix upon 
 the dark portion the melting of the frost. 
 
 53. As before, we clinch this inference by 
 concentrating the dark waves alone upon the 
 Mask. The frost is dissipated exactly as it 
 was by the full. beam. 
 
 54. These effects are rendered strikingly 
 visible by darkening with ink the freezing 
 mixture within the flask. When the hoar- 
 frost is removed, the blackness of the surface 
 from which it had been melted comes out in 
 strong cor- ~ st with the adjacent su.wy 
 'vt!t,eness. f^hen the flask itself, instead of 
 the -freezing mixture, is blackened, the purely 
 luminous waves bein absorbed by the glass. 
 
 w&rm it ; the glass reacts upon the frost nnd 
 melts it. Hence the wisdom of darkening, 
 instead of the ikisk itself, the mixture within 
 the flask. 
 
 t 55. This experiment proves to demonstra- 
 tion the statement in paragraph 30 : that it 
 is the dark waves of the sun that nit-It thy 
 mountain snow and ice, and originate all the 
 rivers derived from glaciers. 
 
 There are writers who seem to regard 
 science as an aggregate of facts, nnd hence 
 doubt its efficacy as an exercise of the rea- 
 soning powers. But all that I have here 
 taught you is the result of reason, taking its 
 stand, however, upon the sure basis of ob- 
 servation and experiment. And this is the 
 spirit in which our further studies are to be 
 pursued. 
 
 o. OCEANIC DISTILLATION. 
 
 56. IN.e sun, you know, is never exactly 
 overhead in England. But at the equator, 
 and within certain limits north and south of 
 it. the sun &t certain periods of the year is 
 directly overhead at noon. These limits are 
 called the Tropics of Cancer and of Capri- 
 corn. Upon the belt comprised between 
 these two circles the sun's rays fall with their 
 mightiest power ; for here they shoot directly 
 downward, and heat both earth and sea more* 
 than when they strike slantingly. 
 
 57. When the vertical sunbeams strike the-- 
 land they heat it, and the air in contact with, 
 the hot soil becomes heated in turn. But. 
 when heated the air expands, and when it 
 expands it becomes lighter. This lighter air- 
 rises, like wood plunged into waler.lh rough 
 the heavier air overhead. 
 
 58. When the sunbeams fall upon the sea 
 the water is warmed, though not so much as. 
 the land. The warmed water expands, be- 
 comes thereby lighter, and therefore continues, 
 to float upon the top. This upper layer of 
 water warms to some extent the air in contact 
 with it, but it also sends up a quantity of 
 aqueous vapor which, being far lighter than 
 air, helps the latter to rise. Thus both from 
 the land and from the sea we have ascending- 
 currents established by the action of the sun! 
 
 59. When they reach a certain elevation in. 
 the atmosphere, these currents divide and 
 flow, part toward the north and part toward 
 the south ; -jvliiie from the north and the south 
 a flow of heavier and colder air sets in to. 
 supply the place of the ascending warm air. 
 
 60. Incessant circulation is thus established 
 in the atmosphere. The equatorial air and 
 vapor flow above toward the north and 
 south poles, while the polar air flows below 
 toward the equator. The two currents of 
 air thus established are called the upper and 
 the lower trade-winds. 
 
 61. But before the air returns from the 
 poles great changes have occurred. For the 
 air as it quitted the equatorial regions was 
 laden with aqueous vapor, which could not 
 subsist in the cold polar regions. It is there 
 precipitated, falling sometimes as rain, or 
 more commonly as snow. The land near the 
 pole is covered with this snow, which gives 
 
00 
 
 THE FORMS OF WATER 
 
 birth to vast glaciers in a manner hereafter to 
 be explained. 
 
 02. It is necessary that you should have a 
 perfectly clear view of this process, for great 
 mistakes have been made regarding the man- 
 ner in which glaciers are related to the heat 
 of the sun. 
 
 63. It was supposed that if the sun's heat 
 were diminished, greater glaciers than those 
 now existing would be produced. But the 
 lessening of the sun's heat would infallibly 
 diminish the quantity of aqueous vapor, and 
 thus cut otf the glaciers at their source. A 
 brief illustration^will complete your knowl- 
 edge here. 
 
 64. lu the process of ordinary distillation, 
 the liquid to be distilled is heated and con- 
 verted into vapor in one vessel, and chilled 
 and reconverted into liquid in another. What 
 has just been stated renders it plain that the 
 earth an 1 its atmosphere constitute a vast 
 distilling apparatus in which the equatorial 
 ocean piays the part of the boiler, and the 
 chill regions of the poles the part of the con- 
 denser. In this process of distillation /teat 
 plays quite as necessary a part as cold, and 
 before Bishop Heber could speak of " Green- 
 land's icy mountains," the equatorial ocean 
 had to be warmed by the sun. We shall 
 have more to say upon this question after- 
 ward. 
 
 ILLUSTRATIVE EXPERIMENTS. 
 
 05. I have said that when heated, air ex- 
 pands. If you wish to verify this for your- 
 self, proceed thus. Take an empty flask, 
 Btop it by a cork ; pass through the cork a 
 narrow glass tube. By heating the tube in 
 a spirit-lamp you can bend it downward, so 
 that when the flask is standing upright the 
 open end of the narrow tuba may dip into 
 water. Now cause the flame of your spirit- 
 lamp to play against the flask, The flam 3 
 heats the glass, the glass heats the air ; the 
 air expands, is driven through the narrow 
 tube, and issues in a storm of bubbles from 
 the water. 
 
 66. Were the heated air unconfmed, it woull 
 rise in the heavier cold air. Allow a sun- 
 beam or any other intense light to fall upon 
 a white wall or screen in a dark room. Bring 
 u heated poker, a candle, or a gas-flame un- 
 derneath the beam. An ascending current 
 rises from the heated body through the beam, 
 and the action of the air upon the light is 
 such as to render the wreathing and waving 
 of the current strikingly visible upon the 
 screen. When the air is hot enough, anJ 
 therefore light enough, if entrapped in a 
 paper bag it carries the bag upward, and 
 you have the fire balloon. 
 
 67. Fold two sheets of paper into two 
 cones, and suspend them with their closed 
 points upward from the end of a delicate 
 balance. See that the cones balance each 
 other. Then place for a moment the flame 
 of a spirit-lamp beneath the open base of one 
 of them ; the hot air ascends from the lamp 
 and instantly tosses upward the cone above it. 
 
 68. Into an inverted glass shade introduce 
 
 a little simks. Let the air come to r-?st. an 1 
 then simply place your hand at (he open 
 mouth of the shade/ Mimic hurricanes nre 
 produced by the air warmed by the hand, 
 which are strikingly visible when the smoka 
 is illuminated by a strong light. 
 
 69. The heating of the tropical air by the 
 sun is indirect. The solar beams have scarce- 
 ly any power to heat the air through which 
 they pass ; but they heat the laud and ocean, 
 and these communicate their heat to the air 
 in contact with them. The air and vapor 
 start upward charged with the heat thus 
 communicated. 
 
 7. TROPICAL RAINS. 
 
 70. But long before the air and vapor from 
 the equator reach the poles, precipitation 
 occurs. Wherever a hurnid warm wind 
 mixes with a cold dry one, rain falls. In- 
 deed the heaviest rains occur at those places 
 where the sun is vertically overhead. We 
 must inquire a little more closely into their 
 origin. 
 
 71. Fill a bladder about two thirds full of 
 air at the sea-level, and take it to the summit 
 of Mont Blanc. As you ascend, the bladder 
 becomes more and more distended ; at the 
 top of the mountain it is fully distended, and 
 has evidently to bear a pressure from within. 
 Returning to the sea-level you find that the 
 tightness disappears, the bladder finally ap- 
 pearing as flaccid as at first. 
 
 72. The reason is plain. At the sea-level 
 the air within the bladder has to bear the 
 pressure of the whole atmosphere, being 
 thereby squeezed into a comparatively small 
 volume. In ascending the mountain, you 
 leave more and more of the atmosphere be- 
 hind ; the pressure becomes less and less, 
 and by its expansive force the air within the 
 bladder swells as the outside pressure is di- 
 minished. At the top of the mountain the 
 expansion is quite sufficient to render the 
 bladder tight, the pressure within being then 
 actually greater than the pressure without. 
 By means of an air-pump we can show the 
 expansion of u balloon partly filled with air, 
 when the external pressure has been in part 
 removed. 
 
 78. But why do I dwell upon this ? Sim- 
 ply to make plain to you that the unconfined 
 air, heated at the earth's surface, and as- 
 cending by its lightness, must expand more 
 and more the higher it rises in the atmos- 
 phere. 
 
 74. And now I have to introduce to you a 
 new fact, toward the statement of which 1 
 have been working for some time. It is 
 this : The ascending air is chilled by its expan- 
 sion. Indeed this chilling is one source of 
 the coldness of the higher atmospheric re- 
 gions. And now fix your eye upon those 
 mixed currents of air and aqueous vapor 
 which lise from the warm tropical ocean. 
 They start with plenty of heat to preserve 
 the vapor as vapor ; but as they rise they 
 come into regions already chilled, and they are 
 still further chilled by their own expansion. 
 .The consequence mio;ht be foreseen. Th 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 load of vapor is in great part precipitated, 
 dense clouds are formed, their particles coa- 
 lesce to rain-drops, which descend daily in 
 gushes so profuse that the word " torren- 
 tial " is used to express the copiousness of 
 the rainfall. I could shoTv you this chilling 
 by expansion, and also the consequent pre- 
 cipitation of clouds. 
 
 75. Thus long before the air from the 
 equator reaches the poles, its vapor is iu 
 great part removed from it, having rede 
 sccnded to the earth as rain. Still a good 
 quantity of the vapor is carried forward, 
 which yields hail, rain, and snow in noUhem 
 and southern lands. 
 
 ILLUSTRATIVE EXPERIMENTS. 
 
 76. I have said that the air is chilled during 
 its expansion. Prove this, if you like, thus. 
 With a condensing syringe, you can force air 
 into an iron box furnished with a stopcock, 
 to which the syringe is screwed. Do so till 
 the density of the air within the box is 
 doubled or trebled. Immediately after this 
 condensation, both the box and the air within 
 it are wajm, and can be proved to be so by 
 a proper thermometer. Simply turn the cock 
 and allow the compressed air to stream into 
 the atmosphere. The current, if allowed to 
 strike a thermometer, visibly chills it ; and 
 with other instruments the chill may to 
 made more evident still. Even the hand 
 feels the chill of the expanding air. 
 
 77. Throw a strong light, a concentrated 
 sunbeam for example, across the issuing cur- 
 rent ; if the compressed nir be oidinary 
 humid air, you sec the precipitation of a liltlc 
 cloud by the chill accompanying the expan- 
 sion. This cloud-formation may, however, 
 be better illustrated in the following way : 
 
 78. In a darkened room send a Mrong 
 beam of light through a glass tube three feet 
 long and three inches wide, stopped at its 
 ends by glass plates. Connect the tube by 
 means of a stopcock with a vessel of about 
 one fourth its capacity, from which the air 
 has been removed by an air-pump. The ex- 
 hausted cylinder of the pump itself will an- 
 s\ver capitally. Pill the glass lube with 
 humid air ; then simply turn on the stopcock 
 which connects it with the exhausted vessel. 
 Having more room the air expands, cold ac- 
 companies the expansion, find, as a conse- 
 quence, a dense and brilliant cloud imme- 
 diately fills the tube. If the expeiiment be 
 made ^for yourself aione, you may see the 
 cloud in ordinary daylight ; indeed, the brisk 
 exhaustion of any receiver filled with humid 
 air is known to produce this condensation. 
 
 79. Other vapors than that of water may 
 be thus precipitated, some of them yielding 
 clouds of intense brilliancy, and displaying 
 iridescences, such as are sometimes, but not 
 frequently, seen in the clouds floating over 
 the Alps. 
 
 8'J. In science, what is true for the small is 
 true for the large. Thus by combining the 
 conditions observed on a large scale in 
 nature we obtain on a small scale the Dhe- 
 nomena of atmospheric clouds. 
 
 8. MOUNTAIN CONDENSERS. 
 
 81. To complete our view of the process of 
 atmospheric precipitation we must take into 
 account the action of mountains. Imagine 
 a south-west wind blowing across the At- 
 lantic towaid Ireland. In its passage it 
 charges itself with aqueous vapor. In the 
 south of Ireland it encounters the mountains 
 of Kerry : the highest of these is Magilli- 
 cuddy's Reeks, mar Killatne}'. Now the 
 lowest stratum of this Atlantic wind is that 
 which is most fully charged with vapor. 
 When it encounters the base of the Kerry 
 mountains it is tilted up and flows bodily 
 over them. Its load of \apor is therefore 
 cairied to a height, it expands on reaching 
 the height, it is chilled in consequence of the 
 expansion, and comes down in copious show- 
 ers of rain. From this, in fact, arises the 
 luxuriant vegetation of Killainey ; to this, 
 indeed, the lakes owe their water supply. 
 The cold crests of the mountains also aid in 
 the work of condensation. 
 
 82. Note the consequence. There is a 
 town called C.diirciveen, to the south-west of 
 Mcigillicudely's Reeks, at which observations 
 of the rainfall have been made, and a good 
 distance further to the north-east, right in 
 the course of the south-west wind, there is 
 another town, called Pottariington, at which 
 observations of rainfall have also been made. 
 But before the wind reaches the latter station 
 it has passed over the mountains of Kerry 
 juid left a great portion of its moisture be- 
 hind it. What is the result? At Cahirci- 
 wen, as shown by Dr. Lloyd, the raini'all 
 amounts to 51) inches in a year, while at 
 Portarlington it is only 21 inches. 
 
 80. Again, you may sometimes descend 
 from the Alps, when the fall of rain and 
 snow is heavy and incessant, into Italy, anil 
 find the sky* over the plains of Lombardy 
 blue and cloudless, the wind at the sam* 
 time blowing over tJie plain toward the Alps. 
 Below the wind is hot enough to keep its 
 vapor in a perfectly transparent state ; but it 
 meets the mountains, is tilted up, expanded; 
 and chilled. The cold of the higher summits 
 also helps the chill. The consequence is that 
 the vapor is precipitated as rain or snow, 
 thus producing bad weather upon the 
 heights, while the plains below, flooded with 
 the same air, enjoy the aspect of the un- 
 clouded summer sun. Clouds blowing from 
 the Alps are also sometimes dissolved over 
 the plains of Lombardy. 
 
 84. In connection with the formation of 
 clouds by mountains, one particularly in- 
 structive effect may be here noticed. You 
 frequently see a streamer of cloud many 
 hundred yards in length drawn out from an 
 Alpine peak. Its steadiness appears perfect, 
 though a strong wind may be blowing at the 
 same time over th3 mountain-head. Why is 
 the cloud not blown away? It is blown 
 away ; its permanence is qnly apparent. At 
 ene end it is incessantly dissolved, at the 
 other end it is incessantly renewed : supply 
 and consumption being thus equalized, the 
 
93 THE FORMS OF WATER 
 
 cloud appears as changeless as tfee mountain duced in calm air, the icy particles build 
 to which it seems to cling. When the red themselves into beautiful stellar shapes, inch 
 sun of the evening shines upon these cloud- star possessing six rays. There is no rievia- 
 Rtreamers they resemble vast torches with tion from this type, though in other respects 
 their flames blown through the air. the appearances of the snow stars are inli- 
 
 9. A RCniTECTUItE OF SttOW. 
 
 8;>. We now resemble persons who have by, who gave numerous drawings of them. 
 climbed a difficult peak, and thereby earned I have observed them in midwinter tilling the 
 the enjoyment of a wide prospect, Having air, and loading the slopes of the Alps. Bi?t 
 made ourselves masters bf the conditions in England they are also to be seen, and no 
 necessary to the production if mountain words of mine could convey so vivid an im- 
 snow, we are able to take a comprehensive pression of their beauty as the annexed cTraw- 
 and intelligent view of the phenomena of ings of a few of them, executed at Green- 
 glaciers. wich by Mr. Glaisher. 
 
 80. A few words are still necessary as to 90. It is worth pausing to think what won- 
 the formation of snow. The molecules and derful woik is going on in the atmosphere 
 atoms of ail substances, when allowed free during the formation and descent of every 
 play, build themselves into definite and, for snow-shower : what building power is 
 the most pait, "beautiful forms called ens- brought into play ! and how imperfect seem 
 tals. Iivm. copper, gold, silver, lead, sulphur, the productions of human minds and hands 
 when melted and permitted to cool gradually, when compared with those formed by Die 
 all show this crystallizing power. The metal blind forces of nature ! 
 
 bismuth shows* it in a particularly stiiking 01. But who ventures to call the forces of 
 manner, and when properly fused and solidi- nature blind? In reality, when we spi-ak 
 iie.'l, self-built crystals of great size and thus we are describing our own condition. 
 beauty are formed of this metal. The blindness is curs ; and what we really 
 
 87. If you dissolve saltpetre in water, and ought to say, and to confess, is that our 
 lhw the solution to evaporate slowly, you powers are absolutely unable to comprehend 
 :nay obtain large crystals, for no portion of cither the origin or the end of the operations 
 the salt is converted into vapor. The water of nature. 
 
 of our atmosphere is fresh, though it is de- $2- But while we thus acknowledge ou 
 lived from the salt sea. Sugar dissolved in limits, there is also reason for wonder at the 
 water, and pei milled to evaporate, yields extent to which science has mastered the 
 crystals of sugar candy. Alum leadily crys- system of nature. From age to age, and 
 tallizes in the same way. Flints dissolved, from generation to generation, fact has bed! 
 as they sometimes are in nature, and permit- added to fact, and law to law, the irue inctb- 
 led to crystallize, yield the prisms and pyra- od and order of the Universe being thereb/ 
 mids of rock crystal. Chalk dissolved *and more and more revealed. In doing this i-ci- 
 crystallized yields Iceland spar. The dia- <nc:e has encountered and overthrown vaiir,us 
 mond is crystallized carbon. All our pre forms of superstition and deceit, of credulity 
 clous stones, the ruby, sapphire, beryl, topaz and imposture. But the world continually 
 emerald, are all examples of this crystallizing produces weak persons and wicked persons; 
 power. and as long as they continue to exist side by 
 
 88. You have heard of the force of gravi- feicje > as th ^7 do in this our day, very deh. 
 tation; and you know that it consists of an ing beliefs will also continue to infest the 
 attraction of every particle of matter for world. 
 
 every other particle. You know that plan- 
 
 ets and moons are held in their orbits by this 10 - ATOMIC POLES. 
 
 attraction. But gravitation is a very simple 93. " What did I mean when, a few nv;, 
 affair compared to the force, or rather forces, ments ago (88), 1 spoke of attracting and re- 
 of crystallization. For litre ihe ultima!- pcllent poles?" Let ine try to answer tl, 5* 
 particles of matter, inr onceivably small as question. You know that astronomers ami 
 they arc, show themselves possessed of at- geographers speak of the earth's poles, an I 
 tractive and rcpelleu poles, by the mulual you have also heard of magnetic poles, tlx.> 
 Action of which the shape and structuie ol poles of a magnet being the points at whi< h 
 the crystal are determined. In the soiid con- the attraction and repulsion of the magm I 
 dition the attracting poles are rigidly locked are as it v/ere concentrated. 
 together ; but if sufficient heat be apolied the 9-J- Every magnet possesses two such 
 b-uid of union is dissolved, and in the slate poles ; and if iron tilings be scattered over * 
 of fusion the poles arc pushed so far asunder magm-t, ear-h particle becomes alo endowed 
 as to be practically out of each other's range, with two poles. Suppose such particles de- 
 The natural tendency of the molecules to v >d of weight and floating in our atmos- 
 bui'ld themselves together is thus neutralized, phere, what must occur when they come 
 
 89. This is the case with water, which as a near <-ac!i other? Manifestly the repellent 
 liquid is to all appearance formless. When poles will retreat from each other, while thy 
 sutliciently cooled the molecules are brought attractive poles will approach and finally lock 
 within the play of the crystallizing force, themselves together. AIM! supposing the 
 and they tbcp -'nnrre themselves in forms of particles, instead of a single pair, to possess 
 ir/'-escribabte i,**. When snow is pio- several pairs of poles arranged at definitj 
 
IK CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 
 FIG. 2. SNOW CRYSTALS. 
 
 points over their surfaces ; you can then pic- 
 ture them, in obedience to their mutual at- 
 tractions and repulsions, building themselves 
 together to form masses of definite shape and 
 structure. 
 
 ( J5. Imagine the molecules of water in 
 cairn cold air to be gifted with poles of this 
 description, which compel the particles to la}' 
 themselves together in a detiaite order, anil 
 you have before your mind's eye the unseen 
 architecture which finally produces the visi-- 
 
 ble and beautiful crystals of tbf r.aow. 
 Thus our first notions and conceptions of 
 poles are obtained from the sight ol' o\ir eyes 
 in looking at the effects of magnetism ; and 
 we then transfer these notions and concep- 
 tions to particles which no eye has ever 
 seen. The power by which we thus picture 
 to ourselves effects beyond the range of the 
 senses is what philosophers call the Imagina- 
 tion, and in the effort of the mind to seize 
 upon the unseen architecture of crystals, we 
 
T,iE FORMS O 
 
 ATER 
 
 have an example of the " scientific use' of 
 this faculty. Without Imagination vye 
 mi<rht have critical power, but not creative 
 oower, in science. 
 
 11. AllCLIITECTUHE OF L.\Ki: ICE. 
 
 OC. We have thus made ourselves acquaint- 
 ed with the beautiful snow-flowera self-con- 
 structeil by the molecules of water in calm 
 cold air. Do the molecules show this archi- 
 tectural power when ordinal y water is 
 frozen ? What, for example, h the structure 
 of the ice over which we skate in winter? 
 Quite as wonderful as the lloweis of the 
 snow. The observation is rare, if not new, 
 but I have seen in water slowly freezing six- 
 rayed ice-stars formed, and floating free on 
 the surface. A six-rayed star, mm cover, b 
 typical of the construction of all our lake ice. 
 It is built up of such forms wonderfully in- 
 terlaced. 
 
 97. Take a slab of lake ice and place ^it in 
 the path of a concentrated sunbeam. Watch 
 the track of the beam through the ice. Part 
 of the beam is stopped, pait of it goes 
 through ; the former produces internal 
 liquefaction, the latter has no cifi-ct what- 
 ever upon the ice. But the liquefaction M 
 not uniformly diffused. From separate spots 
 of the ice little shining points are seen to 
 sparkle forth. Every one of those points is 
 surrounded by a beautiful liquid llower with 
 six petals. 
 
 98. Ice and water are so optically alike that 
 unless the light fall properly upon these 
 tiowers you cannot see them. But what is? 
 the central spot ? A vacuum. Ice swims on 
 water because, bulk for bulk, it is lighter 
 than water ; so that when ice is melted it 
 shrinks in size. Can the liquid flowers then 
 occupy the whole space of the ice melted ! 
 Plainly no. A little empty space is foim-j I 
 with the flowers, and this space, or rather its 
 surface, shines in the sun with the lustre of 
 burnished silver. 
 
 99. In all cases the flowers are formed 
 parallel to the surface of freezing. They are 
 formed when the sun shines upmi the ice of 
 every lake ; sometimes in myriads, and so 
 small as to require a magnify ing-glass to see 
 them. They are always attainable, but then 
 beauty is ofien marred by internal defects of 
 the ice. Even one portion of the same piece 
 of ice may show them exquisitely, while a 
 second portion shows them imperfectly. 
 
 100. Annexed is a very imperfect sketch 
 of these beautiful figures. 
 
 101. Here we have a reversal of the pro 
 cess of crystallization. The searching solar 
 beam is delicate enough to take the molecules 
 down without deranging the order of their 
 architecture. Try the experiment for vour- 
 eelf with a pocket-lens on a sunny day. 
 You will not find the flowers confused ; they 
 till lie parallel to the surface of freezing. In 
 this exquisite w r ay every bit of the u-e over 
 which our skaters glide in AV inter is put to- 
 gether. 
 
 102. I said, in 97, that a portion of the 
 sunbeam was .stopped by, the JQO and lique- 
 
 fied it. AVhat ia this portion? The dark 
 heat of the sun. The great body of the light 
 wavts and even a portion of the d.irk one* 
 pass thiough the ice without losing any of 
 their heating power. When propeily con- 
 centrated on combustible bo-Jies, even after 
 having passed thiough the ice, their burning 
 power becomes manifest. 
 
 103. And the ice itself may be employed 
 to coneenttatu th'-m. With an ice-lens i:> 
 the polar regions Dr. Bcoresby has often con- 
 centrated the sun's rays so as to make then 
 burn won1, fire gunpowder, and melt lead ; 
 thus proving that the limiting power is re- 
 tained by the rays, even after they have 
 passed through so cold a substance. 
 
 104. By rendering the rays of the electric 
 lamp parallel, and then sending them 
 through a lens of ice, we obtain all the 
 effects which Dr. Scoresby obtained with the 
 rays of the sun. 
 
 12. Tnz Gounca OP THE ARVEIKON. ICE 
 PINNACLES, TOWEIIS. AND CHASMS OF THIS 
 GLACIS:! Di:3 Co:3. PASSAGE TO THE 
 
 MONTANVCRT. 
 
 105. Our preparatory studies arc for tho 
 present ended, and thus informed, let us ap- 
 proach the Alps. Through the village of 
 Chamouui, in 'oavoy, a ri^er rushes which U 
 called the Arve. Let us trace this rivjL-r 
 backward from Chamouni. At a little dis- 
 tance from the village the river folks; one 
 of its branches still continues to Le called the 
 Arve, the other is the Arveiron. Following 
 this latter we corne to what is called the 
 " source of the Arveiron" a short hour's 
 walk from Chamouni. Here, as in the case 
 of the Rhone already referred to, you ara 
 fronted by a huge mass of ice, the end of a 
 glacier, and from au arch in the ice tho 
 Arveiron issues. Do not trust the arch ia 
 summer. Its roof falls at intervals with u 
 startling crash, and would infallibly crush 
 any person on whom it might fall. 
 
 106. We must now be observant. Look- 
 ing about us here, we find in front of the ice 
 curious heaps and ridges of debris, which 
 are more or less concentric. These are the 
 terminal moraine* of the glacier. We shall 
 examine them subsequently. 
 
 107. We now turn to the left, and ascend 
 the slope beside the glacier. As we ascend 
 we get a better view, and find that the ice 
 here fills a narrow valley. We come' upon 
 another singular ridge, not of fresh debris 
 like those lower down, but covered in part 
 with trees, and appearing to be literally as 
 "old as the hills." It tells a wonderful 
 tale. We soon satisfy ourselves that the 
 ridge is an ancient moraine, and at once con- 
 clude that the glacier, at some former period 
 of its existence, was vastly larger than it is 
 now. This old moraine stretches right 
 across the main vf.lley, and abuts against the 
 mountains at the opposite side. 
 
 108. Having passed the terminal portion of 
 the glacier, which is covered with stones and 
 rubbish, we find ourselves beside a very 
 wonderful exhibition of ice. The glacier de 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 95 
 
 gcends a steep gorge, and in doing so is riven beautiful pyramid of the Aiguille du Dru 
 and broken in the most extract dinary man- (shown in our frontispiece) ; and to the right 
 ner. Here are towers, and pinnacles, and at the Aiguille des Charrnox, with its sharp 
 fantastic shapes wrought out by the action pinnacles bent as if they were ductile. Look- 
 of the weather, which put one in mind of ing straight up the glacier the view is bound 
 rude sculpture. From deep chasms in the cd by the great crests called La Granao 
 glacier issues a delicate sh:mmer of blue Jorasse, nearly 14,000 feet high. Our object 
 lin-ht. At times we hear a sound like Hum- now is to get into the very heart of the 
 der, which arises either from the falling of a mountains, and to pursua to its origin tne 
 tower of ice or from the tumble of a huge wonderful frozen river which we have just 
 stone into a chasm. Thw * lacier maintains crossed. 
 
 this wild and chactic cbaiaeler for some 114. Starting from the Montanvert with 
 time: and the best iceman would find him- the glacier below in to our left, we soon 
 self defeated in any attempt to get along it. reach some rocks resembling the Mauvais 
 
 109. We reach a place railed i"ne Cbapeau, Pas ; they are called tea Ponts. We cross 
 where, if we wish,, we can have Refreshment them and reacli I 'Annie, where we quit the 
 in a little mountain hut. We then pass the l:md for the ice. We walk up the glacier, 
 MauwiiaPo*, a precipitous rock, on the face but before i caching the promontory called 
 cf which steeps are hewn, and the utiprac- Trfilaporte, we take once more to the moun- 
 tiscd traveller is assisted by a lope. \V<- par- tain-side ; for though the path here has been 
 sue our journey, partly along the mountain- forsaken on account of its danger, for the 
 side, and partly along a ridge of singularly sake of knowledge w; are prepared to incur 
 art.ficial aspect a lateral moraine. We at danger to a reasonable extent. A little gla- 
 lungth face a house perched upon an emi- cier reposes on the slope to our right. Wq 
 nonce at the opposite side of the glacier. may see a huge boulder or two poised on the 
 This is the auberge of the Moritanveit, well CQ d of the glacier, and, if fortunate, also see 
 Known to all visitors to this portion of the the boulder liberated and plunging violently 
 Alps. down the slope. Presence of mind is all that 
 
 110. Here we cross the glacier. I should is necessary to render our safety certain ; but 
 have told you that its lower part, including travellers do not always show presence of 
 the broken portion we have passed, is called mind, and hence the path which formerly 
 the Glacier des Bois ; while the place that led over this slope has been forsaken. The 
 wo are now about to cross is the beginning whole slope is cumbered by masses of rock 
 of the Met de Glace. You feel that this which this little glacier has sent down, 
 term is not quite appropriate, for the glacier These I wished you to see ; by and by they 
 here is much more like a riser of ice 'than a sna11 be fully accounted for. 
 
 sea. The valley which it fills it about half a H5. Above Trelaportc to the right you see 
 mile wide. a most singular cleft in the rocks, in the 
 
 111. The ice maybe riven where we en- middle of which stands an isolated pillar, 
 ter upon it, but with the necessarv care there hewn out by the weather. Our next object 
 is no difficulty in crossing this portion of the is to get to the tower of rock to the left of 
 Mer de Glace. The clefts and chasms in the tn ^t cleft, for from that position we shall 
 ire aie called crevasses; we shall make their gain a must commanding and instructive 
 acquaintance on a grander scale by and by. view of the Mer de Glace and its sources. 
 
 li'2. Look up and down this bide of the 116.. The cleft referred to, with its pillar* 
 glacier. It is considerably riven, but as we maybe seen to the right of the above engrav- 
 advance the crevasses will diminish, and we ing of the Mer de Glace. Below the cleft 
 *h\\ find ven* few of them at the other side, is also seen the little glacier just referred 
 Note this for' future use. The ice is at first to. 
 
 diily ; but the dirt soon disappears, and you 117. We may reach this cleft by a steep 
 come upon the clean crisp suiface of the gla- gully, visible from our present position, and 
 cier. You have already noticed that Ihe leading directly up to the cleft. But these 
 clean ice is while, and thai fiom a distance gullies, or couloirs, are very dangerous, be- 
 it resembles snow rather tlu.n ice. This is ing the path ways,of stones falling from the 
 caused by the breaking up of the surface by heights. We will therefore take the rocks to 
 the solar heat. When you pound trans- the left of the gully, by close inspection as- 
 parent rock-salt into powder it. is as white certain their assailabl'3 points, and there at- 
 as table-salt, and it is the minute fissuring tack them. In the Alps as elsewhere won- 
 </f the surface of the glacier by the sun's rays derful things may bo dona by looking stead- 
 that causes it to appear white. Within the fastly at difficulties, and testing them wher- 
 glacier the ice is transparent. After an ex- ever they appear assailable. We thus reach 
 hilarating passage we get upon the opposite our station, where the glory of tlie prospect, 
 lateral moraine, and ascend the steep slope and the insight that we gain as to the forma- 
 from it to the Montanvert Inn. tion of the Mer de Gl ice. far more than re- 
 
 13. THE MER DE GLACE AND ITS pay us for thy labor of our ascent. 
 
 SOURCES. OUR FIRST CLIMB TO THE I 18 - For we see, the glacier below us, 
 
 CLEFT STATION stretching its frozen tongue elownward past 
 
 the Montanvert. And we now rind this sin- 
 
 113. Here the view before us is very le glacier branching out into three others, 
 grand. We look across the glacier at the som e of them wider than itself. Regard the 
 
THE FORMS OF WATEE 
 
 V- iin.'h to the light, the Glacier du Ge:-mt. 
 It. stretches smoothly up for a long distance, 
 I hen becomes disturbed, and then changes to 
 a great frozen cascade, down which the ice 
 appears to tumble in wild confusion. Above 
 the cascade you see an expanse of shinmg 
 snow, occupying an area of some square 
 miles. 
 
 14. ICE-CASCADE AND SNOWS OP THE 
 
 COL DU GEANT. 
 
 119. Instead of climbing to tho height where 
 we now stand, we might have continued our 
 walk upon the Mer do Gluvc turned round 
 
 the promontory of Teilap^itc, and walked 
 right up the Glacier du Geant. We should 
 have found ice under our feet up to the bottom 
 of the cascade. It is not so compact as the 
 ice lower down, but you would not think of 
 icf using to call it ice. 
 
 120. A we approach the fall, the smooth 
 and unbroken character of the glacier 
 changes more and more. We encounter 
 transverse ridges succeeding each other with 
 augmenting steepness. The ice becomes 
 more and more fissured and confused. We 
 wind through tortuous ravines, climb huge 
 ice-mounds, and creep cautiously along 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 97 
 
 crumbling crests, with crevasses right and 
 left. The confusion increases until further 
 advance along the centre of the glacier is im- 
 possible. 
 
 121. But with the aid of an axe to cut 
 steps in the steeper ice- walls and slopes we 
 might, by swerving to cither side of the 
 glacier, work our way to the top of tne cas- 
 cade. If we ascended to the right, we should 
 have to take care of the ice avalanches which 
 sometimes thunder down the slopes ; if to the 
 left, we should have to take care of the stones 
 let loose from the Aiguille Noire. After we 
 had cleared the cascade, we should have to 
 beware for a time of the crevasses, which for 
 some distance above the fall yawn terribly. 
 But by caution we could get round them, and 
 sometimes cross them by bridges of snow. 
 Here the skill and knowledge to be acquired 
 only by long practice come into play ; and 
 here also the use of the Alpine rope suggests 
 itself. For not only are the snow-bridges 
 often frail, but whole crevasses are some- 
 times covered, the unhappy traveller being 
 first made aware of their existence by the 
 snow breaking under his feet. Many lives 
 have thus been lost, and some quite re- 
 cently. 
 
 122. Once upon the plateau above the ice- 
 fall we find the surface totally changed. Be- 
 low the fall we walked upon ice ; here we are 
 upon snow. After a gentle but long ascent 
 we reach a depression of (he ridge which 
 hounds the snow-fild at the top, and now 
 iook over Italy. We stand upon the famous 
 Col du Geant. 
 
 123. They were no idle soimpcrcrs on the 
 mountains that made these wild recesses first 
 Known ; it was not the desire for health 
 which now brings some, or the desire for 
 grandeur and beauty which brings others, or 
 the wish to be able to say that they have 
 climbed a mountain or crossed a col, which 
 I ft'ar brings a good many more ; it was a 
 desire for knowledge that brought the first ex- 
 plorers here, and on this col the celebrated 
 De Saussurc lived for seventeen days, mak- 
 ing scientific observations. 
 
 15. QUESTIONING THE GLACIERS. 
 
 124. I would now ask you to consider for 
 a moment the facts which such an excur- 
 sion places in our possession. The snow 
 through which we have in idea trudged is 
 iihe snow of last winter and spring. Had we 
 placed last August a proper mark upon the 
 surface of the snow, we should find it this 
 August at a certain depth beneath the sur- 
 fMce. A good deal has been melted by the 
 summer sun, but a good dual of it remains, 
 and it will continue until the snows of the 
 coming winter fall and cover it. This again 
 will be in part preserved till next August, a 
 good deal of it remaining until it is covered 
 by the snow of the subsequent winter. We 
 thus arrive at the certain conclusion that on 
 the plateau of the Col du Geant the quantity 
 of snow that falls annually e.vcmt* the, quan- 
 tity melted. 
 
 125. Had we conic in the month of April 
 
 or May, we should have found the glacier be- 
 low the ice fall also covered with snow, 
 which is now entirely cleared away by the 
 heat of summer. Nay, more, the ice there 
 is obviously melting, forming running brooks 
 w T hich cut channels in the ice, and expand 
 here and there into small blue-green lakes. 
 Hence you conclude with certainty that be- 
 low the ice-fall the quantify of frozen material 
 failing upon the (/lacier is leas than the quan- 
 tity melted. 
 
 126. And this forces upon us another con- 
 clusion ; between the glacier below the ice- 
 fall and the plateau above it there must exist 
 a line where the quantity of snow which falls 
 is exactly equal to the quantity annually 
 melted. This is the snow-line. On some 
 glaciers it is quite distinct, and it woidd be 
 distinct here were the ice less broken and 
 confused than it actually is. 
 
 127. The French term neve is applied to 
 the glacial region above the snow-line, while 
 the word glacier is restricted to the ice below 
 it. Thus the SUDWS of the Col du Gant 
 constitute the nev6 of the Glacier du Geant, 
 and in part, the neve of the Mer de Glace. 
 
 128. But if every year thus leaves a resi- 
 due of snow upon the plateau of the Col du 
 Geant, it necessarily follows that the plateau 
 must get annually higher, provided the snow 
 remain upon it. Equally certain is the con- 
 clusion that the whole length of the glacier 
 below the cascade must sink gradually lowei , 
 if the waste of annual melting be not made- 
 good. Supposing two feet of snow a year to 
 remain upon the Col, this would raise it to & 
 height far surpassing that of Mont Blanc in 
 five thousand years. Such accumulation, 
 must take place if the snow remain upon the 
 Col ; but the accumulation does not take 
 place, hence the snow does not remain on 
 the Col. The question then is, Whither does 
 it go? 
 
 16. BRANCHES AND MEDIAL MORAINES 
 
 OF THE MER DE GLACE FROM THE CLEFT 
 
 STATION. 
 
 1.29. We shall grapple with this question 
 immediately. Meanwhile look at that ice- 
 valley in fiont of us, stretching up between 
 Mont Tacul and the Aiguille de Lechaud, 
 to the base of the great ridge called the 
 Grande Jorasse. This is called the Glacier 
 de Lechaud. It receives t*t its head the 
 snows of the Jorasse and of Mont Mallet, 
 and joins the Glacier du Geunt at the prom- 
 ontory of the Tacul. The glacieis seem 
 welded together where they join, but they 
 continue distinct. Between them you clearly 
 trace a stripe of debris (c on the annexed 
 sketch-plan) ; you trace a similar though 
 smaller stripe (a on the sketch) from the 
 junction of the Glacier du Gaunt with the 
 Glacur des Periades at the foot of the Ai- 
 guille Noire, which you also follow along 
 the Mer de Glace. 
 
 130. We also see another glacier, or a portion 
 of it, to the left, falling apparently in broken 
 fragments through a narrow gorge (Cascade 
 dif Talefre on the sketch) and joining the 
 
98 
 
 THE FORMS OF WATER 
 
 Fia. 5. SKETCH-PLAN, SHOWING THE MORAINES a, b, c, d, o, or THE MER DE GI.ACE. 
 
 Lechaud. and from their point of junction 
 also a, stripe of d6bris (d) runs downward 
 along the Mer de Glace. Beyond this again 
 we notice another stripe (e), which seems to 
 begin at the bottom of the ice-fall, rising as 
 it were from the bodv of the glacier. Beyond 
 all of these \ve can notice the* lateral moraine 
 of the Mer de Glace. 
 
 131. These stripes are the medial moraines 
 of the Mer de Glace. We shall learn more 
 about them immediately. 
 
 l':J2. And now, having informed our 
 minds by these observations, let our eyes 
 'wander over the whole glorious scene, the 
 spHn lured peaks and the hacked end jagged 
 crests, the far-stretching snow-fields, Ihe 
 smaller glaciers which nestle on the heights, 
 the deep blue heaven and the sailing clouds. 
 T-* it not worth some labor to gain command 
 of suHi a scene ? But the delight it imparts 
 is heightened by the fact that we did not 
 come expressly to see it ; we came to instruct 
 ourselves about the glacier, and this high en- 
 1 >yment is an incident of our labor. You 
 will find it thus through life ; without hon- 
 est labor there can be no deep joy. 
 $ 17. THE TALEFRE AND THE JARDIN. 
 
 WOUK AMONG THE CBEVASSES. 
 
 1P>.>. And now let us descend to the Mer 
 de Glace, for I want to take you across the 
 Clacitr to that broken i-e-fall, the origin of 
 
 which we have not yet seen. We aim at the 
 farther side of the glacier, and to reach it we 
 must cross those dark stripes of debris which 
 we observed from the heights. Looked at 
 from above, these moraines seemed flat, but 
 now we find them to be ridges of stones and 
 rubbish, from twenty to thirty feet high. 
 
 134. We quit the ice at a place called the 
 Couvercle, and wind round this promontory, 
 ascending all the time. We squeeze ourselves 
 through the Ecjralets, a kind of natural stair- 
 case in the rock, and soon afterward obtain 
 a full view of the ice-fall, the origin of which 
 we wish to find. The ice upon the fall is 
 much broken ; we have pinnacles and towers, 
 some erect, some leaning, and some, if we 
 are fortunate, falling like those upon the 
 Glacier des Bois ; o nd we have chasms f torn 
 which issues a delicate blue light. With ihe 
 ice-fall to our right we continue to ascend, 
 until at length we command a view of a 
 huge glacier basin, almost level, and on the 
 middle of which stands a solitary island, en- 
 tirely surrounded by ice. We stand at the 
 edge of the lacier du Talefre, and connect 
 it with the ice-fall we have passed. The 
 glacier is bounded by rocky ridges, hacked 
 and torn at the top into teeth and edges, and 
 buttressed by snow fluted by the descending 
 stones. 
 
 135. We cross the basin to the central 
 island, and find grass and floweis at tii 
 
IN CLOUDS AJSTD RIVERS, ICE AND GLACIERS. 
 
 place where we enter upon it. This is the 
 celebrated Jardin, of which you have often 
 bear!. The upper part of the Jardin is bare 
 rock. Close at hand is one of the noblest 
 prvik.s in this portion of the Alps, the Ai- 
 guille Vorie. It is between thirteen and 
 fourteen thousand feet high, and down its 
 sides, after freshly- fallen snow, avalanches 
 incessantly thunder. From one of its pro- 
 jf-ctions a *tieak of moraine starts down the 
 Talc f re ; from the Jardin also a similar streak 
 of moraine issues. Both continue side by side 
 to the top of the ice-fall, where they are en- 
 gulfed in the chasms. But at the bottom of 
 the fall they reappear, as if newly emerging 
 from the body of the glacier, and afterward 
 they continue along tin-; Mer de Glace. 
 
 13G. Walk with me now alongside the mo- 
 raine from the Jardin down toward the ice- 
 fall. For a time our work is easy, such fis- 
 sures as appear offeiing no impediment to 
 our march. But the crevasses become grad- 
 ually wider and wilder, following each other 
 at length so rapidly as to leave merely walls 
 of ice between them. Here perfect steadi- 
 ness of foot is necessary a slip would be 
 death. We look toward the fall, and ob- 
 serve the confusion of walls and blocks and 
 chasms below us increasing. At length pru- 
 dence and reason, cry "Halt!" We may 
 swerve to ihe light or to the left, and mak- 
 ing our way along crests of ice, with chasms 
 on both hands, reach either the right lateral 
 moraine or the left lateial moraine of the 
 glacier. 
 
 18. FIRST QUESTIONS REGARDING GLA- 
 CIER MOTION. DUIFTISG OF BODIES 
 BURIED ix A CREVASSE. 
 
 137. But what arc these lateral moraines? 
 As you and I go from day to day along the 
 efaeiers, their origin is gradually made plain. 
 We see at intervals the stones and rubbish 
 descending from the mountain-sides and ar- 
 rested by the ice. All along the fringe of 
 the glacier the stones and rubbish fall, and it 
 soon becomes evident that we have here the 
 source of the lateral moraines. 
 
 138. But how are the medial moraines to 
 be accounted for? How does the debris 
 range itself upon the glacier in stripes some 
 hundreds of yards from its edge, leaving the 
 space between them and the edge clear of 
 rubbish ? Some have supposed the stones to 
 have rolled over the glacier from the sides, 
 but the supposition will not bear examina- 
 tion. Call to mind now our reasoning re- 
 garding the excess of snow which falls above 
 the snow-line, and our subsequent question, 
 How is the snow disposed of ? Can it be that 
 the entire mass is moving slowly down- 
 ward ? If so, the lateral moraines would be 
 carried along by the ice on which they rest, 
 and when two branch glaciers unite they 
 would lay their adjacent lateral moraines to- 
 gether to* form a medial moraine upon tha 
 trunk glacier. 
 
 139. There is, in fact, no way that we c:in 
 *ee of disposi.ig of the excess of snow above 
 the snow-line ; there is no way of making 
 
 good the constant waste of the ice below the 
 snow-line ; there is no way of accounting for 
 the medial moraines of the glacier, but by 
 supposing that from the highest snow-fields 
 of the Col -In G6ant, the Lechaud, and the 
 Talefre, to the extreme end of the Glacier des 
 Bois, the whob mass of frozen matter i.s 
 moving downward. 
 
 140. If you were older, it wouM give me 
 Treasure to take you up Mont Blanc. Stait- 
 Jig from Chamouai, we shoukl first pass 
 through woods au I pastures, then up tha 
 steep hill-face with tlie Glacier des Bossons 
 to our right, t.-> a rock known as the Pierre 
 Pointae ; thence to a higher rock called th 
 Pierre Vfichelle, because here a- la-Uer is 
 usually placed to assist iu crossing the chasms 
 of the glacier. At the Pierre ]'Echlle we- 
 should strike tin ice, and passing under the 
 Aiguille du Mi li, which towers to the Icjft, 
 and wh'ch sonrrJmes sweeps a portion of 
 the track with stone avalanches, we should 
 cross the Glacier des Bossons ; amid heaped- 
 up mounds and broken towers of ice ; up 
 steep slopes ; over chasms so deep that their 
 bottoms are hid in darkness. 
 
 141. \Ve reach the rocks of the Grands 
 Mulets, which form a kind of barren islet in 
 the icy sea ; thence to the higher-snow -fields, 
 crossing the Petit Plateau, which we should 
 find cumbered by blocks of ice. Looking to 
 the right, we should see whence they came, 
 for rising here with threatening aspect high 
 above us arc the broken ice-crags of the 
 Dome du Goute. The guides wish to pass this 
 place in silence, and it is just as well to hu- 
 mor them, however much you may doubt 
 the competence of the human voice to bring 
 the ice-crags down. From the Petit Plateau 
 a steep snow-slope would carry us to the 
 Grand Plateau, and at day-dawn I know 
 nothing in the whole Alps more grand and 
 solemn than this place. 
 
 143. One object of our ascent would be 
 now attained ; for here at the heail of the 
 Grand Plateau, and at the foot of the final 
 slope of Mont Blanc, I should show you a 
 great crevasse, into which three guides were 
 poured by an avalanche in the ytar 1820. 
 
 143. Is this language correct ? A crevasse 
 hardly to be distinguished from the present 
 one undoubtedly existed here in 1820. But 
 was it the identical crevasse now existing? 
 Is the ice riven here to-day the same as that 
 riven fifty-one years ago? By no means. 
 How is this proved ? By the fact that more 
 than forty years after their interment, the 
 remains of those three guides were found 
 near the end of the Glacier des Bossons, 
 many miles below the existing crevasse. 
 
 144. The same observation proves to dem 
 onstration that it is the ice near the bottom of 
 the higher n6ve that becomes the surface-ice 
 
 of the glacier near its end. The waste of the 
 surface below the snow-line brings the deeper 
 portions of the ice more and more to the 
 light of day. 
 
 145. There are numerous obvious indica- 
 tions of the existence of glacier motion, 
 though it is too sl&w to catch the eyo at, 
 
100 
 
 THE FORMS OF WATER 
 
 once. The crevasses change within certain 
 ]imits from year to year, and sometimes from 
 month to month ; and this could not be if 
 the ice did not move. Rocks and stones also 
 are observed, which have been plainly torn 
 from the mountain-sides. Blocks seen to 
 fall from particular points are afterward ob- 
 served lower down. On the moraines rocks 
 are found of u totally different mineralogical 
 character from those composing the moun- 
 tains right and left ; and in all such cases 
 strata of the same character are found bor- 
 dering the glacier higher up. Hence the 
 conclusion that the foreign boulders have 
 been floated d ,wn by the ice. Further, the 
 ends or " snouts" of many glaciers act like 
 ploughshares on the land in front of them, 
 overturning with slow but merciless energy 
 huts and chalets that stand in their way. 
 Facts like these have been long known to the 
 inhabitants of the High Alps, who were thus 
 made acquainted in a vague and general way 
 with the motion of the glaciers. 
 
 ID. THE MOTION OF GLACIERS. MEASURE- 
 MENTS BY HUGI AND AQASSIZ. DRIFTING 
 OF HUTS ON THE ICE. 
 140. But the growth of knowledge is from 
 vagueness toward precision, and "exact de- 
 terminations of the rate of glacier motion 
 were soon desired. With reference to such 
 measurements, one glacier in the Bernese 
 Oberlaud will remain forever memorable. 
 From the little town of Meyringen in Swit- 
 zerland you proceed up the valley of Hasli, 
 past the celebrated waterfall of Handeck, 
 where the river Aar plunges into a chasm 
 more than 200 feet deep. You approach the 
 Grimsel Pass, but instead of crossing it you 
 turn to the right and follow the course of "the 
 Aar upward. Like the Rhone and the 
 Arveiron, you find the Aar issuing from a 
 glacier. 
 
 147. Get upon the ice, or rather upon the 
 deep moraine shingle which covers the ice, 
 and walk upward. It is hard walking, but 
 af'.er some time you get clear of the rubbish, 
 and on to a wide glacier with a great medial 
 moraine running along its back. This mo- 
 raine is formed by the junction of two branch 
 glaciers, the Lauteraar and the Finsteraar, 
 which unite at a promontory called the Ab- 
 Rchwung to form the trunk glacier of the 
 Unteraar. 
 
 148. On this great medial moraine in 1827 
 an intrepid and enthusiastic Swiss professor, 
 Ilugi, or Solothurm (French Soleure), built 
 a hut with a view to observations upon the 
 glacier. His hut moved, and he measured 
 its motion. In the three years from 1827 
 l:> 1830 it had moved 330 feet downward. 
 In 1833 it had moved 2354 feet ; and in 1841 
 M. Agassiz found it 4712 feet below its first 
 position. 
 
 140. In 1840, M. Agassiz himself and some 
 bold companions took shelter under a great 
 overhanging slab of rock on the same mo- 
 raine, to which they added side-walls and 
 other means of protection. And because he 
 wad his comrades came from Neufchatel, the 
 
 hut was called long afterward the " Hotel 
 des Neuch&telois." Two years subsequent 
 to its erection M. Agassiz found that tke 
 " hotel " had moved 486 feet downward. 
 
 20. PRECISE MEASUREMENTS OF AGASSIZ 
 AND FORBES. MOTION OF A GLACIER 
 PROVED TO RESEMBLE THE MOTION OF A 
 RIVER. 
 
 150. We now approach an epoch in the 
 scientific history of glaciers. Had the first 
 observers been practically acquainted with 
 the instruments of precision used in survey- 
 ing, accurate measurements of the motion of 
 glaciers would probably have been earlier 
 executed. We are now on the point of see- 
 ing such instruments introduced almost si- 
 multaneously by M. Agassiz on the glacier 
 of the Unteraar, and by Professor Forbes on 
 the Mer de Glace. Attempts had been made 
 by M. Escher de la Linth to determine the 
 motion of a series of wooden stakes driven 
 into the Ak-tsch glacier, but the melting 
 was so rapid that the stakes soon fell. To 
 remedy this, M. Agassiz in 1841 undertook 
 the great labor of currying boring tools to 
 his " hotel," &nd piercing the Unteraar 
 glacier at six different places to a depth of 
 ten feet, in a straight line across the glacier. 
 Into the holes six piles were so firmly driven 
 that they remained in the glacier for a year, 
 ami in 1842 the displacements of all six were 
 determined. They were found to be 160 
 feet, 225 feet, 209" feet, 245 feet, 210 feet, 
 and 125 feet, respectively. 
 
 151. A great step is here gained You 
 notice that the middle numbers ar^ the larg- 
 est. Tiiey correspond to the central portion 
 of the glacier. Hence, these measurements 
 conclusively establish, not only the fact of 
 glacier motion, but that tJie centre of the 
 glacier, like that of a river,- moves more rapiily 
 than the xidea. 
 
 152. With the aid of trained engineers M. 
 Agassiz followed up these measurements in 
 subsequent years. His researches are record- 
 ed iu a work entitled " Systeme glaciaire," 
 which is accompanied by a very noble atlas 
 of the Glacier of the Unteraar, published in 
 1847." 
 
 153. These determinations were made by 
 means of a theodolite, of which I will give 
 you some notion feanediately. The same 
 instrument was employed the same year by 
 the late Principal Forbes upon the Mer de 
 Glace. He established independently the 
 greater central motion. He showed, more- 
 over, that it is not necessary to wait a year, 
 or even a week to determine the motion of a 
 glacier ; with a correctly-adjusted theodolite 
 he was able to determine the motion of vari- 
 ous points of the Mer de Glace from day to 
 day. He affirmed, and with truth, that the 
 motion of the glacier might be determined 
 from hour to hour. We shall prove this 
 farther on (162). Professor Forbes also tri- 
 angulated the Mer de Glace, and laid down 
 an excellent map of it. His first observa- 
 tions and his survey are recorded in a cele- 
 brated book published in 1843, and entitled 
 
y CLOUDS AND RIVERS, ICE AND G& 
 
 "Travels in tho Alps." 
 
 154. These observations were also followed 
 up in subsequent years, the results being re- 
 corded in a seties of detached letters and es- 
 says of great interest. These were subse- 
 quently collected in a volume entitled " Oc- 
 casional Papers on the Theory of Glaciers," 
 published in 1859. The labors of Agassiz 
 and Forbes are the two chief sources of our 
 knowledge of glacier phenomena. 
 
 21. THE THEODOLITE AND ITS USE. Oui 
 N MEASUREMENTS. 
 
 155. My object thus far is attained. i 
 have given j r ou proofs of glacier motion, and 
 a historic account of its measurement. And 
 now we must try to add a little to the knowl- 
 edge of glaciers by our own hUws on the 
 ice. Resolution must not be wanting at the 
 commencement of our work, ^or steadfast 
 patience during its prosecution. Look then 
 ac this theodolite ; it consists mainly of a 
 telescope and a graduated circle, the tele- 
 scope capable of motion up an( i down, and 
 the circle, carrying the telescope along with 
 it, capable of motion right and left. When 
 desired to make the motion exceedingly fine 
 an'-l minute, suitable screws, called tangent 
 screws, are employed. The instrument is 
 supported by three legs, movable, but firm 
 when properly planted. 
 
 150. Two spirit-levels are fixed at right 
 angles to each other on the circle just refei- 
 red to. Practice enables one to lake hold of 
 the legs of the instrument, and so to fix them 
 that tbe circle shall be nearly horizontal. By 
 means of four levelling screws we render it 
 accurately horizontal. Exactly under the 
 centre of the instrument is a small hook from 
 which a plummet is suspended ; the point of 
 the bob just touches a rock on which we 
 make a mark ; or if the earth be soft under- 
 neath, we drive a stake into it exactly under 
 the plummet. By re-suspending the plum- 
 met at any future time we can find to a hat"- 
 breadth the position occupied by the instru- 
 ment to-day. 
 
 157. Look through the telescope ; you see 
 it crossed by two fibres of the finest spider's 
 (bread. In actual work we first direct the 
 telescope across the glacier, until the inter- 
 section of the two fibres accurately covers 
 some well-defined pcint of rock or tree at the 
 other side of the valley. This, our fixed 
 standard, we sketch with its surroundings in 
 ft note-book, so as to be able immediately to 
 recognize it oa our return to this place. Irn- 
 iginc a straight line drawn from the centre 
 of the telescope to this point, and that this 
 line in permitted to drop straight down upon 
 the glacier, every point of it falling as a 
 stone would fall ; along such a line we have 
 now to fix a series of stakes. 
 
 158. A trained assistant is already upon 
 the glacier. He erects his staff and stands 
 behind it ; the telescope is lowered without 
 swerving to the right or to the left ; in mathe- 
 matical language it remains in tlie xame zerti- 
 "Mi plane. " The crossed fibres of the tele- 
 scope probably strike the ice a little away 
 
 from the staff of the assistant ; by a wave of 
 the arm he moves right or left ; he may 
 move too much, so we wave him back again. 
 After a trial or two be knows whether he is 
 near the proper point, and if so makes his 
 motions small. He soon exactly strikes the 
 point covered by the intersection of the 
 fibres. A signal is made which tells him 
 that he is right ; he pierces the ice with an 
 auger and drives in a stake. He then goes 
 forward, and in precisely the same manner 
 takes up another point. After one or two 
 stakes have been driven in, the assistant is 
 able to take up the other points very rap- 
 idly. Any requisite number of stakes may 
 thus be fixed in a straight line across tht> 
 glacier. 
 
 159. Next morning we measure the motion 
 of all the stakes. The theodolite is mounted 
 in its former position and carefully levelled 
 The telescope is directed first upon the 
 standard point at the opposite side of the 
 valley, being moved by a tangent screw until 
 the intersection of the spider's threads accu- 
 rately covers the point. The telescope is 
 then lowered to the first stake, beside which 
 our trained* assistant is already standing. 
 He is provided with a staff with feet and 
 inches marked on it. A glance shows us 
 that the stake has moved down. By our sig- 
 nals the assistant recovers the point from 
 which we started yesterday, and then deter- 
 mines the distance from this point to the 
 stake. It is, say, inches ; through this dis- 
 tance, therefore, the stake has moved. 
 
 160. We are careful to note the hour anfl 
 minute at which each stake is driven in, and 
 the hour and the minute when its distance 
 from its first position is measured ; this ena- 
 bles us to calculate the accurate daily motion 
 of the point in question. The distances 
 through which all the other points have 
 moved are determined in precisely the same 
 way. 
 
 161. Thus we shall proceed to work, first 
 making clear to our minds what is to be 
 done, and then making sure that it shall be 
 accurately done. To give our work reality, 
 I will here record the actual measurements 
 executed, and the actual thought suggested, 
 on the Mer de Glace in 1857. The only un- 
 reality that I would ask you to allow, Is that 
 you and I are supposed to be making the ob- 
 servations together. The labor of measuring 
 was undertaken for the most part by Mr. 
 Hirst. 
 
 22. MOTION OP THE MER DE GLACE. 
 
 162. On July 14, then, we find ourselves 
 at the end of the Glacier des Bois, not far 
 from the source of the Arveiron. We direct 
 our telescope across the glacier, and fix the 
 intersection of its spider's threads accurately 
 upon the edge of a pinnacle of ice. We 
 leave the instrument untouched, looking 
 through it from hour to hour. The edge of 
 ice moves slowly, but plainly, past the fibres, 
 and at the end of three hours we assure our- 
 selves that the motion has amounted to sev- 
 eral inches. While standing near the vault 
 
THE FORMS OF WATER 
 
 Grancle Jorasso. 
 
 Col da 
 Geant. 
 
 Chapeau. 
 
 FlG. 6. OUTLINE-Pl^AN, SHOWING THE MEASURED LINES OB 1 THE Ml!U I)!! GLACE AND ITS TmBlTTARIBa. 
 
 of the Arvoiron, and talking about going 
 into it, its roof gives way and falls with the 
 sound of thunder. It is not. therefore, with- 
 out reason that I warned you against enter- 
 log these vaults in summer. 
 
 163. We ascend to the Montanvert Inn, 
 fix on it as a residence, and then descend to 
 the lateral moraine of the glacier a little be- 
 low the inn. Here we erect our theodolite, 
 and mark its exact position by a plummet. 
 We must first make sure that our line is per- 
 pendicular, or nearly so, to the axis or mid' 
 die line of the glacier. Our instructed assist- 
 ant lays down a long staff in the direction 
 of the axis, assuring himself, by looking up 
 and down, that it is the true direction. 
 With another staff in his hand, pointed 
 
 toward our theodolite, he shifts his position 
 until the second staff is perpendicular to the 
 first. Here he gives us a signal. We direct 
 our telescope upon him, and then gradually- 
 raising its end in a vertical plane we find, 
 and note by sketching, a standard point at the 
 other side of the glacier. This point known, 
 and our plummet mark known, we can on 
 any future day find our line. (To render the 
 measurements more intelligible, 1 append an 
 outline diagram of the Mer do Glace, and of 
 its tributaries.) 
 
 164. Along the line just described ten 
 stakes were set on July I'.th, 1857. Their dis- 
 placements were measured on the following 
 day. Two of them had fallen, but here are 
 the distances passed over by the eight re- 
 
IN CLOUDo AND RIVERS, ICE AND GLACIERS. 
 
 103 
 
 that the glacier is retarded not only by its 
 sides but by its bed ; that the upper portions 
 of the ice slide over the lower ones. Now if 
 Ihe bed of the Mar de Glace should have emi- 
 nences here aud there rising sufficiently near 
 to the surface to retard tile motion of the 
 
 malning ones in twenty-four hours. 
 DAILY MOTION OF THE MER DE GLACE. 
 FIRST LINE : A A' UPON TUB SKETCH. 
 Eist West 
 
 Stake 1 2 3 4 5 7 9 10 
 
 Inches 12 17 23 26 25 26 27 33 
 
 165. You have already assured yourself surface, they might produce the small irregu- 
 by actual contact that the body of the glacier larities noticed above. 
 
 is real ice, and you may have read that 169. We note particularly, wliib upon t!ie 
 glaciers move : but the actual observation of ice, that the 26th stake, like the 10th stake 
 the motion of a body apparently so rigid is in our last line, stands much nearer to the 
 strangely interesting. And not only does the eastern than to the western side of the 
 fee move bodily, but one part of it moves glacier; the ni-.MSiiremni s, therefore, off or 
 past another ; the rate of motion augmenting a further proof that the centre of this portion 
 gradually from 12 inches a day at the side to of the glacier is nol the place of swiftest mo- 
 33 inches a day at a distance from the side. ' 
 This quicker movement of the central ice of 
 glaciers had been already observed by Agas- 
 siz and Forbes ; we verify their res-tilts, and 
 now proceed to something new. Crossing 
 the" Glacier du Geant, which occupies more 
 than half the valley, we find that our line of 
 stak?s is not yet at an end. The 10th stake 
 stands on the part of the ice which comes 
 
 tion. 
 
 23. UNEQUAL MOTION OF THE TWO SIDES 
 OF THE ME u DE GLACE. 
 
 170. But in neither the first line nor the 
 second were we able to push our measure- 
 across the glacier. Why? In 
 to do one thing we are often 
 taught another, and thus in science, if ,ve 
 are only steadfast in our wort?, our very de- 
 feats are converted into means of instruc- 
 tion. We at first planted our theodolite on 
 the lateral moraine of the Mer de Glace, ex- 
 
 friction of the sides is least, the motion ought - $ **J^ nowtot- 
 
 from the Talrfre. 
 
 106. Now the motion of the sides is slow, 
 because of the friction of the ice against its 
 boundaries ; but then one would think that 
 midway between the boundaries, where the 
 
 VUSSi. Si? SSftS"Mffi =; U,e eentS ^ ScEp, to U= 
 
 ft5S^^^^.gSrS^tf^g 
 
 inches a da" 81te S1 ? e * lll(i gl acier was intercepted by the 
 
 167. Here we have something to think of ; elevatioa ^ " c < re - T " e 
 but before a natural philosopher can think 
 with comfort he must be perfectly sure of 
 his facts. The foregoing line ran across the 
 
 glacier a little below 
 Will 
 
 the Montanvert. We 
 
 and to multiply our chances of discovery we 
 
 place along it 31 stakes. On the subsequent which we sweep the 
 
 dav five of these were found unfit for IISP 
 
 at the centre. The mountain- 
 slopes, in fact, are warm in summer, ami 
 they melt the ice nearest to them, thus caus- 
 ing a fall from the centre to the sides. 
 
 171. But yonder on the heights at the 
 other side of the glacier we see a likely place 
 for our theodolite. We cross the glacier and 
 plant our instrument in a position from 
 glacier from side to 
 
 side. Our first fine was below the Montan- 
 vert, our second line above it ; this third line 
 is exactly opposite the Montanvert ; in fact, 
 the mark on which we have fixed the fibre- 
 cross of the theodolite is a corner of one of 
 the windows of the little mn. Along this 
 line we fix twelve stakes on July 20th. On the 
 21st one of them had fallen : but the vi-loci 
 ties of the remaining eleven in 24 hours were 
 found to be as follows : 
 
 THIRD LINE : C C' UPON TUB SKETCH. 
 
 East We*t 
 
 Stake, 1 23456789 10 11 
 
 luches 20 23 29 30 34 28 25 23 25 18 9 
 
 172. Both the first stake and the eleventh 
 
 18th ; from 23 inches at the 19th we fall to in this series stor)d near the si(ics cf tbe , a _ 
 from 2o inches at the ----- ^ - - i ., .. . e 
 
 remaining six-and-tvventy in 24 hours. 
 B B' UPON THE SKETCH. 
 
 SECOND LINE 
 West 
 
 3 4 5 
 12 15 15 
 16 17 18 
 23 23 21 
 
 Stake... 2 
 Inches.. 11 
 b take... lo 
 Iaches..23 
 
 East 
 
 168. Look at these numbers. The first 
 broad fact Ihev reveal is the advance in the 
 rate of motion from first to last. There are, 
 however, small irregularities ; from 2'3 inches 
 at the 17th stake we fall to 21 inches at the 
 
 21 inches at the 20th 
 
 21st we fall to 22 inches at the 22d and 
 
 23d ; but notwithstanding these small ups 
 
 cier. On the eastern side the motion is 20 
 inches, while on the western side it is only 0. 
 It rises on the eastern side from 20 to 34 
 
 and downs, the general advance of the rate inchGS at lhe 5th stake wliicu we standillg 
 
 f motion is manifest Now there may have upon thc glacier can see to be much ucare r 
 
 been some slight displacement of the stakes to l tho cas f ern lh ' an to the westem side . T/lc . 
 
 by melting surhcient to account for these unitcd evidence of these three lines places tl* 
 
 small deviations from uniformity in the in- factbeyond doubt, that opposite the Mwitanvert, 
 
 crease of the motion But another solution and for some distance above it and below it, tte 
 
 is also possible. We shall afterward learn ^ole eastern side of the glacier is mooing more 
 
104 
 
 THE FORMS OF WATER 
 
 quickly than the western side. 
 
 24. SUGGESTION OP A NEW LIKENESS OF 
 ^LACIER MOTION TO RIYEU MOTION. 
 CONJECTURE TESTED. 
 
 173. Here we have cause for reflection, and 
 facts arc comparatively worthless it' they do 
 not provoke this exercise of the mind. It is 
 because facts of nature are not isolated hut 
 connected, th:it science, to follow tiiem, must 
 also form a connected whole. The mind of 
 the natural philosopher must, as it were, ba 
 ii web of thought corresponding in all its 
 fibres with the web of fact in nature. 
 
 174. Let us, then ascend to a point whi^li 
 commands a good view of this portion of ihe 
 Mer de Glace. The ice-river we see is not 
 straight hut, curved, and its curvature Isfrotn 
 the Mootauvert ; that is to say, its convex 
 side is east, and its concave side is west (look 
 to the sketch). You have already pondc;ed 
 the fact that a glacier, in gome respect*, moves 
 like a river. How would a river move 
 through a curved channel ? This is known. 
 Were'tha ice of the Mer de Glace displaced 
 by water, the point of swiftest motion at the 
 Montanvert would not be the centre, but a 
 point east of the centre. Can it be then that 
 this "water rock," as ico is sometimes 
 called, acts in this respect also like water ? 
 
 175. This is a thought suggested on the 
 spot ; it may or it may not be true, but the 
 jjieans <>f testing it are at hand. Looking up 
 the glacier, we see that at ks Fonts it also 
 bends, but that there its convex curvature Is 
 toward the western side of the valley (look 
 again to the sketch). If our surmise be tiue, 
 the point of swiftest motion opposite les 
 Fonts ought to lie west of the axis of the 
 glacier. 
 
 176. Lt j t us test this conjecture. On July 
 25th we fix in a line across this portion of the 
 glacier seventeen stakes ; every one of them 
 has remained firm, and on the 2(jth we lind 
 the motion for 24 hours to be follows : 
 
 FOURTH LINE : D D' UPON TIIE SK.ETCU. 
 
 East West 
 
 Htako 1 2 3 456789 10 11 12 13 14 15 
 
 Inches....? 8 13 15. 16 19 20 ^1 fcl 23 23 sil 22 IT 15 
 
 177. Inspected by the naked eye alone, the 
 stakes 10 and 11, where the clacier reaches 
 its greatest motion, are seen to be considera- 
 bly to the west of the axis of the glacier. 
 Thus far we have a perfect verification of 
 Wie guetss which prompted us to make these 
 measurements. You will here observe that 
 the " guesses" of science arc not the work 
 of chance, but of thoughtful pondering over 
 antecedent farts. The guess is the " induc- 
 tion" from the facts, to be ratified or ex- 
 ploded by the test of subsequent experiment. 
 
 178. And though even now we have ex- 
 ceedingly strong reason for holding that the 
 point of maximum velocity obeys the law of 
 liquid motion, the strength of our conclusion 
 will be doubled if we can show that the 
 point shifts back to the eastern side of the 
 axis at another place of flexure. Fortunate- 
 UT such a place exists opposite Trelapqjte. 
 
 Here the convex curvature of tne vallsy 
 turns again to the east. Across this portioa 
 of the glacier a line was set out on -July 28th, 
 and from measurements on the Ulst, the ratu 
 of motion per 24 hours was determined. 
 FIFTH LIN T E : E E' UPON THIS SKKTCII.-] 
 West East 
 
 Stake 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 
 
 Inches.. .11 14 13 15 15 16 17 19 20 19 20 18 16 15 13 
 
 179. Here, again, the mere estimate of 
 distances by the oye would show us that 
 the three ^ takes which moved fastest, viz. 
 the 9th, 10th, and lllli, were all to the east 
 of thu middle line of the glacier. The dem- 
 onstration that the point of swiftest motion 
 wanders to and fro across the axis, as the 
 flexure of the valley changes, is, therefore 
 shall I say complete V 
 
 180. Not yet. For if surer menns are open 
 to us we must not rest content with estimates 
 by the eye. We have with us a surveying 
 chain : let us shake it out and measure these 
 lines, noting the distance of e^ery stake 
 from the side of the glacier. This is no easy 
 work among the crevasses, but 1 confide it 
 confidently to Mr. Hirst and you. \Ve can 
 afterward compare a number of .slakes on the 
 eastern side with the same number of stakes 
 taken at the same distances !r-m the western 
 side. For example, a pair of stakes, one ten 
 yards from the eastern side and the other ten 
 yards fiom the western side; another pair, 
 one fifty yards from the eastern side and the 
 other fifty yaids from the western side, and 
 so on, can v be compared together. For the 
 sake of easy reference, let us call the points 
 thus compared in pairs, eqniraient points. 
 
 181. Tin re were five pairs of such points 
 upon our fourth line, D 1)', and here are 
 their velocities : 
 
 Eastern points ; motion in inches.. 13 15 16 18 SO 
 \Vertern points " " -.15 17 1'2 23 23 
 
 In every rase here the stake at the western 
 side moved moie rapidly than tlu equivalent 
 slaKO at the eastern side. 
 
 182 Applying the same analysis to our 
 fifth line, E E', we have the following series 
 of velocities of three pairs of equivalent 
 points : 
 
 Eastern points ; motion in inches 15 18 19 
 
 Western points " 13 15 17 
 
 183. Hern the three points on the eastern 
 side move more rapidly than the equivalent 
 points on the western side. 
 
 184. It is thus pioved : 
 
 1. That opposite the Montanvert the east- 
 ern half of the Mer de Ulace moves more 
 rapidly than the western half. 
 
 2. That opposite lex Pouts the western half 
 of the glacier moves more rapidly than the 
 eastern half. 
 
 3. That opposite Ti elaporte t .e eastern half 
 of the glacier again moves more rapidly than 
 the western half. * 
 
 4. That these changes in the place of great- 
 est motion are determined by the flexures of 
 the valley through which the Mer de Glace 
 moves. 
 
2T CLOUDS AND RIVERS, ICS AND GLACIERS. 
 
 25. NEW LAW OP GLACIER MOTION. 
 
 185. Let u* express these facts in another 
 va . Supposing the points of swiftest motion 
 m" n VL-CV great number of lines crossing the 
 fler de Glace 1o be determined ; the line 
 oining all those points together is what 
 mathematicians would call the locus of the 
 3oiut, of swiftest motion. 
 
 ISO. At Tielaporte this line would lie cast 
 )f the centre ; at the Pouts it would lie^vest 
 if the centre ; hence in passing from Tiela- 
 porte to the Punts it would cross the centre. 
 But at the Montanvert it would again lie 
 jast of the centre ; hence between the Pouts 
 and the Mont an vert the centre must be 
 crossed a second lime. If there were further 
 sinuosities upon the Mer de Glace there 
 would he further ciossiugs of the axis of the 
 glacier. 
 
 187. The points on the axis which mark 
 the transition from eastern to western bend- 
 ing, and the reverse, may be called jmnts of 
 contrary flexure. 
 
 188. Now what is true of the Mer tie 
 Glace is true of ail other glaciers moving 
 through sinuous valleys : so that the facts 
 established in the Mer (le Glace may be ex- 
 panded into the following general law of 
 gl icier motion : 
 
 YYiien a glacier moves through a sinuous 
 vsiiley. the locus of the point of maximum 
 motion does not coincide with the centre of 
 the glacier, but, on the contrary, always lies 
 on tue convex side of the central line. The 
 locus is therefore a curved line more deeply 
 sinuous than the valley itself, and crosses 
 the axis of the glacier at each point of con- 
 trary flexure. 
 
 Is9. The dotted line on the Outline Plan 
 (Fig. G) represents the locus of the point of 
 maximum inotbn, the iirm line marking the 
 centre of the glacier. 
 
 190. Substituting t he word river for glacier, 
 this law is also true. The motion of the 
 water is ruled by precisely the same condi- 
 tions as the motion of the ice. 
 
 191. Let us now apply our law to the ex- 
 planation of a difficulty. Turning to the 
 careful measurements executed by ftl. Agas- 
 siz on the glacier of the Uuteraar, we notice 
 in the discussion of these measurements a 
 section of the " Syste"me glaciaire" devoted 
 to the "Migrations of the Centre." It is 
 here shown that the middle of the Untcraar 
 glacier is not always the point of swiftest 
 motion. This fact has hitherto remained 
 without explanation ; but a glance at the 
 Unteraar valley, or at the map of the valley, 
 shows the enigma to be an illustration of the 
 law whifh we have just established on the 
 Mer de Glace. 
 
 ^ 23. MOTION OF Axis OP MER DE GLACE. 
 
 192. We have now measured the rate of 
 motion of five different lines across the trunk 
 of the Mer de Glace. Do they all move 
 alike ? No. Like a river, a glacier at differ- 
 ent places moves at different rates. Coin- 
 
 paring together the points of maximum mo- 
 tion of all rive lines, we have this result . 
 
 MOTION OF MER DE GLACE. 
 
 At Treliiporte 20 inches a day 
 
 Mies Punts 
 
 Above the Moutanvcrt. 
 
 At the Montanvert 
 
 lielow the Montanvert 33 
 
 193. There is thus an increase of rapidity 
 as we descend the glacier from Treiaporle tc 
 the Montanvett ; the maximum motion at the 
 Montanvert being fourteen inches a day 
 greater than at Trelaporte. 
 
 27. MOTION OP TRIBUTARY GLACIERS. 
 
 194. So much for the trunk glacier ; lot 
 us now investigate the branches, permitting, 
 as we have hitherto done, reflection OD 
 known facts to precede our attempts to dis^ 
 cover unknown ones. 
 
 195. As we stood upon our " cleft sta- 
 tion," whence we had so capital a view of the 
 Mer de Glace, we were struck by the fact that 
 some of the tributaries of the glacier were 
 wider than the glacier itself. Supposing 
 water to be substituted for the ice, how do 
 you suppose it would behave? You would 
 doubtless conclude that the motion down the 
 broad and slightly inclined valleys of the 
 Geant and the Lechaud wouid be compara- 
 tively slow, but that the water would forca 
 itself with increased rapidity through the 
 "narrows" of Tielaporte. Let us test this 
 notion as applied to ihe ice. 
 
 19(3. Planting our theodolite in the shadow 
 of Mont Tacul,' and choosing a suitable 
 point at the opposite side of the Glacier civ 
 Geant, we fix on July 29th a series of teE 
 stakes across the glacier. The motion of tlu 
 line in twenty-four hours was as follows : 
 MOTION OF CLACIER DU GEANT. 
 SIXTH LINE: II II' UPON SKETCH. 
 
 Stake 1 * 3 4 5 7 8 9 1C 
 
 Indies 11 10 13 1) 1,J U ii 10 9 5 
 
 197. Our conjecture is fully verified. The 
 maximum motion here is seven inches a day 
 less than that of the Mer dc Glace at Treia- 
 porte (192). 
 
 198. And now for the Lechaud branch. 
 On August 1st we lix ten stakes across this 
 glacier above the point where it is joined by 
 the Talefre. Measured on August 3d, and 
 reduced to twenty-four hours, the motion was 
 found to be : 
 
 MOTION OP GLACIER DE LECHAUD. 
 
 SEVENTH LINE: KK'upo* SKETCH. 
 Stake... . 1 a 3 4 5 6 7 8 9 1C 
 Iaca 5 8 10 9 J) S 6 7 b 
 
 j09. Here our conjecture is still further 
 verified, the rate of motion being even less 
 than that of the Glacier du Geant. 
 
 28. MOTION OF TOP AND BOTTOM OF 
 GLACIEU. 
 
 200. We have here the most ample and 
 varied evidence that the sides of a glacier, 
 like those of a river, are retarded by Jrictioa 
 against its boundaries. But the likeness doas 
 
108 
 
 THE FORMS OF WATER 
 
 not end hero. The motion of a river Is re- 
 tarded by the friction against its bed. Two 
 observers, viz., Professor Forbes and M. 
 Charles Martins, concur in showing the same 
 to be the cast? with a glacier. The obser- 
 vations of both have k j en objected to ; hence 
 it is all the more incumbent on us to seek for 
 decisive evidence. 
 
 201. At the Tacul (near the point a upon 
 the sketch plan, Fig. 5) a wall of ice about 
 150 feet high has already attracted our atten- 
 tion. Bending round to joia the Lechaud the 
 G lacier du Geaut is here drawn away from 
 the mountain side and exposes a line section. 
 We try to measure it top, bottom, and mid- 
 dle, and are defeated twice over. ^Vu try it a 
 third time and succeed. A. stake \j fixer! at 
 the summit of the ice-precipice, another 
 at 4 feet from the bottom, and a third at 35 
 feet above the bottom. These lower stakes 
 are tixed at some ri.sk of boulders falling 
 upon us from above ; but by skill and CMI- 
 tion we succeed in measuring the motions 
 of all three. For 2 1 hours the motions aro : 
 
 Top *!ake (5 .relics. 
 
 MiJuh! sU'.;u 4! 
 
 B >;,io.u 3(Ako *'& '' 
 
 202. The retarding influence of the bed of 
 the glacier is reduced to demonstration by 
 these measurements. The bottom does not 
 move with half the velocity of the surface. 
 
 29. LATERAL COMPRESSION OF A GLACIER. 
 
 203. Furnished with the knowledge which 
 those labors and measurements have given 
 us, let us once more climb to our station be- 
 side the Cleft, under the Aiguille de Char- 
 moz. At our first visit we saw the medial 
 moraines of the glacier, but we knew noth- 
 ing about their cause. We now know that 
 they mark upon the trunk its tributary gla- 
 ciers. Cast your eye, then, first upon the 
 Glacier du Geant ; realize its widtii in its 
 own valley, and see how much it is narrowed 
 at Treiaporte. The broad ice-stream of the 
 Lechaud is still more surprising, being 
 squeezed upon the Mer de Glace to a narrow 
 white band between its bounding moraines. 
 The Talefre undergoes similar compression, 
 Let us now descend, shake out our chain, 
 measure, and express in numbers the width 
 of the tributaries, and the actual amount of 
 compression suffered at Tie'aportc. 
 
 204. We find the width of the Glacier du 
 G&mt to be 5155 links, or 1134 yards. 
 
 205. The width of the Glacier de LSchaud 
 we find to be 3725 links, or 825 yards. 
 
 200. The width of the Tatefre we find to 
 be 2900 links, or 638 yards. 
 
 207. The sum of the widths of the three 
 branch glaciers is therefore 2597 yards. 
 
 208. At Treiaporte these three branches 
 are forced through a gorge 893 yards wide, 
 or one third of their previous width, at the 
 rate of twenty inches a day. 
 
 209. If we limit our view to the Glacier dc 
 Lchuud, the facts are still more astonishing 
 Previous to its junction with the Talefre^ 
 this glacier has a width of 825 yards ; in 
 passing through the jaws of the granite vise 
 
 at Treiaporte, its width is reduced to eighty- 
 eight yards, or in round numbers to one tenth 
 of its previous width. (Look to the sketch 
 on page 9.) 
 
 210. Aro we to understand by this that the 
 ice of the Lechaud is squeezed to one tenth 
 of its former volume? By no means. It is 
 mainly a change offonn, not of volume, Unit 
 occurs at Treiaporte. Previous to its com- 
 pression, the glacier resembles a plate of ice 
 lying fiat upon its bed. After its compres- 
 sion, it resc-mblcs a pl.ue faced upon its edge. 
 The squeezing, doubtless, has deepened the 
 ice. 
 
 CO. LONGITUDINAL COMPRESSION OF A 
 GLACIER. 
 
 211. The icj is forced through the gorge at 
 Tn'laporte by a piessurc from behind; in 
 fact i he Glacier du Geant, immediately above 
 Treiaporte. represents a piston or a plug 
 which drives the ice through the gorge. 
 What t'lfect must this pressure have upon 
 the plug itself V Reasoning alone renders it 
 probable that the pressure will shorten the 
 plug ; that the lower part of the Glacier du 
 Geaut will to some extent yield to the pres- 
 sure from behind. 
 
 212. Let us test this notim. About three 
 quarters of a mile above tl-e Taeul, and en 
 the mountain-slope to the le f 'L as we ascend. 
 we observe a patch of verdi/rc. Thither wo 
 climb ; there we plant our theodolite, and set 
 out across the Glacier du GeV.ut, aline, which 
 we will call lino No. 1 (F F' upon sketch, 
 Fig. G.) 
 
 213. About a quarter of a mile lower down 
 we find a practicable couloir on the mountain- 
 side ; we ascend it, reach a suitable platform, 
 plant our instrument, and set out a second 
 line, No. 2 (G G' upon sketch). We must 
 hasten our work here, for along this couloir 
 stones are discharged from a- small glacier 
 which rests upon the slope of Mont Tacul. 
 
 214. Still lower down by another quarter 
 of a mile, which brings us ner.r the Tacul, 
 we set out a third line, No. 3 (II H' upon 
 sketch), across the glacier. 
 
 215. The daily motion of tho centres o-f 
 these three lineaSs as follows : 
 
 Inches. Distances asunder. 
 
 No. 1 20-55 i 
 
 No. 2 13-43 f ^ >' ards - 
 
 437 
 
 No. 3 12-75 f- 
 
 216. The first line here moves five inches 
 a day more than the second ; and the second 
 nearly three inches a day more than the 
 third. The reasoning is therefore confirmed. 
 The ice-plug, which is in round numbers one 
 thousand yards long, is shortened by the 
 pressure exerted on its front at the rate cf 
 about eight inches a day. 
 
 217. A river descending the Valley du 
 Geant would behave in substantially the samo 
 fashion. It would have its motion on ap- 
 proaching Treiaporte diminished, and it 
 would pour through the defile with a velocity 
 greater than that of the water behind^' 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 H)7 
 
 81. SLIDING AND FLOWING. HAKD ICE 
 AND SOFT ICE. 
 
 218. We have thus far confined ourselves 
 to the measurement and discussion of glacier 
 motion ; but in our excursions we have no- 
 ticed many things besides. Here and there, 
 where the ice has retreated fiom the moun- 
 tain side, we have seen the rocks fluted, 
 scored, and polished ; thus proving that the 
 .ice had slidden over them and ground them 
 'down. At the source of the Arveiron we 
 noticed the water rushing from beneath the 
 glacier charged with fine matter. All glacier 
 rivers are similarly charged. The Rhone 
 carries its load of matter into the Lake of 
 Geneva ; the rush of the river is here ar- 
 rested, the matter subsides, and the Rhone 
 quits the lake clear and blue. The Lake of 
 Geneva, and many other Swiss lakes, are in 
 part filled up with this matter, and will, in 
 all probability, finally be obliterated by it. 
 
 219. One portion of the motion of a glacier 
 is due to this bodily sliding of the mass over 
 its bed. 
 
 220. We have seen in our journeys over 
 the glacier streams formed by tiie melting of 
 the ice, and escaping through cracks and 
 crevasses to the bed of the glacier. The fmc 
 matter ground down is thus washed away ; 
 the bed is kept lubricated, and the sliding of 
 the ice rendered more easy than it would 
 otherwise be. 
 
 221. As a skater also you know how much 
 ice is weakened by a thaw. Before it actu- 
 ally melts it becomes rotten and unsafe 
 Test such ice with your penknife : you cau 
 dig the blade readily into if, or cut the ice 
 with ease Try good sound ice in the same 
 way : you find it much more resistant. The 
 one, indeed, resembles soft chalk ; the other 
 hard stone. 
 
 222. Now the ]\Icr ib Glace in summer is 
 in this thawing condition. Its ice is rendered 
 soft and yielding by the sun ; its motion is 
 thereby facilitated. AVe have seen that not 
 only docs the glacier slide over its bed, but 
 that the upper layers slide over the under 
 ones, and that the centre slides past the sides. 
 The softer and more yielding the ice is, the 
 more free will be this motion, and the more 
 readily also will it be forced through a defile 
 like Trelaporle. 
 
 223. But in winter the thaw ceases ; the 
 quantity of water reaching the bed of the 
 glacier is diminished or entirely cut off. The 
 ice also, to a certain depth at least, is frozen 
 hard. These considerations would justify 
 the opinion that in winter the glacier, if it 
 moves at all, must move more slowly than in 
 summer. At all events, the summer meas- 
 urements give no clue to the winter motion. 
 
 221. This point merits examination. I will 
 not, however, ask you to visit the Alps in 
 midwinter ; but, if you allow me, I will be 
 your deputy to the mountains, and report to 
 you faithfully the aspect of the region and 
 the behavior of the ice. 
 
 32. WINTER ON THE MER DE GLACB. 
 
 225. The winter chosen is an inclement 
 one. There is snow in London, snow ia 
 Paris, snow in Geneva ; snow near Champuni 
 so deep that the road fences are entirely 
 effaced. On Christmas night nearly at mid- 
 night 1859, your deputy reaches Chamouni. 
 
 226. The snow fell heavily on December 
 20th ; but on the 2?th, during a lull in the 
 storm, we turn out. There are with me four 
 good guides and a porter. They tie planks 
 to their feet to prevent them from sinking iu 
 the snow ; I neglect this precaution and sink 
 often to the waist. Four or five times during 
 our ascent the slope cracks with an explosive 
 sound, aud the snow threatens to come down 
 in avalanches. 
 
 The freshly-fallen snow \ /as in that partic- 
 ular condition which causes its granules to 
 adhere, and hence every flake fulling on the 
 trees had been retained there. The laden 
 pines presented beautiful and often fantastic 
 forms. 
 
 227. After five hours and a half of arduous 
 work the Montanvert was attained. We un- 
 locked the forsaken auberge. round which 
 the snow was reared in buttresses. I have 
 already spoken of the complex play of crys- 
 tallizing forces. The frost-figures on thu 
 window-panes of the auberge were wonder- 
 ful : mimic shrubs and ferns wrought by the 
 building power while hampered by the ad- 
 hesion between the glass and the film in 
 which it worked. The appearance of the 
 glacier was very impressive ; all sounds were 
 stilled. The cascades which in summer fill 
 the air with their music were silent, hanging 
 from the ledges of the rocks in fluted col- 
 umns of ice. The surface of the glacier was 
 obviously higher than it had been in sum- 
 mer ; suggesting tho thought that while the 
 winter cold maintained the lower end of the 
 glacier jammed between its boundaries, the 
 upper portions still moved downward and 
 thickened the ice. The peak of the Aiguille 
 da Dru shook out a cloud banner, the origin 
 and nature of which have been already ex- 
 plained (84). 
 
 228. On the morning of the 28th this ban- 
 ner was strikingly large and grand, and red- 
 dened by the light of tne rising sun it glowed 
 like a flame. Roses of cloud also clustered 
 round the crests of the Grande Jorasse and 
 hung upon the pinnacles of Charmoz. Four 
 men, well roped together, descended to the 
 glacier. I had trained one of them in 1857, 
 and he was now to fix the stakes. The 
 storm had so distributed the snow as to leave 
 alternate longths of the glacier bare anil 
 thickly covered. Whore much snow lay, 
 great caution was required, for hidden cre- 
 vasses were underneath. The men sounded 
 with their staffs at every step. Once while 
 looking at the party through my telescope 
 the leader suddenly disappeared ; the roof 
 of a crevasse had given way beneath him : 
 but the other three men promptly gathered 
 round and lifted him out of the fissure. The 
 true line was soon picked up by the thoodo 
 
108 
 
 THE FORMS OF WATER 
 
 lite ; one by one the stakes were fixed until 
 a series of eleven of them stood across the 
 glacier. 
 
 229. To get higher up the valley was im- 
 practicable ; the snow was too deep, and the 
 aspect of th'3 weather too threatening ; so the 
 theodolite was planted amid the pines a little 
 way below the Montanvert, whence through 
 a vista I could see across the glacier. _ The 
 men were wrapped at intervals by whirling 
 snow-wreaths, which quite hid them, and we 
 had to take advantage of the lulls in the wind. 
 Fitfully it came up the valley, darkening the 
 air, catching the snow upon the glacier, and 
 tossing it throughout its entire length into 
 high and violently agitated clouds, separated 
 from each other by cloudless spaces corre- 
 sponding to the naked portions of the ice. In 
 the midst of this turmoil the men continued 
 to work. Bravely and steadfastly stake after 
 stake was set until at length a series of ten 
 of them was fixed across the glacier. 
 
 230. Many of the stakes were fixed in the 
 snow. They were four feet in length, and 
 were driven in to a depth of about three 
 feet. But that night, while listening to the 
 wild onset of the storm, I thought it possible 
 that the stakes and the snow which held 
 them might be carried bodily away before 
 the morning. The wind, however, lulled. 
 We rose with the dawn, but the air w T as thick 
 with descending snow. It was all composed 
 of those exquisite six-petalled flowers, or six- 
 rayed stars, which have been already figured 
 and described ( 9). The weather brighten- 
 ing, the theodolite was planted at the end of 
 the first line. The men descended, and, 
 trained by their previous experience, rapidly 
 executed the measurements. The first line 
 was completed before 11 A.M. Again the 
 snow began to fall, filling all the air. Span- 
 gles innumerable were showered upon the 
 heights. Contrary to expectation, the men 
 could be seen and directed through the 
 shower. 
 
 231. To reach the position occupied by the 
 theodolite at the end of our second line, I had 
 to wade breast-deep through snow which 
 seemed as dry and soft as flour. The toil of 
 the men upon the glacier in breaking through 
 the snow was prodigious. But they did not 
 flinch, and after a time the leader stood be- 
 hind the farther stake, and cried, Nous awns 
 fini. I was surprised to hear him so dis- 
 tinctly, for falling snow had been thought 
 very deadening to sound. The work was 
 finished, and I struck my theodolite with the 
 feeling of a general who had won a small 
 Battle. 
 
 28:3. We put the house in order, packed 
 up, and shot by glissade down the steep 
 slopes of JM Film to the vault of the Arvei- 
 ron. We found the river feeble, but not 
 dried up. Many weeks must have elapsed 
 since any water had been sent down from the 
 surface of the glacier. But at the setting in 
 of winter the fissures were in a great measure 
 charged with water ; and the Arveiron of 
 to-day was probably due to the gradual 
 drainage of the glacier. There was now no 
 
 danger of entering the vault, for the ice 
 seemed as firm as marble. In the cavern we 
 were bathed by blue light. The strange 
 beauty of the place suggested magic, and put 
 me in mind of stories about fairy caves which 
 I had read when a boy. At the source of the 
 Arveirou our winter visit to the Mer de Glace 
 ends ; next morning your deputy was on his 
 way to London. 
 
 33. WINTER MOTION OF THE Msii DE 
 GLACE. 
 
 233 Here are the measurements executed 
 in the winter of 1859 : 
 
 LINE No. I. 
 
 Stake t 2 .3 4 5 6 7 8 9 10 11 
 
 Inches 7 11 It 13 14 14 10 13 12 13 7 
 
 LINB No. IT. 
 
 Stake 1 2 3 4 5 7 8 9 10 
 
 Inches 8 10 14 Iti 16 1J 13 17 15 14 
 
 234. Thus the winter motion of the Mer 
 de Glace near the Montanvert is, in round 
 numbers, half the summer motion. 
 
 235. As in summer, the eastern side of the 
 glacier at this place moved quicker than the 
 western. 
 
 34. MOTION OF THE GHINDELWALD AND 
 ALETSCII GLACIEKS. 
 
 236. As regards the question of motion, 
 to no other gfacier have W T C dtvoted ourselves 
 with such thoroughness as to the Mer de 
 Glace ; we are, however, abie to add a few 
 measurements of other celebrated glaciers. 
 Near the village of Gimdelwalcl in the Ber- 
 nese Oberland, there are two great ice- 
 streams called lespectively the Upper and the 
 Lower Grindelvvald glaciers, the second of 
 which is frequently visited by travellers in 
 the Alps. Across it on August 6th, 1800, a 
 series of twelve stakes was fixed by Mr. 
 Vaughan Hawkins and myself. Measured 
 on the 8th and reduced to its daily rate, the 
 motion of these stakes was as follows : 
 
 MOTION OF LOWER GRINDELWALD GLACIER. 
 
 East West 
 
 Stake... 1 23 5 6 7 8 9 19 11 12 
 
 Inches.. 18 11) 20 21 21 21 22 20 19 18 17 14 
 
 237. The theodolite was here planted a 
 little below the footway leading to the higher 
 glacier region, and at about a mile above the 
 end of the glacier. The measurement was 
 rendered difficult by crevasses. 
 
 238. The Ingest glacier in Switzerland is 
 the Great Aletsch, to which further reference 
 shall subsequently be made. Across it on 
 August 14th, 1860, a series of thirty-four 
 stakes was planted by Mr. Hawkins and me. 
 Measured on the 16th and reduced to their 
 daily rate, the velocities were found to be as 
 follows : 
 
 MOTION OF GREAT ALETSCH GLACIER. 
 East 
 
 Stake 12345 6 7 8 9 10 11 13 
 
 Inches 23468 11 13 14 18 17 17 W 
 
 Stake 13 14 15 16 17 13 19 20 21 21 S3 
 
 Inches 19 18 18 17 19 19 19 19 17 17 13 
 
 Stake 24 25 a<i 27 28 29 80 31 83 33 31 
 
 Inches 16 17 K 17 17 17 17 17 16 12 13 
 
 West 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 109 
 
 239. The maximum motion here is nine- 
 teen inches a day. Probably the eastern side 
 of the glacier is shallow, the retardation of 
 the betf making the motion of the eastern 
 slakes inconsiderable. The width of the 
 glacier here is 1)030 links, or about a mile 
 and a furlong. The theodolite was planted 
 high among ^the rocks on the western flank 
 ot' the mountain, about half a mile above the 
 Margelin See. 
 
 35. MOTION OF MORTERATSCII GLACIER. 
 240 Far to the east of the Oberland, and 
 in that interesting part of Switzerland known 
 as the Ober Eugadin, stands a noble group of 
 mountains, less in height than those of the 
 Oberland, but still of commanding elevation. 
 The group derives its name from its most 
 dominant peak, the Piz Bernina. To reach 
 the place we travel by railway from Basel to 
 ZUrich, and from Zurich to Chur (French 
 Coire), whence we pass by diligence over 
 either the Albula pass or the Juliar pass to 
 the village of Pontresina. Here we are in 
 the immediate neighborhood of the Bernina 
 mountains. 
 
 241. From Pontresina we may walk or 
 drive along a good coach road over the Ber- 
 nina pass into Italy. At about an hour above 
 the village you would look from the road 
 into the heart of the mountains, the line of 
 vision passing through a valley, in which is 
 couched a glacier of considerable size. 
 Along its back you would trace a medial 
 moraine, and you could hardly fail to notice 
 how the moraine, from a mere narrow streak 
 at first, widens gradually as it descends, un- 
 til finally it quite covers the lower end of the 
 glacier. Nor is this an effect of perspective ; 
 lor were you to stand upon the mountain 
 slopes which nourish the glacier, you would 
 see thence also the widening of the streak of 
 rubbish, though the perspective here would 
 tend to narrow the moraine as it retreats 
 downward. 
 
 242. The ice-stream here referred to is tho 
 Morteratsch glacier, the end of which is a 
 short hour's walk from the village of Pon- 
 tresina. We have now to determine its rate 
 of motion and to account for the widening of 
 its medial moraine. 
 
 243. In the summer of 18G4 Mr. Hirst and 
 myself set out three lines of stakes across the 
 glacier. The first line crossed the ice high 
 up ; the second a good distance lower down, 
 and t.he third lower still. Even thrj third 
 line, however, was at a considerable distance 
 above the actual snout of the glacier. Tho 
 daily motion of these three lines was as fol 
 lows : 
 
 FIUST LINE. 
 
 Stake 1 2 3 4 5 6 7 8 9 10 11 
 
 Inches 8 12 13 13 14 13 12 ! 1J 7 5 
 
 SECOND LINE. 
 
 Stake. 1 2 3 4 5 6 7 8 9 10 11 
 
 inches 1 4 6 8 10 11 11 11 11 H n 
 
 TIIIRD LINE. 
 
 Stake 1 2 3 4 5 G 7 8 9 n 11 
 
 Inches 1 24 5 V 607755 4 
 
 244. Compare these lines together. You 
 
 notice the velocity of the first is greater than 
 that of the second, and the velocity of tho 
 second greater than that of the third. 
 
 245. The lines were permitted to move 
 downward for 100 hours, at the end of 
 which lime the spaces passed over by the 
 points of swiftest motion of the three lines 
 were as follows : 
 
 MAXIMUM MOTION IN 100 HOURS. 
 
 First line 50 inches. 
 
 Second line 4-4 " 
 
 Third line 30 " 
 
 246. Here then is a demonstration that the 
 upper portions of the Morteralsch glacier are 
 advancing on the lower ones. In 1871 tJie 
 motion of a point on tlie middle of the glacier 
 near its snout was found to be less than two 
 inches a, day ! 
 
 247. What, then, is the consequence of 
 this swifter march of the upper glacier? 
 Obviously to squeeze this medial incline 
 longitudinally, and to cause it to spread out 
 laterally. We have here distinctly revealed 
 the cause of the widening of the medial mo- 
 raine. 
 
 248. It has been a question much dis- 
 cussed, whether a glacier is competent to 
 sc'oop out or deepen the valley through which 
 it moves, and this very Morteratsch glacier 
 has been cited to prove that such is not the 
 case. Observers went to the snout of the^ 
 glacier, and finding it sensibly quiescent, 
 they concluded that no scooping occurred*. 
 But those who contended for the power 01 
 glaciers to excavate valleys never stated, or 
 meant to state, that it was the snout of the* 
 glacier which did the work. In the Morte- 
 ratsch glacier the work of excavation, which* 
 certainty goes on to a greater or less extent*, 
 must be far more effectual high up the val-, 
 ley than at the end of the glacier. 
 
 36. BIRTH OP A CREVASSE : REFLEO 
 
 TIONS. 
 
 240. Preserving the notion that we are- 
 working together, we will now enter upon *k 
 new field of inquiry. We have wrapped up 
 our chain and are turning homeward after 
 a hard day's work upon the Glacier du 
 Geant, when under our feet, as if coming 
 from the body of the glacier, an explosion is 
 heard. Somewhat startled, we look inquir- 
 ingly over the ice. The sound is repeated,, 
 several shots being fired in quick succession. 
 They seem sometimes to our right, some- 
 times to our left, giving the impression that 
 the glacier is breaking all round us. Still 
 nothing is to be seen. 
 
 250. We closely scan the ice, and after an. 
 hour's strict search we discover the cause of 
 the reports. They announce the birth of a 
 crevasse. Through a pool upon the glacier 
 we notice air-bubbles ascending, and find 
 the bottom of the pool crossedtoy a narrow 
 crack, from which the bubbles issue. Right 
 and left from this pool we trace the young 
 fissure through long distances. It is some- 
 times almost too feeble to be seen, and at no 
 Clace is it wide enough to admit a knite- 
 lade. 
 
THE FORMS OF TVATEP 
 
 251. It is difficult to believe that the for. 
 midable fissures, among which you and I 
 havu so often trodden with awe, could com- 
 mence in this small way. Such, however, is 
 the case. The great and gaping chasms on 
 and above the ice-fails of the Geaut and the 
 Talcfre begin as narrow cracks, which open 
 gradually to crevasses. We are thus taught 
 in an instructive and impressive way that up 
 pearances suggestive of very violent action 
 may really be produced by processes so slow 
 as to require refined observations l > detect 
 them. In the production of natural phe- 
 nomena two things always conic into play, 
 the inteiixity of Ihe acting force, and I he time 
 during which it acts. Make the intensity 
 great and the time small, an 1 you have sud- 
 den convulsion ; but precisely the same ap- 
 parent effect may be produced by making the 
 intensity small and the time great. This 
 truth is strikingly illustrated by the Alpine 
 icv-falls and crevasses ; and many geological 
 phenomena, which at first sight suggest vio- 
 lent convulsion, may be really produced in 
 the self-same almost impercepiibb way. 
 
 37. ICICLES. 
 
 2/52. The crevasses are grandest on the 
 higher neves, where they sometimes appear 
 as long yawning fissures, and sometimes as 
 chasms of irregular outline. A delicate blue 
 light shimmers from them, but this is grad- 
 ual ty lost in the darkness of their profounder 
 portions. Over the edges of the chasms, 
 and mostly over the southern edges, hangs a 
 coping of snow, and from this depend like 
 stalactites rows of transparent icicles, 10, 20, 
 80 feet long. These pendent spears consti- 
 tute one of the most beautiful features of the 
 higher crevasses. 
 
 253. How are they produced ? Evidently 
 by the thawing of the snow. But why, 
 Avhen once thawed, should the water freeze 
 again to solid spears ? You have seen icicles 
 pendent from a house-eave, which have been 
 manifestly produced by the thawing of the 
 snow upon the roof. If we understand these 
 we shall also understand the vaster stalactites 
 of the Alpine crevasses. 
 
 254. Gathering up such knowledge as we 
 possess, and reflecting npuii it patiently, let 
 us found on it, if we can, a theory of icicles 
 
 255. First, then, you aie to know that the 
 air of our atmosphere is hardly heated at all 
 by the rays of the sun, whether visible or in- 
 visible. The air is highly transparent to all 
 kinds of rays, and it is only the scanty frac- 
 tion to which it is not transput cut that ex- 
 pend their force in warming it. 
 
 256. Not so, however, with the snow on 
 which the sunbeams fall. It absorbs the 
 solar heat, and on a sunny day you may see 
 the summits of the high Alps glistening with 
 the water of liquefaction. The air above 
 and around the mountains may at the same 
 time be many degrees below the freezing 
 point in temperature. 
 
 257. You have only to pass from sunshine 
 into shade to prove this. A single step 
 suffices to carry you from a place where the 
 
 thermometer stands high to one wneie it 
 stands )ow ; the change being due, not to 
 any difference in the temperature of the air, 
 but simply to the withdrawal of the ther- 
 mometer from the direct action of the solar 
 rays. Nay, without shifting the thermome- 
 ter at all, by interposing a suitable screen, 
 which cuts off the sun's rays, the coldness of 
 the air may be demonstrated. 
 
 258. Look now to the snow upon your 
 house-roof. The sun plays upon it and 
 melts it ; the water trickles to the cave and 
 then drops down. If the eave face the sun 
 the water remains water ; but if the eave do 
 not face the sun, the drop, before it quits its 
 parent snow, is already in shadow. Now the 
 shaded space, as we have learned, may be 
 below the freezing temperature. If so, the 
 drop, instead cf falling, congeals, and the 
 rudiment of an icicle is formed. Other 
 drops and driblets succeed, which trickle 
 over the rudiment, congeal upon it in part 
 and thicken, it at the root. But a portion of 
 the water reaches the free end of the icicle, 
 hangs from it, and is there congealed before 
 it escapes. The icicle is thus lengthened. In 
 the Alps, where the liquefaction is copious 
 and the cold of the shaded crevasse intense, 
 the icicles, though produced in the same way. 
 naturally grow to a greater size. The drain- 
 age of the snow after the sun's power is with- 
 drawn also produces icicles. 
 
 259. It is interesting and important that 
 you should be able to explain the formation 
 of an icicle ; but it is far more important 
 that you should realize the way in which the 
 various threads of what we call Nature are 
 woven together. You cannot fully under- 
 stand an icicle without first knowing that 
 solar beams powerful enough to fuse the 
 snows and blister the human skin, nay, it 
 might be added, powerful enough, when con- 
 centrated, to burn up the human body itself, 
 may pass through the air and still leave it at 
 an icy temperature. 
 
 38. THE BERGSCHRUND. 
 
 260. Having cleared away this difficulty, 
 let us turn once more to the crevasses, taking 
 them in the older of their formation. First 
 then above the neve we have the final Alpine 
 peaks and crests, against which the snow is 
 often reared as a steep buttress. We have 
 already learned that both neves and glaciers 
 are moving slowly downward ; but it usual- 
 ly happens that the attachment of the high- 
 est portion of tho buttress to the rocks is 
 great enough to enable it to hold on while 
 the lower portion breaks away. A very 
 characteristic crevasse is thus formed, called 
 in the German-speaking portion of the Alps 
 a Beryschrund. It often surrounds a peak 
 like a fosse, as if to defend it against the as- 
 saults of climbers. 
 
 261. Look more closely into its formation. 
 Imagine the snow as* yet unbroken. Its 
 higher portions cling to the rocks and move 
 downward with extreme slowness. But its 
 lower portions, whether from their greater 
 depth aud weight or their less perfect at" 
 
CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 iir 
 
 tacnment, arc compelled to move more 
 quick?}'. A.putt\s therefore exerted, tend- 
 ing to separate the lower from the upper 
 snow. For a lime this pull is resisted by the 
 cohesion of the neve ; but this at length 
 gives way, and a crack is formed exactly 
 across the line in which the pull is exerted. 
 In other words, crevasse is for fried at rig/it 
 angles to the line of tension. 
 
 39. TRANSVERSE CREVASSES. 
 
 262. Both on the neve' and on the glacier 
 the origin of the crevasses is the same. 
 Through some cause or other, the ice is 
 thrown into a state of strain, and as it can- 
 not stretch it breaks across the line of tension. 
 Take, for example, the ice-full of the Geant, 
 or of the Talefre, above which you know 
 the ^crevasses yawn terribly. Imagine the 
 neve and the glacier entirely peeled away, 
 so as to expose the surface over which they 
 move. From the Col du Geant we should 
 sue this surface falling gently to the place 
 uo\v occupied by the brow of the cascade. 
 Here the surface would fall steeply down to 
 the bed of the present Glacier du Ge'ant, 
 v/liera the slope would become gentle once 
 more. 
 
 26-J. Think of the neve" moving over such 
 a sin face. It descends from the Col till it 
 reaches the brow just referred to, It crosses 
 the brow, and must bend down to keep upon 
 its bed. Realize clearly what must occur. 
 Tlie surface of the ne"ve is evidently thrown 
 iito a state of strain : it breaks and forms a 
 Crevasse. Each fresh portion of the ne*ve as 
 it passes the brow is similarly broken, and 
 thus a succession of crevasses is sent down 
 the fall. Between every two chasms is a 
 great transverse ridge. Through local strains 
 upon the fall those ridges are also frequently 
 broken across, towers of ice xtfracs being 
 the result. Down the fall both ii<iges and 
 se'racs are borne, the dislocation being aug- 
 mented during the descent. 
 
 204. What must occur at the foot of the 
 fall? Here the slope suddenly lessens in 
 steepness. It is plain that the crevasses 
 must not only cease to open here, but that 
 they must in whole or in part close up. At 
 the summit of the fall, the bending was such 
 as to make the surf ace con vex ; at the bottom, 
 of the fall, the bending renders the surface 
 concave. In the one case we have strain, in 
 the other pressure. In the one case, therefore, 
 we have the opening, and in the other the 
 closing of crevasses. This reasoning corre- 
 sponds exactly with the facts of observation. 
 
 263. Lay bare your arm end stretch it 
 straight. Make two ink dots half an inch or 
 an inch apart, exactly opposite the elbow. 
 Bend your arm, the dots approach each 
 other, and are finally brought together. Let 
 the two dots represent tiie two sides of a 
 crevasse at the bottom of an ice-fall ; the 
 bending of the arm resembles the bending of 
 the ice, and the closing up of the dots re- 
 sembles the closing of the fissures. 
 
 266. The same remarks apply to various 
 portions of the Mer de Glace. At certain 
 
 places the inclination changes from a gentler 
 to a steeper slope, and on crossing the brow 
 between both the glacier breaks its back. 
 Transverse crevasses are thus formed. There 
 as such a change of inclination opposite to the 
 Angle, and a still greater but similar change 
 at the head of the Glacier des Bois. The 
 consequence is that the Mer de Glace at the 
 former point is impassable, and at the latter 
 the rending and dislocation are such as we 
 have seen and described. Below the Angle, 
 and at the bottom of the Glacier des Bois, 
 the steepness relaxes, the crevasses heal up, 
 and the glacier becomes once more continu- 
 ous and compact. 
 
 40. MARGINAL CREVASSES. 
 2G7. Supposing, then, that we had no 
 changes of inclination, should we have no 
 crevasses ? We should certainly have less of 
 them, but they would not wholly disappear. 
 For other circumstances exist to throw the 
 ice into a state of strain, and to determine its 
 fracture. The principal of these is the more 
 rapid movement of the centre of the glacier, 
 
 268. Helped by the labors of an eminent 
 man, now dead, the late Mr. Wm. Hopkins, 
 of Cambridge, let us master the explanation 
 of this point together. But the pleasure of 
 mastering it would be enhanced if we could 
 see beforehand the perplexing and delusive 
 appearances accoun'-jd for by the explana- 
 tion. Could my wishes be followed out, I 
 would at this point of our researches carry 
 you off with me to Basel, thence to Thun, 
 thence to Interlaken, thence lo Grindelwald, 
 where you would rind yourself in the actuat 
 presence of the Wetteihorn arid the Eiger, 
 with all the greatest peaks of the Bernese 
 Oberland, the Finsteraaihorn, the Schreck-r 
 horn, the Mouch, the Jungfrau, at hand. 
 At Grindelwald, as we have already learned, 
 there are two well known glaciers theObe? 
 Grindelwald and the Unter Grindelwald gla- 
 ciers on tlu latter of which our observa- 
 tions should commence. 
 
 269. Dropping down from the village tQ 
 the bottom of the valley, we should breast 
 the opposite mountain, and with the 'great 
 limestone precipices of the Wetterhorn to 
 our left, we should get upon a path which 
 commands a view of the glacier. Here we 
 should see beautiful examples of the opening 
 of crevasses at the summit of a brow, and 
 their closing at the bottom. But the chief 
 point of interest would be the crevasses 
 formed at the side of this glacier the mar- 
 ginal crevasses, as they may be called. 
 
 270. We should find the side copiously fis^ 
 sured, even at those places where the centre 
 is compact ; and we should particularly no- 
 tice that the lissures would neither run in 
 the direction of the glacier nor straight 
 across it, but that they would be oblique to it, 
 inclosing an angle of about 45 degrees with 
 the sides. Starling from the side of the gla- 
 cier the crevasses Avould be seen to point up- 
 ward ; that is to say, the ends of the fis- 
 sures abutting against the bounding moun- 
 tain would appear to be drrjgtd down. Were 
 
113 
 
 THE FORMS OF WATER 
 
 L S 5? C 
 
 
 
 
 m 
 
 B T T D 
 
 FIG. 7. 
 
 you less instructed than you now are, I 
 might lay a wager that the aspect of these 
 fissures would cause you to conclude that the 
 centre of the glacier is left behind by the 
 quicker motion of the sides. 
 
 271. This indeed was the conclusion 
 drawn by M. Agassiz from this very appear- 
 ance, before he had measured the motion of 
 the sides and centre of the glacier of the 
 Unteraar. Intimately versed with the treat- 
 ment of mechanical problems, Mr. Hopkins 
 immediately deduced the obliquity of the 
 lateral crevasses from the quicker How of the 
 centre. Standing beside the glacier with 
 pencil and note-book in hand," I would at 
 once make the matter clear to you thus. 
 
 272. Let A c, in the annexed figure, be one 
 side of Ihe glacier, and B D the other ; and 
 let the direction of motion be that indicated 
 t>y the arrow. Let s T be a transverse slice 
 of the glacier, taken straight across it, say to- 
 day. A few days or weeks hence this slice 
 wi'll have been carried -down, and because the 
 centre moves more quickly than the bides it 
 will not remain straight, but will bend into 
 the form s' T'. 
 
 273. Supposing T i to be a small square of 
 the original slice near the side of the glacier. 
 In its new position the square will be distort- 
 ed to the lozenge-shaped figure T' i'. Fix 
 your attention upon the diagonal T i of the 
 square : in the lower position this diagonal, 
 if the ice could stretch, would be lengthened 
 to T' i'. But the ice does not stretch ; it 
 breaks, and we have a crevasse formed at 
 right angles to T' i. The mere inspection of 
 the diagram will assure you that the crevasse 
 will point obliquely upward. 
 
 274. Along the whole side of the glacier 
 the quicker movement of the centre produces 
 a similar state of strain ; and the conse- 
 quence is that the sides are copiously cut by 
 those oblique crevasses, even at places where 
 the centre is free from. them. 
 
 275. It is curious to see at other places the 
 transverse fissures of the centre uniting with 
 those at the sides, so as to form great curved 
 crevasses which stretch across the glacier 
 A 'rom side to side. The convexity of the 
 curve is turned upward, as mechanical prin- 
 ciples declare it ought to be. But if you 
 were ignorant of those principles, you would 
 never infer frorn the aspect of these curves 
 the quicker motion of the centre. In land- 
 slips, and in the motion of parliaily indurat- 
 ed mud, you may sometimes notice appear- 
 ances similar to those exhibited by the ice. 
 
 41. LONGITUDINAL CREVASSES 
 
 276. "We have thus unravelled the orlgia 
 of both transverse and marginal creva-ses. 
 But where a glacier issues from a steep and 
 narrow defile upon a comparatively level 
 plain which allows it room to expand later- 
 ally, its motion is in part arrested, and the 
 level portion has to bear the thrust of the 
 steeper portions behind. Here the line of 
 thrust is in the direction of the glacier, 
 while the direction at right angles to this is 
 one of tension. Across this latter the glacier 
 breaks, and longitudinal crevasses are formed. 
 
 277. Examples of this kind of crevasse are 
 furnished by the lower pait of the Glacier of 
 the Rhone, when looked down upon from 
 the Gi imsel Pass, or from any commanding 
 point on the flanking mountains. 
 
 42. CREVASSES IN RELATION TO CUIIVA- 
 TUKE OP GLACIER. 
 
 278. One point in addition remains to bo 
 discussed, and your present knowledge will 
 enable you to master it in a moment. You 
 remember at an early period of our researches 
 that we crossed the Mer de Glace from the 
 Chapeau side to the Montanvert side. I then 
 desired you to notice that the Chapeau side 
 of the glacier was more fissured than either 
 the centre or the Montanvert side (75). Why 
 should this be so ? Knowing as we now do 
 that the Chapeau side of the glacier moves 
 more quickly than the other, that the point 
 of maximum motion does not lie on the cen- 
 tre but far east of it, we are prepared to an- 
 swer this question in a peifectly satisfactory 
 manner. 
 
 279. Let AB and c D, in the following dia- 
 gram, represent the two curved sides of the 
 Mer de Glace at the Montanvert, and let m n 
 be a straight line across the glacier. Let o 
 be the point of maximum motion. The me- 
 chanical state of the two sides of the glacier 
 may be thus made plain. Supposing the line 
 m n to be a straight elastic string with its 
 ends fixed ; let it be grasped firmly at tho 
 point o by the finger and thumb, and drawn 
 too, keeping the distance between o' and the 
 side c D constant. Here the length, n o of 
 the string would have stretched to n o\ and 
 the length m o to m o', and you see plainly 
 that the stretching of the short line, in com- 
 parison with its length, is greater than that 
 of the long line in comparison with its length. 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 FIG. 8. 
 
 In other words, the strain upon no' is greater 
 than that upon m o' ; so that if one of them 
 were to break under the strain, it would be 
 the short one. 
 
 280. These two lines represent the condi-, 
 tious of strain upon the two sides of the gla- 
 cier. The sides are held back, and the cen- 
 tre tries to move on, a strain being thus set 
 up between the centre and sides. But the 
 displacement of the point of maximum mo- 
 tion through the curvature of the valley 
 makes the strain upon the eastern ice greater 
 than that upon the western. The eastern 
 s..".c of the glacier is therefore more crevassed 
 than the western. 
 
 281. Here indeed resides the difficulty of 
 getting along the eastern side of the Mer de 
 Glace : a difficulty which was one reason for 
 our crossing the glacier opposite to the Mon- 
 tanvert. There are two convex sweeps on 
 the eastern side to one on the western side, 
 hence on the whole the eastern side of the 
 Mer de Glace is most riven. 
 
 g 43. MORAINE- RIDGES, GLACIER TABLES, 
 AND SAND CONES. 
 
 282. When you and I first crossed the "Mer 
 de Glace from Trelapoite to the Couvercle, 
 we I'ound that the stripes of rocks and rub- 
 bish which constituted the medial moraines 
 were ridges raised above the general level of 
 the glacier to a height at some places of 
 twenty or thirty feet. Oil examining these 
 ridges we found the rubbish to be superficial, 
 and that it rested upon a great spine of ice 
 which ran along the back of the glacier. By 
 what means has this ridge of ice been raised V 
 
 283. Most boys have read the story of Dr. 
 Franklin's placing bits of cloth of various 
 colors upon snow on a sunny day. The bits 
 of cloth sank in the snow, the dark ones 
 jnost. 
 
 284. Consider this experiment. The sun's 
 rays first of all fail upon the upper surface 
 of the cloth and warm it. The neat is then 
 conducted through the cloth to the under 
 surface, and the under surface passes it oil to 
 the snow, which is finally liquefied by the 
 heat. It is quite manifest that the quantity 
 of snow melted will altogether depend upon 
 the amount of heat sent from tiie upper to 
 the under surface of the cloth. 
 
 285. Now cloth is what is called a bad 
 conductor. It does not permit heat to travel 
 freely through it. B .i where it has merely 
 to pass through the thickness of a single bit 
 of cloth, a jjood quantity of the heat y;eta 
 
 through. But if you double or treble or 
 quintuple the thickness of the cloth ; or, 
 what is easier, if you put several pieces one 
 upon the other, you come at length to a 
 point where no sensible amount of heat could 
 get through from the upper to the under sur- 
 face. 
 
 286. What must occur if such a thick 
 piece, or such a series of pieces of cloth, 
 were placed upon snow on which a strong 
 sun is falling? The snow round the cloth is 
 melted, but that underneath the cloth is pro- 
 tected. If the action continue long enough 
 the inevitable result will be that the level of 
 the snow all round the cloth will sink, and 
 the cloth will be left behind perched upon 
 an eminence of snow. 
 
 287. If you understand this, you have al- 
 ready mastered the cause of the moraine- 
 ridges. They are not produced by any 
 swelling of the ice upward. But the ice 
 underneath the rocks and rubbish being pro- 
 tected from the sun, the glacier right and 
 left melts away and leaves a ridge behind. 
 
 288. Various other appearances upon the 
 glacier are accounted for in the same way. 
 Here upon the Mer de Glace we have flat 
 slabs of rock sometimes lifted up on pillars 
 of ice. These are the so-called Glacier Ta- 
 bks. They are produced, not by the growth 
 of a stalk of ice out of the glacier, but by 
 the melting of the glacier all round the icu 
 protected by the stone. Here is a sketch of 
 one of the Tables of the Mer de Glace. 
 
 289. Notice moreover that a glacier table 
 is hardly ever set square upon its pillar. It 
 generally leans to one side, and repeated ob- 
 servation teaches you that it so leans as to 
 enable you always to draw the north and 
 south line upon the glacier. For the sun be- 
 ing south of the zenith at noon pours its rays 
 against the southern end of the table, while 
 the northern end remains in shadow. The 
 southern end, therefore, being most warmed 
 does not protect the ice underneath H so ef- 
 fectually as the northern end. The table be- 
 comes inclined, and ends by sliding bodily 
 olf its pedestal. 
 
 290. In the figure opposite we have what 
 may be called an ideal table. The oblique 
 lines represent the direction of the sunbeams, 
 and the consequent tilting of the table here 
 shown resembles that observed upon t^ 
 glaciers. 
 
 291. A pebble will not rise thus : like 
 Franklin's single bit of cloth, a dark-colored 
 pebble sinks in the ice. A spot of black 
 mould will not rest upon the surface, but 
 will sink ; and various parts of the Glacier du 
 Geant are honeycombed by the sinking of 
 such spots of dirt into the ice. 
 
 292. "But when the dirt is of a thickness 
 sufficient to protect the ice the case is differ- 
 ent. Sand is often washed away by a stream 
 from the mountains, or from the moraines, 
 and strewn over certain spaces of the glacier. 
 A most curious action follows . the sanded 
 surface rises, the part on which the sand lies 
 thickest rising highest. Little peaks and 
 eminences jut forth, and when the distribi*- 
 
114 
 
 THE FORMS OF WATER 
 
 Fro. 9. 
 
 tkm of the sand is favorable, and the action 
 wifficiently prolonged, you have little moun- 
 tains formed, sometimes singly, and some- 
 times grouped so as to mimic the Alps 
 themselves. The Sand Cones of the Mer de 
 Glare are not striking ; but on the Gorncr, 
 the Aletseh, the Morteratsch, and other gla- 
 cieis, they form singly and in groups, reach- 
 ing sonic-limes a height of ten or twenty 
 feet. 
 
 44. THE GLACIER MILLS OR MOULINS. 
 
 29:J You and I have learned by long ex 
 poiience the character of the Mer de Glace. 
 We have marched over it daily, with a defi- 
 nite object in view, but we have not closed 
 our eyes to nher objects. It is from side 
 glimpses of things which are not at the mo 
 ineut occupying our attention that fresh sub- 
 jects of inquiry arise in scientific investiga- 
 tion. 
 
 21)4. Thus in marching over the ice near 
 Trclaporte we were often struck by a sound 
 resembling low rumbling thunder. We 
 subsequently sought out the origin of this 
 sound, and found it. 
 
 25)5. A large area of this portion of the 
 glacier is unbroken. Driblets of water have 
 room to form rills, rills to unite and form 
 Rtifjims, streams to combine to form rush- 
 iiur brooks, which sometimes cut deep chan- 
 nels in the ice. Sooner or later these streams 
 reach a strained portiou of the glacier, where 
 a crack is formed across the stream. A way 
 is thus opened for the water to the bottom of 
 the glacier. By long action the stream 
 hollows out a shaft, the crack thus becoming 
 the starting-point of a funnel of unseen 
 depth, into which the water leaps with the 
 sound of thunder. 
 
 290. This funnel and its cataract form p 
 glacier Mill or Moulin. 
 
 v 2i7. Let me grasp your hand firmly while 
 you stand upon the edge of this shaft and 
 'look into it. The hole, with its pure blue 
 shimmer, is beautiful, but it is terrible. Ia- 
 
 Fio. 10. 
 
 cautious persons have fallen into these 
 shafts, a second or two of bewilderment bo 
 ing followed by sudden death. But caution 
 upon the glaciers and mountains ought, by 
 habit, to be made a second nature to explor- 
 ers like you and me. 
 
 298. The crack into which the stream first 
 descended to form the moulin, moves down 
 with the glacier. A succeeding portion of 
 the ice reaches the place where the breaking 
 strain is exerted. A new crack is then 
 formed above the moulin, which is thence- 
 forth forsaken by the stream, and moves 
 downward as an empty shaft. Here upon 
 the Mer de Glace, in advance of the Grand 
 Moulin we see no less than six of these for- 
 saken holes. Some of them we sound to a 
 depth of 90 feet. 
 
 299. But you and I both wish to deter- 
 mine, if possible, the entire depth of the Mer 
 de Glace. The Grand Moulin offers a chance 
 of doing this which we must not neglect 
 
IN CLOUDS AND RIVERS. ICE AND GLACIERS. 
 
 Our tlrst effort to sound the moulin fails 
 through the breaking of our cord by the im- 
 petuous plunge of tho water. A lump of 
 grease in the hollow of a weight enables a 
 mariner to judge of a sea bottom. We em- 
 
 C" ' such a weight, but cannot reach the 
 of the g'uc er. A depth of 163 feet is 
 the utmost rtajhed by our plummet. 
 
 300. From July 28th to August 8th we have 
 watched the progress of the Grand Moulin. 
 On the former date the position of the mou- 
 lin was fixed. On the 31st it had moved 
 down 50 inches ; a little more than, a day 
 afterward it had moved 74 inches. On 
 August 8lh it had moved 198 inches, which 
 gives an average of about 18 inches in 
 twenty four hours. No doubt next summer 
 upon the Mer de Glace a Grand Moulin will 
 be found thundering near Trolaporte ; but 
 like the crevasse of Hie Grand Plateau, al- 
 ready referred to ( 1(5), it will not be our 
 moulin. This, or i at her the ice which it 
 penetrated, is now probably more than a 
 mile lower down than it was in 1857. 
 
 45. THE CHANGES OF VOLUME OF WATER 
 
 BY HEAT AND COLD. 
 
 SOI. We have noticed upon the glacier 
 shafts and pits filled with water of the most 
 delicate blue. In some cases these have been 
 the shafts of extinct moulins closed at the 
 bottom. A. theory has been advanced to ac- 
 count for them, which, though it may be un- 
 tenable, opens out considerations regarding 
 '.he properties of water that ought to be 
 familiar to inquirers like you and me. 
 
 302. In our dissection of lake ice by a 
 beam of heat ( 11) we noticed little vacu- 
 ous spots at the centres of the liquid flowers 
 formed by the beam. These spots we re- 
 ferred to the fact that when ice is melted the 
 water produced is less in volume than the 
 ice, and that hence the water of the flower 
 was not able to occupy the whole space 
 covered by the flower. 
 
 303. Let us more fully illustrate this sub- 
 ject. Stop a small flask water-tight with a 
 cork, and through the cork introduce a 
 narrow glass tube also water-tight. It is 
 easy to fill the flask with water "so that the 
 liquid shall stand at a certain height in the 
 glass tube. 
 
 304. Let us now warm the flask with the 
 flame of a spirit-lamp. On first applying the 
 flame you notice a momentary sinking of the 
 liquid in the glass tube. This is due to the 
 momentary expansion of the flask by ha.t ; 
 it becomes suddenly larger when the flame is 
 first applied. 
 
 305. But the expansion of the water soon 
 overtakes that of the flask and surpasses it. 
 We immediately see the rise of the liquid 
 column in the glass tube, exactly as mercury 
 rises in the tube of a watmed thermometer. 
 
 800. Our glass tube is ten inches long, and 
 at starting the water stood in it at a height of 
 five inches. We will apply the spirit-lamp 
 flame until the water rises quite to the top of 
 the tube and trickles over. This experiment 
 suffices to show the expansion of the wale? 
 
 by heat. 
 
 307. We now take a common finger-glass 
 and put into it a little pounded ice and "salt. 
 On this we place the flask, and ihen build 
 round it the freezing mixture. The liquid 
 column retreats dosvn, the tube, proving the 
 contraction of the liquid by cold. We allow 
 the shrinking to continue for some minutes, 
 noticing that the downward retreat of the 
 liquid becomes gradually slower, and that it 
 finally ceases altogether. 
 
 308~. Keep your eye upon the liquid col- 
 umn ; it remains quiescent for a fi action of 
 a minute, and then moves once more. But 
 its motion is now upward instead of down- 
 ward. The freezing mixture now acts exactly 
 li/ce theflatne. 
 
 309. It would not be difficult to pass a 
 thermometer through the cork inio the 
 flask, and it would tell us the exact tempera- 
 ture at which the liquid ceased to coii tract 
 and began to expand. At that mumtnt we 
 should find the temperatuic of the liquid a 
 shade over 39 Fahr. 
 
 310. At this temperature, then, water at- 
 tains its maximum density. 
 
 311. Seven degrees below this temperature, 
 or at 32 Fahr., the liquid begins to turn into 
 solid crystals of ice, which you know swims 
 upon water because it is bulkier for a given 
 weight. In fact, this halt of the apprracb- 
 ing molecules at the temperature of 39", is 
 but the preparation for the subsequent act of 
 crystallization, in which the expansion by 
 cold culminates. Up to the point of solidin> 
 cation the increase of volume is slow and 
 gradual ; while in the act of solidification it 
 is sudden, and of overwhelming strength. 
 
 312. By this force of expansion the Floren- 
 tine Academicians long ago burst a sphere 
 of copper nearly three quaiters of an inch in 
 thickness. By the same force the celebrated 
 astronomer Huyghens burst in 1GG7 iron can- 
 nons a finger breadth thick. Such experi- 
 ments have been frequently made since. 
 Major Williams during a severe Quebec win- 
 ter filled a mortar with water, and closed it 
 
 i by driving into its muzzle a plug of wood. 
 
 i Exposed to a temperature 50 Fahr. below 
 the freezing point of water, the mKal resist- 
 ed the strain, but the plug gave wy.y, b( ing 
 projected to a distance of 400 feet. At 
 Warsaw howitzer shells bave been thus ex- 
 ploded ; and you and I have shivered thick 
 bomb-shells to fragments by placing them 
 for half an hour in a freezing m'xture. 
 
 313. The theory of the shafts and pits re- 
 ferred to at the beginning of inis section is 
 this : The water at the surface of the shaft 
 is warmed by the sun, say to a temperature 
 of 39 Fahr. The water at the I ottom, in 
 contact with the ice, must be at 32 or near 
 it. The heavier water is therefore at the 
 top ; it will descend to the bottom, melt the 
 ice there, and thus deepen the shaft. 
 
 314. The circulation here refeired to un- 
 doubtedly goes on, and some curious ejects 
 are due to it ; but not, I think, the one here 
 ascribed to it. The deepening of a shaft im- 
 plies a quicker melting of its bottom than of 
 
115 
 
 THE FORMS OF WATER 
 
 the surface of the glacier. It is not easy to 
 see how the fact of the solar heat being first 
 absorbed by water, and then conveyed by it 
 10 the bottom of the shaft, should make the 
 melting of the bottom more rapid than that 
 of the ice which receives the direct impact 
 of the solar rays. The surface of the glacier 
 must sink at least as rapidly as the bottom of 
 the pit, so that the circulation, though actu- 
 ally existing, cannot produce the cited as- 
 cribed to it. 
 
 40. CONSEQUENCES FLOWING FROM THE 
 FOREGOING PROPERTIES OF WATER. 
 CORRECTION OF ERRORS. 
 
 315. I was not much above your age when 
 the pioperly of water ceasing to contract by 
 cold at a temperature of 39" Fahr. was made 
 known to me, and 1 still remember the im- 
 pression it made upon me. For I was asked 
 to consider what would occur in case this 
 solitary exception to an otherwise universal 
 law ceased to exist. 
 
 316. I was asked to reflect upon the con- 
 dition of a lake stored with fish and offering 
 its surface to very cold air. It was made 
 clear to me that the water on being first 
 chilled would shrink in volume and become 
 heavie/, that it would theiefore sink and 
 have its; p'ate supplied by the warmer and 
 lighter wati-r fiom the deeper portions of the 
 lake. 
 
 317. It was pointed out to mi; that without 
 the law referred to this process of circulation 
 would go on until th^ whole watir of the 
 lake had been lowered to the free/ing tem- 
 per ami e. Congelation would then begin, 
 and would continue as long as any waler re- 
 mained to be solidified. One consequence of 
 this would be to destroy every living thing 
 contained in the lake. Other calamities 
 weie added, all of which weie said to be 
 prevented by the perfectly exceptional ar- 
 rangement, that after a certain time the colder 
 water becomes the lighter, floats on the sur- 
 face of the lake, is there congealed, thus 
 throwing a protecting roof over the life 
 below. 
 
 318. Count Rumford, one of the most solid 
 of scientific men, writes in the following strain 
 about this question : '' It does not appear to 
 me that there is anything which human 
 sagacity can fathom, within the wide-extend- 
 ed bounds of the visible creation, which 
 affoidsa more striking or more palpable 
 proof of the wisdom of the Creator, and of 
 the special care He has taken, in the general 
 arrangement of the uuiveise, to preserve ani- 
 mal M'e, than this wonderful contrivance. 
 
 31S). " Let me beg the attention of my 
 leaders while 1 endeavor to investigate this 
 imobt interesting subject ; and let me at the 
 :*umc time bespeak his candor and indul- 
 gence. I feel the danger to which a mortal 
 exptMses himself who has the temerity to ex- 
 plain the designs of Infinite Wisdom. The 
 enterprise is adventuious, but it surely can- 
 not be improper. 
 
 320. "Had not Providence interfered on 
 iiua occasion in a manner which may well 
 
 be considered as m^'acul<ms t all the fresh 
 water within the polar circle must, inevitably 
 have been frozen to a veiy great depth iu 
 winter, and every plant ami tree destroyed." 
 
 321. Through many pages of his book 
 Count Rumford continues in \\\\ strain to 
 expound the ways and intentions of the Al- 
 mighty, and he does cot hesitate to apply 
 very harsh words to those who cannot share 
 his notions. He calls them hardened and de- 
 graded. \Ve aie here .warned :>f the fact, 
 which is too often forgotten, that tlie pleas- 
 ure or comfort of a belief, or the warmth or 
 exaltation of feeling which it produces, is no 
 guarantee of its truth. For the whole of 
 Count Rumford's delight and enthusiasm in 
 connection with this subjact, and the wholo 
 of his ire against those who did not share his 
 opinions, were founded upon an erroneous 
 notion. 
 
 322. Water is not a solitary exception to 
 an otherwise general law. There are other 
 molecules than those of this lieniid which re- 
 quire more room in the soliel c-ijstalline con- 
 dition than in the adjacent molten condition. 
 Iron is a case in point. Solid iron floats 
 upon molten iron exactly as ice floats upon 
 water. Bismuth is a still more impressive 
 case, and we could shiver a bomb as cer- 
 tainly by the solidification of bismuth as by 
 that of water. There is no fish to be taken 
 care of here, still the " contrivance" is the 
 same. 
 
 323. I am reluctant to mention (hem iu 
 the same biealh with Count liumioid, but I 
 am told that in our own day theie are people 
 who profess to find the comforts of a religion 
 in a superstition lower than any that has 
 hitherto degraded the civilized human mind. 
 So that the happiness of a faith and the truth 
 of a failh are two totally different things. 
 
 324. Life and the conditions of life are in 
 necessary haimony. This is a truism, for 
 without the suitable conditions life could not 
 exist. But both life and its conditions set 
 forth the operations of inscrutable Power. 
 We know not its origin ; we know not its 
 end. And the presumption, if not the eleg- 
 radation, rests with those who place upon 
 the throne of the universe a magnified image 
 of themselves, and make its doings a mere 
 colossal imitation of their own. 
 
 47. THE MOLECULAR MKC/TANISM OF 
 WATER-CONGELATION. 
 
 82.1. But let us return to our science. 
 How are we to picture this act of expansion 
 on the part of freezing water ? By what oper- 
 ation do the molecules demand wilh such 
 irresistible emphasis more room in the solirl 
 than in the adjacent liquid condition ? In all 
 cases of this kiud we must derive our con. 
 ceptions from the world of the. senses, and 
 transfer them afterward to a world transcend- 
 ing the range of the senses. 
 
 32(>. You have not forgotten our conver- 
 sation regarding " atomic poles" (^ 10), and 
 how the notion of polar force came to be ap- 
 plied to crystals. With this fiesh in^ your 
 memory, you will have no great 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 117 
 
 Ft? 
 
 In understanding how expansion of volume 
 may accompany tho act of crystallization. 
 
 327. I place a number of magnets before 
 , ju. Thpy, as matter, are affected by grav- 
 ty, and, if perfectly free, they would move 
 
 toward each other in obedience to the attrac- 
 tion of gravity. 
 
 328. But they are not only matter, but 
 magnetic matter. They not only act upon 
 each other by the simple force of gravity, 
 but by the polar force of magnetism. Im- 
 agine them placed at a distance from each 
 other, and perfectly free to move. Gravity 
 first makes itself felt and draws them to- 
 gether. For a time the magnetic force issu- 
 ing from the poles is insensible ; but when a 
 certain nearness is attained, 1he polar force 
 comes into play. The mutually attracting 
 points close up, the mutually repellent points 
 retreat, and it is easy to see that this action 
 may produce an arrangement of the magnets 
 which requires more room. Suppose them 
 surrounded by a box which exactly incloses 
 them at the moment the polar force first 
 comes into play. It is easy to see that in ar- 
 ranging themselves subsequently the repelled 
 corners and ends of the magnets may be 
 caused to nress against the Bides of the box, 
 and even to burst ii, if the forces be suffi- 
 ciently strong. 
 
 329. Here then we have a conception which 
 may be applied to the molecules of water. 
 They, like the magnets, are acted upon by 
 two distinct forces. For a time, while thr. 
 
 
 FIG. 11. 
 
 liquid ia being coole.l they approach each 
 other, in obedience to their general uttractioo 
 for each other. But at a certa! i point new 
 "**ces, some attractive, some repulsive, ema^ 
 
 noting from special points of the molecules, 
 come into plav. The attracted points closa 
 up, the repelled points retreat. Thus tLo 
 molecules turn and rearrange themselves, 
 demanding, as they do so, more space, and 
 overcoming all ordinary resistance by the 
 energy of their demand. This, in general 
 terms, is an explanation of the expansion of 
 water in solidifying : it would be easy to 
 construct an apparatus for its illustration. 
 
 48. THE DIKT BANDS OF THE MEII DE 
 GLACE. 
 
 330. Pass from bright sunshine into a 
 moderately lighted room ; for a time all ap- 
 pears so dark that the objects in the room are 
 not to be clearly distinguished, Hit violent- 
 ly by the waves of light ( 3) the optic nervo 
 is numbed, and requires time to recover its 
 sensitiveness. 
 
 331. It is for this reason that I choose the 
 present hour for a special observation on iho 
 Mer de Glace. The sun has sunk behind the 
 ridge cf Charmoz, and the surface of the 
 glacier is in sober shade. The main portion 
 of our day's work is finished, but we have 
 still sufficient energy to climb tho slopes ad- 
 jacent to the Montanvert to a height of a 
 thousand feet or thereabout above the ice. 
 
 332. We now look fairly down upon the 
 glacier, and see it less foreshortened than 
 from the Montanvert. We notice the dirt 
 overspreading its eastern side, due to the 
 crowding together of its medial moraines. 
 We see the comparatively clean surface of 
 the Glacier du Geant ; but we notice upon 
 tills surface an appearance which we have 
 not hitherto seen. It is crossed by a series 
 of gray bent bands, which follow each other 
 in "succession, from Trelaporte downward. 
 We count eighteen of theso from our present 
 position. (See sketch. Fig. 12.) 
 
 333. These are the Dirt Bands of the Mcr 
 <le Glace ; they were first observed by Pro- 
 fessor Forbes in 18i2. 
 
 334. They extend down theglackr further 
 than we can sec : and if we cross the valley 
 of Chamouni, r,ml climb the mountains 
 ;U the opposite side, to a point near the 
 little auberge, called La Fle'gere, we shall 
 command a view of the end cf the glacier 
 and observe the completion of the series cf 
 bands. We notice that they are confined 
 throughout to the portion of the glacier do- 
 rived from the Col du Geant. (See sketch, 
 Fig. 11.) 
 
 335. We must trace them to their source. 
 You know how noble and complete a view 
 is obtained of the glacier and Col de Gt'*iut 
 from the Cleft Station above Trelaporte. 
 Thither we must once more climb : <uul 
 thence we can see the succession of Lands 
 stretching downward to the Montanvert, 
 and upward to the base of tho ice-casciuile 
 upon the Glacier du Ge'ant. The cascade is 
 evidently concerned in their formation. (See 
 Bketch, Fig. 13.) 
 
 336. And how ? Simply enough. The gla- 
 cier, as we know, is broken transversely a,t the 
 summit of the ice- fall, and descends the do 
 
1115 
 
 THE FOKMS OF WATEIi 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 119 
 
 clivity in a series of great transverse ridges. 
 At the base of the fall, the chasms are 
 closed, but the ridges in part remain, forming 
 protuberances, which run like vast wrinkles 
 across the glacier. These protuberances are 
 more and more bent because of the quicker 
 motion of the centre, and the depressions be- 
 tween them form receptacles for the fine 
 mud and ddbris washed by the little rills from 
 tLe adjacent slopes. 
 
 '337. The protuberances sink gradually 
 through the wasting action of the sun. so 
 that long before Tre'laporte is reached they 
 have wholly disappeared. Not so the dirt of 
 which they were the collectors : it continues 
 to occupy, in transverse bands, the flat sur- 
 face of the glacier. At Trelaporte, moreover, 
 where the valley becomes narrow, the bands 
 are much sharpened, obtaining there the 
 character which they afterwaul preserve 
 throughout the Mer de Glace. Other glaciers 
 with cascades also exhibit similar bands. 
 
 49. SEA ICE AND ICEBERGS. 
 
 338. We are now equipped intellectually 
 for a campaign into another territory. Water 
 becomes heavier and more difficult to freeze 
 when salt is dissolved in it. Sea water is 
 therefore heavier than fresh, and the Green- 
 land Ocean requires to freeze it a temperature 
 3 degrees lower than frish water. When 
 concentrated till its specific gravity reaches 
 1.1045, sea water requires for its congelation 
 a temperature 1S degrees lower than the or- 
 dinary freezing-point. 
 
 339. But even when the water is saturated 
 with salt, the crystallizing force studiously 
 rejects the salt, and devotes itself to the con- 
 gelation of the water alone. Hence the ice 
 of sea water, when melted, produces fresh 
 water. The only saline particles existing in 
 such ice are those entangled mechanically in 
 its pores. They have no part or lot in the 
 structure of the crystal. 
 
 340. This exclusiveness, if I may uso the 
 term, of the water molecules ; this entire re- 
 jection of all foreign elements from the edi- 
 fices which they build, is enforced to a sur- 
 prising degree. Sulphuric acid has so strong 
 an affinity for water that it is one of the most 
 powerful agents known to the chemist for 
 the removal of humidity from air. Still, as 
 shown by Faraday, when a mixture of sul- 
 phuric acid and water is frozen, the crys- 
 tal formed is perfectly sweet and free from 
 acidity. The water alone has lent itself to 
 tho crystallizing force. 
 
 341. Every winter in the Arctic regions 
 the sea freezes, roofing itself with ice of 
 enormous thickness and vast extent. By the 
 summer heat, and the tossing of the waves, 
 this is broken up ; the fragments are drifted 
 by winds and borne by currents. They 
 clash, they crush each other, they pile them- 
 selves into heaps, thus constituting the chief 
 danger encountered by mariners in the polar 
 seas. 
 
 342. But among the drifting masses of flat 
 sea-ice, vaster masses sail, which spring from 
 * totally different source These are the lc&- 
 
 bergs of the Arctic seas. They rise some- 
 times to an elevation of hundreds of fet't 
 above the water, while the weight of ice sub- 
 merged is about seven times that seen above. 
 
 343. The first observers of striking natural 
 phenomena generally allow wonder and im- 
 agination more than their due place. But to 
 exclude all error arising from this cause, I 
 will refer to the journal of a, cool and intrepid 
 Arctic navigator, Sir Leopold McClintock. 
 He describes an iceberg 250 feet high, which 
 was aground in 500 feet of water. This 
 WoUld make the entire height of the berg 75$ 
 feet, not an unusual altitude for the greater 
 icebergs. 
 
 344. From Baffin's Bay 1 hese mighty masses 
 come sailing down through Davis' Straits into 
 the broad Atlantic. A vast amount of heat 
 is demanded for the simple liquefaction of ice 
 ( 48) ; and the melting of icebergs is on this 
 account so slow, that when large they some- 
 times maintain themselves till they have been 
 drifted 2000 miles from their place of birth. 
 
 345. What is their origin? The Arctic 
 glaciers. From the mountains in the interior 
 the indurated snows slide into the valleys and 
 fill them with ice. The glaciers thus formed 
 move like the Swiss ones, incessantly down- 
 ward. But the Arctic glaciers reach the sea, 
 enter it, often ploughing up its bottom into 
 submarine moraines. Undermined by the 
 lapping of the waves, and unable to resist the 
 strain imposed by their own weight, they 
 break across, and discharge vast masses into 
 the ocean. Some of these run aground on 
 the adjacent shores, and often maintain 
 themselves for years. Others escape south- 
 ward, to be finally dissolved in the warm 
 waters of the Atlantic. The first engraving 
 on this pasje is copied from a photograph 
 taken by Mr. Bradford during a recent ex- 
 pedition to the Northern seas. The second 
 represents a mass of ice upon the Glacier 
 des Bossons. Their likeness suggests their 
 common origin. 
 
 50. THE ^GGISCHHORN, THE MABGELIN 
 AND ITS ICEBEHGS. 
 
 346. I am, however, unwilling that you 
 should quit Switzerland without seeing such 
 icebergs as it can show, ?nd indeed there are 
 other still nobler glaciers than the Mer de 
 Glace with which you ought to be ac- 
 quainted. In tracing the Rhone to its 
 source, you have already ascended the valley 
 of the Rhone. Let us visit it again together ; 
 halt at the little town of Viesch, and go from 
 it straight up to the excellent hostelry on the 
 slope of the ^Eggischhorn. This we shall 
 make our headquarters while we explore 
 that monarch of European ice-streams the 
 great Aletsch glacier. 
 
 347. Including the longest of its branches, 
 this noble ice-river is about twenty miles 
 long, while at the middle of its trunk it meas- 
 ures nearly a mile and a quarter from side 
 to side. The grandest mountains of the Ber- 
 nese Oberland, the Jun.irf'rau, the Monch, the 
 Trusberg, the Aletschhorn, the Breithorn, 
 the Gletscherhorn, and many another noble 
 
130 
 
 THE FORMS OF WATER 
 
 peak and ridge, arc the collectors of its neves. 
 From three great valleys formed in the heart 
 of the mountains these neves are poured, 
 uniting together to form the trunk of the 
 Aletsch at a place named by a witty moun- 
 taineer, the " Place de la Concorde of Na- 
 ture." If the phrase be meant to convey 
 the ideas of tranquil grandeur, beauty of 
 form, and purity of hue, it is well bestowed. 
 JB4.S. Our hotel is not upon the peak of the 
 ,/Eiggischhorn, but a brisk morning walk 
 soon places us upon the top. Thence we see 
 the glacier like a broad river stretching up- 
 ward to the roots of the Jungfrau, and 
 downward past the Bel Alp toward its end. 
 Prolonging the vision downward, we strike 
 the noblest mountain group in all the Alps 
 the Dora and its attendant peaks, the Matter- 
 
 horn and the Weisshorn. The scene indeed 
 is one of impressive grandeur, a multitude of 
 peaks and crests here unearned contributing 
 to its glory. 
 
 840. But low down to our right, and sur- 
 rounded by the sheltering mountains, is an 
 object the beauty of which startles those who 
 are unprepared for it. Yonder we see the 
 naked side of the glacier, exposing glistening 
 ice-cliffs sixty or seventy feet high. It 
 would seem as if the Aletsch here were en- 
 gaged in the vain attempt to thrust an arm 
 through a lateral valley. It once did so ; but 
 the arm is now incessantly broken off close 
 to the body of the glacier, a great space for- 
 merly covered by the ice being occupied by 
 its water of liquefaction. A lake of the love- 
 liest blue is thus formed, which reaca^ quit 
 
 Fio. 14. 
 
 Fro. 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 13Z 
 
 to the base of the ice-cliffs, saps them, as the 
 Arctic waves sap the Greenland glaciers, and 
 receives from them the broken masses which 
 it has undermined. As we look down upon 
 the lake, small icebergs sail over the tranquil 
 surface, each resembling a snowy swan ac- 
 companied by its shadow. 
 
 350. This is the beautiful little lake of 
 Margeliu, or, as the Swiss here call it, the 
 Margolin See. You see that splash, and imme- 
 diately afterward hear the sound of the plung- 
 ing ice. Tho glacier has broken before our 
 eyes, and dropped an iceberg into the lake. 
 All over the lake the water is set in commo- 
 tion, thus illustrating on a small scale the 
 swamping waves produced by the descent of 
 vast islands of ice from the Arctic glaciers. 
 Look to the end of the lake. It is cumbered 
 with the remnants of icebergs now agiouad, 
 which have been in part watted thither by 
 the wind, but in part slowly borne by the 
 water which moves gently in this direction. 
 
 851. Imagine us below upon the margin of 
 the lake, as I happened to be on one occasion. 
 There is one large and lonely iceberg about 
 the middle. Suddenly a sound like that of a 
 cataract is heard ; we look toward tha ice- 
 berg and see water teeming from its sides. 
 Whence comes the water ? the berg has be- 
 come top-heavy through the melting under- 
 neath ; it is in the act of performing a somer- 
 sault, and in rolling over carries with it a 
 vast quantity of water, which rushes like a 
 waterfall down its sides. And notice that the 
 iceberg, which a moment ago was snowy- 
 white," now exhibits the delicate blue color 
 characteristic of compact ice. It will soon, 
 however, be rendered white again by the ac- 
 tion of the sun. The vaster icebergs of the 
 Northern seas sometimes roll over in the 
 same fashion. A week may be spent with 
 delight and profit at the jEggischhorn. 
 
 51. THE BEL ALP. 
 
 35.2. From the ,Eggisehhorn I might lead 
 you along tlu: mountain ridge by the Bel.ten 
 See, tha fish of which we have already 
 tasted, to the Rieder Alp, and thence across 
 the Aletsch to the Bel Alp. This is a fine 
 mountain ramble, but you and I prefer mak- 
 ing the glacier our 'highway downward. 
 Easy at some places, it is by no means child's 
 play at others to unravel its crevasses. But 
 the steady constancy and close observation 
 which we have bitherto found availing in 
 difficult places do not forsake us here. We 
 clear the fissures ; and, after four hours of 
 exhilarating work, we find ourselves upon the 
 slope lending up to the Bel Alp hotel. 
 
 853. This is one of the finest halting-places 
 in the Alps. Stretching before us up to the 
 ^Eggischhorn and Margelin See is the long 
 last reach of the Aletsch, with its great me- 
 dial moraine running along its back. At hani 
 is the wild gorge of the M-issa, in which the 
 snout of the glacier lies couched like the head 
 of a serpent. The beautiful system of tha 
 Oberaletsch glaciers is within easy reach. 
 Above us is a peak called the Sparrenhorn, 
 "ccessible t.o the most moderate climber, and 
 
 on the summit of which little more than an 
 hour's exertion will place you and me. Below 
 us now is the Oberaletsch glacier, exhibiting 
 the most perfect of medial moraines. Near us 
 is the great mass of the Aletsch horn, clasped 
 hy its neve's, and culminating in brown rock. 
 It is supported by other peaks almost as noble 
 as itself. The Nestliorn is at hand ; while 
 sweeping round to the west we strike the 
 glorious triad already referred to, the Weiss- 
 horn, the Maiterhorn, and the Dom. Take 
 one glance at the crevasses of the glacier im- 
 mediately below us. It tumbles at its end 
 down a Bteep incline, and is greatly riven. 
 But the crevasses open before the steep part 
 is reached, ami you notice the coalescence of 
 marginal and transverse crevasses, produc- 
 ing a system of curved fissures with the con- 
 vexities of the curves pointing upward. 
 The mechanical reason of this is now known 
 to you. The glacier-tables are also numerous 
 and line. I should like to linger with you 
 here for a week, exploring the existing gfa- 
 ciers. and tracing out the evidences of others 
 that have passed away. 
 
 52. THE RIFFELBEKG AND GORNER 
 GLACIER. 
 
 354. And though our measurements and 
 observations on the Mer de Glace are more 
 or less representative of all that can be made 
 or solved elsewhere, I am unwilling to leave 
 you unacquainted with the great system of 
 glaciers which stream from the northern 
 slopes of Monte R^sa and the adjacent moun- 
 tains. From the Bel Alp we can descend to 
 Brieg, and thence drive to Visp ; but you 
 anl Lprefer the breezy heights, so we sweep 
 round 'the promontory of the Nessel, until we 
 stand over the Rhone valle3 r , in front of Visp. 
 From this village an hour's walking carries 
 us to Stalden, where the valley divides into 
 two branches : the one leading through Saaa 
 over the Monte Moro, and the other througij 
 St. Nicholas to Zeimatt. The latter is our 
 route. 
 
 355. We reach Zermatt, but do not halt 
 there. On the mountain ridge, 4000 feet 
 above the valley, we discern the Riffelberg 
 hotel. This we reach. Right in front of us 
 is the pinnacle of the Matterhorn, upon the 
 top of which it must appear incredible to you 
 that a human foot could ever tread. Con- 
 stancy and skill, however, accomplished this, 
 but in the first instance at a terrible price. In 
 the little churchyard of Zermatt we have seen 
 the graves of two of the greatest mountaineers 
 that Savoy and England have produced ; and 
 who, with two gallant young companions, fell 
 from the Matterhorn in 1865. 
 
 356. At the Riffelberg we are within an 
 hour's walk of the famous Gorner Grat, 
 which commands so grand a view of the 
 glaciers of Monte Rosa. But yonder huge 
 knob of perfectly bare rock, which is called 
 the Riffelhorn, must be our station. What 
 the Cleft Station is to the Mer de Glace, the 
 Riffelhorn is to the Gorner glacier and its 
 tributaries. From its lower side the rock, 
 easy as it may seem, is inaccessible. Here, 
 
THE FORMS OF WATER 
 
 indeed, in 1865, a fifth good man met his ena, 
 and he also lies beside his fellow-countrymen 
 ia the churchyard of Zermatt. Passing a 
 little tarn, or lake, called the Riffel See, we 
 assail the Riffelhorn on its upper side. It is 
 capital rock-practice to reach the summit; 
 and from it we command a most extraordi- 
 nary scene. 
 
 357. The huge and many-peaked mass of 
 Monte Rosa faces us, and we scan its snows 
 fn/m bottom to top. To the right is the 
 mighty ridge of the Lyskamm, also laden 
 with snow ; and between both lies the West- 
 ertt G-lacier of Monte Rosa. This glacier 
 meets another from the vast snow-fields of 
 the Oima di Jazzi ; they join to form the 
 Gorner glacier, and from their place of junc- 
 tion stretches the customary medial moraine. 
 On this side of the Lyskamm rise two beau- 
 tiful snowy eminences, the Twins Castor and 
 Pollux ; then come the brown crags of the 
 Brei thorn, then the Little :viatteihorn, and 
 then the broad snow-field of trie Theodule, 
 out of which cp rings the Great Matterhorn, 
 and which you and I will cross subsequently 
 into Italy. 
 
 o->3. The valleys and depressions between 
 these mountains are filled with glaciers. 
 D )wn the flunks 01 the Twin Castor comes 
 the Glacier des Jumeaux, from Pollux comes 
 the Schwartze glacier, from the Breithorn 
 the Trifti glacier, then come the Little Mat- 
 terhorn glacier and the Theodule glacier, 
 each, as it welds itself to the trunk, carrying 
 with it its medial moraine. We can count 
 nine such moraines from our present posi- 
 tion. And to a still more surprising degree 
 than on the Mer de Glace, we notice Hie 
 power of the ice to yield to pressure ; the 
 broad neves being squeezed on the trunk of 
 the Goraer into white stripes, which become 
 ever narrower net ween their bounding mo- 
 raines, and finally disappear under their own 
 shingle. 
 
 859. On the two main tributaries we also 
 notice moraines which seem in each case to 
 rise from the body of the glacier, appearing 
 in the middle of the ice without any apparent 
 origin higher up. These at their sources 
 are sub-glacial moraines, which have b^cn 
 rubbed away from rocky promontories en- 
 tirely covered with ice. They lie hidden for 
 a time in the body of the glacier, and appear 
 at the surface where the ice above them has 
 been melted away by the sun. 
 
 SCO. This is the place to mention a notion 
 long entertained by the inhabitants of the 
 high Alps, that glaciers possess the power of 
 thrusting out all impurities from them. On 
 the Mer de Glace you and I have noticed 
 iargxi patches of clay and black mud which 
 evidently came from the body of the glacier, 
 and we can therefore understand how natural 
 was this notion of extrusion to people unac- 
 customed to close observation. But the 
 power of the glacier in this respect is in 
 reality the power of the sun, which fuses the 
 ice above concealed impurities, and, hke the 
 bodies of the guides on the Glacier des Bos- 
 sons (143) brings them to the light of dav 
 
 361. On no other glacier will you find more 
 objects of interest than on the Gorner. Sand 
 cones, glacier-tables, deep ice-gorges cut by 
 streams and bridged fantastically by bould- 
 ers, moulins, sometimes arched ice-caverns 
 of extraordinary size and beauty. On the 
 lower part of the glacier we notice the par- 
 tial disappearance of the medial moraine in 
 the crevasses, ami its reappearance at the foot 
 of the incline. For many years this glacier 
 was steadily advancing on the meadow in 
 front of it, ploughing up the soil and over- 
 turning the chalets in its way. It now shares 
 in the general reticat exhibited during the 
 last fifteen years among the glaciers of the 
 Alps. As usual, a river, the Visp, rushes 
 from a vault at the extremity of the Gorner 
 glacier. 
 
 53. ANCIENT GLACIERS OP SWITZERLAND, 
 
 362. You have not lost the memory of the 
 old moraine, which interested us so much in 
 our first ascent from the source of Ihe Arvei- 
 ron ; for it opened our minds to the fact that 
 at one period of its history the Mer de Glace 
 attained far greater dimensions than it now 
 exhibits. Our experience since that time 
 has enabled us to pursue these evidences of 
 ice action to an extent of which we had tiiea 
 no notion. 
 
 363. Close to the existing glacier, for ex- 
 ample, we have repeatedly seen the mountain- 
 side laid bare by the retreat of the ice. This 
 is especially conspicuous just now, because 
 for the last fifteen or sixteen years the glaciers 
 of the Alps have been steadily shrinking ; so 
 that it is no uncommon thing to see the mar- 
 ginal rocks laid bare for a height of fifty, 
 sixty, eighty, or even one hundred feet above 
 the present glacier. On the rocks thus ex- 
 posed we see the evident marks of the slid- 
 ing ; and our e} 7 es and minds have been so 
 educated in the observation of these appear- 
 ances that we are now able to detect, with 
 certainty, icemarks, or moraines, ancient or 
 modern, wherever they appear. 
 
 364. But the elevations at which we have 
 found such evidence might well shake be-lief 
 in the conclusions to which they point. 
 Beside the Massa Goige, at 1000 feet above 
 the present Aletsch, we found a great old 
 moraine. Descending the meadows between 
 the Be! Alp and Plat ten, we found another, 
 now clothed with grass, and bearing a village 
 on its back. But I wish to carry you to a 
 region which exhibits these evidences on a 
 still grander and more impressive scale. We 
 have already taken a brief flight to the valley 
 of Hasli and the Glacier of the Aar. Let us 
 make that glacier our starting-point. Walk- 
 irg from it downward toward the Grimsel, 
 we pass everywhere over rocks singularly 
 rounded, an 1 fluted, and scarred. These 
 appearances are manifestly the work of the 
 glacier in recent times. But we approach the 
 Grimsel, and at the turning of the valley 
 stand before the precipitous granite flank of 
 the mountain. The traces of the ancient ice 
 are here as plain as they are amazing. The 
 rocks are so hard that not only the fluting 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 129 
 
 and polishing, but even the fine scratches 
 '^hich date back unnamable thousands of 
 years are as evident as if they had been made 
 yesterday. We may trace these evidences to 
 a height of two thousand feet above the pres- 
 ent valley bed. It is indubitable that an 
 ice-river of this astounding depth once flowed 
 through the vale of Hasli. 
 
 365. Yonder is the summit of the Siedel- 
 h rn ; and if we gain it the Unteraar glacier 
 \v :.'! lie like a map below us. From this 
 commanding point we plainly see marked 
 upon the mountain-sides the height to which 
 the ancient ice extended. The ice ground 
 pail of the mountains is clearly distinguished 
 from the splintered crests which in those dis- 
 tant clays rose above the surface of the glacier, 
 and which must have then appeared as island 
 peaks and crests in the midst of an ocean of 
 ice. 
 
 366. We now scamper down the Siedelhora, 
 get once more into the valley of Hasli, along 
 which we follow for more than twenty miles 
 the traces of the ice. Fluted precipices, pol- 
 ished slabs, and beautifully-rounded granite 
 domes. Right and left upon the mountaiu 
 flanks, at great elevations, the evidences ap- 
 pear. We follow the footsteps of the glacier 
 to the Lake of Brientz ; and if we prolonged 
 our inquiries, we should learn that all the 
 lake beds of this region, at the tinia now re- 
 ferred to, bore the burden of immense masses 
 of ice. 
 
 367. Instead of the vale of Hasli, we might 
 take the valley of the Rhone. The traces of 
 a mighty glacier, which formerly rillei it, 
 may be followed all the way to Martiguy, 
 which is 0(J miles distant from the present 
 ice. At Martigny the Rhone glacier was re- 
 inforce t by another from Mont Blanc, and 
 the welJed masses moved onward, planing 
 the mountains right and left, to the lake of 
 Geneva, the basin of which they entirely 
 Billed. Oilier evidences prove that the glacier 
 did not end hare, but pushed across the low 
 country until it encountered the limestone 
 barrier of the Jura Mountains. 
 
 54. ERRATIC BLOCKS. 
 
 368. What are these other evidences ? We 
 have seen mighty rocks poised on the mo- 
 raines of the Mer'de Glace, and we now know 
 that, unless they are split and shattered by 
 the frost, these' rocks will, at some distant 
 day, be landed bodily by the Glacier des Bois 
 in the valley of Chamouni. You have al- 
 ready learned that these boulders often reveal 
 the rnin era logical nature of the mountains 
 among which the glacier has passed ; that 
 specimens are thus brought clown of a char- 
 acter totally different from the rocks among 
 which they are finally landed ; this is striK- 
 ingly the case with the erratic block* strandeJ 
 along the Jura. 
 
 361). For the Jura itself, as already stated, 
 ia limestone ; there is no trace of native 
 granite to be found among these hills. Still 
 along the breast of the mountain above the 
 town of Neufchatel, and at about 800 feet 
 above the lake of Neufchatel, we find 
 
 stranded a belt of granite boulders from 
 Mont Blanc. And when we clear the soil 
 away from the adjacent mountain-side, we 
 find upon the limestone rocks the scarrings 
 of the ancient glacier which brought the 
 boulders here. 
 
 370. The most famous of these rocks, 
 called the Pierre il Bot, measures 50 feet in 
 length, 40 in height, and 20 in width. Mul- 
 tiplying these three numbers together, we 
 obtain 40,000 cubic feet as the volume of the 
 boulder. 
 
 371. But this is small compared to som 
 of the rocks which constitute the freight of 
 even recent glaciers. Let us visit another of 
 them. We have already been to Stalden, 
 where the valley divides into two branches, 
 the right branch running to St. Nicholas and 
 Zermatt, and the left one to Saas and the 
 Monte Moro. Three hours above f:iaas we 
 come upon the end of the Allelein glacier, 
 not filling the main valley, but thrown 
 athwart it so as to sto-p its drainage like a dam. 
 Above this ice-dam we have the Matt mark 
 Lake, and at the head of the lake a small inn 
 well known to travellers over the Monte Moro. 
 
 372. Close to ihis inn is the greatest bould- 
 er that we have ever seen. It measures 
 240,000 cubic feet. Looking across the val- 
 ley we notice a glacier with its present end 
 half a mile from the boulder. The stone, I 
 believe, is serpentine, and were you and I to 
 explore the Schwartzberg glacier to its upper 
 fastnesses, we should find among them* the 
 birthplace of this gigantic stone. Four-and- 
 forty years ago, when the glacier reached the 
 place now occupied by the boulder, it landed 
 there its mighty freight, and then retreated. 
 There is a second ice-borne rock at hand, 
 which would be considered vast were it not 
 dwarfed by the aspect of its huger neighbor. 
 
 373. Evidence of this kind might be multi. 
 plied to any extent. In fact, at this moment, 
 distinguished men, like Professor Favre of 
 Geneva, are determining from the distribu- 
 tion of the erratic blocks the extent of the 
 ancient glaciers of Switzerland. It was, 
 however, an engineer named Venctz that first 
 brought these evidences to light, and an- 
 nounced to an incredulous \vorld the vast 
 extension of the ancient ice. M. Agassiz 
 afterward developed and wonderfully ex- 
 panded the discovery. Pehaps the most in- 
 teresting observation regarding ancient gla- 
 ciers is that of Dr. Hooker, who, during a 
 recent visit to Palestine, found the celebrated 
 Cedars of Lebanon growing upon ancient 
 moraines. 
 
 55. ANCIENT GLACIERS OF ENGLAND, 
 IRELAND. SCOTLAND, AND WALES. 
 
 374. At the time the ice attained this extra- 
 ordinary development in the Alps, many 
 other portions of Europe, where no glaciers 
 now exist, were covered with then). In the 
 Highlands of Scotland, among the mount.uins 
 of England, Ireland, and Wales, the ancient 
 glaciers have written their story as plainly 
 as in the Alps themselves I should like te 
 wander with you through Borrodale in Cuia- 
 
124 
 
 THE FOKMS OF WATER. 
 
 berland, or through the valleys near Beth- 
 gellert in Wales. Under all tlie beauty of the 
 present scenery we should discover the me- 
 morials of a time when the whole region was 
 locked in the embrace of ice. Professor 
 Ramsay is especially distinguished by his 
 writings oil the- ancient glaciers of Wales. 
 
 875. We have made the acquaintance of 
 the Keeks ot Magillicuddy as the great con- 
 densers of Atlantic vapor. At the time now 
 referred to, this moisture did not fall as soft 
 and fructifying rain, but as snow, which 
 formed the nutriment of great glaciers. A 
 chain of lakes now constitutes the chief at- 
 traction of Killarney, the Lower, the Middle, 
 and the Upper Luke. Let us suppose our- 
 selves rowing toward the head of the Upper 
 Lake with the Purple Mountain to our left. 
 Remembering our travels ill the Alps, you 
 would infallibly call my attention to the 
 planing of the rocks, and declare the action 
 to be unmistakably that of glaciers With 
 our attention thus sharpened, we land at the 
 heal of the lake, and walk up the Black 
 Valley to the bas of Magillicuddy 's Keeks. 
 Your conclusion would be, that this valley 
 tells a tale as wonderful as that of Hasli. 
 
 876. We reaeli our boat and row home- 
 ward along the Upper Lake. Its islands 
 now possess a new interest for us. Some of 
 them are bare, others are covered wholly or 
 in part with luxuriant vegetation ; but both 
 the naked and clothed islands are glaciated. 
 The weathering of ages has not altered their 
 forms ; there are the Cannon Rock, the 
 Giant's Coffin, the Man-of-War, all sculp- 
 tured as if tlie chisel had passed over them in 
 our own lifetime. These lakes, now fringed 
 with tender woodland beauty, were all occu- 
 pied by the ancient ice. It has disappeared, 
 and seeds from other regions have been 
 wafted thither to sow the trees, the shrubs, 
 the ferns, and the grasses which now beau- 
 tify Killarney. Mun himself, they say, lias 
 made his appearance in the world since that 
 time of ice ; but of the real period and manner 
 of man's introduction little is professed to be 
 known since, to make them square with sci- 
 ence, new. meanings have been found for the 
 beautiful myths and stories of the Bible. 
 
 377. It is the nature and tendency of the 
 human mind to look backward and lor ward ; 
 to endeavor to restore the past and predict 
 the future. Thus endowed, from data pa- 
 tiently and painfully won, we recover in 
 idea a state of things which existed thou- 
 sands, it may be millions, of years before the 
 history of the human race began. 
 
 56. THE GLACIAL Erocn. 
 
 378. This period of ice-extension lias been 
 named the Glacial Epoch. In accounting 
 for it great minds have fallen into grave er- 
 rors, as we shall presently see. 
 
 379. The substance on which we have 
 thus far been working exists in three differ- 
 ent states : as a solid in ice ; as a liquid in 
 water ; as a gas in vapor. To cause it to 
 pass from one of these states to the next fol- 
 lowing one, heat is necessary. 
 
 330. Dig a hole in the ice of the Mer do 
 Glare in summer, and p ace si thermometer 
 in the hole ; it will suuid ut 32 Fa.hr. Dip 
 your thermometer into one of the glacier 
 streams; it will still mark 32. The water is 
 therefore as cold us ice. 
 
 381. Hence the whole of the. heat j-oured 
 by the sun upon the glacier, and which has 
 I een absorbed by the glacier, is expended ia 
 simply liquefying the ice, and not in render- 
 ing either ice or water a single degree warmer. 
 
 382. Expose water to a lire ; it becomes 
 hotter for a time. It boils, and from that 
 moment it ceases to g t h< tter. After it has 
 begun to boil, all the hrni communicated by 
 the fire is carried away by the steam, t?iouyh 
 the steam itself is not the least fraction -fa de- 
 gree hotter than the wafer. 
 
 883. In fact, simply to liquefy ice a large 
 quantity of heat is necessary, and to vaporize 
 water a still larger quantity is necessary. 
 And inasmuch as thi> heat does not render 
 the water warmer than the ice, nor the steam 
 warmer than the water, it was at, one time 
 supposed to be hidden in the water and in the 
 Fleam. And it was therefore called late/it 
 heat. 
 
 884. Let us ask how much heat must the 
 sun expend in order to convert a pound 
 weight of the tropical ocean into \upor? 
 This problem has been ticcurately solved by 
 experiment. It would require in round num- 
 bers 1000 times the amount of heat necessary 
 to raise one pound of water one degree in 
 temperature. 
 
 383. But the quantity of heat which wou'.d 
 raise the tempi rature of a pound of water 
 one degree would raise the tempt rature of a 
 pound of iron ten degrees. This has been 
 also proved by experiment. IK-nee to con- 
 vert one pound of the tropical ocean into 
 vapor i he sun must expend 10,000 times as 
 much heat as would laise one pound of iron 
 one degree in temperature. 
 
 386. This quantity of heat wr.uld raise the 
 temperature of 5 Ibs. of iron 20UO degrees, 
 which is the fusing point of cast iron ; at 
 this temperature die metal would not only be 
 white hot, but would be passing into the mol- 
 ten condition. 
 
 387. Consider the conclusions at which we 
 have now arrived. For every pound of 
 tropical vapor, or for every pound of Alpine 
 ice produced by the congelation of that va- 
 por, an amount of heat has been expended 
 by the sun sufficient to raise 5 Ibs. of cast- 
 iron to its melting-point. 
 
 388. It would not be difficult to calculate 
 approximately the weight of the Mer do 
 Glace and its tributaries to say, fbr exam- 
 ple, that they contained so many millions of 
 millions of tons of ice and snow. Let the 
 place of the ice be taken by a mass of white- 
 hot iron of quintuple the weight ; with such 
 a picture before your mind you get some 
 notion of the enormous amount of heat paid 
 out by the sun to produce the present glacier. 
 
 389. You must think over this, until it is 
 as clear as sunshine. For you must never 
 henceforth fall into the error already referred 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 125 
 
 to, and which has entangled so many. So 
 natural was the association of ice and cold, 
 that even celebrated men assumed that all 
 that is needed to produce a great extension 
 of our glaciers is a diminution of the sun's 
 temperature. Had they gone through the 
 foregoing reflections and calculations, they 
 would probably have demanded more heat 
 instead of less for the production ot a " gla- 
 cial epoch." What they really needed were 
 condensers sufficiently powerful to 'congeal 
 the vapor generated by the heat of the sun. 
 
 57. GLACIER THEORIES. 
 390. You have not forgotten, and hardly 
 ever can forget, our climbs to the Cleft Sta- 
 tion. Thoughts were then suggested which 
 we have not yet discussed. We saw the 
 branch glaciers coming down from, their 
 neves, welding themselves together, pushing 
 through Trelaporte, and afterward moving 
 through the sinuous valley of the Mer de 
 Glace. These appearances alone, without 
 taking into account subsequent observa- 
 tions, were sufficient to suggest the idea that 
 glacier ice, however hard and brittle it may 
 appear, is really a viscous substance, resem- 
 bling treacle, or honey, or tar, or lava. 
 
 53. DILATATION AMD SLIDING THEORIES. 
 
 891. Still this was not the notion expressed 
 by the majority of writers upon glaciers. 
 Scheuchzer of Zurich, a great naturalist, vis- 
 ited the glaciers in 1705, nnd propounded a 
 theory of their motion. Water, he knew, 
 expands in freezing, nnd the force of expan- 
 sion is so great that thick bomb-shells filled 
 with water, and permitted to freeze, are, as 
 we know (812), shattered to pieces by the ico 
 within. Scheuchzer supposed that the wa- 
 ter in the fissures of the irlacier-s, freezing 1 
 there and expanding with resistless force, 
 was the power which urged the glacier 
 downward. He added to this theory other 
 notions of a less scientific kind. 
 
 392. Many years subsequently, De Char- 
 pentier of Bex renewed and developed this 
 theory with such ability and completeness 
 that it was long known as Ciiarpemier'a 
 Theory of Dilatation. M. Agassiz lor a time 
 espoused this theory, and it was aijo more or 
 less distinctly held by other wriieis. The 
 glacier, in fact, was considered to be a mag- 
 azine of cold, capable of freezing all water 
 percolating through it. The theory was 
 abandoned when this notion of glacier cold 
 was proved by M. Agassiz to be untenable. 
 
 393. In 17b'0, Altmanu and Griiner pro- 
 pounded the view that glaciers moved by 
 sliding over their beds. Nearly lorty years 
 subsequently, this notion was revived by De 
 Saussure, and it has therefore been called 
 "De Saussure's Theory," or the "Sliding 
 Theory," of glacier motion. 
 
 394. There was, however, but little reason 
 to connect the name of De Saussure with this 
 or any other theory of glaciers. Incessantly 
 occupied in observations of another kind, 
 this celebrated man devoted very little time 
 or thought to the question of glacier motion. 
 
 What he has written upon the subject reads 
 less like the elaboration of a theory than the 
 expression of an opinion. 
 
 59. PLASTIC THEORY. 
 
 395. By none of these writers is the prop- 
 erty of viscosity or plasticity ascribed to gla- 
 cier ice ; the appearances of many glaciers 
 are, however, so suggestive of this idea that 
 we may be sure it would have found Ujore 
 frequent expression, were it not in such ap- 
 parent contradiction with our every day ex- 
 perience of ice. 
 
 396. Still the idea found its advocates. In 
 a little book, published in 1773, and entitled 
 "Picturesque Journey to the Glaciers of 
 Savoy," Bordier of Geneva wrote thus : 
 " It is now time to look at all these objects 
 with the eyes of reason ; to study, in the 
 first place, the position and the progression 
 of glaciers, and to seek the solution rf their 
 principal phenomena. At the first aspect of 
 the ice-mountains an observation presents it- 
 self, which appears sufficient to explain all. 
 It is that the entire mass of ice is connected 
 together, and presses from above downward 
 after the manner of fluids. Let us then re- 
 gard the ice, not as a mass entirely rigid and 
 immobile, but as a heap of coagulated 
 matter, or as softened wax, flexible and duc- 
 tile to a certain point." Here probably for 
 the first time the quality of plasticity is as- 
 cribed to the ice of glaciers. 
 
 397. To us, familiar with the aspect of tho 
 glaciers, it must seem strange that this idea 
 once expressed did not at once receive recog- 
 nition and development. But in those early 
 days explorers were few, and the " Pictur- 
 esque Journey" probably hut little known,, 
 so that the notion of plasticity lay dormant 
 for more than half a century. But Bordier 
 was at length succeeded by a man of far 
 greater scientific rapp and insight than him- 
 self. This was Rendu, a Catholic priest and, 
 canon when he wrote, and afterward Bishop < 
 of Annecy. In 1841 Rendu laid before the 
 Royal Academy of Sciences of Savoy his- 
 "Theory of the Glaciers of Savoy," a con- 
 tribution forever memorable in relation to; 
 this subject. 
 
 398. Rendu seized the idea of glacier plas- 
 ticity with great power and clearness, and-; 
 followed it resolutely to its consequences. 
 It is not known that ho had ever seen the 
 work of Bordier ; probably not, as he never- 
 mentions it. Let me quote for you some of 
 Rendu's expressions, which, however, fail to 
 give an adequate idea of his insight and -pre- 
 cision of thought: "Between the Mer de 
 Glace and a river there is a resemblance BO 
 complete that it is imposiible to find in the. 
 glacier a circumstance which does not exist 
 in the river. In currents of water the mo- 
 tion is not uniform either throughout their 
 width or throughout their depth. The fric- 
 tion of the bottom and of the sides, with the 
 action of local hindrances, causes the motion 
 to vary, and only toward the middle of the 
 surface do we obtain the full motion." 
 
 399. This reads like a prediction of what 
 
126 
 
 THE FORMS OF WATER 
 
 has since been established by measurement. 
 Looking at the glacier of Mont Dolent, which 
 resembles a sheaf in form, wide at both ends 
 and narrow in the middle, and reflecting tbat 
 *he upper wide part had become narrow, and 
 the narrow middle part again wide, Rcndu 
 observes, " There. is a multitude of facts 
 which seem to necessitate the belief that gla- 
 eiar ice enjoys a kind of ductility which en- 
 ables if, to 'mould itself to its locality, to thin 
 out, to swell, and to contract as if it were a 
 soft paste." 
 
 400. To fully test his conclusions, Rendu 
 required the accurate measurement of glacier 
 motion. Had he added to his other endow- 
 ments the practical skill of a land-surveyor, 
 he would now be regarded as the prince of 
 glacialists. As it was he was obliged to be 
 content with imperfect measurements. In 
 one of his excursions he examined the guides 
 regarding- the successive positions of a vast 
 rock which he found upon the ice close to 
 the side of the glacier. The mean of five 
 years gave him a motion for this block of 40 
 feet a year. 
 
 401.' Another block, the transport of which 
 he subsequently measured more accurately, 
 gave him a velocity of 400 feet a year. Note 
 his explanation of this discrepancy : '* The 
 enormous difference of these two ol)eerva- 
 tions arises from the fact that one block 
 stood near the centre of the glacier, which 
 moves most rapidly, while the other stood 
 noar the side, where the ice is held back by 
 friction. " So clear and definite were Rcndu's 
 ideas of the plastic motion of glaciers, that 
 had the question of curvature occurred to 
 him, I entertain no doubt that he would 
 have enunciated beforehand the shifting of 
 the point of maximum motion from side to 
 side across the axis of the glacier ( 25). 
 
 402. It is right that you should know that 
 scientific men do not always agree in their 
 estimates of the comparative value of facts 
 ?md ideas ; and it is especially right that you 
 should know that your present tutor attaches 
 a very high value 'to ideas when they spring 
 from the profound and persistent pondering 
 of superior minds, and are not, as is too 
 often the case, thrown out without the war- 
 rant of either deep thought or natural capac- 
 ity. It is because I believe Rendu's labors 
 fulfil this condition that I ascribe to them so 
 high a value. But when you become older 
 and better informed, you may differ from 
 i-iie ; and I write these words lest you should 
 too readily accept my opinion of Rendu. 
 Judge me,' if you care to do so, when your 
 knowledge is matured. I certainly shall not 
 ! ear your verdict. 
 
 403. But, much as I prize the prompting 
 idea, and thoroughly as I believe that often 
 in it the force of genius mainly lies, it would, 
 in my opinion, be an error of omissioa of the 
 gravest kind, and which, if habitual, would 
 insure the ultimate decay of natural knowl- 
 lge, to negL-ct verifying our ideas, and giv- 
 ng them outward reality and substance when 
 '.lie means of doing so are at hand. In sci- 
 j nce, thought, as far as possible, ought to foe 
 
 wedded to fact, This was attempted by 
 Rendu, and in great part accomplished by 
 Agassiz and Forbes. 
 
 GO. Viscous THEORY. 
 
 404. Here indeed the merits of the distin- 
 guished Racialist last named rise conspicu- 
 ously to view. From the able and earnest 
 advocacy of Professor Forbes, the public 
 knowledge of this doctrine of glacial plastic- 
 ity is aUnost wholly derived. He gave the 
 doctrine a more distinctive form ; he first 
 applied the term viscous to glacier ice, and 
 sought to found upon precise measurements 
 a "viscous Theory" of glacier motion. 
 
 405. I am here obliged to state facts in 
 their historic sequence. Professor Forbes 
 when he began his investigations was ac- 
 quainted with the labors of Rendu. In his 
 earliest work upon the Alps he refers to 
 those labors in terms of flattering recogni- 
 tion. But though as a ma te:- of f act lien- 
 du's ideas were there to prompt him, it would 
 be too much to say that he needed their in- 
 spiration. Had Rcndu not preceded him, 
 he might none the less have grasped the idea 
 of viscosity, executing his measurements and 
 applying his knowledge to maintain it. Be 
 that as it may, the appearance of Professor 
 Forbes on the Unteraar glacier in 1841, and 
 on the Mer cle Glace in 1842, and his labors 
 then and subsequently, have given him a 
 name not to be forgotten in the scientific his- 
 tory of glaciers. 
 
 406. The theory advocated by Professor 
 Forbes was enunciated by himself in these 
 words : "A glacier is an imperfect fluid, or 
 viscous body, which is urged down slopes of 
 certain inclination by the natural pressure of 
 its parts." In 1773 Bordier wrote thus: 
 "As the glaciers always advance upon the 
 plain, and never disappear, it is absolutely 
 essential that new ice shall perpetually take 
 the place of that which is melted : it must 
 therefore be pressed forward from above. 
 One can hardly refuse then to accept the as- 
 tonishing truth, that this vast extent of haul 
 and solid ice moves as a single piece down- 
 ward." In the passage already quoted he 
 speaks of the ice being pressed as a fluid 
 from abore. Tirap constitute, I believe, 
 Bordier's contributions to this subject. The 
 quotations show his sagacity at an early 
 date ; but, in point of completeness, his 
 views are not to be compared with those of 
 Rendu nnd Forbes. 
 
 407. I must not omit to state here that 
 though the idea of viscosity has not been es- 
 poused by M. Agassiz, his measurements, 
 and maps of measurements, on the Unteraar 
 glacier have been recently cited as the most 
 clear and conclusive illustrations of a quality 
 which, at all events, closely resembles vis- 
 cosity. 
 
 408. But why, with proofs before him 
 mem; copious and characteristic than those of 
 any other observer, does M. Agassiz hesitate 
 to accept the idea of viscosity as applied to 
 ice ? Doubtless because he believes the no- 
 tion to be contradicted by our cvery-day ex- 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 perience of the substance. 
 
 409. Take a mass of ice ten or even fifteen 
 cubic feet in volume ; drpw a saw across it 
 to a depth of half an inch or an inch ; and 
 strike a pointed pricker, not thicker than a 
 very small round tile, into the groove ; the 
 substance will split from top to bottom with 
 a clean crystalline fracture. How is this 
 brittleness to be; reconciled with the notion 
 of viscosity ? 
 
 410. We have, moreover, been upon the 
 glacier and have witnessed the birth of cre- 
 vasses. We have seen* them beginning as 
 narrow cracks suddenly formed, days being 
 iL'Ljuired to open them a single inch. In 
 many glaciers fissures may be traced narrow 
 and profound for hundreds of yards through 
 the ice. What does this prove? Did the 
 ice possess even a very small modicum of 
 that power of stretching, which is character- 
 istic of a viscous substance, such crevasses 
 coul'i not be formed. 
 
 411. Slill it is undoubted that the glacier 
 moves like a viscous body. The centre ilows 
 past the sides, the top flows over the bottom, 
 and the motion through a curved valley cor- 
 responds to fluid motion. Mr. Mathews, 
 Mr. Froude, and above all Signer Bianconi, 
 h;ive, moreover, recently made experiments 
 on ice which strikingly illustiate the flexibil- 
 ity of the substance. These experiments 
 merit, und will doubtless receive, lull atten- 
 tion at a future time. 
 
 61. REGELATIOX THEORY. 
 
 412. I will now describe to you an attempt 
 that has been made of late years to reconcile 
 the brittleness of ice with its motion in gla- 
 ciers. It is founded on the observation, 
 made by Mr. Faraday in 1850, that when 
 two pieces of thawing ice are placed to- 
 gether they freeze together at the place of 
 contact. 
 
 ii:. This fact may not surprise ^^d ; still 
 it ;U (.idsed Mr. Faraday and others, and 
 in (i or very great distinction in science have 
 differed in their interpretation of the fact. 
 The difficulty is to explain where, or how, in 
 ice already thawing the cold is to be found 
 requisite to freeze tne film of water between 
 the two touching surfaces. 
 
 414. The word Regelaiion was proposed by 
 Dr. Hooker to express the freezing together 
 of t\vo pieces of thawing ice observed by 
 Faraday ; and the memoir in which the term 
 was first used was published by Mr. Huxley 
 an;! ilr. Tyncla'.l in the Philosophical Trans- 
 actions for I8,"i7. 
 
 415. The fact of regulation, and its appli- 
 cation irrespective of the cause of rcgelation, 
 may be tnus illustrated : Saw two slabs 
 from a block of ice, and bring their flat sur- 
 faces into contact, ; the}' immediately freeze 
 together. Two platen of ice, laid one upon 
 the other, with flannel round them over 
 night, ate sometimes so firmly frozen in the 
 morning that they will rather break else- 
 whei'e Uum along their surtV.ee of junction. 
 If you erter one of the dripping ice-eaves of 
 Switzerland, you have only, to press for a 
 
 'moment a slab of ice against the roof of tLe 
 uave to cause it to freeze there and stick to 
 tfhe roof. 
 
 416. Place a number of fragments of ice 
 in a basin of water, and cause them to touch 
 each other ; they freeze together where they 
 touch. You can form a chain of such frag- 
 ments ; and then, by taking hold of one end 
 of the chain, you can draw the whole series 
 after it. Chains of icebergs are sometimes 
 formed in this way in the Arctic seas. 
 
 417. Consider what follows from these ob- 
 servations. Snow consists of small particles 
 of ice. Now if by pressure we squeeze out 
 the air entangled in thawing snow, and bring 
 the little ice-granules into close contact, they 
 may be expected to freeze together ; and if 
 the expulsion of the air be complete, the 
 squeezed snow may be expected to assume 
 the appearance of compact ice. 
 
 418. We arrive at this conclusion by rea- 
 soning ; let us now test it by experiment, 
 employing a suitable hydraulic press, and a 
 mould to hold the snow. In exact accord- 
 ance with our expectation, we convert by 
 pressure the snow into ice. 
 
 419. Place a compact mass of ice in a 
 proper mould, and subject it to pressure. It 
 breaks in pieces ; squeeze the pieces forcibly 
 together ; they reunite by regelation, and a 
 compact piece of ice, totally different in 
 shape from the first one, is taken from the 
 rress. To produce this effect the ice must bo 
 in a thawing condition. When its tempera- 
 ture is much below the melting-point it is 
 crashed by pressure, not into a pellucid 
 mass of another shape, but into a white 
 powder. 
 
 430. By means of suitable moulds you 
 may in this way change the shape of ice to 
 any extent, turning out spheres, and cups ; 
 and rings, and twisted ropes of the sub- 
 stance ; the change of form in these cases 
 being effected through rude fracture and re- 
 gelation. 
 
 421. By applying the pressure carefully, 
 rude fracture may be avoided, and the ice 
 compelled slowly to change its form as if it 
 were a plastic body. 
 
 422. Now our first experiment illustrates 
 the consolidation of the snows of the higher 
 Alpine regions. The deeper layers of tha 
 neve have to bear the weight of all above 
 them, and are thereby converted into more or 
 less perfect ice. And our last experiment 
 illustrates the changes of form observed upon 
 the glacier, where, by the slow and constant 
 application of pressure, the ice gradually 
 moulds itself to the valley which it fills. 
 
 423. In glaciers, however, we have also 
 ample illustrations of rude fracture and re-ge- 
 lation. The opening and closing of crevasses 
 illustrate this. The glacier is broken on the 
 cascades and mended at their bases. When 
 two branch glaciers lay their sides together, 
 the regelatiou is so firm that they begin im- 
 mediately to flow in the trunk glacier as a 
 single stream. The medial moraine gives n 
 indication by its slowness of motion that it ib 
 derived from the sluggish ice of the sides o 
 
128 
 
 THE FORMS OF WA^ER 
 
 the branch glaciers. 
 
 424. The gist of the Revelation Theory is 
 that the ice of glaciers changes its form and 
 preserves its continuity under pressure -which 
 keeps its particles together. But when sub- 
 jected to tension, sooner than stretch it breaks, 
 and behaves no longer as a viscous body. 
 
 62. CAUSE OF REGELATION. 
 436. Here the fact of regelation is applied 
 to explain the plasticity of glacier ice, no 
 attempt being made to assign the cause of 
 regelation itself. They are two entirely dis- 
 tinct questions. But a little time will be 
 well spent in looking more closely into the 
 cause of regelation. You may feel some 
 surprise that eminent men shoukl devote their 
 attention to so small a point, but we must 
 not forget that in nature nothing is small. 
 Laws and principles interest the scientific 
 student most, and these may be as well illus- 
 trated by small things as by large ones. 
 
 426. The question of regelation immediate- 
 ly connects itself with that of " latent heat," 
 already referred to (383), but which we must 
 now subject to further examination. To 
 melt ice, as already stated, a large amount 
 of heat is necessary, and in the case of the 
 glaciers this heat is furnished by the sun. 
 Neither the ice so melted nor the water which 
 results from its liquefaction can fall below 
 32 Fahrenheit. The freezing-point of water 
 and the melting-point of ice touch each 
 other, as it were, at this temperature. A 
 hair's-breadth lower water freezes ; a hair's- 
 breadth higher ice melts. 
 
 427. But if the ice could be caused to melt 
 without this supply of solar heat, a tempera- 
 ture lower than that cf ordinary thawing ice 
 would result. When gnow and salt, or 
 pounded ice and salt, are mixed together, the 
 salt causes the ice to melt, and in this way a 
 cold of 20 or 30 degrees below the freezing- 
 point may be produced. Here, in fact, the 
 ice consumes its own warmth in the work of 
 liquefaction. Such a mixture of ice and salt 
 is called " a freezing mixture." 
 
 428. And if by any other means ice at the 
 temperature of 32* Fahrenheit could be 
 liquefied without access of heat from with- 
 out, the water produced would be colder than 
 the ice. Now Professor James Thomson has 
 proved that ice may be liquefied by mere 
 pressure, and his brother. Sir William Thom- 
 son, has also shown tbat water under press- 
 ure requires a lower temperature to freeze it 
 than when the pressure is removed. Pro- 
 fessor Mousson subsequently liquefied large 
 masses of ice by a hydraulic press ; and by 
 a beautiful experiment Professor Helmholtz 
 has proved that water in a vessel from which 
 the air has been removed, and which is 
 therefore relieved from the pressure of the 
 atmosphere, freezes and forms ice-crystals 
 when surrounded by melting ice. All these 
 facts are summed up in the brief statement 
 that the freezing-point of water is lowered b$ 
 pressure. 
 
 429. For our own instruction we may pro- 
 duce the liquefaction of ice by -pressure in 
 
 the following way : You remember the 
 beautiful flowers obtained when a sunbeam 
 is sent through lake ice ( 11), and you have 
 not forgotten that the flowers always foirn 
 parallel to the surface of freezing. Let us 
 cut a prism, or small column of ice with <be 
 planes of freezing lunning across it at right 
 angles ; we place that prism between two 
 slabs of wood, and bring carefully to bear 
 upon it the squeezing force of a 'small hy- 
 draulic press. 
 
 430. It is well to converge by means of a 
 concave mirror a good light upon the ice, 
 and to view it through a magnifying lens. 
 You already see Ihe result. Hazy surfaces 
 are formed in the very body ct the ice, 
 which gradually expand as ttie pressure is 
 slowly uugmenltu. Heie an-d there you 
 notice something resembling crystallization ; 
 fern-shaped figures run with consideiable 
 rapidity through the ice, and when you look 
 carefully at their points and edges you find 
 them in visible motion. These hazy sur- 
 faces are spaces of liquefaction, and the 
 motion you see is that of the ice falling to 
 water under the pressure. That water is 
 colder than the ice was before the pressure 
 was applied, and if the pressure be relieved 
 not only does the liquefaction cease, but the 
 water re-freezes. The cold produced by its 
 liquefaction under pressure is sufficient to 
 re-congeal it when the pressure is removed. 
 
 431. If instead of diffusing the pressure 
 over surfaces of consideiable extent, we con- 
 centi ate it on a small surface, the liquefac- 
 tion will of course be more rapid, and this is 
 what Mr. Bottomley has recently done in an 
 experiment of singular beauty and interest. 
 Let us support on blocks of wood the two 
 ends of a bar of ice 10 inches long, 4 inches 
 deep, and 3 wide, and let us loop over its 
 middle a copper wire one twentieth, or even 
 one tenth, of an inch in thickness. Con- 
 necting the two ends of the wire together, 
 and suspending from it a weight of 12 or 14 
 pounds, the whole pressure of this weight is 
 concentrated on the ice which supports the 
 wire. What is the consequence ? The ice 
 underneath the wire liquefies ; .the water of 
 liquefaction escapes round the wire, but the 
 moment it is relieved from the pressure it 
 freezes, and round about the wire, even be- 
 fore it has entered the ice, you have a frozen 
 casing. The wire continues to sink in the 
 ico ; the water incessantly escapes, freezing 
 as it does so behind the wire. In half an 
 hour the weight falls ; the wire has gone 
 clean through the ice. You can plainly see 
 where it has passed, but the two severed 
 pieces of ice are so firmly frozen together that 
 they will break elsewhere as soon as along 
 the surface of regelation. 
 
 432. Another beautiful experiment bear, 
 ing upon this point has recently been made 
 by M. Boussingault. He filled a hollow steel 
 cylinder with water and chilled it. In pass- 
 ing to ice, water, as you know, expands 
 ( 45) ; in fact, room for expansion is a nec- 
 essary condition of solidification. But in the 
 present case the strong steel resisted the ex- 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 139 
 
 pansion, the water in consequence remaining 
 liquid at a temperature of more than 30 
 Fahr. below the ordinary freezing-point. A 
 bullet within the cylinder rattle"! about at 
 this temperature si owing that the water was 
 sti!. 1 liquid. On opening the tap the liquid, 
 relieved of the pressure, was instantly con- 
 verted into ice. 
 
 433. It is only substances which expand on 
 solidifying that behave in this manner. The 
 metal bismuth, as we know, is an example 
 similar to water ; while lead, wax, or sulphur, 
 all of which contract on solidifying, have 
 their point of fusion heightened by pressure. 
 
 434. And now you are piepared to under- 
 stand Professor James Thomson's theory of 
 regelation. When two pieces of ice are 
 pressed together, liquefaction, he contends, 
 results. The water spreads out around the 
 points of pressure, and when released re- 
 freezes, thus forming a kind of cement be- 
 tween the pieces of ice. 
 
 63. FARADAY'S VIEW OP REGELATION. 
 
 435. Faraday's view of regelation is not 
 so easily expressed, still I will try to give, 
 you some notion of it, dealing in the iirst 
 place with admitted facts. Water, even in 
 open vessels, may be lowered many degrees 
 below its freezing temperature, and still re- 
 main liquid ; it. may also be raised to a tem- 
 perature far higher than its boiling-point, 
 and still resist boiling. This is due to the 
 mutual cohesion of the water particles, which 
 resists the change of the liquid either into 
 the solid or the vaporous condition. 
 
 436. But if into the over-chilled water you 
 throw a particle of ice. the cohesion is rup- 
 tured, and congelation immediately sets in- 
 And if into the superheated water you intro- 
 duce a bubble of air or of st^am, cohesion is 
 iikwise ruptured, and ebullition immediate- 
 ly commences. 
 
 437. Faraday concluded that in the interior 
 of any body, whether solid or liquid, where 
 every particle is grasped, so to speak, by the 
 surrounding particles, and grasps them in 
 turn, the bond of cohesion is so strong as to 
 require a higher temperature to change the 
 stat? of aggregation than is necessary at the 
 surface. At the surface of a piece of ice, for 
 example, the molecules arc free on one side 
 from the control of other molecules ; and 
 they therefore yield to heat moie reaclily than 
 in the interior. The bubble of air or steam 
 in overheated water also frees the molecules 
 on one side ; hence the ebullition consequent 
 upon its introduction. Practically speaking, 
 then, the point of liquefaction of the interior 
 ice is higher than that of the superficial ice. 
 Faraday also refers to the special solidifying 
 power which bodies exert upon their own 
 molecules. Camphor in a glass bottle fills 
 the bottle with an atmosphere of camphor. 
 In such an atmosphere large crystals of the 
 substance may grow by the incessant deposi- 
 tion of camphor molecules upon camphor, at 
 a temperature too high to permit of the 
 slightest deposit upon the adjacent glass. A 
 similar remark applies to sulphur, phospho- 
 
 rus, and the metals in a state of fusion. 
 They are deposited upon solid portions of 
 their own substance at temperatures not low 
 enough to cause them to solidify against 
 Other subst3nces. 
 
 488. Water furnishes an eminent example 
 of this special solidifying power. It may be 
 cooled ten degrees and more below its freez- 
 ing-point without freezing. But this is not 
 possible if the smallest fragment of ice be 
 floating in the water. It then freezes accu- 
 rately at 32 Fahr., depositing itself, how- 
 ever, not upon th sides of the containing 
 vessel, but upon the ice. Faraday observed 
 in a freezing apparatus thin crystals of ice 
 growing in ice-cold water to a length of six, 
 eight, or ten inches, at a temperature incom- 
 petent to produce their deposition upon the 
 sides of the containing vessel. 
 
 430. And now we are prepared for Fara- 
 day's view of regelation. When the surfaces 
 of two pieces of ice, covered with a film of 
 the water of liquefaction, are brought to- 
 gether, the covering film is transferred from 
 the surface to the centre of the ice, where 
 the point of liquefaction, as before shown, 
 is higher than at the surface. The special 
 solidifying power of ice upon water is now 
 brought into play on both sides of the film. 
 Under these circumstances, Faraday held 
 that the film would congeal, and freeze the 
 two surfaces together. 
 
 440. The lowering of the freezing-point 
 by pressure amounts to no more than one 
 seventieth of a degree Fahrenheit for a whole 
 atmosphere. Considering the infinitesimal 
 fraction of this pressure which is brought 
 into play in some cases of regelation, Fara- 
 day thought its effect insensible. He sus- 
 pended pieces of ice, and brought them into 
 contact without sensible pressure, still they 
 froze together. Professor James Thomson, 
 however, considered that even the capillary 
 attraction exerted between two such masses 
 would be sufficient to produce regelatiou. 
 You may make the following experiments, 
 in further illustration of this subject : 
 
 441. Place a small piece of ice on water, 
 and press it underneath the surface by a 
 second piece. The submerged piece may 
 be so small as to render the pressure infini- 
 tesimal ; still it will freeze to the under sur- 
 face of the superior piece. 
 
 443. Place two pieces of ice in a basin of 
 warm water, and allow them to come to- 
 gether ; they freeze together when they 
 touch. The parts surrounding the place of 
 contact melt away, but the pieces continue 
 for a time united by a narrow bridge of ice. 
 The bridge finally melts, and the pieces for a 
 moment are separated. But capillary attrac- 
 tion immediately draws them together, and 
 regelation sets in once more. A new bridgo 
 is formed, which in its turn is dissolved, the 
 separated pieces again closing up. A kind 
 of pulsation is thus established between the 
 two pieces of ice. They touch, they freeze, 
 a bridge is formed and melted ; and thus the 
 rhythmic action continues until the ice di& 
 appears. 
 
130 
 
 THE FORMS OI WAT.LR 
 
 443. According to Professor James Thom- 
 son's theory, pressure is necessary to lique- 
 fy the ice. The heat necessary for liquefac- 
 tion must be drawn from the ice itself, and 
 the cold water must escape from the pressure 
 to be re-frozen. Now in the foregoing ex- 
 periments the cold water, instead of being 
 allowed to freeze, issues into the warm water, 
 still the floatiEg fragments regelate in a mo- 
 ment. The touching surfaces may, more- 
 over, be convex ; they may be reduced prac- 
 tically to points, clasped all round by the 
 warm water, which indeed rapidly dissolves 
 them as they approach each other ; still they 
 freeze immediately when they touch. 
 
 444. You may learn from this discussion 
 that in scientific matters, as in all others, 
 there is room for differences of opinion. 
 The frame of mind to be cultivated here is a 
 suspension of judgment as long as the mean- 
 ing remains in doubt. It may be that Fara- 
 day's action and Thomson's action come both 
 into play. I cannot do better than finish 
 these remarks by quoting Faraday's own con- 
 cluding words, which show how in his mind 
 scientific conviction dwelt apart from dog- 
 matism : "No doubt," he says, "nice ex- 
 perknents will enable us hereafter to criticise 
 such results as these, and separating the true 
 from the untrue will establish the correct 
 theory of regelation. ' ' 
 
 Gi. THE BLUE VEINS OF GLACIERS. 
 
 445. We now approach the end, one im- 
 portant question only remaining to be dis- 
 cussed. Hitherto we have kept it back, for 
 a wide acquaintance with the glaciers was 
 necessary to its solution. We had also to 
 make ourselves familiar by actual experiment 
 with the power of ice, softened by thaw, to 
 yield to pressure, and to liquefy under such 
 pressure. 
 
 446. Snow is white. But if you examine 
 Its individual particles you would call them 
 transparent, not white. The whiteness 
 arises from the mixture of the ice particles 
 with small spaces of air. In the case of all 
 transparent bodies, whiteness results from 
 such a mixture. The clearest glass or crys- 
 tal when crushed becomes a white powder. 
 The foam of champagne is w r hitc through 
 the intimate admixture of a transparent 
 liquid with transparent carbonic acid gas. 
 The whitest paper, moreover, is composed of 
 fibres which are individually transparent. 
 
 447. It is not, however, the air or the gas, 
 but the optical severance of the particles, giv- 
 ing rise to a multitude of reflections of the 
 white solar light at their surfaces, that pro- 
 duces the whiteness. 
 
 48. The whiteness of the surface of a clean 
 glacier (112), and of the icebergs of the 
 Margelm See (357), has been already referred 
 lo a similar cause. The surface is broken 
 into innumerable fissures by the solar heat, 
 the reflection of solar light from the sides of 
 the little fissures producing the observed ap- 
 pparance. 
 
 449. In like manner if you freeze water in 
 a test-tube by plunging it into a freezing 
 
 mixture, the ice produced is white. For the 
 most part also the ice formed in freezing ma- 
 chines is white. Examine such ice, and you 
 will find it filled with smaU air-bubbles. 
 When the freezing is extremely slow the 
 crystallizing force pushes the air effectually 
 aside, and the resulting ice is transparent 
 when the freezing is rapid, the air is entan- 
 gled before it can escape, and the ice is 
 translucent. But even in the case of quick 
 freezing Mr. Faraday obtained transparent 
 ice by skilfully removing the air-bubbles, as 
 fast as they appealed, with a feather. 
 
 45U. In the case of lake ice the freezing is 
 not uniform, but intermittent. It is some- 
 times slow, sometimes rapid. When slow 
 the air dissolved in the water is effectually 
 squeezed out and forms a layer of bubbles on 
 the under surface of the ice! An act of sud 
 den freezing entangles this air, and hence we 
 find lake ice usually composed of layers alter- 
 nately clear, and filled with bubbles, buci? 
 layers render it easy to detect the planes of 
 freezing in lake ice. 
 
 451. And now for the bearing of these facts. 
 Under the fall of the Geant, at the base of 
 the Talefre cascade, and lower down the Mer 
 de Glace ; in the higher regions of the Grin- 
 delwald, the Aar, the Aletsch and the Gor- 
 ner glaciers, the ice does not possess the 
 transparency which it exhibits near the ends 
 of the glaciers. It is white, or whitish. 
 Why ? Examination shows it to be filled 
 with small air-bubbles ; and these, as we 
 now learn, are the cause of its whiteness. 
 
 452. They are the residue of the air orig- 
 nally entangled in the snow, and connected, 
 as before stated, with the whiteness of the 
 snow. During the descent of the glacier, 
 the bubbles are gradually expelled 'by the 
 enormous pressures brought to bear upon the 
 ice. Not on4y is the expulsion caused by the 
 mechanical yielding of the soft thawing ice, 
 but the liquefaction of the substance at 
 places of violent pressure, opening, as it 
 does, fissures for the escape of the air, must 
 play an important part in the consolidation 
 of the glacier. 
 
 453. The expulsion of the bubbles is, how- 
 ever, not uniform ; for neither ice nor any 
 other substance offers an absolutely uniform 
 resistance to pressure. At the base of every 
 cascade that we have visited, and on the 
 walls of the crevasses there formed, we have 
 noticed innumerable blue streaks drawn 
 through the white translucent ice, and giv- 
 ing the whole mass the appearance of lamina, 
 tion. These blue veins turned out upon ex- 
 amination to be spaces from which the air- 
 bubbles had been almost wholly expelled, 
 translucency being thus converted into trans 
 parency. 
 
 454. This is the veined or ribboned structure 
 of glaciers, regarding the origin of which 
 diverse opinions are now entertained. 
 
 455. It is now our duty to take up the 
 problem, and to solve it if we can. On the 
 neves of the Col du Geant, and other gla- 
 ciers, we have found great cracks, and 
 faults, and Bergschrunds, exposing deep sec- 
 
IN CLOUDS AND RIVERS, ICE AND GLACIEK8. 
 
 231 
 
 ttons of the neve ; aiirl on these sections we 
 have found marked the edges of half-consol- 
 idated strata evidently produced by succes- 
 sive falls of snow. The neve is stratified 
 because its supply of material from the at- 
 mosphere is intermittent, and when we first 
 observed the blue veins we were disposed to 
 regard them as due to this stratification. 
 
 450. But observation and reflection soon 
 dispelled this notion. Indeed, it could hard- 
 ly stand in the presence of the single fact 
 that at the ba^es of the ice falls the veins are 
 always verttctd, or neaily so We saw no 
 way of explaining how the hoiizontal strata 
 of the lien; could be eo tilted up at the base 
 of the fall as to be set on edge. Nor is the 
 aspect of the veins that of stratification. 
 
 457. On the central portions of the cas- 
 cades, moreover, there are no signs of the 
 veins. At the bases they first appear, reach- 
 ing in . each case tliei'* maximum develop- 
 ment a little below the ba.se. As you and I 
 stood upon the heights above the Zasenberg 
 And scrutinized the cascade of the Strahlcck 
 
 tranch of the Grmaelwald glacier, we could 
 jjpt doubt that the base of the fall was the 
 birthplace of the veins. "We called this por- 
 tion of the glacier a " Structure Mill, " inti- 
 
 mating that here, and not on l? : ?, ieve, tne 
 veined structure was manufactured. 
 
 458. This, however, is, at bottom, the lan- 
 guage of strong opinion merely, not tha-t of 
 demonstration ; and in science opinion ought 
 to content us only so long as positive proof 
 is unattainable. " The love of repose must 
 not prevent us from seeking this proof. 
 There is no sterner conscience than the sci- 
 entific conscience, and it demands, in every 
 possible case, the substitution for private 
 conviction of demonstration which s^all 1m 
 conclusive to all. 
 
 459. Let us, for example, be shown r, case 
 in which the stratification of the neve is pro- 
 longed into the glacier ; let us see the planes 
 of bedding and the planes of lamination ex- 
 isting side by side, and still indubitably dis- 
 tinct. Such an observation would effectual- 
 ly exclude stratification from the problem of 
 the veined -structure, and through the re- 
 moval of this tempting source of. error we 
 should be icnd^ea more free to pursue the 
 truth. 
 
 , 460. We sought for this conclusive test 
 upon the Mer de Glace, but did nut find it. 
 VVe sought it on the Grindehvald, and the 
 Aar glaciers, with an equal want of suc- 
 cess. On Hie Aletsch glacier, for the first 
 time, we observed the apparent coexistence 
 of bedding and structure, the one cutting the 
 other upon the walls of the same crevasse. 
 Still the case was not sufficiently pronounced 
 to produce entire conviction, and we visited 
 the Goiner glacier with the view of follow- 
 ing up our quest. 
 
 461. Here day after day added to the con- 
 viction that the bedding and the structure 
 were two different things. Still day after 
 day passed without revealing to us the final 
 proof. Surely we have not let our own ease 
 stand in the way of its attainment, and if we 
 retire baffled we shall do so with the con- 
 sciousness of having done our best. Yon- 
 der, however, at the base of the Matterhorn, 
 is the Furgge glacier that we have not }-et 
 explored. Upon it our final attempt must bo 
 made. 
 
 462. We get upon the glacier near its end, 
 and ascend it. We are soon fronted by a 
 barrier composed of three successive walls of 
 neve, th3 one rising above the other, and 
 each ictreating behind the other. The bot- 
 tom of each wall is separated from the top of 
 the succeeding one by a ledge, on which 
 threatening masses of broken neve now rest. 
 We stand amid blocks and rubbish which 
 have been evidently discharged from these 
 ledges, on which other masses, ready appar- 
 ently to tumble, are now poised. 
 
 463. On the vertical walls of this barrier 
 we see, marked with the utmost plainness, 
 the horizontal lines of stratification, while 
 something exceedingly like the veined struc- 
 ture appears to cross the lines of bedding at 
 nearly a right angle. The vertical surface 
 is, howe/er, weathered, and the lines of struc- 
 ture, if aiey be such, are indistinct. The 
 problem now is to remove the surface, and 
 expose the ice underneath. It is one of the 
 
132 
 
 THE FORMS OF WATER 
 
 many cases that have come before us, where 
 the value of au observation Is to be balanced 
 against the danger which it involves. 
 
 464. We do nothing rashly ; but scanning 
 the ledges and selecting a point of attack, 
 we conclude that the danger is not too great 
 to be incurred. We advance to the wall, 
 remove the surface, and are rewarded by the 
 discovery underneath it of the true blue 
 veins. They, moreover, are vertical, while 
 the bedding is horizontal . Bruce, as you 
 know, was defeated in many a battle, but he 
 persisted and won at last. Hero, upon the 
 Furgge glacier, you also have fought and 
 won your little Bannockburn. 
 
 465. But let us not use the language of 
 victory too soon. The stratification theory 
 has been removed out of the field of expla- 
 nation, but nothing has as yet been offered 
 in its place. 
 
 05. RELATION OF STRUCTURE TO 
 PRESSURE. 
 
 460. This veined structure was first de- 
 scribed by the distinguished Swiss naturalist, 
 Guyot, now a resident in the United States. 
 From *he Grimsel Pass I have already 
 pointed out to you the Giles glacier over- 
 spreading the mountains at Ihe opposite side 
 of the valley of the Rhone. It was on this 
 glacier that M. Guyot made his observation. 
 
 467. "I saw," he said, "under my feet 
 the surface of Ihe entire glacier covered with 
 regular furrows, from one to two inches 
 wide, hollowed out in a half-snowy mass, 
 and separated by protruding plates of harder 
 and more transparent ice. It was evident 
 that the glacier here was composed of two 
 kinds of ice, one that of the furrows, snowy 
 and more easily melted ; the other of the 
 plates, more perfect, crystalline, glassy, and 
 resistant ; and that the unequal resistance 
 which the two kinds of ice presented to the 
 atmosphere was the cause of the ridges. 
 
 468. " After having followed them for sev- 
 eral hundred yards, I reached a crevasse 
 twenty or thirty feet wide, which, as it cut 
 the plates and furrows at right angles, ex- 
 posed the interior of the glacier to a depth of 
 thirty or forty feet, and gave a beautiful 
 transverse section of the structure. As far as 
 my eyes could reach, I saw the mass of the 
 glacier composed of layers of snowy ice, each 
 two of which were separated by one of the 
 hard plates of which 1 have spoken, the 
 whole forming a regularly laminated mass, 
 which resembled certain calcareous slates." 
 
 469. I have not failed to point out to you 
 upon all the glaciers that we have visited the 
 little superficial furrows here described ; and 
 you have, moreover, noticed that in the fur- 
 rows mainly is lodged the finer dirt which is 
 scattered over the glacier. They suggest the 
 passage of a rake over the ice. And when- 
 ever these furrows were interrupted by a cre- 
 vasse, the veined structure invariably re- 
 vealed itself upon the walls of the fissure. 
 The surface grooving is indeed an infallible 
 indication of the interior lamination of Has 
 ice. 
 
IN CLOUDS AND RIVERS, ICE AND GLACIERS. 
 
 470. We have tracked the structure through 
 the various parts of the glaciers at which its 
 appearance was most distinct ; and we have 
 paid particular attention to the condition of 
 the ice at these places. The very fact of its 
 cutting the crevasses at right angles is sig- 
 nificant. We know the mechanical origin of 
 the crevasses ; that they are cracks formed 
 at right angles to lines of tension. But since 
 the crevasses are also perpendicular to the 
 planes of structure, these planes must be 
 parallel to the lines of tension. 
 
 471. On the glaciers, however, tension 
 rarely occurs alone. At the sides of the gla- 
 cier, for example, where marginal crevasses 
 are formed, the tension is always accom- 
 panied by pressure ; the one force acting at 
 right angles to the other. Here, therefore, 
 the veined structure, which is parallel to the 
 lines of tension, is perpendicular to the lines of 
 pressure. 
 
 472. That this is so will be evident to you 
 in a moment. Let the adjacent figure rep- 
 
 FIG. 1!). 
 
 resent the channel of the glacier moving in 
 the direction of the arrow. Suppose three 
 circles to be marked upon the ice. one at the 
 centre and the two others at the sides. In a 
 glacier of uniform inclination all these circles 
 would move downward, the central one only 
 remaining a circle. By the retardation of 
 the sides the marginal circles would be drawn 
 out to ovals. The two circles would be 
 elongated in one direction, and compressed in 
 another^ Across the long diameter, which 
 is the direction of strain, we have the mar- 
 ginal crevasses : across the short diameter 
 m n, which is the direction of pressure, we 
 have the marginal veined structure. 
 
 473. This association of pressure and struc- 
 ture is invariable. At the bases of the cas- 
 cades, where the inclination of the bed of the 
 glacier suddenly changes, the pressure in 
 many cases suffices not only to close the cre- 
 vasses but to violently squeezs the ice. At 
 such places the structure always appears, 
 sweeping quite across the glacier. When 
 two branch glaciers unite, their mutual 
 thrusi i i tensities the pie-existing marginal 
 utructure of the branches, and develops new 
 planes of lamination. Under the medial mo- 
 raines, therefore, we have usually a good de- 
 velopment of the structure. It is finely dis- 
 played, for example, under the great medial 
 moraine of the glacier of the Aaf. 
 
 474. Upon this glacier, indeed, the blue 
 veins were observed independently three 
 years after M. Guyot had first described 
 them. I say independently, because M. 
 Guyot's description, though written in 1838, 
 remained unprinted, and was unknown in 
 
 1841 to the observers on the Aar. These 
 were M. Agassi z and Professor Forbes. To 
 the question of structure Professor Forbes 
 subsequently devoted much attention, and it 
 was mainly his observations and reasoning** 
 that gave it the important position now as- 
 signed to it in the phenomena of glaciers. 
 
 475. Thus without quitting the glaciers 
 themselves, we establish the connection bo 
 tween pressure and structure. Is there any- 
 thing in our previous scientific experience 
 with which these facts may be connected ? 
 The new knowledge of nature must always 
 strike its roots into the old, and spring from 
 it as an organic growth. 
 
 GG. SLATE CLEAVAGE AND GLACIER 
 LAMINATION. 
 
 476. M. Guyot threw out an exceedingly- 
 sagacious hint, when he compared the 
 veined structure to the cleavage of slate 
 rocks. We must learn something of this 
 cleavage, for it really furnishes the key to 
 the problem which now occupies us. Let us 
 go then to the quarries of Bangor or Cum- 
 berland, and observe the quarrymen in their 
 sheds splitting the rocks. With a sharp 
 point struck skilfully into the edge of the 
 slate, they cause it to divide into thin plates, 
 fit for roofing or ciphering, as the case may 
 be. The surfaces along which the rock 
 cleaves are called its planes of cleavage. 
 
 477. All through the quarry you notice the 
 direction of these planes to be perfectly con- 
 stant. How is this laminated structure to be 
 accounted for ? 
 
 478. You might be disposed to consider 
 that cleavage is a case of stratification or 
 bedding ; for it is true that in various parts 
 of England there are rocks which can be 
 cloven into thin flags along the planes of 
 bedding. But when we examine these slate 
 rocks we verify the observation, first I be- 
 lieve made by the eminent and venerable 
 Professor Sedgwick, that the planes of bed- 
 ding usually run across the planes of cleav- 
 age^ 
 
 479. We have here, as you observe, a case 
 exactly similar to that of glacier lamination, 
 which we were at first disposed to icgard as 
 due to stratification. We afterward, how- 
 ever, found planes of lamination crossing the 
 layers of the neve, exactly as the planes of 
 cleavage cross the beds of slate rocks. 
 
 480. But the analogy extends further. 
 Slate cleavage continued to be a puzzio to 
 geologists lill the late Mr. Daniel Sharpe 
 made the discovery that shells and other fos- 
 sils and bodies found in slate rocks are inva- 
 riably flattened out in the planes of cleavage. 
 
 481. Turn into any well-arranged museum 
 for example, into the School of Mines in 
 Jermyn Street, and observe the evidence 
 there collected. Look particularly to the 
 fossil trilobites taken from the s4ate rock. 
 They are in some cases squeezed to one third 
 of their primitive thickness. Nurner.ju~ 
 other specimens show in the most striking 
 manner the flattening out of shells. 
 
 482. To the evidence adduced by Mr. 
 
ISi 
 
 THE FORMS OF WATER 
 
 Sharpe, Mr. Sorby added other powerful evi- 
 dence, founded upon the microscopic exam- 
 ination of slate rock. Taking both into ac- 
 count, the conclusion is irresistible that such 
 rocks have suffered enormous pressure at 
 right angles to the planes of cleavage, exactly 
 as the glacier has demonstrably suffered 
 great pressure at right angles to its planes of 
 lamination. 
 
 488. The association of pressure and cleav- 
 age is thus demonstrated ; but the question 
 arises, Do they stand to each other in the re- 
 lation of cause and effect ? The only way of 
 replying to this question is to combine artifi- 
 cially the conditions of nature, and see 
 whether we cannot produce her result*. 
 
 484. The substance of slate rocks was once 
 a plastic mud, in which fossils were imbed- 
 ded. Let us imitate the action of pressure 
 upon such mud by employing, instead of it, 
 softened while wax. Placing a ball of the 
 wax between t\\o glass plates, wetted to pre- 
 vent it from slicking, we apply pressure and 
 flatten out the wax. 
 
 485. The flattened mass is sit first too soft 
 to cleave sharply ; but you can see, by tear- 
 ing, that it is laminated. Let us chill it with 
 ice. We find aflerwaid that no slate rock 
 ever exhibited so fine a cleavage. The lam- 
 ina?, it need hardly be said, are perpendicular 
 to the pressure. 
 
 486. One cause of this laminnticn is that 
 the wax is an aggregate of gtanules the sur- 
 faces of which are places of weak cohesion ; 
 and that by the pressure these granules are 
 squeezed flat, thus producing planes of weak- 
 ness at right angles to the pressure. 
 
 487. But the main cause of the cleavage I 
 take to be the lateral sliding of the particles 
 of wax over each other. Old attachments 
 are thereby severed, which the new ones fail 
 to make good. Thus the tangential sliding 
 produces lamination, as the rails near a sta- 
 tion are caused to exfoliate by the gliding of 
 the wheel. 
 
 488. Instead of wax we may take the slate 
 itself, grind it to fine powder, add water, and 
 thus reproduce the piistine mud. By the 
 proper compression of such mud, in one 
 direction, the cleavage is restored. 
 
 489. Call now to mind the evidences we 
 have had of the power of thawing ice to yield 
 to pressure. Recollect the shortening of the 
 Glacier du Geant, and the squeezing of the 
 Glacier de Lechaud, at Trelaporte. Such a 
 substance, slowly acted upon by pressure, 
 
 will yield laterally. Its part ides wih slide 
 over each other, the severed attachments be- 
 ing immediately made good by legi-lation 
 It will not yield uniformly, but along special 
 planes. It will also liquefy, not uniformly, 
 but along special surfaces. Both the sliding 
 and the liquefaction will take place princi- 
 pally at right angles to the pressure, and gla- 
 cier lamination is the result. 
 
 490. As long as it is sound Ihe laminated 
 glacier ice resists cleavage. Regelatiou, as I 
 have said, makes the severed "attachments 
 good. But when such ice is exposed to ike 
 weather the structure is revealed, and the ice 
 can then be cloven into tablets a square foot, 
 or even a square yard in area. 
 
 67. CONCLUSION. 
 
 491. Here, my friend, our labors close. It 
 has been a true pleasure to me to have you 
 at my side so long In the sweat of our 
 brows we have often reached the heights 
 where our work lay, but you have been 
 steadfast and industrious throughout, using 
 in all possible cases your own muscles in- 
 stead of relying upon mine. Here and there 
 I have stretched an arm and helped you to a 
 ledge, but the work of climbing has been al- 
 most exclusively your own. It is thus that 
 I should like to teach you all things ; show- 
 ing you the way to profitable exertion, but 
 leaving the exertion to you more anxious to 
 bring out your manliness in the presence of 
 difficulty than to make your way snioclh by 
 toning difficulties down. 
 
 492. Steadfast, prudent, without terror, 
 though not at all times without awe, I have 
 found you on rock and ice,- and you have 
 shown the still rarer quality of steadfastness 
 in intellectual effort. As here set forth, our 
 task seems plain enough, but you and I know 
 how often we have had to wrangle resolutely 
 with the facts to bring out their meaning. 
 The work, however, is now done, and you 
 are master of a fragment of that sure an;i 
 certain knowledge which is founded on the 
 faithful study of nature. Is it not worth th 
 price paid for it ? Or rather, was not tho 
 paying of the price the healthful, if some 
 times hard, exercise of mind and body, upon 
 alp and glacier a portion of our delight ? 
 
 493. Here then we part. And should we 
 liot meet again, the memory of these days 
 will still unite us. Give rue your hand. 
 Good-by. 
 
LESSONS IN ELECTRICITY. 
 
 BY 
 JOHN TYNDALL. 
 
LESSONS ON ELECTRICITY. 
 
 CONTENTS: 
 
 Preface, - - 287 
 
 Introduction. 
 
 Historic Notes, 
 
 The Art of Experiment, 
 
 Materials for Experiment, 
 
 Electric Attractions, - 290 
 
 Disco s^ery of Conduction and Insulation, 
 
 The Electroscope Further Inquiries on Conduction and Insulation, - 
 
 Electrics and Non-Electrics, - ~9~> 
 
 Electric Repulsions. Discovery of Two Electricities, - 296 
 
 Fundamental Law of Electric Action, 29? 
 
 Electricity of the Rubber. Double or " Polar " Character of Electric; Force, 29'J 
 
 What is Electricity ? 301 
 
 Electric Induction, Definition of the Term, - 303 
 
 Experimental Researches on Electric Induction, 30:3 
 
 The Electrophorus, 307 
 
 Action of Points and Flames. 30S 
 
 The Electrical Ma.chine, - 309 
 
 Further Experiments on the Action of Points. The Electric- Mill The Golden 
 
 Fish. Lightening Conductors, - Oil 
 
 History of the Ley den Jar. The Leydeii Battery, - 314 
 
 Explanation of the Ley den Jar, - 31.) 
 
 Franklin's Cascade Battery, - 31? 
 
 Novel Leydeh Jars of the Simplest Form, 
 Seat of Charge in the Leydeii Jar, 
 
 Ignition by Electric Spark. Cottrel's Rubber. The Tube Machine. 320 
 
 Duration of the Electric Spark, 
 Electric Light in Vacim, 
 Lichtenberg's Figures, 
 
 Surface Compared with Mass. Distribution of Electricity in Hollow Conductors, 32(J 
 Physiological Effects of Electric Discharges, 
 Atmospheric Electricity, 
 Returning Stroke, 
 
 The Leyden Battery, Its Currents, aiid some of their Effects, 
 Conclusion, 
 Appendix. An Elementary Lecture on Magnetism, 
 
LESSONS IN ELECTRICITY; 
 
 TO WHICH IS ADDED 
 
 AN ELEMENTARY LECTURE ON MAGNETISM. 
 
 BY 
 
 JOHN TYNDA'LL, D.C.L., LL.D., F.R.S., 
 
 PROFESSOR OF NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION OF GREAT BRITAIN. 
 
 WITH SIXTY ILLUSTRATIONS. 
 
 PREFACE. 
 
 MORE than fifty years ago the Board 
 of Managers of the Royal Institution re- 
 solved to extend its usefulness, as a centre 
 of scientific instruction, by giving, during 
 the Christmas and Easter holidays of each 
 year, two courses of Lectures suited to 
 the intelligence of boys and girls. 
 
 On December 12th, 1825, a Commit- 
 tee appointed by the Managers reported 
 " that they had consulted Mr. Faraday 
 on the subject of engaging him to take a 
 part in the juvenile lectures proposed to 
 be given during the Christmas and Easter 
 recesses, and they found his occupations 
 were such that it would be exceedingly 
 inconvenient for him to engage in such 
 lectures. ' ' 
 
 Faraday's holding aloof was, however, 
 but temporary, for at Christmas 1827 wf 
 find him giving a " Course of Six Ele- 
 mentary Lectures on Chemistry, adapted 
 to a Juvenile Auditory." 
 
 The Easter lectures were soon aban- 
 doned, but f rcm the date mentioned to the 
 present time the Christmas lectures havn 
 been a marked feature of the Royal In- 
 stitution.* 
 
 Last Christmas it fell to my lot to give 
 one of these courses. I had heard doubts 
 expressed as to the value of science-teach- 
 ing in schools, and I had heard objec- 
 tions urged on the score of the expensive- 
 ness of apparatus. Both doubts and 
 
 * These brief historic references have al- 
 ready appeared in the preface to the " Forms 
 of Water." 
 
ELECTRICITY. 
 
 objections would, I considered, be most 
 practically met by showing what could 
 be done, in the way of discipline and in- 
 struction, by experimental lessons involv- 
 ing the use of apparatus so simple and 
 inexpensive as to be within everybody's 
 reach. 
 
 With some amplification, the substance 
 of .our Christmas Lessons is given in the 
 present little volume. 
 
 LESSONS IN ELECTRICITY. 
 
 1. Introduction. 
 
 MANY centuries before Christ, it had 
 been observed that yellow amber (elck- 
 tron\ when rubbed, possessed the power 
 of attracting light bodies. 
 
 Thales, the founder of the Ionic philos- 
 ophy (B.C. 580), imagined the amber to 
 be endowed with a kind of life. 
 
 This is the germ out of which has 
 grown the science of electricity, a name 
 derived from the substance in which this 
 pr.ver of attraction was first observed. 
 
 It will be my aim, during six hours of 
 those Christinas holidays, to make you, 
 to some extent, acquainted with the his- 
 tory, facts, and principles of this science, 
 and to teach you how to work at it. 
 
 The science has two great divisions : 
 the one called " Frictional Electricity," 
 the other "Voltaic Electricity." For 
 the present, our studies will be confined 
 to the first, or older portion of the sci- 
 ence, which is called " Frictional Elec- 
 tricity," because in it the electrical 
 power is obtained from, the rubbing of 
 bodies together. 
 
 2. Historic Note*. 
 
 The attraction of light bodies by 
 rubbed amber was the sum of the world's 
 knowledge of electricity for more than 
 2000 years. In 1600 Dr. Gilbert, phy- 
 sician to Queen Elizabeth, whose atten- 
 tion had been previously directed with 
 great success to magnetism, vastly ex- 
 panded the domain of electricity. He 
 showed that not only amber, but various 
 spars, gems, fossils, stones, glasses, and 
 rosins, exhibited, when rubbed, the same 
 power as amber. 
 
 Robert Boyle (1675) proved that a 
 suspended piece of rubbed amber, which 
 
 attracted other bolie.* to itself, was in 
 turn attracted by a body brought near it. 
 He also observed the light of electricity, 
 a diamond, with which he experimented, 
 being found to emit light when rubbe I 
 in the dark. 
 
 Boyle imagined that the electrified 
 body threw out an invisible, glutinous 
 substance, which laid hold of light bodie*, 
 and, returning to the source from which 
 it emanated, carried them along with it. 
 
 Otto von Guericke, Burgomaster of 
 Magleburg, contemporary of Boyle, and 
 inventor of the air-pump, intensified the 
 electric power previously obtained. He 
 devised what may be called the first elec- 
 trical machine, which was a ball of 
 sulphur, about the size of a child's head. 
 Turned by a handle, and rubbed by the 
 dry hand, the sulphur sphere emitted 
 light in the dark. 
 
 Von Guericke also noticed, and this is 
 important, that a feather, having been 
 first attracted to his sulphur globe, was 
 afterward repelled, and kept at a dis- 
 tance from it, until, having touched 
 another body, it was again attracted. 
 He heard the hissing of the " electric 
 lire," and also observed that an unelec- 
 trified body, when brought near his ex- 
 cited sphere, became electrical and capa- 
 ble of being attracted. 
 
 The members of the Academy del 
 Cimento examined various substances 
 electrically. They proved smoke to be 
 attracted, but not flame, which, they 
 found, deprived an electrified body of its 
 power. 
 
 They also proved liquids to be sensible 
 to the electric attraction, showing that 
 when rubbed amber was held over the 
 surface of a liquid, a little eminence was 
 formed, from which the liquid was finally 
 discharged against the amber. 
 
 Sir Isaac Newton, by rubbing a flat 
 glass, caused light bodies to jump be- 
 tween it and a table. He also noticed 
 the influence of the rubber in electric ex- 
 citation. His gown, for example, was 
 found to be much more effective than a 
 napkin. 
 
 Newton imagined that the excited body 
 emitted an elastic fluid which penetrated 
 glass. 
 
 In the ^ efforts of Thales, Boyle, and 
 Newton to form a mental picture of elec- 
 tricity we have an illustration of the ten- 
 
LESSONS IN ELECTRICITY. 
 
 289 
 
 dency of the human mind, not to rest 
 satisfied with the facts of observation, but 
 to pass beyond the facts to their invisible 
 causes. 
 
 Dr. Wall (1708) experimented with 
 large, elongated pieces of amber. lie 
 found wool to be the best rubber of am- 
 ber. " A prodigious number of little 
 cracklings" was produced by the fric- 
 tion, every one of them beinjj accom- 
 panied by a flash of light. " This light 
 and crackling," says Dr. Wall, " seem 
 in some degree to represent thunder and 
 lightning." This is the first published 
 allusion to thunder and lightning in con- 
 nection with electricity. 
 
 Stephen Gray (1729) also observed the 
 electric brush, {-mappings, and sparks. 
 He made the prophetic remark that 
 " though these effects are at present only 
 minute, it is probable that in time theie 
 may be found out a way to collect a 
 greater quantity of the electric fire, and, 
 consequently, to increase the force of 
 that power which by several of those ex- 
 periments, if we are permitted to com- 
 p'-irc great things with small, seems to be 
 of the same nature with that of thiindcF 
 and lightning." This, you will ob- 
 serve, is far more definite than the re- 
 mark of Dr. Wall. 
 
 3. The Art of Experiment. 
 
 We have thus broken ground with a 
 few historic notes, intended to show the 
 gradual growth of electrical science. Our 
 next step must be to get some knowledge 
 of the facts referred to, and to learn how 
 they may be produced and extended. 
 The art of producing and extending such 
 facts, and of inquiring into thorn by prop- 
 er instruments, is the art of experiment. 
 It is an art of extreme importance, for 
 by its means we can, as it were, converse 
 with Nature, asking her questions and 
 receiving from her replies. 
 
 It was the neglect of experiment, and 
 of the reasoning based upon it, which 
 kept the knowledge of the ancient world 
 confined to the single fact of attraction 
 by amber for more than 2000 years. 
 
 Skill in the art of experimenting docs 
 not come of itself ; it is only to be ac- 
 quired by labor. When you first take a 
 billiard cue in your hand, your strokes 
 are awkward and ill-directed. When 
 
 you learn to dance, your first movements 
 are neither graceful nor pleasant. By 
 practice alone, you learn to dance and 
 to play. This also is the only way of 
 learning the art of experiment. You 
 must not, therefore, be daunted by your 
 clumsiness at first : you must overcome 
 it, and acquire skill in tho art by repeti- 
 tion. 
 
 In this way you will come into direct 
 contact with natural truth you will 
 think and reason not on what has been 
 said to you in books, but on \rhat has 
 been said to you by Nature. Thought 
 springing from this source has a vitality 
 not derivable from mere book-knowledge. 
 
 4. Materials for Experiment. 
 
 At this stage of our labors wo arc to 
 provide ourselves with the fallowing 
 materials : 
 
 FIG. 1. 
 
 a. Some sticks of sealing-wax ; 
 
 5. Two pieces of gutta-percha tubing, 
 about 18 inches long and f of an inct 
 outside diameter ; 
 
 c. Two or three glass tubes, about 18 
 inches long and f of an inch wide, closed 
 at one end, and not too thin, lest they 
 should break in your hand and cut it ; 
 
 d. Two or three pieces of clean flannel, 
 capable of being folded into pads of two 
 ar three layers, about eight or ten inches 
 square ; 
 
290 
 
 LESSORS IN KI7ECTRIOITY. 
 
 . A couple of pads, composed of 
 three or four layers of silk, about eight 
 or ten inches square ; 
 
 /. A board about 18 inches square, 
 and a piece of india-rubber ; 
 
 <7. Some very narrow silk ribbon, n, 
 and a wire loop, w, like that shown in 
 fur. 1, in which sticks of sealing-wax, 
 tubes of gutta-percha, rods of glass, or n 
 Walking-stick, may be suspended. I 
 cl)ooso a narrow ribbon because it is con- 
 venient to have a suspending cord that 
 will neither twist nor untwist of itself. 
 
 (I usually employ a loop with the two 
 ends, which arc here shown free, soldered 
 together. The loop would thus be un- 
 broken. But you may not be skilled in 
 the art of soldering, and I therefore 
 choose the free loop, which is very easily 
 constructed. For the purpose of suspen- 
 sion an arrangement resembling a towel- 
 horse, with a single horizontal rail, will 
 be found convenient). 
 
 FIG. 2. 
 
 h. A straw, 1 i', fig, 2, delicately sup- 
 ported on the point of a sewing needle N. 
 This is inserted in a stick of sealing-wax 
 A, attached below to a little circular plate 
 of tin, the whole forming a stand. In 
 fig. 3 the straw is shown on a larger 
 scale, and separate from its needle. The 
 shore bit of straw in the middle, which 
 serves as a cap, is stuck on by sealing- 
 wax. 
 
 t. Tke name "amalgam" is given to 
 a mixture of mercury with other metals. 
 Experience has shown that the efficacy of 
 a silk rubber is vastly increased when it 
 is smeared over with an amalgam formed 
 of 1 part by weight of tin, 2 of zinc, and 
 6 f mercury. A littlo lard is to be first 
 
 smeared on the s-ilk, and the amalgam is 
 to be applied to the lard. The amalgam. 
 if hard, must be pounded or biuised.with 
 a pestle or a hammer until it is soft. 
 You can purchase sixpenny- worth of it at 
 a philosophical instrument maker's. It 
 is to be added to your materials. 
 
 fc. I should like to make these pages 
 suitable for boys without much pocket- 
 money, and, therefore, aim at economy 
 hi my list of materials. But provide by 
 all means, if you can, a fox's brush, such 
 
 as those usually employed in dusting 
 furniture. 
 
 5. Electric Attractions. 
 
 Place your sealing-wax, gutta-percha 
 tubing, and flannel and silk rubbers be- 
 fore a fire, to insure their dryncss. Be 
 specially careful to make your glass 
 tubes and silk rubbers not only warm, 
 but hot. Pass the diied flannel briskly 
 once or twice over a stick of sealing-wax 
 or over a gutta-percha tube. A very 
 small amount of friction will excite the 
 power of attracting the suspended straf 
 as shown in fig. 2. Repeat the experi- 
 ment several times and cause the straw to 
 follow the attracting body round and 
 round. Do the same with a glass tube 
 rubbed with silk. 
 
 I lay particular stress on the heating of 
 the glass tube, because glass has the 
 
LESSONS IN ELECTRICITY. 
 
 231 
 
 power, wliich it exercises, of condensing 
 upon its surface into a liquid film, the 
 {i < I neons vapor of the surrounding air. 
 This film must be removed. 
 
 1 would also insist on practice, in order 
 to render you expert. You will therefore 
 
 attract bran, scraps of paper, gold leaf, 
 soap bubbles, and other light bodies by 
 rubbed glass, sealing-wax, and gutta- 
 perciia. Faraday was fond of making 
 empty egg shells, hoops of paper, and 
 other light objects roll after his excited 
 tubes. 
 
 It is only when the electric power is 
 very weak, that you require your deli- 
 cately suspended straw. With the sticks 
 of wax, tubes, and rubbers here men- 
 tioned, even heavy bodies, when properly 
 suspended, may be attracted. Place, for 
 instance, a common walking stick in the 
 wire loop attached to the narrow ribbon, 
 fig. .1, ;md let it swing horizontally. The 
 glass, rubbed with its silk, or the sealing- 
 wax, or gutta-percha, nibbed with its 
 flannel, will pull the stick quite round. 
 
 Abandon the wire loop ; place an egg 
 in an (gg-cup, and balance a long lath 
 upon the egg, as shown in fig. 4. The 
 lath, though it may be almost a plank, 
 
 will obediently follow the rubbed glass, 
 gutta-percha, or sealing-wax. 
 
 Nothing can be simpler than this lath 
 and egg arrangement, and hardly any- 
 thing could be more impressive. The 
 more you work with it, the better you 
 will like it. 
 
 Pass an ebonite comb through the 
 hair. In dry weather it produces a 
 crackling noise ; but its action upon the 
 lath may be made plain in any weather. 
 It is rendered electrical by friction against 
 the hair, and with it you can pull the lath 
 quite round. 
 
 If you moisten the hair with oil, the 
 comb will still be excited and exert at- 
 traction ; but if you moisten it with 
 water, the excitement ceases ; a comb 
 passed through wetted hair has no power 
 over the lath. You will understand the 
 meaning of this subsequently. 
 
 After its passage through, dry or oiled 
 hair, balance the comb itself upon the 
 egg : it is attracted by the lath. You 
 thus prove the attraction to be mutual : 
 the comb attracts the lath, and the lath 
 attracts the comb. Suspend your rubbed 
 glass, rubbed gutta-percha, and rubbed 
 sealing-wax in jour wire loep. They are 
 all just as much attracted by the lath as 
 the lath was attracted by them. This is 
 an extension of Boyle's experiment witk 
 the suspended amber (2). 
 
 IIovv it is that any unelectrificd body 
 attracts, and is attracted by the excited 
 glass, sealing-wax, and gutta-percha, we 
 shall learn by and by. 
 
 A very striking illustration of electric 
 attraction may be obtained with the board 
 and india-rubber mentioned in our list of 
 materials (4). Place the board before 
 the fire and make it hot ; heat also a 
 sheet of foolscr.p paper and place it on 
 the board. There is no attraction be- 
 tween them. Pass the india-rubber brisk- 
 ly over the paper. It now clings firmly 
 to the board. Tear it away, arid hold jt 
 at arm's length, for it will move to your 
 body if it can. Bring it near a door or 
 wall, it will cling tenaciously to either. 
 The electrified paper also powerfully at- 
 tracts tho balanced lath from a great dis- 
 tance. 
 
 The friction of the hnnd, of a cam- 
 bric handkerchief, or of wash-leather 
 fails to electrify the paper in any high 
 degree. It requires friction by a 
 
233 
 
 LESSONS IN ELECTRICITY. 
 
 special substance to make the excitement 
 strong. This \vo learn by experience. 
 It is also experience that has taught 
 us that resinous bodies are best ex- 
 cited by tianiic-], and vitreous bodies by 
 Bilk; 
 
 Take nothing- for granted in this in- 
 quiry, and neglect no effort to render 
 your knowledge complete and sure. Try 
 various rubbers, r.nd satisfy yourself that 
 differences like that first observed by 
 Newton exist between them. 
 
 Vary also the body rubbed. Excite 
 by friction paraftine and composite can- 
 dies, resin, sulphur, beeswax, ebonite, 
 and shellac. Also rock-crystal and other 
 vitreous substances, and attract with all 
 of them the balanced lath. A film of 
 collodion, a sheet of vulcanized india- 
 rubber, or brown paper heated before 
 the (ire, rubbed briskly with the dry 
 hand, attracts and is attracted by the 
 lath. 
 
 Lay bare also the true influence of 
 heat in the case of our rubbed paper. 
 Spread a cold sheet of foolscap on a cold 
 board on a table, for example. If the 
 air be not very dry, rubbing, even with 
 the india-rubber, will not make them 
 cling together. Bat is it because they 
 were hot that they attracted each other 
 in the first instance ? No, for you may 
 heat your board by plunging it into boil- 
 ing water, and your paper by holding it 
 in a cloud of steam. Thus heated they 
 cannot be made to cling together. The 
 heat really acts by expelling the moisture. 
 Cold weather, if it be only dry, is highly 
 favorable to electric excitation. During 
 frost the whisking of the hand over silk 
 or flannel, or over a cat's back, renders 
 it electrical. 
 
 The experiment of the Florentine 
 academicians, whereby they proved the 
 electric attraction of a liquid, is pretty, 
 and worthy of repetition. Fill a very 
 small watch-glass with oil, until the 
 liquid forms a round curved surface, ris- 
 ing a little over the rim of the glass. A 
 strongly excited glass tube, held over the 
 oil, raises not one eminence only, but 
 several, each of which finally discharges 
 a shower of drops against the attracting 
 glass. The effect is shown in fig. 5, 
 where G is the watch-glass on the stand 
 T, and R the excited glass tube.* 
 
 Cause the excited glass tube to pass 
 
 FIG. 5. 
 
 close by your face, without touching it. 
 You feel, like Hauksbee, as if a cobweb 
 were drawn over the face. You also 
 sometimes smell a peculiar odor, due to 
 a substance developed by the electricity, 
 and called ozone. 
 
 Long ere this, while rubbing your 
 tubes, you will have heard the " hiss- 
 ing'' and *' crackling" so often referred 
 to by the earlier electricians ; and if you 
 have rubbed your glass tube briskly in 
 the dark, you will have seen what they 
 called the "electric fire." Using, in- 
 stead of a tube, a tall glass jar, rendered 
 hot, a good warm rubber, and vigorous 
 friction, the streams of electric fire are 
 very surprising in the dark. 
 
 6. Discovery of Conduction and Insu- 
 lation. 
 
 Here I must again refer to that most 
 meritorious philosopher, Stephen Gray. 
 In 1729 he experimented with a gfoss 
 tube stopped by a cork. When the tubo 
 was rubbed, the cork attracted light bod- 
 ies. Gray states that he was " much 
 surprised "at this, and he " concluded 
 that there was certainly an attractive vir- 
 tue communicated to the cork." This 
 was the starting point of our knowledge 
 of electric Conduction. 
 
 A fir stick 4 inches long, stuck into 
 the cork, was also found by Gray to at- 
 tract light bodies. He made his sticks 
 
 * As a practical measure the watch-glass 
 ought to rest upon a small stand, and not 
 upon a surface of large area. The experi- 
 ment is particularly well suited for projection 
 on a screen. 
 
S IX ELECTRICITY. 
 
 29,1 
 
 longer, but f-*,ill found a power of attrac- 
 tion at thmi' ends. lie then passed on 
 to pack-thread and wire. Hanging ;i 
 thread s, fig. G, from the top window of 
 a house, so that the lower end nearly 
 touched the ground, and twisting the up- 
 per end of the thread round his glass 
 tube R, on briskly rubbing the tube, light 
 bodies were attracted by the lower end 
 B of the thread. 
 
 But Gray's most remarkable experi- 
 ment was this : IIo suspended a long 
 hempen line horizontally by loops of pack- 
 thread, but failed to transmit through 
 it tlm electric power. He then suspend- 
 ed it by ioops of silk and succeeded in 
 
 FIG. 6. 
 
 sending the " attractive virtue" through 
 765 feet of thread. He at first thought 
 the silk was effectual because it was thin ; 
 but on replacing a broken silk loop by a 
 t-till thinner wire, he obtained no action. 
 Finally, he came to the conclusion that 
 his loops were effectual, not because they 
 wore thin but because they were silk. 
 This was the starting-point of our knowl- 
 edge of Insulation. 
 
 It is interesting to notice the devotion 
 of some men of science to their \\<uk. 
 Dr. Wells, who wrote a beautiful CHK.-.V, 
 wherein he explained the oriuiu of dew, 
 
 finished it when he was on the brink of 
 the grave. Stephen Gray was so near 
 dying when his last experiments were 
 made, that he was unable lo write out an 
 account of them. On his death-bed, 
 and, indeed, the very day before his 
 death, his description of them was taken 
 from his lips by Dr. Mortimer, Secretary 
 of the Royal Society, and afterward 
 printed in the " Philosophical Transac- 
 tions." 
 
 One word of definition will be useful 
 here. Some substances, as proved by 
 Stephen Gray, possess in a very high de- 
 gree the power of permitting electricity 
 to pass through them ; other substances 
 ptop the passage of the electricity. 
 Bodies of the first class are called con- 
 ductors ; bodies of the second class arc 
 called insulators. 
 
 You cannot do better than repeat here 
 the experiments of Gray. Push a cork 
 into the open end of your glass tube ; 
 rub the tube, carrving the friction up 
 to the end holding the cork. The cork 
 will attract the balanced lath, shown in 
 fig. 4, with which you have already 
 \vorked so much. 
 
 I5ut the excited glass is here so near 
 the end of the cork that you may not 
 feel certain that the observed attraction is 
 that of the cork. You can, however, 
 prove that the cork attracts by its action 
 upon light bodies which cling to it. 
 Stick a pen-holder into the cork and rub 
 the glass tube as before. The free end 
 of the holder will attract the lath. Stick 
 a deal rod three or four feet long into 
 the cork ; its free end will attract the 
 lath when the glass tube is excited. In 
 this way you prove to demonstration 
 that the electric power is conveyed along 
 the rod. 
 
 7. The Electroscope. Further Inquir- 
 ies on Conduction and Insulation. 
 
 A little addition to our apparatus will 
 now be desirable. You can buy a book 
 of " Dutch metal " for fourpence ; an 1 
 a globular flask like thst shown in fi.j. 
 ?, for sixpence, or at the most a shilling. 
 Find a cork, <:, which tin the fl ik ; pn-a 
 a wire, w, through the cork an 1 bend it 
 near one end at, a right ati-jfle. Attach 
 by means of w:ix to the bent arm, which 
 ought to be about three (ju.-irters of an. 
 inch Jong, two strips, i, of the Dutch 
 
294 
 
 LESSONS IN ELECTRICITY. 
 
 FIG. 7. 
 
 metal, about three inches long and from 
 half an inch to three quarters of an inch 
 wide. The strips will hang down face 
 to face, in contact with each other. 
 Stic;c by sealing-wax upon the other end of 
 the wire a little plate of tin or sheet-zinc, 
 T, about two inches in diameter. In all 
 eases you must be careful so to use your 
 wax as not to interrupt the metallic con- 
 nection of the various parts of your ap- 
 paratus, which we will name an electro- 
 scope. Gold-leaf, instead of Dutch metal, 
 is usually employed for electroscopes. 
 I recommend the " metal " because 
 it is cheaper, and will stand rougher 
 usage. 
 
 See that your globular flask is dry and 
 free from dust. Bring your rubbed seal- 
 ing-wax, R, or your rubbed glass, near 
 the little plate of tin, the leaves of Dutch 
 metal open out ; withdraw the excited 
 body, the leaves fall together. We- shall 
 inquire into the cause of this action im- 
 mediately. Practise the approach and 
 withdrawal for a little time. Now draw 
 \our lubbed sealing-wax or glass along 
 the edge of the tin plate, T. The leaves 
 diverge, and after the sealing-wax or 
 glass is withdrawn they remain divergent. 
 In the first experiment you communi- 
 cated no electricity to the electroscope ; 
 in the second experiment you did. At 
 present I will only ask you to take the 
 opening out of the leaves as a proof that 
 electricity has been communicated to 
 them. 
 
 And now we are ready for Gray's ex- 
 periments in a form different from his. 
 Connect the end of a lung wire with the 
 tin plate of the electroscope ; coil the 
 other end round your glass tube. Rub 
 the tube briskly, carrying the friction 
 close to the coiled wire. A sino-le stroke 
 of your rubber, if skilfully given, will 
 cause the leaves to diverge. ' The elec- 
 tricity has obviously passed through the 
 wire to the electroscope. 
 
 Substitute for the wire a string o f 
 common twine, rub briskly and youwill 
 cause the leaves to diVerge ; but 'there is 
 a notable difference us regards the 
 promptness of the divergence. '.You soon 
 satisfy yourself that the electricity , a-ses 
 with greater facility through the wire 
 than through the string. Substitute for 
 the twine a string of silk. No matter 
 how vigorously you rub, you can now 
 produce no divergence. The electricity 
 cannot get through the silk at all. 
 
 This is the place to demonstrate in a 
 manner never to be forgotten the influ- 
 ence of moisture. Wet your dry silk 
 string throughout, and squeeze it a little 
 so that the water from it may not trick 1 . 3 
 over your glass tube. Coil it round the 
 tube as before, and excite the tube. The 
 leaves of the electroscope immediately 
 diverge. The water is here the con- 
 ductor. The influence of moisture was 
 first demonstrated by Du Fay (1733 to 
 1737), who succeeded in sending elec- 
 tricity through 1256 feet of moist pack- 
 thread. 
 
 It is hardly necessary to point out the 
 meaning of Gray's experiment where he 
 found that, with loops of wire or of 
 pack-thread, he could not send the elec- 
 tricity from end to end of his suspended 
 string. Obviously the electricity escaped 
 in each of these cases through the con- 
 ducting support to the earth. 
 
 My assistant, Mr. Cotixell, who has 
 been working very hard for you and me, 
 has devised an electroscope which we 
 shall frequently employ in our lessons. 
 M, fig. 8, is a little plate of metal, or of 
 wood covered with tin-foil, supported on 
 a rod, G, of glass or of sealing-wax. N 
 is another plate of Dutch metal paper, 
 separated about an inch from M, and at- 
 tached by sealing wax to the long straw 
 i i' (broken off in the figure) ; A A' is a 
 horizontal pivot formed by a sewing 
 
LESSONS IN ELECTRICVfV. 
 
 295 
 
 needle, and supported on a bent strip of 
 mdal, as shown in the figure. By weight- 
 ing the straw with a little wire near j', 
 you so balance it that the plate N shall be 
 just lifted away from M. The wire w, 
 which may be 100 feet long, proceeds 
 from M to your glass tube, round which 
 it is coiled. A single vigorous stroke of 
 the tube, by the rubber, sends electricity 
 along w to M ; N is attracted downward, 
 the other end of the long straw being 
 lifted through a considerable distance. 
 In subsequent figures you will see the 
 complete straw-index, and its modes of 
 application. 
 
 A few experiments with cither of these 
 instruments will enable you to classify 
 bodies as conductors, se'mi-conductors, 
 and insulators. Here is a list of a few 
 of each, which, however, differ much 
 among themselves. 
 
 Conductors. 
 The common metals. 
 Well- burned charcoal. 
 Concentrated acids. 
 {Solutions of tails. 
 Rain water. 
 Li IK n. 
 Living vegetables and animals. 
 
 Semi-conductor*. 
 Alcohol anJ ether. Paper. 
 Dry wood. 8liuw. 
 
 Mmble. 
 
 Insulators. 
 
 Fattv oils. Silk. 
 
 Chalk. Gh;s. 
 
 India-rubber. Wax. 
 
 Dry paper. Sulphur. 
 
 Haii 1 . Shellac. 
 
 A little reflection will enable you to 
 vary these experiments indefinitely. Rul> 
 your excited sealing-wax or glass ajjf.inst 
 the tin plate of your electroscope, and 
 cause the leaves to diverge. Touch the 
 plate with any one of the conductors 
 mentioned in the list ; the electroscope is 
 immediately discharged. Touch it with 
 a semi-conductor ; the leaves fail as le 
 fore, but less promptly. Touch the 
 plate finally with an insulator, the elt-c- 
 tricity cannot pass, and the leaves remain 
 unchanged. 
 
 8. Electrics and Non- Electrics. 
 
 For a long period, bodies were divided 
 into tlcctrics and non-electrics, the former 
 deemed capable of beinnr olectriti >d. the 
 latter not. Thus the amber of the an- 
 cients, and t\ic spars, gems, fossils, stones, 
 glasses, and resins, operated on by Dr. 
 Gilbert, were called electrics, while all the 
 metals were called non-electrics. We 
 must now determine the true meaning of 
 this distinction. 
 
 Take hi succession a piece of brass, of 
 wood coated with tin-foil, a lead bullet, 
 apples, pears, turnips, carrots, cucum- 
 bers uncoatcd wood not very dry will 
 also answer in the hand, and strike them 
 briskly with flannel, or the fox's brush ; 
 none of them will attract the balanced 
 lath, fig. 4, or show any other symptom 
 of electric excitement. All of them 
 therefore would have been onoe called 
 non-electrics. 
 
 But suspend them in succession by a 
 string of silk held in the hand, and strike 
 them again ; every one of them will now 
 attract the lath. 
 
 Reflect upon the meaning of this ex- 
 periment. We have introduced an insu- 
 lator the silk string between the hand 
 and the body struck, and we find that by 
 its introd.iction the non-electric has been 
 converted into an electric. 
 
 The meaning is obvious. When held 
 in the hand, though electricity was devel- 
 oped in each case by the friction, it pass- 
 ed immediately through the hand and 
 body to the caith. This transfer being 
 prevented by tho silk, the electricity, 
 
LESSOVS IN ELECTRICITY. 
 
 once excited, is retained, and the attrac- 
 tion of the lath is the consequence. 
 
 In like manner, a brass tube, held in 
 the hand and struck with a fox's brush, 
 shows no attractive power ; but when a 
 stick of sealing-wax, ebonite, or gutta- 
 percha is thrust into the tube as a han- 
 dle, the striking of the tube at once de- 
 velops the power of attraction. 
 
 And now you see more clearly than 
 you did at first the meaning of the ex 
 periment with the heated foolscap anj 
 india-rubber. Paper and wood always 
 imbibe a certain amount of moisture 
 from the air. When the rubber was 
 passed over the cold paper electricity was 
 excited, but the paper, being rendered a 
 conductor by its moisture, allowed the 
 electricity to pass away. 
 
 Prove all things. Lay your coll fools- 
 cap on a cold board supported by dry- 
 tumblers ; pass your india-rubber over 
 the paper ; lift it by a, loop of silk which 
 has been previously attached to it, for if 
 you touch it it will discharge itself. 
 You will find it electric ; and with it you 
 can charge your electroscope, or attract 
 from a distance your balanced lath. 
 
 The human body was ranked among 
 the non-electrics. Make plain to your- 
 self the reason. Stand upon the iloor 
 and permit a friend to strike you briskly 
 with the fox's brush. Present your 
 knuckle to the balanced lath, you will 
 find no attraction. Here, however, yon. 
 stand upon the earth, so that even if 
 electricity had been developed, there is 
 nothing to hinder it from passing away. 
 
 But, place upon the ground four warm 
 glass tumblers, and upon the tumblers a 
 board.* Stand upon the board and pre- 
 sent your knuckle to the lath. A single 
 stroke of the fox's fur, if skilfully given, 
 will produce attraction. If you stand 
 upon a cake of resin, of ebonite, or upon 
 a sheet of good india-rubber, the effect 
 will be the same. You can also charge 
 your electroscope with this electricity. 
 
 Throw a mackintosh over your shoul- 
 ders and let a friend strike it with the 
 fox's brush, the attractive force is greatly 
 augmented. 
 
 After brisk striking, present your 
 
 * Some caution is necessary hero. A large 
 class of cheap glass tumblers conduct s.i 
 freely that they are unfit for this and similar 
 expeiiineiits. See 19. 
 
 knuckle to the knuckle of your friend. 
 A spark will pass between you. 
 
 This experiment with the mackintosh 
 further illustrates what you have already 
 frequently observed namely, that it i> 
 not friction alono, but the friction of 
 special substances against each other, that 
 produces electricity. 
 
 Thus we prove that non-electrics, like 
 electrics, can be excited, the condition of 
 success being, that an insulator shall be 
 interposed between the non-electric and 
 the earth. It is obvious that the old di- 
 vision into electrics and non-electrics, 
 really meant a division into insulators 
 and conductors. 
 
 9. Electric Repulsions. Discovery of 
 two Electricities. 
 
 Wo have hitherto dealt almost exclu- 
 sively with electric attractions, but in an 
 experiment already referred to (2), 
 Otto von Guericke observed the repulsion 
 of a feather by his sulphur globe. I also 
 anticipated mutters in the use of our 
 Dutch metal electroscope ( 7), where 
 the repulsion of the leaves informed us 
 of the arrival of the electricity. 
 
 Du Fay, who was the real discoverer 
 here, found a gold-leaf floating in the air 
 to be first attracted and then repelled by 
 the same excited body. He afterward 
 proved that when -the floating leaf was 
 repelled by rubbed glass, it was attracted 
 by rubbed resin and that when it was 
 repelled by rubbed resin it was attracted 
 by rubbed glass. Hence the important 
 announcement, by Du Fay, that there are 
 two kinds of electricity. 
 
 The electricity excited on glass was for 
 a time called vitreous electricity, while 
 that excited on sealing-wax was called res- 
 inous electricity. These terms are how- 
 ever improper ; because, by changing the 
 rubber, we can obtain the electricity of 
 sealing-wax upon glass, and the electric- 
 ity of glass upon sealing-wax. 
 
 Roughen, for example, the surface of 
 your glass tube with a grindstone, and 
 rub it with flannel, the electricity of seal- 
 ing-wax will be found upon the vitreous 
 surface. Rub your sealing-wax with vul- 
 canized india-rubber, the electricity of 
 glass will be found upon the resinous sur- 
 face. You will be able to prove this im- 
 mediately. 
 
LESSONS IN ELECTRICITY. 
 
 897 
 
 We now use the term positive or plus 
 electricity to denote that developed on 
 glass by the friction of silk ; and negative 
 or minus electricity to denote that devel- 
 oped on sealing-wax by the friction of 
 flannel. These terms are adopted purely 
 for the sake of convenience. There is 
 no reason in nature why the resinous 
 electricity should not be called positive, 
 and the vitreous electricity negative. 
 Once agreed, however, to apply the terms 
 as here fixed, we must adhere to this 
 agreement throughout. 
 
 10. Fundamental Law of Electric 
 Action. 
 
 In all the experiments which we have 
 hitherto made, one of the substances op- 
 erated on has been electrified b friction, 
 and the other not. But once engaged in 
 inquiries of this description, questions 
 incessantly occur to the mind, the an- 
 swering of which extends our knowledge 
 and suggests other questions. Suppose, 
 instead of exciting only one of the bod- 
 ies presented to each other, we were to 
 vxcite both of them, what would occur ? 
 This is the question which was asked and 
 answered by Du Fay, and which wo must 
 now answer for ourselves. 
 
 Here your wire loop, fig. 1, comes 
 again into play. Place an unrubbed 
 gutta-percha tube, or a stick of sealing- 
 wax, in the loop, and be sure that it is 
 unrubbed that no electricity adheres to 
 it from former experiments. If it fail to 
 attract light bodies, it is unexcited ; if it 
 attract them, pass your hand over it sev- 
 eral times, or, better still, pass it over 
 or through the flame of a spirit lamp. 
 This will remove every trace of electric- 
 ity. Satisfy yourself that the unrubbed 
 gutta-percha tube is attracted by a rubbed 
 one. 
 
 Remove the unrubbed tube from the 
 loop, and excite it with its flannel rub- 
 ber. One end of the tube is held in your 
 hand and is therefore unexcited. Return 
 ihe tube to the loop, keeping your eye 
 upon the excited end. Bring a second 
 rubbed tube near the excited end of the 
 suspended one : strong repulsion is the 
 consequence. Drive the suspended tube 
 round and round by this force of repul- 
 sion 
 
 Bring a rubbed glass tube near the ex- 
 cited end of the gutta-percha tube : 
 strong attraction is the result. 
 
 Repeat this experiment step by step 
 with two glass tubes. Prove that the 
 rubbed glass tube attracts the unrubbed 
 one. Remove the unrubbed tube from 
 the loop, excite it by its rubber, return it 
 to the loop, and establish the repulsion 
 of glass by glass. Bring rubbed gutta- 
 percha or sealing-wax near the rubbed 
 glass : strong attraction is the conse- 
 quence. 
 
 These experiments lead you directly to 
 the fundamental law of electric action, 
 which is this : Bodies charged with the 
 same electricity repel each other, while 
 bodies charged with opposite electricities 
 attract each other. Positive repels posi' 
 five, and attracts negative. Negative 
 repels negative and attracts positive.^ 
 
 Devise experiments which shall still 
 further illustrate this law. Repeat, for 
 example, Otto von Guericke's experi- 
 ment. Hang a feather by a silk thread 
 and bring your rubbed glass tube near it : 
 the feather is attracted, touches the 
 tube, charges itself with the electricity of 
 the tube, and is then repelled. Cause it 
 to retreat from the tube in various direc- 
 tions. Du Fay's experiment with the 
 gold-leaf will be repeated and explained 
 further on. See 18. 
 
 Hang your feather by a common 
 thread ; if no insulating substance inter- 
 venes between the feather and the earth, 
 you can get no repulsion. Why ? Ob- 
 viously because the charge of positive 
 electricity communicated by the rod is 
 not retained by the feather, but passes 
 away to the earth. Hence, you have not 
 positive acting against positive at all. 
 Why the neutral body is attracted by 
 the electrified one, will, as already 
 stated, appear by and by. 
 
 PIG. 11. 
 
LESSOKS 
 
 Attract your straw needle by your rub- 
 bed glass tube. Let the straw strike the 
 tul>e, so that the one shall rub against 
 the other. The straw accepts the elec- 
 tricity of the tube, and repulsion immedi- 
 ately follows attraction, as shown in fig. 
 9. 
 
 Mr. Cuttrcll has devised the simple 
 electroscope represented in fig. 10 to 
 snow repulsion. A is a stem of sealing- 
 wax with a small circle of tin, T, at the 
 top. w is a bent wire proceeding from 
 T, with a small disk attached to it by 
 wax. i i' is a little straw index, support- 
 ed by the needle N, as shown in fig. 10. 
 The stem A', also of sealing-wax, is not 
 quite vertical, the object being to cause 
 the bit of paper, i', to rest close to w 
 when the apparatus is not electrified. 
 When electricity is imparted to T, it 
 f.ows through the wires w and w, over 
 both disk and index : immediate repul- 
 sion of the straw is the consequence. 
 
 No better experiment can be made to 
 illustrate the self-repulsive character of 
 electricity than the following one. Heat 
 your square board ( 5), and warm, as 
 before, your sheet of foolscap. Spread 
 the paper upon the board, and excite it 
 by the friction, of india-rubber. Cut 
 from the sheet two long strips with your 
 
 penknife. Hold the stiips together at 
 one end. Separate them from the board, 
 and lift them into the air : they forcibly 
 diive each other apart, producing a wide 
 divergence. 
 
 Cut r.-.rveral stiips. so as to form a kind 
 of tassel. Hold them together at one 
 end. Separate them from the board, and 
 lift them into the air : they are driven 
 asunder by the self-repellent electricity, 
 presentini*" an appearance which may ie- 
 mind you of the hair of Medusa. The 
 effect is represented in fig. 11.* 
 
 Another very beautiful experiment fits 
 in here. Let fine silver sand, s, fig. 12, 
 issue in a stream from a glass funnel, 
 through an aperture one eighth of an inch 
 in diameter. Connect the sand in the 
 funnel by a fine wire w, fig. 13, with your 
 warm g-lass tube. Unelectrified, the 
 
 * In one of my earliest lectures at the 
 Royal Institution, having rubbed a sheet of 
 foulscap, I was about lo lii't it bodily from 
 the hot board, and to place it against the 
 wall, when the thought of cutting it into 
 strips, and allowing them to net upon each 
 other, orciirnd to me. The itsult, of 
 course, was that above described. Simple 
 and obvious as it was. it gave Faraday, who 
 was present at the time, the most lively 
 pleasure. The simplest experiment, if only 
 suited to its object, delighted him. 
 
LESSONS IN ELECTRICITY. 
 
 299 
 
 FIQ. 12. 
 
 FIG. 11. 
 
 RflTKl particles descend as a continuous 
 si i cam, s s', fig. 12, but at every stroke 
 of die rubber they fly asunder, as in fig. 
 13, through self-repulsion. f 
 
 Or let three or four fine fillets of water 
 issue from three or four pin-holos in the 
 bottom of a vessel close to each other. 
 Connect the water of the vessel with 
 your glass tube, and rub as before. The 
 liquid veins are scattered into spray by 
 every stroke of the rubber. 
 
 These experiments are best made with 
 " CottrelPs rubber," described in 24. 
 
 And now you must learu to determine 
 with certainty the quality cf the elec- 
 tricity with which any body presented to 
 you may be charged. You see immedi- 
 ately that attraction is no sure test, be- 
 cause unelectrified bodies are attracted. 
 Further on ( 14) you will be able to 
 grapple with another possible source of 
 error in the employment of attraction. 
 
 In determining quality, you must 
 ascertain, by trial, the kind of electricity 
 by which the charged body is repelled ; 
 if, for example, any electrified body re- 
 pel, or is repelled by, sealing-wax rubbed 
 with flannel, the electricity of the body 
 
 f For these, and also for experiments with 
 thu electroscope, the teacher of a large class 
 will find the lime light shadows upon awhile 
 screen (or better still, those of the electric 
 light) exceedingly useful. The effects arc 
 thus rendered visible to all at once. 
 
 is negative ; if it repel, or is repelled by, 
 glass, rubbed with silk, its electricity is 
 positive. Du Fay had the sagacity to 
 propose this mode of testing quality. 
 
 Apply this test to the strips of fools- 
 cap paper excited by the india-rubber. 
 Bring a rubbed gutta-percha tube near 
 the electrified strips, you have strong at- 
 traction. Bring a rubbed glass tube 
 between the strips, you have strong re- 
 pulsion and augmented divergence. 
 Hence, the electricity, being repelled by 
 the positive glass, is itself positive. 
 
 11. Electricity of the Rubber. Doubh 
 or " Polar" Character of the Electric 
 Force. 
 
 We have examined the action of each 
 kind of electricity upon itself, and upon 
 the other kind ; but hitherto we have 
 kept the rubber out of view. One of 
 the questions which inevitably occur to 
 the inquiring scientific mind would be, 
 How is the rubber affected by the act of 
 friction ? Here, as elsewhere, you must 
 examine the subject for yourself, and 
 base your conclusions on the facts you 
 establish. 
 
 Test your rubber, then, by your bal- 
 anced lath. The lath is attracted by the 
 flannel which has rubbed against gutta- 
 percha ; and it is attracted by the silk, 
 which has rubbed against glass. 
 
300 
 
 LESSOMS IN FLTCTRTCITY. 
 
 Regarding the quality of the electricity 
 of the flannel or of the silk rubber, the 
 attraction of the lath teaches you nothing. 
 But, suspend your rubbed glass tube, and 
 bring the flannel rubber near it : repul- 
 sion follows. The silk rubber, on the 
 contrary, attracts the glass tube. Sus- 
 pend your rubbed gutta-percha tube, and 
 bring the silk rubber near it ; repulsion 
 follows. The flannel, on the contrary, 
 attracts the tube. 
 
 The conclusion is obvious : the elec- 
 tricity of the flannel is positive, that of 
 the silk is negative. 
 
 But the flannel is the rubber of the 
 gutta-percha, whose electricity is nega- 
 tive ; and the silk is the rubber of the 
 glass, whose electricity is positive. Con- 
 sequently, wo have not only proved the 
 rubber to be electrified by the friction, 
 but also proved the electricity of the 
 rubber to be opposite in quality to that 
 of the body rubbed. 
 
 All your subsequent experience will 
 verify the statement that the two electric- 
 ities always go together ; that you can- 
 not excite one of them without at the 
 wine time exciting the other, and that 
 the lectricity of the rubber, though op- 
 posite in quality, is in all cases precisely 
 equal in quantity to that of the body 
 rubbed. 
 
 And now we will test these principles 
 by a new experiment. In 5 we learned 
 that an ebonite comb is electrified by its 
 passage through dry hair. You can 
 readily prove the electricity of the comb 
 to be negative. But the hair is here the 
 rubber, and, in accordance with the 
 principle just laid down, an equal quan- 
 tity of positive electricity has been excit- 
 ed in the hair. If you stand on the 
 Ground uninsulated, the electricity of the 
 hair passes freely through your body to 
 the earth. 
 
 But stand upon an insulating stool* 
 on your board, for example, supported 
 by four warm tumblers while I, stand- 
 ing on the ground, pass the comb briskly 
 through your hair. I pass it ten, twenty, 
 tliiity times, and then ask you to attract 
 
 * A stool with glass legs wnieh, to protect 
 them f:om the moisture of the air. are 
 usually coated with a solution of shellac. 
 Regarding the attraction of glass for atmos- 
 pheric humidity, you will call to mind what 
 Las been said in 5. 
 
 your balanced lath. You present your 
 knuckle, but there is no attraction. 
 
 Here the comb and the hair soon reach 
 their maximum excitement, bevond which 
 no further development of electricity oc- 
 curs. Now, though the comb, as shown 
 in 5, is competent to attract the lath, 
 while your body is here incompetent to 
 do so, this may be because the small 
 quantity of electricity existing in a con- 
 centrated form upon the comb becomes, 
 when dirfused over the body, too feeble 
 to produce attraction. 
 
 Can we not exalt the electricity of your 
 body ? Guided by the principles laid 
 down, let us try to do so. First I pass 
 the unelectrified comb through your 
 hair ; it comes away electrified. After 
 discharging the comb by passing my hand 
 closely over it, I pass it again through 
 the hair. As before, it quits the hair 
 electrified, and I again discharge it. I 
 do this ten or twenty times, always de- 
 priving the comb of its electricity after it 
 has quitted the hair. Now present your 
 knuckle to the balanced lath. It is pow- 
 erfully attracted. 
 
 Here, as I have said, the unelectrified 
 comb carried in each case electricity away 
 with it ; but, in accordance with the fore- 
 going principles, it left an equal quantity 
 of the opposite electricity behind it. 
 And though the amount of electiicity 
 corresponding to a single charge of the 
 comb, when diffused over the body, 
 proved insensible to our tests, that 
 amount ten or twenty times multiplied 
 became not only sensible, but strong. 
 Indeed, by discharging the comb, and 
 passing it in each case undeetrified 
 through the hair, the insulated human 
 body cair be rendered highly electrical. 
 
 Near the beginning of this section I 
 said, in rather an off-hand way, thai 
 rubbed flannel repels rubbed glass, while 
 rubbed silk repels rubbed gutta-percha. 
 Now, while it is generally easy to obtain 
 the repulsion by the flannel, It is by no 
 means always easy to obtain the repulsion 
 by the silk. Over and over again I have 
 been foiled in my attempts to show this 
 repulsion. I wish you, therefore, to be 
 awaro cf an infallible method of obtain- 
 ing it. 
 
 Stand on your insulating stool, and rub 
 your glass tube briskly with the amalga- 
 mated silk ; hand me the tafcc. I pass 
 
LESSONS IN ELECTRICITY. 
 
 301 
 
 my hand closely over its surface, rcmor- 
 iug from that surface nearly the whole of 
 its electricity. I hand you the tube 
 again, and you again excite it. You 
 hand it to me, and I again discharge it. 
 In each case, therefore, you excite an un- 
 clectrified glass tube, and in each case 
 the tube leaves behind upon the rubber 
 an amount of negative electricity equal in 
 quantity to the positive carried away. 
 By thus adding charge to charge, the 
 rubber is rendered highly electrical ; and 
 even should its insulating power bo im- 
 paired by the amalgam, it can now afiord 
 to yield a portion of its electricity to your 
 hand and body, and still powerfully repel 
 rubbed gutta-percha. The principle, 
 which might be further illustrated, is ob- 
 tiously the same as that applied in the 
 case of the comb. 
 
 12. What is Electricity ? 
 
 Thurs far we have proceeded from fact 
 (o fact, acquiring knowledge of a very 
 valuable kind. But facts alone cannot 
 satisfy us. We seek a knowledge of the 
 principles which lie behind the facts, and 
 which aie to be discerned by the mind 
 alone. Thus, having spoken as we have 
 done, of electricity passing hither and 
 thither, and of its being prevented from 
 passing, hardly any thoughtful boy cr 
 giri can avoid asking what is it that thus 
 passes ? what is electricity ? Boyle and 
 Newton betrayed their need of an answer 
 to this question when the one imagined 
 his unctuous threads issuing from and re- 
 turning to the electrified body ; and \\hen 
 the other imagined that an elastic fluid 
 existed which penetrated his rubbed glass. 
 
 When I say " imagined " I do not in- 
 tend to represent the notions cf these 
 great mew as vain fancies. "Without im- 
 agination we can do nothing here. By 
 imagination I mean the power of pictur- 
 ing mentally things which, though they 
 have an existence as real as that of the 
 world around us, cannot be touched 
 directly by the organs of sense. I mean 
 the purified scientific imagination, with- 
 out the exercise of which we cannot take 
 a, single step into the region of causes and 
 principles. 
 
 It was by the exercise of the scientific 
 imagination that Franklin devised the 
 theory of a single electric fluid to explain 
 electrical phenomena. This fluid he sup- 
 
 posed to be self-repulsive, and diffused 
 in definite quantities through all bodies. 
 lie supposed that when a body has more 
 than its proper share it is positively, 
 when less than its proper share it is nega- 
 tively electrified. It was by the exercise 
 of the same faculty that Symmer devised 
 the theory of two electric fluids, each 
 self-repulsive, but both mutually attrac- 
 tive. 
 
 At first sight Franklin's theory seems 
 by far the simpler of the two. But its 
 simplicity is only apparent. For though 
 Franklin assumed only one fluid, he was 
 obliged to assume three distinct actions. 
 Firstly, he had the self -repulsion of the 
 electric particles. Secondly, the mutual 
 atraction of the electric particles and the 
 oonderablc particles of the body through 
 which the ciectnoitv was diffused. 
 Thirdly, these two assumptions when 
 strictly followed out ica^ 10 the unavoid- 
 able conclusion that the material particles 
 also mutually repel each other. Thus the 
 theory is by no means so simple as it ap- 
 pears. 
 
 The theory of Symmer, though at first 
 pight the most complicated, is in reality 
 by far the simpler of the two. Accord- 
 ing to it electrical actions are produced 
 by two fluids, each .self-repulsive, but 
 both mutually attractive. These fluids 
 cling to the atoms of matter, and carry 
 the matter to which they cling along with 
 them. Every body, in its natural condi- 
 tion, possesses both fluids in equal quan- 
 tities. As long {is the fluids are mixed 
 together they neutralize each other, the 
 body in which they arc thus mixed being 
 in its natural or unelectrical condition. 
 
 By friction (and by various other 
 means) these two fluids may be torn 
 asunder, the one clinging by preference to 
 the rubber, the other to the body rubbed. 
 
 According to this theory there must 
 always be attraction between the rubber 
 and the body rubbed, because, as we 
 have proved, they arc oppositely electri- 
 fied. This is in fact the case. And 
 mark what I now say. Over and above 
 the common friction, this electrical at- 
 traction has to be overcoaao v /fene^ r< wa 
 rub glass with silk, cr 88fiiL^g~tfi* .^itli. 
 flannel. 
 
 You arc too young to fully grasp this 
 subject yet ; and indeed it would lead us 
 too far away to enter fully into it. But 
 
802 
 
 LESSORS IN ELECTRICITY. 
 
 I will throw out for future reflection the 
 remark, that the overcoming of the ordi- 
 nary friction produces heat then and there 
 upon the surfaces rubbed, while the force 
 expended in overcoming the electric at- 
 traction may be converted into heat 
 which sLal! appear a thousand miles away 
 from the place whore it was generated. 
 
 Theoretic conceptions are incessantly 
 checked and corrected by the advance of 
 knowledge, and this theory of clectiic 
 fluids is doubted by many eminent scien- 
 tific men. It will, at till events, have to 
 be translated into a form which shall con- 
 nect it with heat and light, before it can 
 be accepted as complete. Nevertheless, 
 keeping ourselves unpledged to the the- 
 ory, we shall find it of exceeding service 
 both in unravelling and in connecting to- 
 gether electrical phenomena. 
 
 13. Electric Induction, 
 the Term. 
 
 Definition, of 
 
 We have now to apply the theory of 
 electric fluids to the important subject of 
 electric induction. 
 
 It was noticed by early observers that 
 contact was not necessary to electrical ex- 
 citement. Otto von Gueiickc, as we 
 have already seen ( 2), found that a 
 body brought near his sulphur globe be- 
 came electrical. By bringing his excited 
 glass tube near one end of a conductor, 
 Stephen Gray attracted light bodies at 
 the other end. lie also obtained attrac- 
 tion through the human body. From 
 
 FIG. 15. 
 
 the human body also Du Fay, to his 
 astonishment, obtained a spark. Can- 
 ton, in 1753, suspended pith-balls by 
 thread, and holding an excited glass 
 tube, at a considerable distance from 
 them, caused them to diverge. On re- 
 moving the tube the balls fell together, 
 no permanent charge being impaited to 
 them. Such phenomena were further 
 studied i:nd developed by Wilcke and 
 ^pinus, Coulomb and Poisson. 
 
 These and all similar results are cm- 
 braced by the law, that when an electri- 
 fied body is brought near an unelectrified 
 conductor, the neutral fluid of the latter 
 is decomposed ; one of its constituents 
 being attracted, the other repelled. 
 When the electrified body is withdrawn, 
 the separated electricities flow again to- 
 gether and render the conductor unelec- 
 tric. 
 
 This decomposition of the neutral fluid 
 by the mere presence of an electrified 
 body is called induction. It is also called 
 electrification by influence. 
 
 If, whilf 't is under the influence of 
 the electrified bodv, the bodv influenced 
 be touched, the free electricity (which is 
 always of the same kind as that of the 
 influencing body) passes away, the 
 opposite electricity being held cap- 
 tive. 
 
 On removing the electrified body the 
 captive electricity is set free, the conduc- 
 tor being charged with electricity oppo- 
 site in kind to that of the body which 
 
^LESSOXS IN ELECTRICITY. 
 
 303 
 
 electrified it. 
 
 You cannot do better lierc than repeat 
 Stephen Gray's experiment. Suppoit :i 
 small plank or lath, L L', Fig. 14, upon a 
 wnnn tumbler, G, and bring under one of 
 its ends, L, and within four or five inches 
 of that end, scraps of light paper or of 
 gold leaf. Excite your glass tube, R, 
 vigorously, and bring it over the other 
 end of the plank, without touching it. 
 The ends may be six or eight feet apart ; 
 the light bodies will be attracted. The 
 experiment is easily made, and you are 
 not to rust satisfied till you can make it 
 with case and certainty. 
 
 This is a fit place to repeat that you 
 must keep a close eye upon the tumblers 
 you employ for insulation. Some of 
 thorn, made of common glass, are hardly 
 to be accounted insulators at all. 
 
 14. Experimental Researches on 2Jlcc- 
 tric Induction. 
 
 Our mastery over this subject of in- 
 duction must be complete ; for it under- 
 lies all o';r subsequent inquiries. "With- 
 out reference to it nothing is to be ex- 
 plained ; possessed of it you will enjoy 
 not only a wonderful power of explana- 
 tion, but of prediction. We will attack 
 it, therefore, with the determination to 
 exhaust it. 
 
 Arid here a slight addition must be 
 made to our apparatus. We must be in 
 a condition to take samples of electricity, 
 and to convey them, with the view of 
 testing them, from place to place. For 
 this purpose the little "carrier," shown 
 in fig. 15, will be found convenient. T 
 i.5 a bit of tin-toil, two or three inches 
 square. A 9*. raw stem is stuck on to it 
 by sealing-wax, the lower end of the stem 
 being covered by sealing-wax. To make 
 the inflation sure, the part between R 
 and s' is wholly of sealing-wax, You 
 v,an have sterns of ebonite, which are 
 stronger, for a few pence ; but you can 
 have this one for a fraction of a penny. 
 The end u' is to be held in the hand ; the 
 electrified body is to be touched by T, 
 
 id the electricity conveyed to an clec- 
 
 osoopo to be tested. 
 Touch your rubbed glass rod with T, 
 
 id then touch your electroscope : the 
 leaves diverge with positive electricity. 
 ~?oacli your rubbed gutta-percha or seal- 
 ing-wax \vitIiT, and then touch your elec- 
 
 troscope : the leaves diverge with nega- 
 tive electricity. If the electricity of any 
 body augment the divergence produced 
 by the glass, the electricity of that body 
 is positive. If it augment the divergence 
 produced by the gutta-percha, the elec- 
 tricity is negative. And now we ar^ 
 ready for further work. 
 
 Place an egg, E, fig. 16, on its sid* 
 
 FIG. 16. 
 
 upon a dry wine-glass \ bring your ciciV- 
 cd glass tube, o, within an inch or so of 
 the end of the egg. What i.s the condi- 
 tion of the egg 1 Its electricity is decom- 
 posed ; the negative fluid covering the 
 end a adjacent to the glass, the positive 
 covering the other end b. Remove the 
 glass tube : what occurs ? The two elec- 
 tricities flow together and neutrality is 
 restored. Prove this neutrality. Neither 
 a carrier touching the egg, nor the egg it- 
 self, has any power to pffect your elec- 
 troscope, or to attract your balanced lath. 
 Again, bring the excited tube near tho 
 egg. Touch its distant part b with your 
 carrier. The carrier now attracts the 
 straw (fig. 2) or the balanced lath (fig. 4). 
 It also causes tho leaves of your electro- 
 scope to diverge. What is the quality 
 of the electricity ? It repel* and i-i rc- 
 
 rlled by rubbed glass ; the electricity at 
 is therefore positive. Discharge tho 
 carrier by touching it, and bring it into 
 contact with the end a of the cg.-j nearest 
 to the glass tube. The electricity you 
 take away repels and \s> repelled by gutta- 
 percha. It is therefore negative. Test 
 the quality also by the electroscope. 
 
 While the tube o is near the egg touch 
 the end b with your finger ; now try to 
 charge the carrier by touching b : you 
 cannot do so the positive electricity has 
 disappeared* Has the negative disap- 
 
804 
 
 LESSONS IN ELECTRICITY. 
 
 peared also ? No. Remove the glass 
 tube, and once more touch the egg at b 
 by the carrier. It is charged, not with 
 positive, but with negative electricity. 
 nearly understand this experiment. The 
 neutral electricity of the egg is first de- 
 composed into negative and positive ; the 
 former attracted, the latter repelled by 
 the excited glass. The repelled elec- 
 tricity is free to escape, and it has escaped 
 on your touching the egg with your fin- 
 ger. But the attracted electricity cannot 
 escape as long as the influencing tube is 
 held near. On removing the tube which 
 holds the negative fluid in bondage, that 
 fluid immediately diffuses itself over the 
 whole egg. An apple, or a turnip, will 
 answer for these experiments at least as 
 well as an egg. 
 
 Discharge the egg by touching it. Rc- 
 cxcite the glass tube and bring it again 
 near. Touch the egg wth a wire or with 
 your finger at a. Is it the negative at a, 
 into which you plunge your finger, 
 tlutt escapes ? No such thing. The free 
 positive fluid passes through the nega- 
 tive, and through your finger to the 
 earth. Prove this by removing, first, 
 your finger, and then the glass tube. The 
 egg is charged negatively. 
 
 Again ; place two eggs, E E, fig. 17, 
 
 PIG. 17. 
 
 lengthwise on two dry wine-glasses, g g, 
 and cause two of their ends to touch 
 each other, as at c. Bring your rubbed 
 glass rod near the end a, and while it is 
 there separate the eggs by moving one 
 glass away from the other. Withdraw 
 the rod and test both eggs, a repels 
 rubbed sealing-wax, and b repels rubbed 
 glass ; a is therefore negative, b is posi- 
 
 tive. The two charges, moreover, exact- 
 ly neutralize each other in the oiectro- 
 scope. Again l>rinj the eggs together 
 and restore the rubbed tube to its plao 
 near a. Touch a and then separate . tho 
 eggs. Remove the glass rod an-1 t^st the 
 eggs. a is negative, b is reutral. Its 
 electricity lias escaped through the lin- 
 ger, though placed ;.t a. 
 
 Equally good, if not indeed more 
 handy, for theso experiments arc two 
 apples A A, fig. 10, supported on stems 
 cf sealing-wax. A needle is healed L;. \ 
 sni-k in each case into the rtick of wax at 
 the top, and on to the nccilc the apple is 
 pushed. The sealing-wax stems are stuck 
 on by melting to little foot-boards. By 
 arrangements of this kind you rnako ex- 
 periments which are more instructive than 
 those usually made with instruments 
 twenty times more expensive. 
 
 Push vour researches still farther, and 
 instead of bringing the eggs or apples to- 
 gether place them six feet or so apart, 
 
 FIG. 18. 
 
 and let a light chain, c, fig. 19, or a wire, 
 stretch from one to the other. Two 
 brass balls, or wooden balls covered with 
 
 tin-foil, supported by tall drinking glasses, 
 ' will be better than the eggs for this 
 
 GG 
 
 experiment, for they will bear better the 
 strain of the chain ; but you can make 
 the experiment with the eggs, or very 
 readily with the two apples or two tur- 
 nips. For the present we will suppose 
 the straw-index 1 1' not to be there. Rub 
 
LESSORS IN ELECTRICITY. 
 
 305 
 
 FIG. 19. 
 
 yonr glass tube B, and bring it near one 
 of the balls ; test both : the near one, T', 
 is negative, the distant one, T, positive. 
 Touch the near one, the positive elec- 
 tricity, which had been driven along the 
 chain to the remotest part of the system, 
 returns along the chnin, passes through 
 (he negative, which is held captive by 
 the tube, and escapes to the earth. 
 When the tube R is removed, negative 
 electricity overspreads both chain and 
 balls. 
 
 In fig. 8 you made the acquaintance of 
 the plate N, and the straw-index i i y , 
 shown on a smaller scale in fig. 19. By 
 their means you immediately see both 
 the effect of the first induction, and the 
 consequence of touching any part of tha 
 system with the finger. The plate K 
 rests over the ball or turnip T, the posi- 
 tion of the straw-index being that shown 
 by the dots. Bring the rubbed tube neat 
 T' : the end N of the index immediately 
 descends and the other end rises along 
 the graduated scale. Remove the glass 
 rod; the index 1 1' immediately falls. 
 Practice this approach and withdrawal, 
 and observe how promptly the index de- 
 clares the separation and rccomposition 
 of the fluids. 
 
 While the tube is near T', and the end 
 S T of the index is attracted, let T' be 
 touched by the finger. The end N is im- 
 mediately liberated, for the electricity 
 which pulled it down escapes along the 
 chain and through the finger to the 
 earth. Now remove your excited tube. 
 The captive negative electricity diffuses 
 itself over both balls, arid the index is 
 again attracted. 
 
 Instead of the chain you may interpose 
 between the balls 100 feet of wire sup- 
 
 by silk loops. This is done m 
 fig. 20, which shows the wire w support- 
 ed by the silk strings 8 s s. For the bafl 
 or turnip T', fig. 19, the cylinder c, on a 
 glass support a, is substituted, the little 
 table M taking the place of the ball T. 
 Every approach and withdrawal of the 
 rubbed glass tube R is followed obedient- 
 ly by the attraction and liberation of N, 
 and the corresponding motion of the in- 
 dex N I. 
 
 Repeat here an experiment, first 
 
306 
 
 LESSONS IN ELECTRICITY. 
 
 by a great electrician named ^Epinns. I 
 wish you to make these historic experi- 
 ments. Insulate an elongated metal con- 
 ductor, c c', fig. 21, or one formed of 
 wood coated with tin-foil even a carrot, 
 cucumber, or parsnip, so that it be insu- 
 lated, will answer. Let a small weight, 
 W, suspended from a silk string, s, re^t 
 en one end of the conductor, and hold 
 
 FLO. 2L 
 
 your rubbed glass tube, R, over the other 
 end. You can predict beforehand what 
 will occur when you remove the weight. 
 It carries away with it electricity, which 
 repels rubbed glass, and attracts your 
 balanced lath. 
 
 Stand on an insulating stool ; or make 
 one by placing a board on four warm 
 tumblers. Present the knuckles of your 
 riirht hand to the end of tho balanced 
 lath, and stretch forth your left arm. 
 There is no attraction. But let n friend 
 or an assistant bring the rubbed glass 
 tube over the left arm ; the kUi immedi- 
 ately follows the right Land. 
 
 Touch the lath, or any (.th'-r uninsulat- 
 ed body ; the ** attractive virtue," as it 
 was called by Gray, disappears. After 
 this, as long r.& the "excited tul is held 
 
 over the arm there in no attraction. But 
 when the tube is removed the attractive 
 power of the hand is restored. Here the 
 first attraction was by positive -electricity 
 driven to the right Land from the left, 
 and the second attraction by negative 
 electricity, liberated by the removal of 
 the glass rod. Experiment proves the 
 logic of theory to be without a flaw. 
 
 Stand on an insulating stool, and place 
 your right hand on the electroscope ; 
 there is no action. Stretch forth the left 
 arm and permit an assistant alternately 
 to bring near, and to withdraw, an excit- 
 ed glass tube. The gold leaves open ?;nd 
 collapse i.i similar alternation. At every 
 approach, positive electricity is driven 
 over the gold leaves ; at every with- 
 drawal, the equilibrium is restored. 
 
 We arc now in a condition to repeat, 
 with case, the experiment of Du Fay- 
 mentioned in 1.,. A board is support- 
 ed by four silk ropes, and on the board 
 is stretched a boy. Bring his fore Lend, 
 or better still his nose, under the end of 
 your straw-index 1 i', fig. 22. Then 
 bring down over his legs your rulbed 
 glass tube ; instantly the end i' is attzact- 
 ed and the end i rises along the graduat- 
 ed scale. Before the end i' comes into 
 contact with the nose or forehead a spark 
 passes between it and the boy. 
 
LESSONS IN ELECTRICITY. 
 
 307 
 
 1 will now ask you to charge your 
 Dutch metal electroscope (fig. 7) posi- 
 tively by rubbed gutta-percha, and to 
 charge it negatively 7 by rubbed glass. A 
 moment's reflection will enable you to do 
 it. You bring your excited bodv near : 
 the same electricity as that of the excited 
 body is driven over the leaves, and they 
 diverge by repulsion. Touch the elec- 
 troscope, the leaves collapse. Withdraw 
 your finger, and withdraw afterwards the 
 excited body : the leaves then diverge 
 with the opposite electricity. 
 
 The simplest way of testing the quality 
 of electricity is to charge the electroscope 
 with electricity of a known kind. If, on 
 the approach of the body to be tested, 
 the leaves diverge still wider, Ilio lvr<* 
 and the body are similarly electrified. 
 The reason is obvious. 
 
 Omitting the last experiment, the 
 wealth of knowledge which these re- 
 searches involve might be placed within 
 any intelligent boy's reach by the wise 
 expenditure of half-a-crovvn. 
 
 Once firmly possessed of the principle 
 of induction and versed in its application, 
 the difficulties of our subject will melt 
 away before us. In fact oar subsequent 
 work will consist mainly in unravelling 
 phenomena by the aid of this principle. 
 
 Without a knowledge of this principle 
 wo could render no account of the attrac- 
 tion of neutral bodies by our excited 
 tubes. In reality the attracted bodies 
 arc not neutral : they are first electrified 
 by influence, and it is because they are 
 thus electrified that they are attracted. 
 
 This is the place to refer more fully 
 to a point already alluded to. Neutral 
 bodies, as just stated, arc attracted, be- 
 cause they arc really converted into elec- 
 trified bodies by induction. Suppose a 
 body to be feebly electrified positively, 
 and that you bring your rubbed glass 
 tubo t:> bear upon the body. You clear- 
 ly sec that the induced negative electricity 
 may be strong enough to mask and over- 
 come the weak positive charge possessed 
 by the body. We should thus have two 
 bodies electrified alike, attracting each 
 other. This is the danger against which 
 I promised to warn you in 10, where 
 the test of attraction was rejected. 
 
 Wo will now apply the principle of in- 
 duction to explain a very beautiful inven- 
 tion, made known by the celebrated 
 
 Voltain 1775. 
 
 15. The Mectrophorus. 
 
 Cut a circle, T, fig. 23, 6 inches in 
 diameter out of sheet zinc, or out of 
 common tin. Heat it at its centre by the 
 
 FIG. 23. 
 
 flame of a spirit-lamp or of a candle. 
 Attach to it there a stick of sealing-wax, 
 H, which, when the metal cools, i* to 
 serve as an insulating handle. You have 
 now the lid of the electrophorus. A 
 resinous surface, or what is simpler a 
 sheet of vulcanized india-rubber, p, or 
 even of hot brown paper, will answer for 
 the plate of the electrophorus. 
 
 Rub your " plate" with flannel, or 
 whisk it briskly with a fox's brush. It 
 is thereby negatively electrified. Place 
 thy " lid " of your olectrophorus on the 
 excited surface : it touches it at a few 
 points only. For the most pait lid and 
 plate arc separated by a film of air. 
 
 The excited surface acts by induction 
 across this film upon the lid, attracting 
 its positive and repelling its negative 
 electricity. You Lnvo in fact in the lid 
 two layers of electricity, the lower one, 
 which is " bound," positive ; tho upper 
 one, which is " free," negative. ^Lift 
 the lid : the electricities iiow again to- 
 gether ; neutrality is restored, and your 
 lid fails to attract your balanced lath. 
 
 Once more plnce the lid upon the ex- 
 cited surface : touch it with the finger. 
 What occurs ? You ought to know. 
 Tho free electricity, , which is negative, 
 will escape through your body to tho 
 earth, leaving the chained positive be- 
 hind. 
 
 Now lift the lid by the handle : what 
 is its condition ? Again I say you ought 
 
LESSORS IN ELECTRICITY. 
 
 Fw. 24. 
 
 to know. It is covered with free posi- 
 tive electricity. If it be presented to the 
 lath it will strongly attract it : if it be 
 presented to the knuckle it will yield a 
 spark. 
 
 A iniooth half-crown, or a penny, will 
 answer for this experiment. Stick to tho 
 coin an inch of sealing-wax as an insulat- 
 ing handle : bring it down upon the ex- 
 cited india-rubber : touch it, lift it, and 
 present it to your lath. The lath may 
 be six or eight feet long, three inches 
 wide and half an inch thick ; the little 
 electrophorus lid, formed by the half 
 crown, will pull it round and round. The 
 experiment is a very impressive one. 
 
 Scrutinize your instrument still further. 
 Let the end of a thin wire rest upon the 
 lid of your electrophorus, under a little 
 weight if necessary ; and connect the other 
 end of the wire with the electroscope. 
 As you lower the lid down toward the 
 excited plate of the electrophorus, what 
 must occur ? The power of prevision 
 now belongs to you and you must exer- 
 cise it. The repelled electricity will flow 
 over the leaves of the electroscope, caus- 
 ing them to diverge. Lift the lid, they 
 oollapse. Lower and raise the lid several 
 times, and observe the corresponding 
 rhythmic action of the electroscope leaves. 
 
 A little knob of sealing-wax, B, coated 
 with tin-foil, or indeed any knob with a 
 conducting surface, stuck to the lid of 
 the electrophorus, will enable you to ob- 
 tain a better spark. The reason of this 
 will immediately appear. 
 
 More than half the ralne of your pres- 
 ent labor consists in arranging each ex- 
 periment in thought before it is realized 
 in fact ; and more than half the delight 
 of your work will consist in observing 
 the verification of what you have foreseen 
 and predicted. 
 
 16. Action of Points and Flames. 
 
 The course of exposition proceeds 
 naturally from the electrophorus to the 
 electrical machine. But before we take 
 up the machine we must make our minds 
 clear regarding the manner in which elec- 
 tricity diffuses itself over conductors, and 
 more especially over elongated and point- 
 ed conductors. 
 
 Rub your glass tube and draw it over 
 an insulated sphere of metal of wood 
 covered with tin-foil, or indeed any other 
 insulated spherical conductor. Repeat 
 the process several times, so as to impart 
 a good charge to the sphere. Touch the 
 charged sphere with your carrier, and 
 transfer the charge to the electroscope. 
 Note the divergence of the leaves. Dis- 
 charge the electroscope, and repeat tho 
 experiment, touching, IIOWCVPV, some 
 other point of the sphere. The electro- 
 scope shows sensibly the same amount of 
 divergence. Even when the greatest ex- 
 actness of the most practised experi- 
 menter is brought into play, the spherical 
 conductor is found to be equally charged 
 at all points of its surface. You may 
 figure the electric fluid as a little ocean 
 encompassing the sphere, and of the same 
 depth everywhere. 
 
 But supposing the conductor, instead 
 of being a sphere, to be a cube, an elon- 
 gated cylinder, a cone, or a disk. The 
 depth, or as it is sometimes called the 
 density, of the electricity, will not be 
 everywhere the same. The corners of 
 the cube will impart a stronger charge to 
 your carrier than the sides. The end of 
 the cylinder will impart a stronger charge 
 than its middle. The edge of the disk 
 will impart a stronger chargo than its flat 
 surface. The apex or point of the cone 
 will impart a stronger chargo than its 
 curved surface or i:s base. 
 
 You can satisfy yourself of the truth of all 
 this in a rough, but certain way, by charg- 
 ing, after the sphere, a turnip cut into the 
 
LESSOST3 m ELECTRICITY. 
 
 form of a cube ; or a cigar-box coated with 
 tin-foil ; a metal cylinder, or a wooden 
 one coated with tin-foil ; a disk of tin or 
 of sheet zinc ; a carrot or parsnip with its 
 natural shape improved so as to make it 
 a sharp cone. You will find the charge 
 Imparted to the carrier by the sharp cor- 
 ners and points of such bodies, when elec- 
 trified, to be greater than that communi- 
 cated by the gently rounded or flat sur- 
 faces. The difference may not be great, 
 but it will be distinct. Indeed an egg 
 laid on its side, as we have already used 
 it in our experiments on induction (fig. 
 1C), yields a stronger charge from its 
 ends than from its middle. 
 
 
 Fia. 25. 
 
 Let me place before you an example of 
 this distribution, taken from the excellent 
 work on " Frictional Electricity" by Pro- 
 fessor Riess of Berlin. Two cones, fig. 
 24, are placed together base to base. 
 Calling the strength of the charge along 
 the circular edge where the two bases join 
 eacli other 100, the charge at the apex of 
 the blunter cone is 133 ; and at the apex 
 of the sharper one 202. The other num- 
 bers give the charges taken from the 
 points where they are placed. Fig. 25, 
 moreover, represents a cube with a cone 
 placed upon it. The charge on the faco 
 of the cube being 1, the charges at fho 
 corners of the cube and at the apex of 
 the cone are given by the other numbers; 
 they are all far in excess of the electricity 
 on the flat surface. 
 
 Iliess found that he fould deduce with 
 great accuracy the sharpness of a point, 
 from the charge which it imparted, lie 
 compared iu this way the sharpness of 
 various thoins, with that of a firio Knolish 
 Rcwing'needle. The following is the re- 
 sult : -Euphorbia thorn was si mi per than 
 the needle ; gooseberry thorn of ilu same 
 
 sharpness as the needle ; while cactus, 
 blackthorn, and rose, fell more and more 
 behind the needle in sharpness. Calling, 
 i^r example, the charge obtained from 
 c iphorbia 90 ; that obtained from the 
 needle was 80, and from the rose only 
 53. 
 
 Considering that each electricity is 
 self-repulsive, and that it heaps itself up 
 upon a point, in the manner here shown, 
 you will have little difficulty in conceiv- 
 ing that when the charge of a conductor 
 carrying a point is sufficiently strong, the 
 electricity will finally disperse itself by 
 streaming from the point. 
 
 The following experiments are theoret- 
 ically important : Attach a stick of 
 sealing-wax to a small plate of tin or of 
 wood, so that the stick may stand 
 upright. Heat a needle and insert it into 
 the top of the stick of wax ; on this 
 needle mount horizontally a carrot. You 
 have thus an insulated conductor. Stick 
 into your carrot at one of its ends a sew- 
 ing needle ; and hold for an instant your 
 rubbed glass tube in front of this needle 
 without touching it. AVhat occurs? The 
 negative electricity of the carrot is imme- 
 diately discharged from the point against 
 the glass tube. Remove the tube, test 
 the carrot : it is positively electrified. 
 
 And now for another experiment, not 
 BO easily made, but still certain to succeed 
 if you are careful. Excite your glass rod, 
 turn your needle away from it, and bring 
 the rod near the other end of the carrot. 
 What occurs ? The positive electricity 
 is now icpe'llcd to the point, from which 
 it will stream into the air. Remove the 
 rod and test the carrot : it is negatively 
 electrified. 
 
 Again turn the point toward you, and 
 place in front of it a plate of dry glass, 
 wax, resin, shellac, paraffin, gutta-percha 
 or any other insulator. Pass your rubbed 
 glass tube once downwards or upwards, 
 the insulating plate being between the ex- 
 cited tabe and the point. The point will 
 discharge its electricity against the insu- 
 lating plate, which on trial will be found 
 negatively electrified. 
 
 17. The Electrical Machine. 
 
 An electrical machine consists of two 
 principal parts : the insulator which is 
 
SiO 
 
 LESSOXS IN ELECTRICITY. 
 
 PIG. 26. 
 
 excited by friction, and the * ' prime con 
 ductor." 
 
 The sulphur sphere of Otto von Guc- 
 rickc was, as already stated, the first 
 electrical machine. The hand was the 
 rubber, and indeed it long continued to 
 be so. For the sulphur sphere, Ilauks- 
 bcc and Vv'incLlcT substituted globes cf 
 glass. Doze of Wittenberg (1741) add- 
 ed the prime conductor, which was at f:r t 
 a tin tube supported by resin, orouspcnu- 
 ed by sill:. Soon afterward Gordon 
 substituted a glass cylinder for the globe. 
 It "was sometimes mounted vertically, 
 sometimes horizontally. Gordon so in- 
 tensified l.i.'i discharges as to be able to 
 kill email birds with them. In 17CO 
 Planta introduced the plate machine now 
 commonly in r.se. 
 
 Mr. Cottrell has constructed for these 
 Lessons the email cylinder machine shown 
 in fig. 20. The glass cylinder is about 7 
 inches long and 4 inches in diameter ; 
 its cost is eighteen pence. Through the 
 cylinder passes tightly, as an axis, a 
 pidcc of lath, rendered secure by sealing- 
 wax where it enters and where it quits 
 the cylinder. G is a glass rod supporting 
 the conductor c, which is a piece of lath 
 coated with tin-foil. Into the lath is 
 driven the scries of pin points, r, r. The 
 rubber n, is cccn at the further side cf 
 the cylinder, supported by the upright 
 lath r/, and caused to press against the 
 glass. G' ha flap of u'lk attached to the 
 rubber. Y/hen the handle 13 turned 
 
 FIG. 27. 
 
 sparks may be taken, or a Leyden jar 
 charged at the knob c. 
 
 A plato machine h thown i:i fig. 27. 
 p b the plate, which t;:rn3 on an axi.i 
 passing through its centre ; r. and r/ arc 
 two rubbers which clasp tho plate, with 
 the flips of cilk G G' attached to them. A 
 and A' arc rows of points forming part cf 
 the prime conductor, c. G G' is an insu- 
 lating rod cf glass, which cuts off tho 
 connection between the conductor and 
 the handle of the machine. 
 
 The prime conductor is charged in the 
 following manner. When the glass plato 
 is turned, as it passes each rubber it is 
 positively electrified. Facing tho elec- 
 trified glass is the row of points, placed 
 midway between the two rubbers. On 
 these points the glass acts by induction, 
 attracting the negative and repelling the 
 positive. In accordance with the princi- 
 ples already explained in 10, tho nega- 
 tive electricity streams from tho points 
 against the excited glass, which then 
 passes on neutralized to the next rubber, 
 where it is again excited. 
 
 Thus the prime conductor is charged, 
 not by tho direct communication to it of 
 positive electricity, but by depriving it 
 of its negative. 
 
 If when tho conductor is charged you 
 bring tho knuckle near it, the electricity 
 passes from the conductor to tho knuckle 
 in the form of a spark. 
 
 Take this spark with the blunt knuckle 
 while the machine is being turned ; and 
 
LESSON'S IN "ELECTRICITY. 
 
 311 
 
 FIG. 28. 
 
 then try the effect of presenting the finger 
 ends, instead of the knuckle, to the con- 
 ductor. The spark falls exceedingly in 
 brilliancy. Substitute for the finger ends 
 a needle point : you fail to get a spark 
 at nil. To obtain a good spark the elec- 
 tricity upon the prime conductor must 
 reach a sufficient density (or tension as it 
 i* sometimes called) ; and to secure this 
 no points from which thu electricity can 
 stream out must exist on the conductor, 
 or be presented to it. All parts of tho 
 conductor are therefore carefully rounded 
 off, sharp points and edges being avoided. 
 It u usual to attach to the conductor 
 an electroscope consisting of an upright 
 metal stem, A c, fig. 28, to which a straw 
 with a pith ball, u, at its free end, is at- 
 tached. The straw turns loosely upon a 
 pivot at c. The electricity passing from 
 the conductor is diffused over the whole 
 electroscope, and the straw and stem be- 
 ing both positively electrified, repel each 
 other. The straw, being the movable 
 body, flies away. The amount of the 
 divergence is measured upon a graduated 
 arc. 
 
 18, Further Experiments on the Action 
 of Points. The Electric Mill. The 
 Golden Fish. Lightning Conductors. 
 
 If no point exist on the conductor, Q 
 single turn of the handle of the machine 
 usually suffices to cause the straw to stand 
 out at a large angle to the stem. If, on 
 the contrary, a point be attached to the 
 conductor, you cannot produce a large 
 divergence, because the electricity, as 
 
 FIG. 29. 
 
 fast as it is generated, is dispersed by the 
 point. The same effect is observed when 
 you present a point to the conductor. 
 The conductor acts by induction upon the 
 point, causing the negative electricity to 
 stream from it against the conductor, 
 which is thus neutralized almost as fast as 
 it is charged. Flames and glowing em- 
 bers act like points ; they also rapidly 
 discharge clectiicity. 
 
 The electricity escaping from a point 
 or flame into the air renders the air self 
 repulsive. The consequence is that when 
 the hand is placed over a point mounted 
 on the prime conductor of a machine in 
 pood action, a cold blast is distinctly felt. 
 Dr. Watson noticed this blast from n 
 flame placed on an electrified conductor ; 
 while Wilson noticed the blast from a 
 point. Jallabert 'and the Abbe Nollet 
 also observed and described the influence 
 of points and flames. The blast is called 
 the " electric wind." Wilson moved 
 bodies by its action : Faraday caused it 
 to depress the surface of a liquid : Ham- 
 ilton employed the reaction of the electric 
 wind to make pointed wires rotate. The 
 " wind " was also found to promote 
 evaporation. 
 
 Hamilton's apparatus is called the 
 " electric mill." Make one for yourself 
 thus : Place two straws s s', 88% fig. 29, 
 about eight inches long, across each other 
 ht a right angle. Stick them together at 
 their centres by a bit of sealing-wax. 
 Pass a fine wire through each straw, and 
 bend it where it issues from the straw, 
 so as to form a little pointed arm perpea- 
 
312 
 
 LESSONS IN ELECTRICITY. 
 
 FIG. 30. 
 
 dicnlar to the straw, and from half an 
 inch to three quarters of an incli long. 
 It is easy, by means of a bit of cork or 
 sealing-wax, to. fix the wire so that the 
 little bent arms shall point not upward or 
 downward, but sideways, when the cross 
 is horizontal. The points of sewing 
 needles may also be employed for the bent 
 arms. A little bit of straw stuck into the 
 cross at the centre forms a cap. This 
 slips over a sewing needle, N, supported 
 by a stick of sealing-wax, A. Connect 
 the sewing needle with the electric 
 machine, and turn. A wind of a certain 
 force is discharged from every point, and 
 the cross is urged round with the same 
 force in the opposite direction. 
 
 Place your left hand on the prime con- 
 ductor of your machine. Let the hand) 
 
 You 
 
 of course, so 
 
 arrange the points that the wind from 
 some of them would neutralize the wind 
 from others. But the little pointed arms 
 are lo be so bent that the reaction in 
 every case shall not oppose but add itself 
 to the others. 
 
 The following experiments will yield 
 you important information regarding the 
 action of points. Stand, as you have so 
 often done before, upon a board support- 
 ed by four warm tumblers. Hold a small 
 sewing needle, with its point defended 
 by the forefinger of your right hand, 
 toward your Dutch metal electroscope. 
 
 31. 
 
 be turned by a friend or an assistant : tho 
 leaves of the electroscope open out a lit- 
 tle. Uncover the needle point by the re- 
 moval of your finger ; the leaves at once 
 fly violently apart. 
 
 Mount a stout wire upright on the con- 
 ductor, c, fig. 30, of your machine ; or 
 sapport the wire by sealing-wax, gutta- 
 
LESSONS IK ELECTRICITY. 
 
 813 
 
 FIG. 32. 
 
 Fio. 33. 
 
 FIQ. 34. 
 
 pcrcha or glass, at a distance from the 
 conductor, and connect both by a fmo 
 wire. Bend your stout wire into a hook, 
 and hang from it a tassel, T, composed 
 of many strips of light tissue paper. 
 Work the machine. Electricity from the 
 conductor flows over the tassel, and the 
 strips diverge. Hold your closed fist 
 toward the tassel, the strips of paper 
 stretch toward it. Hold the needle, de- 
 fended by the finger, toward the tassel : 
 attraction also ensues. Uncover the needle 
 without moving the hand ; the strips 
 retreat as if blown away by a wind. 
 Holding the needle, N, fig. 31, upright 
 underneath the tassel, its strips discharge 
 themselves and collapse utterly. 
 
 And now repeat Du Fay's experiment 
 which led to the discovery of two electric- 
 ities. Excite your glass tube, and hoi 1 
 it in readiness while a friend or an assist- 
 ant liberates a real gold or silver leaf in 
 the air. Bring the tube near the leaf : 
 it plunges toward the tube, stops sud- 
 denly, and then flies away. You may 
 chase it round the room for hours with- 
 out permitting it to reach the ground. 
 The leaf is first acted upon inductively 
 by the tube. It is powerfully attracted 
 
 for a moment, and rusnrs toward the 
 tube. But from its thin edges and cor- 
 ners the negative electricity streams forth, 
 leaving the haf positively electrified. 
 Repulsion then sets in, because tube and 
 leaf are electrified alike, as shown in fig. 
 32. The retreat of the tassel in the last 
 experiment is due to a similar cause. 
 
 There is also a discharge of positive 
 electricity into the air from the more dis- 
 tant portions of the gold -leaf, to which 
 that electricity is repelled. Both dis- 
 charges are accompanied by an electric 
 wind. It is possible to give the gold- 
 leaf a shape which shall enable it to float 
 securely in the air, by the reaction of the 
 two winds issuing from its opposite ends. 
 This is Franklin's experiment of the 
 Golden Fish. It was first made with the 
 charged conductor of an electrical ma- 
 chine. M. Srtsczek revived it in a more 
 convenient form, using instead of the 
 conductor the knob of a charged Leyden 
 jar. You may walk round a room with 
 the jar in your hand ; the " fish " will 
 obediently follow in the air an inch or 
 two, or even throe inches, from the 
 knob. Sec A B, fig. 33. Even a hasty 
 motion of the jar will not shake it away, 
 
814 
 
 "LESSONS tff ELECTRICITY. 
 
 Well - pointed lightning conductors, 
 when acted on by a thunder cloud, dis- 
 charge their induced electricity against 
 the cloud. Franklin raw this with great 
 clearness, and illustrated it with great in- 
 genuity. The under side of a thunder 
 cloud, when viewed horizontally, he 
 observed to be inched, composed, in 
 fact, of fragments one below the other, 
 sometimes reaching near the eaith. 
 These he regarded is so many stepping- 
 stones Y/hich assist in conducting the 
 stroke of the cloud. To represent those 
 by experiment he took two or three locks 
 of fine loose cotton, tied them ir, a row, 
 and hiing them from his pr.' n .ic con- 
 ductor. When this was excited the locks 
 stretched downward toward tho earth ; 
 but by presenting a sharp point erect 
 under the lowest bunch of cotton, it 
 shrunk upward to that above it, nor did 
 the shrinking cease till all the locks had 
 retreated to the prime conductor itself. 
 " May not," SMVS Franklin, "the small 
 ek'Ctrified cloud, whose equilibrium with 
 the earth is so soon restored by the point, 
 rise up to the main body, and by that 
 means occasion so large a vacancy that 
 the grand cloud cannot strike in that 
 placet" 
 
 19. History of the Ley den Jar. The 
 Ley den Battery. 
 
 The next discovery which we have to 
 master throws all former ones into the 
 shade. It was first announced in a letter ad- 
 dressed on the 4th of November, 1V45, 
 to Dr. Liebcrkuhn, of Berlin, by Kleist, 
 a clergyman of Cammin, in Fomerania. 
 By means of a cork, c, fig. 34, he fixed a 
 r.ail, N, in a phial, G, into which he had 
 poured a little mercury, spirits, or water, 
 w. On electrifying tho nail he was ablo 
 to pass from one room into another wit: 
 the phial in his hand nd to ignite spirits 
 of wine with it. " If," said he, * 4 while 
 it is electrifying I put my finger, or a 
 piece of gold which I hold in my hand, 
 to the nail, I receive a shock which stuns 
 my arms and shoulders." 
 
 In the following year Cunaeus of Ley- 
 den made substantially the ssme discov- 
 ery. It caused great wonder #nd dread, 
 which arose chietly from the excited im- 
 agination. Musschcnbroekfclt the shock, 
 and declared in a letter to a friend that 
 
 lie would not take a second one for th 
 crown of France. Bleeding at the nose, 
 ardent fever, a heaviness of head which 
 endured for days, were all ascribed to the 
 shock. Boze wished that he might die 
 of it, so that he might enjoy the honor 
 of having his death chronicled in the 
 Paris " Academy of Sciences." Kleist 
 missed the explanation of the phenome- 
 non ; while the Leyden philosophers cor- 
 rectly stated the conditions necessary to 
 the success of the experiment. Hence 
 tlio paial received the name of the Ley- 
 den phial, or Leyden jar. 
 
 Tho discovery of Kleist and Cunasus 
 excited the most profound interest, and 
 the subject was explored in all directions. 
 Wilson in 174G filled a phial partially 
 with water, and plunged it into water, so 
 as to bring the water surfaces, within 
 and without the phial, to the same level. 
 On charging such a phial the strength of 
 the shock was found greater than had 
 oeen observed before. 
 
 Two years subsequently Dr. Watson 
 And Dr. Bevis noticed how the charge 
 grew stronger as the area of the conduct- 
 or in contact with the outer surface of 
 the phial increased. They substituted 
 shot for water inside the jar, and ob- 
 tained substantially the same effect. Dr. 
 Bevis then coated a plate of glass on both 
 sides with silver foil, to within about an 
 inch of the edge, and obtained from it 
 discharges as strong as those obtained 
 from a phial containing half a pint of 
 water. Finally Dr. Watson coated his 
 phial inside and out with silver foil. By 
 these steps the Leyden jar reached the 
 form which it possesses to-day. 
 
 It is easy to repeat the experiment of 
 Dr. Bevis. Procure a glass plate nine 
 inches square ; cover it 0:1 both sides, as 
 he did, with tin-foil seven inches square, 
 leaving the rirn uncovered. Connect one 
 side with tho earth, and the other with 
 tho machine. Charge and discharge : 
 you obtain a brilliant spark. 
 
 In our experiment with the Golden 
 Fish. (fig. 33), we employed a common 
 form of the Leyden j:',r, only with the 
 difference that to get to a cufiicient dis- 
 tance from tho glass, so as to avoid the 
 attraction of the fish by the jaritsolf, the 
 knob was placed higher than usual. But 
 with a good flint-glass tumbler, a piece of 
 tin-foil, akd a bit of stout wire, you can 
 
LESSORS IX E.ECTKICITr. 
 
 make a jar for yourself. Bad glass, re- 
 member, is not rare. In fig. 35 you 
 have such a jar. T is the outer-, T' the 
 inner coating, reaching to within irn inch 
 c-f the edge of the tumbler G. ^' is the 
 
 FIG. 33. 
 
 Fia.36. 
 
 wire fastened below by wax, and sur- 
 mounted by a knob, which may be of 
 metal, or of wax or wood, coated with 
 tin-foil. In charging the jar you con- 
 nect the outer coating with the earth 
 ay with a gas-pipe or a water-pipe and 
 present the knob to the conductor of 
 your machine. A few turns will charge 
 the jar. It is discharged by laying one 
 knob of a ** discharger" against the 
 outer coating, and causing the other knob 
 
 to approach the knob of the jar. Be- 
 fore contact, the electricity flies from 
 knob to knob in the form of a spark. 
 
 A " discharger" suited to our means 
 and purposes is shown in fig. 36. n is a 
 stick of sealing-wax, or, better still, of 
 ebonite ; w w a stout wire bent as in the 
 figure, and ending in the knobs B B'. 
 These may be of wax coated with tin- 
 foil. Any other light conducting knobs 
 would of course answer. The insulating 
 handle n protects you effectually from 
 the shock. 
 
 You must render yourself expert in the 
 use of the discharger. The mode of 
 using it is shown in rig. 37. 
 
 By augmenting the size of a Leydsn 
 jar we render it capable of accepting a. 
 larger charge of electricity. But there 
 is a limit to the size of a jar. When 
 therefore, larger charges are required 
 than a single jar can furnish, we make 
 use of a number of jars. In tig. 38 nine 
 of them are shown. All their interior 
 coatings are united together by brass 
 rods, while all Um outer coatings rest 
 upon a metal suiface in free communica- 
 tion with the eaith. 
 
 This combination of Leydcn jars con- 
 stitutes the Leydcn Battery, the effect of 
 which is equal to that of a single jar ,f 
 nine times the size of one of the jars. 
 
 20. Explanation of the Leydcn Jar. 
 The principles of electrical induction 
 
LESSONS IN ELECTRICITY. 
 
 FIG. 38. 
 
 "with which you arc now so familiar will 
 enable you to thoroughly analyze and 
 understand the action of the Leydcn jar. 
 In charging- the jar the outer coating is 
 connected with the earth, and the inner 
 coating with the electrical machine. Let 
 the machine, as usual, be of glass yield- 
 ing positive electricity. When it is 
 worked the electricity poured into the jar 
 acts inductively across the glass upon the 
 outer coating, attracting its negative and 
 repelling its positive to the earth. Two 
 mutually attractive electric layers are thus 
 in presence of each other, being separa- 
 ted merely by the glass. When the ma- 
 chine is in good order and the glass of 
 the jar is thin, the attraction may be ren- 
 dered strong enough to perforate the jar. 
 By means of the discharger the opposite 
 electricities are enabled to unite in the 
 form of a F par If. 
 
 Franklin saw and announced with clear- 
 ness the escape of the electricity from 
 the outer coating of the jar. His state- 
 ment is that whatever be the quantity of 
 the " electric fire" thrown into the jar, 
 an equal quantity was dislodged from the 
 outside. We have now to prove by ac- 
 tual experiment that this explanation is 
 correct. 
 
 Place your Leydcn jar upon a, table, 
 and connect the outer coating with your 
 electroscope. There is no divergence of 
 the leaves when electricity is poured into 
 the jar. 
 
 But here the outer coating is connect- 
 
 ed through the table with the earth. Let 
 us cut off this communication by an in- 
 sulator. Place the jar upon a board up- 
 portcd by warm tumblers, or upon a 
 piece of vulcanized india-rubber cloth, 
 and again connect the outer coating with 
 the electroscope. The moment electric- 
 ity is communicated to the knob of the 
 jar the leaves of Dutch metal diverge. 
 Detach the wire by your discharger and 
 test the quality of the electricity it is 
 positive, as theory declares it must be. 
 
 Consider now the experiment of Klcist 
 and Cumcus (fig. 04). You will, I doubt 
 not, penetrate its meaning. You will see 
 that in their case the liand formed the 
 outer coating of the jar. When elec- 
 tricity was communicated through the 
 nail to the water within, that electricity 
 acted across the glass inductively upon 
 the hand, attracting the ono fluid and 
 repelling the other to the earth. 
 
 Again, I say, prove all things ; and 
 what is here affirmed may be proved by 
 the following beautiful and conclusive ex- 
 periment : Stand on your board, i i' fig. 
 39, insulated by its four tumblers ; or 
 upon a sheet of gutta-percha, or vulcan- 
 ized india-rubber. Seize the old Leyden 
 phial, j, with your left hand, and pre- 
 sent the knuckle of your right hand to 
 your balanced lath, L' L. When electric- 
 ity is communicated to the nail, the lath 
 i* immediately attracted by the knuckle. 
 Or touch your electroscope with your 
 right hand ; when the phial is charged 
 
LESSORS IN ELECTRICITY. 
 
 PICK 39. 
 
 the leaves immediately diverge, by the 
 electricity driven from your left hand to 
 the electroscope. 
 
 Here the nail may be electrified either 
 by connecting it with the prime conduct- 
 or oft he machine, or by rubbing it with 
 an excited glass rod. Indeed, I should 
 prefer your resorting to the simplest and 
 cheapest means in making these experi- 
 ments. 
 
 21. Franklin's Cascade Battery. 
 
 As a thoughtful and reflective boy or 
 girl you cannot, I think, help wondering 
 at the power which your thorough mas- 
 tery of the principles of induction gives 
 you over these wonderful and complica- 
 ted phenomena. By those principles the 
 various facts of our science are bound to- 
 gether into an organic whole. But we 
 have not yet exhausted the f ruitf ulness of 
 this principle. 
 
 Consider the following problem. 
 Usually we allow the electricity of the 
 outer coating to escape to the earth. 
 Suppose we try to utilize it. Place, then, 
 your jar, A B, fig. 40, upon vulcanized 
 india-rubber, and connect by a wire B c 
 its outer coating with the knob or inner 
 coating of a second jar c D. "What will 
 occur when the first jar is charged ? 
 Why, the second one will be charged also 
 
 by the electricity which has escaped from 
 the outer coating of the first. And sup- 
 pose you connect the outer coating of 
 the second insulated jar with the inner 
 
518 
 
 LESSONS IN ELECTRICITY. 
 
 coating of a ifiird, E F ; what occurs ? 
 The third jar will obviously be charged 
 with the electricity repelled from the 
 outer coating 1 of tho second. Of course 
 we need not stop here. We may have a 
 long series of insulated jars, the outer 
 coating of each being connected with the 
 inner coating of the next succeeding one. 
 Connect the outer coating of the last jar 
 i K by a wire c with the caith, and charge 
 the first jar. You charge thereby the 
 entire series of jars. In this simple way 
 you master practically, and grasp tho 
 theory of Franklin's celebrated " cascade 
 battery." 
 
 You must see that before making this 
 important experiment you could really 
 have predicted what would occur. This 
 power of prension is one of the most 
 striking characteristics of science. 
 
 22. Novel Ley den Jars of the Simplest 
 J^orm. 
 
 Possessed of its principles, we can re- 
 duce the Lcydon jar to far simpler forms 
 than any hitherto dealt with. Spread a 
 sheet of tin-foil smoothly upon a table, 
 and Jay upon the foil a* pane of glass. 
 Remember that the glass, as usual, must 
 be dry. Stick on to the glass by seal- 
 ing-wax two loops of narrow silk ribbon, 
 by which the pane may be lifted ; and 
 then lay smoothly upon the glass n sec- 
 ond sheet of tin-foil, less than the pane 
 in size, leaving a rim of uncotercd glass 
 all round. Carry a fine wire from the 
 upper sheet cf tin-foil to your electro- 
 scope. A little weight will keep the end 
 of the wire attached to the tin-foil. 
 
 Rub this weight with your excited 
 glass tube, two or three times if neces- 
 sary, until you see a slight divergence of 
 the Dutch metal leaves. Or connecting 
 the weight with the conductor of your 
 machine, turn very carefully until the 
 slight divergence is observed. What is 
 the condition of things here ? You have 
 poured, say positirc electiicity on to the 
 upper sheet of metal. It acts "inducti vo ly 
 across the glass upon the under sheet, the 
 positive fluid of which escapes to the 
 earth, leaving the negative behind. You 
 see before your mind's eye twx> layers 
 holding each other in bondage. Now 
 take hold of your loops and lift the glass 
 plate, so as to separate the upper tin-foil 
 from the lower. What would you ex- 
 
 pect to occur ? Freed from the grasp of 
 the lower layer, the electricity of the up- 
 per one will diffuse itself over the elec- 
 troscope so promptly and powerfully, 
 that if you are not careful you will de- 
 stroy the instrument by the mutual repul- 
 sion of its leaves. 
 
 Practise this experiment, which is a 
 very old one of mine, by lowering and 
 lifting the glass plate, and observing the 
 corresponding rhythmic action of the 
 leaves of the electroscope. 
 
 Common tin-plate may be used in this 
 experiment instead of tin-foil, and a 
 sheet of vulcanized india-rubber instead 
 of the pano of glass. Or simpler still, 
 for the tin-foil a sheet of common un- 
 warmed foolscap may be employed. 
 Satisfy yourself of this. Spread a sheet 
 of foolscap on a table ; lay the plate of 
 glass upon it, an I spread a leaf of fools- 
 cap, less than the glass in size, on the 
 plate of glass. Connect tho loaf with 
 the electroscope, and charge it, exactly 
 as you charged the tin-foil. On lifting 
 tho glass with its leaf of foolscap, the 
 leaves of the electroscope instantly fly 
 apart ; on lowering the glass they again 
 fall together. Abandon the under sheet 
 altogether, and make the table the outer 
 coating ; if it be not of very dry wood, 
 or corered by an insulating varnish, you 
 will obtain with it the results obtained 
 with the tin-foil, tin, and foolscap. 
 Thus by the simplest means we illustrate 
 great principles. 
 
 The withdrawal of the electricity from 
 the electroscope, by lowering the plate of 
 glass, so as to bring the electricity of the 
 upper coating within the grasp of the 
 
 FIG. 41. 
 
LESSONS IN ELECTRICITY. 
 
 31$ 
 
 lower one, is sometimes called ** conden- 
 sation. ' ' The electricity on one plate or 
 sheet was figured ai squeezed together, 
 ^r condensed, by the attraction of the 
 other. A special instrument called a 
 condenser is constructed by instrument 
 makers to illustrate the action here ex- 
 plained. 
 
 You may readily make a condenser for 
 yourself. Take two circles, p p', fig. 41, 
 of tin or of sheet zinc, and support the one, 
 p', by a stick of sealing-wax or glass, G : 
 the other, p, by a metal stem, connected 
 with the earth. The insulated plate, p', 
 is called the collecting plate ; the unin- 
 sulated one, P, the condensing plate. 
 Connect the collecting plate with your 
 electroscope by the wire w, and bring tha 
 condensing plate near it, leaving, how 
 ever, a thin space of air between them. 
 Charge the collector, p', or the wire, w, 
 with your glass rod, until tho leaves of 
 the electroscope begin to diverge. 
 Withdraw the condensing plate, the 
 leaves fly asunder ; bring tho condensing 
 plate near, the leaves again collapse. 
 
 Or vary your constraction, Rnd make 
 your condenser thus. Employing the 
 table, or a sheet of foolscap if the table 
 V>e an insulator, as one plate of the con- 
 denser, sproad upon it the sheet of india- 
 
 rubber, P, fig. 42, and lay upon the 
 rubber the sheet of block-tin, A B. Con- 
 nect the tin by the wire, w, with the 
 electroscope, T. Impart electricity to 
 the little weight, A, till the leaves, L, W- 
 gin to diverge ; then lift the tin plate by 
 ks two silk loops ; tho leaves at once fly 
 csander. 
 
 Finally, show your complete knowledge 
 of the Ley den jar, ami your freedom 
 from the routine of the instrument makers, 
 by making a "jar" in the following 
 novel way. Stand upon a board sup- 
 ported by warm tumblera. Hold in your 
 right hand a sheet of vulcanized india- 
 rubber, and clasp, with it between you, 
 tho left hand of a friend in connection 
 with the earth. Place your left hand on 
 tho conductor of the machine, and let it 
 bo worked. You and your friend sooa 
 feel a crackling and a tickling of the 
 hands, due to the heightening attraction 
 of the opposite electricities across the in- 
 dia-rubber. The " hand- jar" is then 
 charged. To discharge it you have onJy 
 to bring your other hands together : ths 
 shock of the Leyden jar is then felt and 
 its spark seen and heard. 
 
 By the discharge of the hand-jar you 
 can tire gunpowder. But this will be re- 
 ferred to more particukirly further OB. 
 (See 25.) 
 
 23. Seat of Charge in the Ley den Jar. 
 
 Franklin sought to determine how tho 
 charge was hidden in the Leyden jiir. 
 He charged with electricity a bottle half 
 filled with water and coated on the out- 
 side with tin-foil ; dipping the finger oJ 
 one hand into the water, and touching 
 the outside coating with the other, he 
 received a shock. He was thus led to 
 inquire, Is the electricity in the water I 
 He poured the water into a second bot- 
 tle, examined it, and found that it had 
 carried no electricity along with it. 
 
 His conclusion wa3 '* that the electric 
 fire must either have been lost in tho d&- 
 canting, or must have remained in the 
 bottle. The latter he found to be true ; 
 for, filling the charged bottle with fresii 
 water, he obtained the shock, and vrra 
 therefore satisfied that the power of giv- 
 ing it resided in the glass itself."* 
 
 * Priestley's " History of Electricity," 
 Sa edition, p. 149 
 
320 
 
 (An account of Franklin's discoveries 
 was given by him in a series of letters 
 addressed to "Peter Collmson, Esq.* 
 F.R.S., from 1747 to 1754). 
 
 So much for history ; but yon arc to 
 verify the history by repeating Franklin's 
 experiments. Place water in a wide 
 glass vessel ; place ;i second glass vessel 
 within the first, and till it to the same 
 height with water. Connect the outer 
 water by a wire with the earth, and the 
 inner water by a wire with the electric 
 machine. One or two turns furnish a suf- 
 ficient charge. Removing the inner wire, 
 and dipping one finger into the outside 
 and the- other into the. inside water, a 
 smart shock is felt. This was Franklin's 
 first experiment. 
 
 Pass on to the second. Coat a, glass 
 jar uiih tin-foil (not too high) ; fill it to 
 the same height with water, and place it 
 on india-rubber cloth. Charge it by 
 connecting the outside coating with the 
 earth, and the water inside (by means of 
 a stem cemented to the bottom of the vir 
 and ending above in a knob) with an 
 electric machine. You obtain a bright 
 spark on discharging. This proves your 
 apparatus to be in good order. 
 
 Recharge. Take hold of the charged 
 jar with the india-rubber, and pour the 
 water into a second similar jar. No sen- 
 sible charge is imparted to the latter. 
 Pour fresh unelectrified water into the 
 first jar, and discharge it. The retention 
 of the charge is shown by a brilliant spaik. 
 Be careful in these experiments, or yon 
 will fail, as I did at first. The edge of 
 the jar out of which the water is poured 
 has to be surrounded by a band of bibu- 
 lous paper to catch the final drop, which, 
 trickling down, would discharge the jar. 
 
 Experiments like those of Franklin are 
 now made by rendering the coatings of 
 the Leydcn jar movable. Such a jar be- 
 ing charged, the interior coating may be 
 lifted out and proved unelectric. The 
 glass may then be removed from the outer 
 coating and the latter proved unelectric. 
 Restoring the jar and coatings, on con- 
 necting the two latter, the discharge 
 passes in a brilliant spaik. 
 
 Make a jar with movable coatings 
 thus : Roll cartridge paper round a good 
 flint-glass tumbler, c, fig. 43, to within 
 about an inch of the top. Paste down 
 the lower edge of the paper, and put a 
 
 LE33O:73 IN ELECTRICITY. 
 
 paper bottom to it corresponding to the 
 bottom of the glass. Coat the paper, T, 
 inside and out with tin-foil. Make a 
 similar coating, T', for the inside of the 
 tumbler, attaching to it an upright 
 
 PIG. 43. 
 
 w, ending in a hook. You have then to 
 all intents and purposes a Leyden jar. 
 
 Put the pieces together and charge the 
 jar. By means of a rod of glass, seal- 
 ing-wax, or gutta-percha, lift out the in- 
 terior coating. It will carry .a little elec- 
 tricity away with it. Place it upon a 
 tnble and discharge it wholly. Then by 
 the hand lift the glass out of the outer 
 coating. Neither of the coatings now 
 shows the slightest symptom of electric- 
 ity. Restore the tumbler to its outer 
 coating, and by means of the hook and 
 insulating rod, restore the inner coating 
 to its place. Discharge the jar : you 
 obtain a brilliant spark. The electricity 
 which produces this spark must have 
 been resident in and on the glass. 
 
 Here, as in all other cases, you can 
 charge your jar with a rubbed glass tube, 
 though a machine in good working order 
 will do it more rapidly. With *' Cot- 
 trell's rubber," described in the next 
 section, you may greatly exalt the per- 
 formance of your glass tube. 
 
 24. Ignition by the Electric Spark. 
 CottrelVs Rubber. The Tube-ma- 
 chine. 
 
 Various attempts had been vainly made 
 by Nollet and others to ignite inflam- 
 
LESSONS IN ELECTRICITY. 
 
 821 
 
 FIG. 44. 
 
 rnable substances by the electric spark. 
 This was first effected by Ludolf, at the 
 opening of the Academy of Sciences by 
 Frederick the Great at Berlin, on the 
 23d of January, 1744. With a spark 
 from the sword of one of the court cav- 
 aliers present on the occasion, Ludolf ig- 
 nited sulphuric ether. 
 
 Dr. \\atson also made numerous ex- 
 periments on the ignition of bodies by 
 the electric spark. He fired gunpowder 
 and discharged guns. Causing, more- 
 over, a spoon containing ether to be held 
 by an electrified person, he ignited the 
 ether by the finger of an unelectrified per- 
 son. He also noticed that the spark va- 
 ried in color when the substances between 
 which it passed varied. 
 
 These, and numerous other experi- 
 ments may be made with a far simpler 
 " machine" than any hitherto described. 
 It was devised for your benefit by Mr. 
 Cottrell. In the electric machine, as we 
 have learned, the prime conductor is 
 flooded with positive electricity through 
 the discharge of the negative from the 
 points against the excited glass. Your 
 gloss tube and rubber may be similarly 
 turned to account. A strip of sheet- 
 brass or copper, p, fig. 44, is sewn on to 
 the edge of the silk pad, R, employed as 
 a rubber. Through apertures in the strip 
 about twenty pin-points are introduced, 
 and soldered to the metal. When the 
 tube is clasped by the rubber, the rnetal 
 strip and points quite encircle the tube. 
 
 When a fine wire, ID, connects the strip 
 of metal with the knob of a Leyden jar, 
 by every downward stroke of the rubber 
 the glass tube is powerfully excited, and 
 
 hotly following the exciting rubber is t^ 
 circle of points. From these, against ths 
 rod, negative electricity is discharged,, 
 the free positive electricity escaping along; 
 the wire to the jar, which is thus rapid- 
 ly charged. 
 
 The ignition of gas is readily effected! 
 by Cottrell's rubber. Connecting thee 
 strip of metal, R, fig. 45, with an insulat- 
 ed metallic knob, B, placed within a* 
 quarter or an eighth of an inch of an 
 uninsulated argand burner connected with 
 the earth, at every downward stroke of" 
 the rubber a stream of sparks passes be- 
 tween the knob and burner. If gas bo* 
 turned on, it is immediately ignited by 
 the stream of sparks. Blowing out the- 
 fiame and repeating the experiment, every 
 stroke of the rubber infallibly ignites the- 
 as. 
 
 Sulphuric ether, in a spoon which ha*. 
 been previously warmed, is thus ignited ;; 
 but the ether soon cools by evaporation ; 
 its vapor is diminished by the cold, and 
 it is then less easy to ignite. Bisulphide 
 of carbon may be substituted for the 
 ether, with the certainty that every stroke 
 of the rubber will set it ablaze. The 
 spark thus obtained also fires a mixture 
 of oxygen and hydrogen. The two gasei 
 
LESSONS IF ELECTRICITY. 
 
 unite with explosion to form water, when 
 an electric spark is pnssed through them. 
 
 Mr. Cottrell has also mounted his glass 
 -tube so as to render friction in both direc- 
 jtions available. The tube-machine is 
 represented in fig. 46. A D is the glass 
 tube, ciasped by the rubber, n. p p 1 arc 
 two strips of metal furnished with rows 
 of points. From p p' wires proceed to 
 the knob c, which is insulated by the 
 horizontal stem, o. This insulating stem 
 may be abolished with advantage, the 
 wires from p and p' being rendered strong 
 enough to support the ball c. At c 
 sparks may be taken, a Ley den jar 
 charged, the electric mill turned, while 
 wires carried from it may be employed 
 in experiments on ignition. I however 
 strongly recommend to your attention 
 the more simple rubber shown in fig. 44. 
 
 " Seldom," says Hies*, " has an ex- 
 periment done so innch to 'develop the 
 science to which it belongs a* this of the 
 ignition of bodies by the electric sparks." 
 It aroused universal interest ; and was 
 repeated in all Royal houses. Money 
 was ready for the further prosecution of 
 electrical research. The experiment 
 afterward spread among the people. 
 Jliesw considers it probable tlu-it the gen- 
 eral interest thus excited led to th dis- 
 covery of the Ley de u jar, which was mado 
 soon after vr&rd. 
 
 Fio. 47. 
 
 Klingenstierna astonished King Fred- 
 erick of Sweden by igniting a spoon of 
 alcohol with a piece of ice. With Cot- 
 trell's rubber and bisulphide of carbon 
 this striking experiment is easily made, 
 and you ought to render your knowl- 
 edge complete by repeating it. At 
 every stroke of the rubber the spark 
 from the end of a pointed rod of ice in- 
 fallibly sets the bisulphide) on tire. 
 
 Cadogan Morgan, in 1785, sought to pro- 
 duce the electric spark in the interior of 
 solid bodies. He inserted two wires into 
 wood, and caused the spark to pass be- 
 tween them: the wood \rns illnminated with 
 blood-red light, or with yellow light, ac- 
 cording as the depth at which the spark 
 was produced wa~s greater or less. Tlia 
 spark of the Leyden jar produced within an 
 ivory ball, an orange, an apple, or under tin; 
 thumb, illuminates these bodies through- 
 out. A lemon is especially suited to this 
 
 ffra, 48. 
 
LESSONS IN ELECTRICITY. 
 
 experiment, flashing forth at every spark 
 as a spheriod of brilliant golden Jight. 
 The manner in which the lemon is mount- 
 ed on the brass stem B is shown in fig. 
 47. The spark occurs at 5, in the interval 
 between the stems A and B. A row of 
 eggs in a glass cylinder is also brilliantly 
 illuminated at the passage of every spark 
 from a Ley den jar. 
 
 25. Duration of the Electric Spark. 
 
 The duration of the electric spark is 
 very brief ; in a special case Sir Charles 
 Whcatstone found it to be jrj^^tli of a 
 second. This, however, was the maxi- 
 mum duration. In other cases it was 
 less than the millionth of a second. 
 
 When a body is illuminated for a in- 
 stant, the image of the body remains 
 upon the retina of the eye for about one- 
 fifth of a second. If, then, ji body in 
 swift motion be illuminated by an instan- 
 taneous flash, it will be seen to stand 
 motionless for one-fifth of a second at 
 the point where the flash falls upon it. A 
 riiie bullet passing through the air, and 
 illuminated by an electric flash, would 
 be seen thus motionless ; a, circle like D 
 D', fig. 48, divided into black and white 
 sectors, and rotating so quickly as to 
 cause the sectors to blend to a uniform 
 gray, appears, when illuminated by the 
 spark of a Leyden jar, perfectly motion- 
 less, with all its sectors revealed. A fall- 
 ing jet of water, which appears contin- 
 
 uous, is resolved by 'the electric flash 
 into its constituent drops. Lightning, 
 as shown by Professor Dove, is similarly 
 rapid in its discharge. 
 
 For a long time it was found almost 
 impossible to ignite gunpowder by the 
 electric spark. Its duration is so brief 
 that the powder, when the discharge oc- 
 curred in its midst, was simply scattered 
 violently about. In 1787 Wolff intro- 
 duced into the circuit through which the 
 discharge passed a glass tube wetted on 
 the inside. He thereby rendered the 
 ignition certain. This was owing to the 
 retardation of. the spark by the imperfect 
 conductor. Gun-cotton, phosphorus, and 
 amadou, which are torn asunder by the 
 unrelarded sparks are ignited when the 
 discharge is retarded by a tube of water. 
 A wetted string is the usual means re- 
 sorted to for retardation when gunpow- 
 der is to be discharged. 
 
 The instrument usually employed for 
 the ignition of powder is the universal 
 discharger. We make our own dis- 
 charger thus : i and ^-(fig. 49) are in- 
 sulating rods of glass or sealing-wax, 
 supporting two metal arms, the ends of 
 which can be brought down upon the 
 little central table s. One of the metal 
 arms of the discharger being connected 
 by a wire e with the eartL, the separated 
 ends of the two arms are surrounded 
 ^Yith powder B. Sending through it the 
 unretarded charge, the powder ia scatter- 
 
 FIQ. 49. 
 
824 
 
 LESSONS IN ELECTRICITY. 
 
 FIG. 50. 
 
 ed mechanically. Introducing the wet 
 string w into the circuit, ignition infalli- 
 bly occurs when the spark passes. 
 
 This is the place to fulfil our promise 
 to ignite gunpowder by the " hand- jar. " 
 Fig. 50 explains the arrangement. H 11' 
 are the hands of the insulated person. 
 F the hand of the uninsulated friend, i 
 (he india-rubber between both hands. 
 The lead ball B is suspended by a wet 
 string s. On the little stand P, connect- 
 ed with the earth, is placed the powder. 
 The charging of the hand- jar is described 
 in 22. When charged, it is only nec- 
 essary to bring the ball n down upon 
 the powder to cause it to explode. 
 
 26. Electric Light in Vacuo. 
 
 The electric light in vacuo was first 
 observed by Picard in 1675. While 
 carrying a barometer from the Observa- 
 tory to the Porte St. Michel in Paris, he 
 saw light in the upper portion of the 
 lube. Sebastien and Cassini observed it 
 Afu?r wards in other barometers. John 
 iicrnouilli devised a " mercurial phos- 
 phorus," by shaking mercury in a tube 
 vhlch had been exhausted by an air- 
 pump. This was handed to the King of 
 Prussia Frederick I. who awarded for 
 it a medal of forty ducats value. The 
 great mathematician wrote a poem in 
 noiior of the occasion. 
 
 Fia. 51. 
 
 Bernouilli failed to explain the effect. 
 The explanation was reserved for Ilauks- 
 bee, who in 1705 took up the subject 
 and experimented upon it before the 
 Royal Society. On tke plate of an air- 
 pump he placed two bell-jars, one over 
 the other. Tin outer and larger jar was 
 open at Hie top. Into the opening 
 llauksbee iixod, air-tight, a funnel, 
 which he stopped with a plug of wood 
 and filled with mercury, lie exhausted 
 the space between the two jars, withdrew 
 the wooden plug and allowed the mer- 
 cury to stream against the outer surface i 
 of the inner jar. lie thus obtained aJ 
 shower of lire. This is a truly beautiful 1 
 experiment when witnessed by an ob- 
 server close at hand. 
 
 A copy of Hauksbee's own figure il- 
 lustrating this experiment is annexed, 
 fig. 51. M is the funnel containing the 
 mercury, ithe plug of wood, 8 the. outer 
 and s' the inner bell-jar. Instead of the 
 plug P, an india-rubber tube, held by a 
 clip, may be employed with advantage to 
 connect the funnel with the exhausted 
 jar. By gradually relaxing the clip the 
 mercury may be made to fall at a rate 
 corresponding to the maximum luminous 
 effect. The streams of light produced 
 are very beautiful, but they arc more 
 continuous than they arc shown to be by 
 Uauksbuo, 
 
 In 1706 llauksbee referred the phe- 
 
LESSONS IN ELECTRICITY. 
 
 nomenon to its true cause, namely, the 
 friction between mercury and glass in the 
 highly rarefied air. John Bernoulli! ridi- 
 culed Hauksbee's explanation. But 
 truth outlives ridicule, and it is now uni- 
 versally admitted that Hauksbee was 
 right. 
 
 Hauksbee also made the following ex- 
 periment, which, as shown by Riess, is 
 explained by reference to the principle of 
 induction. A hollow glass globe was 
 mounted so as to be capable of quick 
 rotation. It was exhausted, and while 
 it rotated the hand was placed against it 
 in the dark. It was positively electri- 
 fied by the hand. This positive electricity 
 acted inductively on the gla?s itself, at- 
 tracting its negative, but discharging its 
 positive as a luminous g'ow through the 
 rarefied air within. Haukshee was able 
 to read by the light thus produced. 
 
 By such experiments it was shown that 
 rarefied air favored the passage of elec- 
 tricity. Dry air is in fact an insulator, 
 which must be broken through to pro- 
 duce the electric spark. Through an ex- 
 hausted glass tube six feet long a dis- 
 charge freely passes which would be in- 
 competent to leap over the fiftieth part 
 of this interval in air. But whereas the 
 spark in air is dense and brilliant, the 
 discharge in vacuo fills the exhausted 
 tube with a diffuse light. 
 
 (It is here worthy of remark that at 
 a very early period Grummert, a Pole, 
 proposed the employment 'of this diffuse 
 electric light to illuminate coal mines a 
 notion which has been revived in our 
 day. The light in this form is not com- 
 petent to ignite the explosive gases which 
 produce such terrible disasters in mines.) 
 
 Priestley, in his " History of Electric- 
 ity," thus describes the light in vacuo. 
 " Take a tall receiver, very dry, and in the 
 top of it insert with cement a wire not 
 very acutely pointed, then exhaust the 
 receiver and present the knob of the wire 
 to the conductor, and every spark will 
 pass through the vacuum in a broad 
 stream of light, visible through the whole 
 length of the receiver, be it ever so tall. 
 This stream often divides itself into a 
 variety of beautiful rivulets, which are 
 continually changing their course, uniting 
 and dividing again in the most pi-easing 
 manner. If a jar be discharged through 
 
 this vacuum, it gives the appearance of a 
 very dense body of fire, darting directly 
 through the centre of the vacuum with- 
 out ever touching the sides." 
 
 Cavendish employed a double barome- 
 ter-tube, bent into a form of a horseshoe, 
 with its curved portion empty, to show 
 the passage of electricity through a 
 vacuum. It is really not the vacuum 
 which conducts the electricity, but the 
 highly attenuated air and vapor which 
 fill the space above the barometric 
 columns. When the mercury employed 
 is carefully purged of air and moisture 
 by previous boiling, the space above 
 the mercury, as proved by Walsh, 
 De Luc, Morgan, and Davy, is wholly 
 incapable of conducting electricity. 
 Similar experiments have been made in 
 the laboratory of Mr. Gassiot, to whom 
 we are indebted for so many beautiful 
 electrical experiments. Professor Dewar 
 has also brought his experimental skill to 
 bear with success upon this subject. 
 
 Electricity, therefore, dees not pass 
 through a true vacuum ; it requires pon- 
 derable matter to carry it. If a gold- 
 leaf electroscope be kept at a distract 
 from al] conductors, it may be kept 
 charged for an almost indefinite period 
 in a good air-pump vacuum. 
 
 The matter rendered thus luminous by 
 the electrical discharge is attracted and 
 repelled like other electrified matter. " A 
 finger," says Priestley, " put on the out- 
 side of the glass will draw it [the lumi- 
 nous stream] wherever a person pleases. 
 If the vessel be grasped with both hands, 
 every spark is felt like the pulsation of a 
 great artery, and all the fire makes to- 
 wards the hands. This pulsation is felt 
 at some dirtance from the receiver ; and 
 in the dark a light is seen betwixt the 
 hands and glass." 
 
 " All this," continues the historian of 
 electricity, " while the pointed wire is 
 supposed to be electrified positively ; if 
 it be electrified negatively the appearance 
 is remarkably different. Instead of 
 streams of fire, nothing is seen but one 
 uniform luminous appearance, like a 
 white cloud, or the milky-way on a clear 
 starlight night. It seldom readies the 
 whole length of the vessel, but is gen- 
 erally only like a lucid ball at the end of 
 the wire." 
 
 Of the two appearances here described 
 
320 
 
 LESSOVS IN ELECTRICITY. 
 
 FKI. &1 
 
 tho fonriGr is now known n tho ehctrie 
 brush, and the latter rs the electric glow. 
 Both c*n be produced in unconfined air. 
 Tho glow is sometimes seen on the masts, 
 of clips, and it is mentioned by tho an- 
 ci-euta as appearing on the points of 
 kacea. It is called St. Ermo's or St. 
 Elmo's fire, ?:ftcr the .*ailoiV saint, Eras- 
 inu*, \*ho nuHered martyrdom at Gaeta, 
 at the beginning of the fourth century. 
 
 The purple color of the diffused light 
 in attenuated air was noticed by Hauks- 
 boc. The color depends upon the resi- 
 due of attenuated gas, or vapor, through 
 which the discharge passes. If it be an 
 oxygen-residue the light is whitish, if it 
 be a hydrogen-residue the light is red, if 
 a nitrogen-residue the light is purple, 
 fj;act!y resembling that displayed at times 
 by vlro aurora borealis a color doubtless 
 due to tho discharge of electricity 
 through the attenuated nitrogen of the air. 
 
 Electric light in vacuo is readily pro- 
 duced by the friction of an amalgamated 
 FH I -^cr against the outside of an exhausted 
 tube. The light is also produced by the 
 friction of mercury within a barometric 
 vacuum. The discharges through tube* 
 
 many feet in length and exhausted by 
 an air-pump are very fine. The double 
 barometer tube of Cavendish also yields 
 a truly splendid bow of light, when a 
 strong electric discharge is sent through 
 it. For this experiment fig. 52 shows 
 the best arrangement. p is the prime 
 conductor of an electrical machine, i an 
 insulated metal ball, connected by a wire 
 with the mercury trough A. The 
 trough u is connected by a wire with the 
 earth, c and c' mark the height of the 
 mercurial columns. When the machine 
 is worked sparks pass from p to i, a 
 vivid bow of light at each passage stretch- 
 ing from o to c'. By causing i to ap- 
 proach P, the discharges become more 
 frequent, t>nt more feeble ; by augment- 
 ing the distance P i, the sparks become 
 rarer, but more strong. When very 
 tro.ng, a bow of dazzling brilliancy ac- 
 companies every spark.* 
 
 Small tubes tor these experiments are 
 hd*t obtained from philosophical instru- 
 ment makers 
 
 27. Lichlenbtry's Figures. 
 
 Licht.cnberg deriasd a me-xis of rc- 
 vaaling tho condition of an electrified 
 surface by dusting it with powder. It'sd 
 lead, in passing through mr-slin, is p</i- 
 tivcly electrified ; flower of sulphur M 
 negatively electrified. Whisking a fox's 
 brush over a cake of resin, and drawing 
 over the surface the knob of a Lcyden j*r, 
 positively charged, tho re^.n is rendered 
 in part negafivc and in part positive. 
 Dusting the mixed powder over the sur- 
 face, the Rulphur arrange* itself over the 
 positive places, and the red lead ovc? 
 the negative places, a verv beautiful pat- 
 tern being the result. 
 
 This experiment of LicMenberg's con- 
 stituted the germ of Chladni's important 
 acoustical researches. k Chladni's fig- 
 ures " were the direct offspring of 
 ** Lichtcnbcrg's figures." 
 
 28. Surface Compared with Mass. 
 Distribution cf Electricity in Hollow 
 Conductors. 
 
 Monnier proved that the charge of a 
 
 * It is well ta have tho interval p i fit 
 s"me distance from the how, srvthr.t Ihu liirht 
 of the spark shall not impair thu efl(x:t of ih 
 discharge upon the eye. 
 
LESSONS IN ELECTRICITY. 
 
 327 
 
 conductor depended upon its surface, 
 and not upon its solid content*. An 
 anvil weighing 200 pounds gave a smaller 
 sp::rk tlan a speaking trumpet weighing 
 10 pounds. A solid ball of lead gave a 
 spark only of the same force as that ob- 
 tained from a piece of thin lead of tho 
 same superficies, bent into the form of a 
 hoop. Finally Monnier obtained a strong 
 spark from a long strip of sheet lead, 
 but a very small one when it was rolled 
 into a, lump. 
 
 Le Roi and D'Arcy showed that a hol- 
 low sphere accepted the same charge 
 when empty as when filled with mercury, 
 which augmented its weight sixty-fold. 
 And this proves the influence of surface 
 ad distinguished from mass. 
 
 The distribution of electricity is well 
 illustrated by the deportment of hollow 
 bodies. Impart by your carrier (fig. 15) 
 successive measures of electricity to the 
 interior of an insulated ice-pail, or a 
 pewter pot. On testing the interior of 
 the vessel with the carrier and an elec- 
 troscope no electricity is found there ; 
 but it is found on the external surface. 
 A hat suspended by silk strings answers as 
 well as the ice-pail. 
 
 Thi* ^Tperiment with the hat is a very 
 inatractire one. Tho hat may be charged 
 cither with Cottrell's rubber or with your 
 rubbed glass tube. 
 
 Notice, Trhen testing, that you take 
 your strongest charges from the cd^ss 
 and not from tho round or flat .smface cf 
 the hat. The strongest charge of all U 
 communicated to the earlier by the IG? 
 of the hat. 
 
 The successive charges may be cow- 
 muni cated to the hat by a metal ball :u-:> 
 pcndcd by silk. The charged bll, o* 
 touching the interior surface, beeorr>4 
 completely unclectric. 
 
 Franklin placed a long chain in -A sti- 
 ver tea-pot which he electrified. Con- 
 necting his teapot with a pith-ball ch -.- 
 troscope he produced a diverge*.:*. 
 Then lifting the chain by a silk string**. 
 found that over the portion outside v~$ 
 teapot the electricity diffused itself, T!M 
 withdrawal of the electricity from !*t* 
 electroscope being announced by the par- 
 tial collapse of the divergent pith-balls. 
 
 The mode of repeating lhi experi y iiicjj 
 is shown in fig. 53, where T is the tea- 
 pot, supported on a ^ood glass tumbler 
 o, and connected by the wire w with the 
 
328 
 
 LESSONS IN ELECTRICITY. 
 
 electroscope E. The effect is small, but 
 distinct. 
 
 The greatest experiment with hollow 
 conductors was made by Faraday, who 
 placed himself in a cubical chamber built 
 of laths and covered with paper and wire 
 gauze. It was suspended by silk ropes. 
 Within this chamber he could not detect 
 tho slightest sign of electricity, however 
 delicate his electroscope, and however 
 strongly the sides of the chamber might 
 t>e electrified. 
 
 29. Physiological Effects of the Elec- 
 tric Discharge. 
 
 The physiological effect of the electric 
 shock lias been studied in various ways. 
 Graham caused a number of persons to 
 lay hold of the same metal plate, which 
 was connected with the outer coating of 
 a charged Leyden jar, and also to lay 
 hold of a rod by which the jar was dis- 
 charged. The shock divided itself 
 equally among them. 
 
 The Abbe Nollet formed a lino of one 
 hundred and eighty guardsmen, and sent 
 the discharge through them all. He also 
 killed sparrows and fishes by the shock. 
 The analogy of these effects with those 
 produced by thunder and lightning could 
 not escape attention, nor fail to stimulate 
 inquiry. 
 
 Indeed, as experimental knowledge in- 
 creased, men's thoughts became more def- 
 inite and exact as regards the relation of 
 electrical effects to thunder and lightning. 
 The Abbe Nollet thus quaintly expresses 
 himself : " If any one should take upon 
 him to prove, from a well-connected 
 comparison of phenomena, that thunder 
 is, in the hands of Nature, what electric- 
 ity is in ours, and that tho wonders 
 which we now exhibit at our pleasure are 
 little imitations of those great effects 
 which frighten us ; I avow that this idea, 
 if it was well supported, would give me 
 a great deal of pleasure.' lie then 
 points out the analogies between both, 
 and continues thus : " All those points 
 of analogy, which I have been some time 
 meditating, begin to make me believe 
 that one might, by taking electricity as 
 the model, form to one's self in relation to 
 thunder and lightning, more perfect and 
 more probable ideas than what have been 
 -offered hitherto."* 
 
 * Priestley's " Ilistorvof Electricity," pp. 
 151-52. 
 
 These views were prcralent at tho time 
 now referred to, and o.ut of them grew 
 the experimental proof by tne great 
 physical philosopher, Franklin, of the 
 substantial identity of the lightning Gash 
 and the electric spark. 
 
 Franklin was twice struck senseless by 
 the electric shock. lie afterwards sent 
 the discharge of two large jars through 
 six robust men ; they fell to the ground 
 and got up again without knowing what 
 had happened ; they neither heard nor 
 felt the discharge. Priestley, who made 
 many valuable contributions to elec- 
 tricity, received the charge of two jars, 
 but did not find it painful. 
 
 This experience agrees with mine. 
 Some timo ago I stood in this room with 
 a charged battery of fifteen large Leyden 
 jars beside me. Through some awkward- 
 ness on my part I touched the wire lead- 
 ing from the battery, and the discharge 
 went through me. Fur a sensible inter- 
 val life was absolutely blotted out, but 
 there was no trace of pain. After a life- 
 tie time consciousness returned ; I saw 
 confusedly both the audience and the ap- 
 paratus, arid concluded from this, and 
 from my own condition, that I had re- 
 ceived the discharge. To prevent the 
 audience from being alarmed, I made 
 the remark that it had often been my de- 
 sire to receive such a shock accident- 
 ally, and that my wish had at length 
 been fulfilled. But though th intellect- 
 ual consciousness of my position return- 
 ed with exceeding rapidity, it was not so 
 with the optical consciousness. For, 
 while making the foregoing remark, my 
 body presented to my eyes the appear- 
 ance of a number of separate pieces. 
 The arms, for example, were detached 
 from the trunk and suspended in the air. 
 In fact, memory and the power of rea- 
 soning appeared to be complete, long be- 
 fore the restoration of the optic nerve to 
 healthy action. 
 
 This may be regarded as an experi- 
 mental proof that people killed by light- 
 ning suffer no pain. 
 
 80. Atmospheric Electricity. 
 
 The air at all times can bo proved to 
 be a reservoir of electricity, which un- 
 dergoes periodic variation. We have 
 seen that ingenious men began soon to 
 suspect u common on^is. for the crack- 
 
LESSONS IN ELECTRICITY. 
 
 820 
 
 Fio. 54. 
 
 ling and light of the electric spark, and 
 thunder and lightning. The greatest in- 
 vestigator in this field is the celebrated 
 Dr. Franklin. He made an exhaustive 
 comparison of the effects of electricity 
 and those of lightning. The lightning 
 flash he saw was of the same shape as the 
 elongated electric spark ; like electricity, 
 lightning strikes pointed objects in pref- 
 erence to others ; lightning pursues the 
 path of least resistance ; it burns, dis- 
 solves metals, rends bodies asunder, and 
 strikes men blind. Franklin imitated all 
 these effects, striking a pigeon blind, and 
 killing a hen and turkey by the electrical 
 discharge. I place before you in fig. 
 - ? -, with a view to its comparison with a 
 ;*charge of forked lightning, the long 
 spark obtained from an effective ebonite 
 machine, furnished with a conductor of a 
 special construction, which favors length 
 of spark. 
 
 Having completely satisfied his mind 
 by this comparison of the identity of 
 both agents, Franklin proposed to draw 
 electricity from the clouds by a pointed 
 rod erected oa a high tower. But be- 
 fore the tower could be built he succeed- 
 ed in his object by means of a kite with 
 a pointed wire attached to it. The 
 electricity descended by the hempen 
 string which held the kite, to a key at 
 the end of it, the key being separated 
 from the observer by a silken string held 
 in the hand. Franklin thus obtained 
 sparks, and charged a Leyden phial with 
 atmospheric electricity. 
 
 But, spurred by Franklin's researches, 
 an observer in France had previously 
 proved the electrical character of light- 
 ning. A translation of Franklin's writ- 
 ings on the subject fell into the hands of 
 the naturalist Buffon, who requested his 
 friend D'Alibard to revise the transla- 
 tion. D'Alibard was thus induced to 
 erect an iron rod 40 feet long, supported 
 by silk strings, and ending in a" sen try- 
 
 box. It was watched by an old dragoon 
 named Coiffier, who on the 10th of May, 
 1752, heard a clap of thunder, and im- 
 mediately afterwards drew sparks from 
 the end ot the iron rod. 
 
 The danger of experiments with metal 
 rods was soon illustrated. Professor 
 Richmann of St. Petersburg had a rod 
 raised three or four feet above the tiles 
 of his house. It was connected by a 
 chain with another rod in his room ; the 
 latter rod resting in a glass vessel, and 
 being therefore insulated from the earth. 
 On the Gth of August, 1753, a thunder 
 cloud discharged itself against the exter- 
 nal rod ; the electricity passed down- 
 wards along the chain ; on reaching the 
 tod below, it was stopped by the glass 
 vessel, darted to Richmann's head, which 
 was about a foot distant, and killed him 
 on the spot. Had a perfect communica- 
 tion eiisted between the lower rod and 
 the earth, the lightning in this case would 
 hav expended itself harmlessly. 
 
 In 1749 Franklin proposed lightning 
 conductors. He repeated his recom- 
 mendation in 1753. He was opposed on 
 two grounds. The Abbe Nollct, and 
 those who thought with him, considered 
 it as impious to ward off heaven's light- 
 nings, as for a child to ward off the 
 chastening of its father. Others thought 
 that the conductors would " invite" the 
 lightning to break upon them. A long 
 discussion was also carried on as to 
 whether the conductors should be blunt 
 or pointed. Wilson advocated blunc 
 conductors against Franklin, Cavendish, 
 and Watson. lie so influenced George 
 III., hinting that the points were a re- 
 publican device to injure his Majesty, 
 that the pointed conductors on Bucking- 
 ham House were changed for others end- 
 ing in balls. Experience of the most 
 varied kind has justified the employment 
 of pointed conductors. In 1769 St. 
 
880 
 
 LESSONS IN ELECTRICITY. 
 
 Fio. 55. 
 
 Paul's Cathedral was first protected. 
 
 The most decisive evidence in favor of 
 conductors was obtained from ships ; and 
 such evidence was needed, to overcome 
 the obstinate prejudice of seamen. Case 
 after case occurred in which ships un- 
 protected by conductors were singled 
 out from protected ships, and shattered 
 or destroyed by lightning. ' The con- 
 ductors were at first made movable, be- 
 ing ioisted on the approach of a thun- 
 derstorm ; hut these were finally aban- 
 doned for the fixed lightning conductors 
 devised by the late Sir Snow Harris. 
 The saving of property and life by this 
 obvious outgrowth of electrical research is 
 incalculable. 
 
 31. TJte Returntny Stroke. 
 
 In the year 1779 Charles, Viscount Ma- 
 hon, afterward Earl Stanhope, pub- 
 lished his " Principles of Electricity." 
 On the title-page of the book stands the 
 following remark : " This treatise com- 
 prehends an explanation of an electrical 
 returning stroke, by which fatal effects 
 may be produced even at a vast distance 
 from the place where the lightning 
 falls." 
 
 Lord Mahon's experiments, which are 
 models Df scientific clearness and pre- 
 cision, will be readily understood by ref- 
 
 erence to the principles of electric induc- 
 tion, with which you are now so familiar. 
 It need only be noted here that whenever 
 he speaks of a body being plunged in an 
 " electrical atmosphere," he means that 
 the body is exposed to the inductive ac- 
 tion of a second electrified body, which 
 latter he supposed to be surrounded by 
 such an atmosphere. 
 
 A few extracts from his work will gir* 
 a clear notion of the nature of his dis- 
 covery : 
 
 " I placed an insulated metallic cylin- 
 der, A B, fig. 55, within the electrical at- 
 mosphere of the prime conductor [p c] 
 when charged, but beyond the striking 
 distance. The distance between the near 
 end A of the insulated metallic bodj 
 and the side of the prime conductor 
 was 20 inches. The body A u wsa 
 of brass, of a cylindrical form, .18 inch- 
 es long, by two inches in diameter. 
 I then placed another insulated brass 
 body E F, 40 inches long by about 3j 
 inches in diameter, with its end E at ths 
 distance of about one-tenth of an inch 
 from the en-l B of the other metallic 
 body A B. I electrified the prime con- 
 ductor. All the time that it was receiv- 
 ing its plus charge of electricity there 
 passed a great number of weak (red or 
 
LESSONS IN ELECTRICITY. 
 
 331 
 
 "mrple) sparks from the end u of tlio near 
 jodv A B into the end E of the remote 
 body EI-." 
 
 Make clear to your mind tiio origin of 
 thi.s stream of weak red or purple sparks. 
 It i.i obviously dua. to the inductive 
 action of the prime conductor P c upon 
 tlio body A c. The positive electricity 
 of A c being repelled by the prims con- 
 ductor, passed as a stream, of sparks to 
 
 E F. 
 
 " When the prime conductor, Laving 
 received its full charge, came suddenly 
 to discharge, with an explosion, its super- 
 abundant electricity on a large brass ball 
 L, which was made to communicate with 
 lihe earth, it always happened that the 
 electrical fluid, which had been gradually 
 expelled from the body A B and driven into 
 the body E r, did suddenly return from 
 the body E F iuto the body A n, in a 
 strong and bright spark, at the very in- 
 stant that the explosion took place upon 
 the ball L. 
 
 " This I call the electrical returning 
 stroke." 
 
 For the two conductors Lord Mali on 
 then substituted his own body and that 
 of another person, both of them standing 
 upon insulating stools. He continues 
 thus : 
 
 ** I placed myself upon an insulating 
 stool E (fig. 56), so as to have my right 
 arm A at the distance of about 20 inches 
 from a large prime conductor ; another 
 person, standing upon another insulating 
 
 es. 
 
 stool K, brought his right hand F within 
 ono- quarter of an inch of my left hand n. 
 
 41 When the prime conductor began to 
 receive its plus charge of electricity, we 
 felt the electrical fluid running out of my 
 hand B into his hand F. 
 
 " V> T hen we separated our hand? B and 
 F a little, the electricity passed between 
 us in small sparks, which sparks increased 
 in sharpness the farther we removed our 
 hands c and F asunder, until we had 
 brought them quite out of a striking dis- 
 tance. The interval* of time between 
 these departing sparks increased also the 
 more the distance between our hands B 
 and Y was increased, as must necessarily 
 be the case. 
 
 "As soon as the prime conductor came 
 suddenly to discharge iU electricity upon 
 the ball L, the superabundant electricity 
 which the other person had received from 
 my body did then r turn from him to me 
 in a sharp spark, which issued from his 
 hand F at the very iiiftaut that the explo- 
 sion of the prime conductor took place 
 upon the ball L. 
 
 44 I still continued upon tho insulating 
 stool E, and I dc&ircd the other person to 
 stand upon the floor. The returning 
 stroke between us was still stronger than 
 it had yet been. The reason of it was 
 this : The other person being no longer 
 insulated, transmitted his superabundant 
 electricity freely into the earth. I conse- 
 quently became still more negative than 
 before. 
 
 " Now, when the returning stroke 
 
332 
 
 LESSONS IN ELECTRICITY. 
 
 came to take place, not only the elec- 
 tricity which had passed from my body 
 into the body of the other person, but 
 also the electricity which had passed 
 from my body into the earth (through 
 the other person), did suddenly return 
 upon inc from his hand F to my 
 hand B, at the same instant that the 
 discharge of the prime conductor took 
 place upon the ball L. This caused the 
 returning stroke to be stronger than be- 
 fore." 
 
 Lord Mahon fused metal?, and pro- 
 duced strong physiological effects by the 
 return stroke. 
 
 In nature disastrous effects may be pro- 
 duced by the return stroke. The earth's 
 surface, and animals or men upon it, may 
 be powerfully influenced by one end of 
 an electrified cloud. Discharge may oc- 
 cur at the other end, possibly miles away. 
 The restoration of the electric equilib- 
 rium by the return shock may be so 
 violent as to cause death. 
 
 This was clearly seen and illustrated by 
 Lord Mahon. Fig. 57 is a reduced copy 
 of his illustration. JL B c is the electri- 
 fied cloud, the two ends of which, A and 
 c, come near the earth. The discharge 
 occurs at c. A man at F is killed by the 
 returning stroke, while the people at D, 
 nearer to the place of discharge, but far- 
 ther from the cloud, are uninjured. 
 
 TVith the viow of still further testing 
 your knowledge of induction, I have here 
 copied a portion of this admirable essay ; 
 but the entire memoir of Lord Manor, 
 would constitute a most useful and inter- 
 esting lesson in electricity. 
 
 For our own instruction we can illus- 
 trate the return shock thus : Connect 
 one arm of your universal discharger, fig. 
 49, with a conductor like c, fig. 20, and 
 the other arm with tho earth. Bring c 
 within a few inches of your prime con- 
 ductor, but not within striking distance ; 
 on working tho machine a stream of fee- 
 ble sparks will pass from point to point 
 of the discharger. Let the prime con- 
 ductor be discharged from time to thtoe 
 by an assistant ; at every discharge the 
 returning stroke is announced by a flash 
 between the points of the discharger at *. 
 If gun-cotton with a little fulminating 
 powder scattered on it, or a fine silver 
 wire, be introduced between the points 
 of tho discharger, the one is exploded 
 and the other deflagrated. 
 
 The stream of repelled sparks first seem 
 may be entirely abolished by establishing 
 an imperfect connection between the con- 
 ductor c and tho earth : a chain resting 
 upon the dry table on which the conduct- 
 or stands will do. The chain permits 
 the feebler sparks to pass through it in 
 
 Fw. 58. 
 
LESSORS IN ELECTRICITY. 
 
 8*3 
 
 preference to crossing the space s ; but 
 the returning stroke is too strong and 
 sudden to find a sufficiently open channel 
 through the table and chain, and on the 
 discharge of the prime conductor the 
 spark is seen. 
 
 It was tho action of the return shock 
 upon a dead frog's limbs, observed in the 
 laboratory of Professor Galvani, that led 
 to Galvani's experiments on animal elec- 
 tricity ; and led further to the discovery, 
 by Volta, of the electricity which bears 
 his name. 
 
 32. The Leyden Battery, its Currents, 
 and some of their Effects. 
 
 In the ordinary Leyden battery de- 
 scribed in 19 all the inner coatings are 
 connected together, and all the outer 
 coatisgs arc also connected together. 
 Such a battery acts as a single large jar 
 of extraordinary dimensions. 
 
 Wires are warmed by a moderate elec- 
 tric discharge ; by augmenting the charge 
 they arc caused to glow ; with a strength- 
 ened charge the metal is torn to pieces ; 
 fusion follows ; and by still stronger 
 charges the wires are reduced to metallic 
 dust and vapor. 
 
 For such experiments the wire must be 
 thin. Without resistance we can have 
 no heat, and when the wire is thick we 
 have little resistance. The mechanism 
 of the discharge, as shown by the figures 
 produced, is different in different wires. 
 The figure produced by the dust of a def- 
 lagrated silver wire on white paper is 
 shown in fig. 58. 
 
 When the discharge of a powerful bat- 
 tery is sent through a long steel chain 
 with the ends of its links unsoldered, the 
 sparks between the unsoldered links carry 
 the incandescent particles of the steel 
 along with them. These are consumed 
 in the air, a momentary blaze occurring 
 along the entire chain. Chain cables 
 have been fused by being made the chan- 
 nels of a flash of lightning. 
 
 Retaining our conception of an electric 
 fluid, at this point we naturally add to it 
 the conception of a current. It is the 
 electric current which produces the effects 
 just described. In many of our former 
 experiments we had electricity at rest 
 (static electricity), here we have electric- 
 ity in motion (dynamic electricity). 
 Sending the current from a battery 
 
 through a fiat spiral (the primary) formed 
 of fifty or sixty feet of copper wire, and 
 placing within a little distance of it a 
 second similar spiral (the secondary) with 
 its ends connected ; the passage of tho 
 current in the first spiral excites in tho 
 second a current, \vhich is competent to 
 deflagrate wires, and to produce all the 
 other 5 effects of the electrical discharge. 
 Even when the spirals are some feet asun- 
 der, the shock produced by the second- 
 ary current is still manifest. 
 
 The current from tbe secondary spiral 
 may be carried lound a third ; and this 
 third spiral may be allowed to act upon 
 u fourth, exactly as the primary did upon 
 the secondary. A tertiary current is thus 
 evoked by the secondary in. the fourth 
 spiral. 
 
 Carrying this tcitiary current round a 
 fifth spiral, and causiig it to act induc- 
 tively upon a sixth, we obtain in the lat- 
 ter a current of the fourth order. In this 
 way we generate a long progeny of cur- 
 rents, all of them having the current sent 
 from the battery through the first spiraJ 
 for a common progenitor. To Prof. 
 Ilenry of the United States, and to Prof. 
 Riess of Berlin, we are indebted for the 
 investigation of the laws of these cur- 
 rents. These researches, bow eve*,, were 
 subsequent to, and were indeed suggest- 
 ed by, experiments of a similar character 
 previously made by Faraday with Voltaic 
 electricity. 
 
 Besides the electricity of friction and 
 induction we have the following sources 
 and forms of this power. 
 
 The contact of dissimilar metals pro- 
 duces electricity. 
 
 The contact of metals with liquids pro- 
 duces electricity. 
 
 A mere variation of the character of the 
 contact of two bodies produces electricity. 
 
 Chemical action produces a continuous 
 flow of electricity (Voltaic electricity). 
 
 Heat, suitably applied to dissimilar 
 metals, produces a continuous flow of 
 electricity (thermo-electricity). 
 
 The heating and cooling of certain 
 crystals produce electricity (pyro-electric- 
 ity). 
 
 The motion of magnets, and of bodies 
 carrying electric currents, produces elec- 
 tricity (magneto-electricity). 
 
 The friction of sand against a metal 
 
31 
 
 LESSONS IN ELECTKICITY. 
 
 plate produces electricity. 
 
 The friction <-f condi-nscd water-parti- 
 cles against a safety valve, or better etill 
 against a box-wood nozzle through which 
 Attain is driven, produces electricity 
 (Armstrong's hydro-c lectiic machine). 
 
 These aro different manifestations of 
 one and the same power ; arid they are 
 all evoked by an equivalent expenditure 
 of some other power. 
 
 Conclusion. 
 
 Onr experimental researches end here. 
 I would now bespeak your attention for 
 five minutes longer. The cxpcn?iveness 
 of apparatus is sometimes urged as an 
 obstacle to the introduction of science 
 into schools. I hope it has been shown 
 that the obstacle 13 not a real one. Leav- 
 ing out of account the few larger experi- 
 ments, which have contributed but little 
 to our knowledge, it is manifest that the 
 wise expenditure of a couple of guineas 
 would enable any competent teacher to 
 place the leading facts and principles of 
 Jrictional electricity completely at the 
 command of his pupils ; giving them 
 UiwreDy precious knowledge, arni still 
 more precious intellectual discipline a 
 discipline which invokes observation, re- 
 flection, prevision by the exercise of rea- 
 son, and experimental verification. 
 
 And here, if I might venture to do so, 
 I would urge upon the science teachers 
 of our public and other schools that the 
 immediate future of science as a factor in 
 English education depends mainly upon 
 them. I would respectfully submit to 
 them whether it would not bo a, mistake 
 to direct their attention at present to the 
 collection of costly apparatus. Their 
 principal function just now is to arouse a 
 lovo for scientific study. This is best 
 done by the exhibition of the needful 
 faeta and principles with the simplest 
 possible appliances, and by bringing their 
 pupils into contact with actual experi- 
 mental work. 
 
 The very time and thought spent in 
 devising such simple instruments will give 
 tko teacher himself a grasp and mastery 
 of his subject which ho could not other- 
 wise obtain ; but it ought to be known 
 by the head meters of our schools that 
 time is needed, not only for devising such 
 instruments, but also for preparing tho 
 experiment* to be made with them after 
 
 they have been devised. No scienco 
 teacher is f;t to meet his class without 
 this distinct and special prenaration be- 
 fore every lesson. His experiments aro 
 part and parcel of his language, and they 
 ought to be as strict in logic, and as free 
 from stammering, as his spoken words. 
 To rnuko them so may imply an expendi- 
 ture of time which few head masters now 
 contemplate, but it is a necessary expen- 
 diture, and they will act wisely in mak- 
 ing provision for it. 
 
 To them, moreover, in words of 
 friendly warning, I would say, make 
 room for science by your own healthy 
 and spontaneous action, and do not wait 
 until it is forced upon you by revolution, 
 ary pressure from without. Tho condi- 
 tion of things now existing cannot con- 
 tinue. Its simple statement suffices to 
 call down upon it the condemnation of 
 every thoughtful mind. With reference 
 10 the report of a Commission appointed 
 .ast year to inquire into the scientific in- 
 struction of this country, Sir John Lub- 
 Dock writes as follows : " Tnc Com- 
 missioners have published returns from 
 moro than a hundred and twenty of the 
 larijer endowed schools. In more th.nn 
 half of these no science whatever i 
 taught ; only thirteen have a laboratory, 
 and only eighteen possess any scientific; 
 apparatus. Out of the whole number, 
 I-j.ss than twenty schools devote as much 
 * > four hours a week to science, and only 
 thirteen attach any weight at all to scien- 
 tific subjects in the examinations." 
 
 Well may the Commissioners pronounce 
 such a state of things to be nothing less 
 than a national calamity ! If persisted 
 in, it will assuredly be followed by a re- 
 action which the truest friends of classi- 
 cal culture in England will have the great- 
 est reason to deplore. 
 
 APPENDIX. 
 
 AN ELEMENTARY LECTURE ON 
 MAGNETISM.* 
 
 WE have no reaa.--a t:> believe thsit the 
 sheep or tho dnjj. or, indeed, any of the 
 bwer animals, feel aa interest hi the 
 
 * From the author's volume, 
 Science. 1 " 
 
 Fra-jmeate of 
 
LESSONS IN ELECTRICITY. 
 
 885 
 
 by \vhich natural phenomena are regulated. 
 A V I r.\ iy I) !(T'i!i'>(l \>\ r ;i thunder-storm ; 
 bi.-iU r.viv .<: i l > IO.J-M, ;irrl e t \->. return to 
 llioi stall-* <ii!rin<: a* -1-r ei lir^- ; bill neither 
 birds n >r can!", in f;ir r-t wo know, ever 
 think of inquiiing int > tie causes of th^c 
 things. It is ntnerwise with man. Tho 
 presence of natti-al object?, the. occurrence 
 of mtii'-al event- 1 , l!n?va*ied apnearances of 
 lh". universe in which he dwi 1's, pent Irate 
 beyond his organs of sense, and appeal to an 
 inner power of which the scnsts aie the mere 
 instrum r nts an'l excitants. No fact, is to 
 him either final or original. II u cannot limit 
 hinnelf i) the contemplation of it alone, but 
 endeavors t-> ascertain its position in a series 
 to which the con.-.tit;ili M n of his mini assures 
 him il must bcl ing. H* icgarels all that he 
 witnesses in the present as the efflux and 
 sequence of s m^thina; that has gone before, 
 an I a< the source of a system of events 
 whifh is to follow. The notion of spon- 
 tam'ily, v which ia his ruder state ho ao- 
 countcd for natural events, is abandoned ; 
 the i'lfu that Nature is an aggregate of inde- 
 pendent pruts also disappears, as the connec- 
 tion an 1 mr.tunl dependence of physical pow- 
 ers b'v.mo mo'e and rmre manifest ; until 
 h'i i.-> finally led. and that chiefly by the sci- 
 
 1 cncf! of which 1 happen this evening to be 
 the exponent, to regaul Nature as an organic 
 whole, as u body each of whose members 
 sympathizes with the rest, changing, it is 
 true, from ages to ages, but without one real 
 bieak ' f continuity, or a single interruption 
 of the fixed relations of cause and effect. 
 
 The system of things which we call Nature 
 is, however, too vast and various to be studied 
 first-han I by any single mind. As knowledge 
 exten Is there is always a tendency to sub- 
 divide the fiel.l of investigation, its various 
 paits being taken up by different individuals, 
 and thus receiving a greater amount of atten- 
 tion than could possibly be bestowed on them 
 if each investigator aimed at the mastery of 
 the whole. East, west, north, and south, the 
 huaian mind pushes its conquests ; but the 
 centripetal form in which knowledge, as a 
 whole, advances, spreading evor wider on all 
 sides, is due in reality to the exertions of in- 
 dividuals, each of whom directs his efforts, 
 more or less, along a single line. Accepting, 
 in many respects, his culture from his fellow- 
 
 . xmro, taking it from spoken words and from 
 written books, in some one direction, the stu- 
 dent of nature mut actually touch his work, 
 lie may otherwise be a distributor of knowl- 
 edge, but not a creator, and fails to attain 
 that vital it v r-f tli ought and correctness of 
 
 judgment w 7 hich direct and habitual contact 
 with nitiirfil truth can alone impart. 
 
 One large department of the system of Na- 
 ture which forms the chief subject of my own 
 I'.tudie.s, and to which it is n:/ duty to call 
 your attention this evening. '... thai (.f PU\MC.-. 
 or natural philosophy. This term is "large 
 
 ! uiougtito cover tiie study of Nature gen- 
 erally, but it is usually lestricted to a dcpert 
 mem which, perhaps, l.es closer to our per-, 
 tnaa any oilier. It deals with the 
 
 phenomena and laws of light and heat with 
 the phenomena and laws of magnetism and 
 electricity with those of sound with the 
 pressures and motions of liquids and gases, 
 whether in a state of translation or of undula- 
 tion. The science of mechanics is a portion 
 of natural philosophy, though at present so 
 large as to ne,ed the exclusive attention of him 
 who would cultivate it profoundly. Astron- 
 omy is the application of physics to the mo- 
 tions of the heavenly bodies, the vastness of 
 the field causing if, however, to be regarded 
 as a department in itself. In chemistry 
 physical agents play important pa*ts. By 
 heat and light we cause bodies to combine, 
 and by heat and light we decompose them. 
 Electricity tears asunder the locked atoms of 
 compounds, through their p^wer of separat- 
 ing carbonic acid into its constituents ; tine 
 solar beams build up the whole vegetable 
 world, and by it the animal, v/hile the" touch 
 of the self-same beams causcn hydrogen and 
 chlorine to unite with sudden explosion and 
 form by their combinal ion a powerful acid. 
 Thus physics and chemistry inter mingle, 
 physical agents being empk.yid by the chem- 
 ist as a means to an ind ; while in physics 
 proper the laws and phenomena of the agents 
 themselves, both qualitative and quantitative, 
 are the primary objects of htlention. 
 
 JVly duty heie to-night is !o t-pind an hour 
 in telling how this* subject is t > bj studied, 
 and how a knowledge of it is lj bo imparted 
 toothers. W hen first invited to do ihis, I 
 hesitated before accepting the icsponsibil.iy. 
 It would be easy to tnteitaiu you with an ac- 
 count of what natural philosophy hap accom- 
 plished. I might point lo those application* 
 of science regarding which we hear so much 
 in the newspapers, "and which we often find 
 mistaken for science itself. I might, of 
 course, ling changes on the .steam-engine and 
 the telegraph, the electrotype and the photo- 
 graph, the medical applications of physics, 
 and the million other inlets by which tcieo- 
 tific thought filters into practical life. That 
 would be easy computed with the task of in- 
 forming you how ycu arc to make the study 
 of physics t he instrument of your own culture, 
 how you are to possess its factu and make 
 them living seeds which shall take io:.t and 
 grow in the mind, and not lie like de:ul lum- 
 ber in the storehouse of memory. This is a 
 task much heavier than the mere cataloguing 
 of scientific achievements ; and it is one 
 which, feeling my own w:ini cf time and 
 power to execute it aright, 1 might well hesi- 
 tate to accept. 
 
 But let me sink excuses, and attack the 
 work to the best of my ability. First and 
 foremost, then, I would advise you to get a 
 knowledge of facts from actual oboei vuii^n. 
 Facts looked at directly are vital ; when they 
 pass into words half the sap is taken out cf 
 them. Ycu wish, for example, to get a 
 knowledge of magnetism ; wul, provide your- 
 S( If with a good bock on the subject, if ycu 
 can, but d) not be content with what the 
 book tells you ; do not be satisfied with its 
 descriptive wood-cuts ; see the actval thing 
 
306 
 
 LESSORS IN ELECTRICITY. 
 
 yourstlf. Half of our book-writers describe 
 experiments which they never made, and 
 Uieir descriptions often luck both force and 
 truth ; but no matter how clever or conscien- 
 tious they may be, their written words cannot 
 supply the place of actual observation. 
 Every fact has numerous radiations, which 
 are shorn off by the man who describes it. 
 Go, then, to a philosophical instrumeut- 
 maker, and give, according to your means, 
 for a straight bar-magnet say, half a crown, 
 or, if you can afford it, five shillings for a 
 pair of thorn ; or get a smith to cut a length 
 of ten inches from a bar of steel an inch wide 
 and half an inch thick ; rile its ends decently, 
 harden it, and get somebody like myself to 
 magnetize it. Two bar-magnets are better 
 than one. Procure some darning-needles 
 such as these. Provide yourself also with a 
 little unspun silk ; which will give you a sus- 
 pending tibre void of torsion ; make a little 
 loop of paper or of wire, thus, and attach 
 your fibre to it. Do it neatly. In the loop 
 place your darning-needle, and bring the two 
 ends or poles, as they are called, of your 
 magnet successively up to cither end of the 
 needle. Both the poles, you find, attract both 
 ends of the needle. Replace the needle by a 
 bit of annealed iron wire, the same effects en- 
 su<\ Suspend successively little rods of lead, 
 copper, silver, or brass, of wood, glass, 
 ivory, or whalebone ; the magnet produces 
 no sensible effect upon any of these suh- 
 itances. You thence infer a special property 
 in the case ot steel and iron. Multiply your 
 experiments, however, nndyou will find that 
 some other substances besides iron are acted 
 upon by your magnet. A rod of the metal 
 nickel, or of the metal cobalt, from which the 
 blue color used by painters is derived, ex- 
 kibits powers similar to thoee observed with 
 tho iron and steel. 
 
 In studying the character of the force you 
 may, howeve^r, confine yourself to iron and 
 steel, which are always at hand. Make your 
 experiments with the darning-necdlo over 
 and over again ; operate on both ends of tho 
 needle ; try both ends of the magnet. Do 
 not think the work stupid ; you are convers- 
 ing with Nature, and must acquire a certain 
 grace and mastery over her language ; and 
 these practice can alone impart. Let every 
 movement be made with care,- and - avoid 
 slovenliness from the outset. In every one 
 of your experiments endeavor to feel the re- 
 sponsibility of a moral agent. Experiment, 
 as I have said, is the language by which we 
 address Nature, and through which she sends 
 her replies ; in the use cf this language a 
 lack of straightforwardness is as possible and 
 as prejudicial as in the spoken language of 
 the tongue. If you w ish to become acquaint- 
 ed with the truth of Nature, you must from 
 the first resolve to deal with her sincerely. 
 
 Now remove your iicedlefrorn its loop, and 
 draw it from end to end along one of the 
 ends of thomngnet ; re-suspend'it, tnd repeat 
 your former experiment. You fliul the result 
 different. You now find that each extremity 
 f the magnet attract* one end of the needle 
 
 nnrt repels the other. The simple attraction 
 observed in the first instance is now replaced 
 by a dual force. Repeat the experiment till 
 you have thoroughly observed the ends which 
 attract and those which repel each other. 
 
 Withdraw the magnet entirely from tho 
 vicinity of your needle, and leave the loUer 
 freely suspended by its fibre. Shelter ii as 
 well as you can from currents of air, and if 
 you have iron -buttons on you-r coa't or a slee? 
 penknife in your pocket, beware of their ac 
 tion. If you work at iiight, beware of iron 
 candlesticks, or of brass ones with .iron rod.-} 
 inside. Freed from such disturbaaces, the 
 needle takes up a certain determinate po- 
 sition. It sets its length nearly north an'f 
 south. Draw it aside from this position and 
 let it go. After several oscillations it will 
 again come to it. If you have obtained your 
 magnet from a philosophical instrument- 
 maker, you will see a mark on one of its emls. 
 Supposing, then, that you drew your needle 
 along the end thus marked, and that the eye- 
 eud of your needle was the last to quit the 
 magnet, you will find that the eye turns to 
 the south, the point of the needle turning 
 toward the north. Make suie of this, and do 
 not take this statement on my authority. 
 
 Now lake a second darning-needle like the 
 first, and magnetize it in precisely the same 
 manner : freely suspended it also will turn its 
 point to the north and its eye to the south. 
 Your next step is to examine the action ot" the 
 two needles which you have thus magnelizvri 
 upon each other. 
 
 Take one ot them in your hand, and leave 
 the other suspended ; bring the eye-end of 
 the former near the eye-end of the latter ; the 
 suspended needle retreats : it is repelled. 
 Make the same experiment with the two 
 points, you obtain the same result, the sus- 
 pended needle is repelled. Now cause tho 
 dissimilar ends to acton each other you 
 have attraction point attracts eye and eye 
 attracts point. Prove the reciprocity of this 
 action by removing the suspended needle, 
 and putting the other in its place. You ob- 
 tain the same result. The attraction, then, 
 is mutual, and the repulsion is mutual, and 
 you have thus demonstrated in the clearest 
 manner the fundamental law of magnetism, 
 that like poles repel, and that, .unlike poles at- 
 tract each other. You may say that this is 
 all easily understood without doing ; but do 
 it, and your knowledge will not be confined 
 to what I have uttered here. 
 
 I have said that one end of your magnet 
 has a mark upon it ; lay several silk fibres 
 together, sa as to get sufficient strength, or 
 employ a thin silk ribbon, and form a loop 
 large enough to hold your magnet. Suspend 
 it ; it turns its marked end toward the north. 
 This marked end is that which in England is 
 called the north pole. If a common smith 
 has made your magnet, it will be convenient 
 to determine its noith pole yourself, and to 
 mark it with a file. You vary your experi- 
 ments by causing your magneti/ed darning- 
 needle to attract and repel your larpe mag- 
 net ; it is guile competent to do so. In 
 
LESSONS IN ELECTRICITY. 
 
 337 
 
 netlzlng Iho needle I hare supposed the rye- 
 end to be the last to quit tho marked end of 
 tho magnet ; that end of th.' needle is a south 
 pole. TJie end which la 4 quits the magnet 
 is always opposed in p'-larity to the end of 
 I ho magnet with which it lias been in contact. 
 BiMuglit near each other they mutually at- 
 iract, and thus demons. rate that they ate uu- 
 liki 1 p >!cs. 
 
 You may perhaps learn all this in a single 
 hour; but spend seveial at it, if necessary; 
 an 1 remember, understanding it is not sulli- 
 cient : you mu^t obtain a manual aptitude in 
 addressing Nature. If you speak to your fel- 
 low-man, you are not entitled to use jargon. 
 Bad experiments nre juigon addressed to Na- 
 ture, and just as much tj be deprecated. A 
 manual dexterity in ill astral ing the interac- 
 titin of magnetic poles is of the utmost impor- 
 tance at tuis stage of your progress, and you 
 must not neglect attaining this power over 
 your implements. As yru proceed, more- 
 over, you will be tempted to do more than I 
 can possibly suggest. Thoughts will cccur 
 to you which you will endeavor to follow 
 out ; questions will arise which you will try 
 to answer. The same experiment may be 
 twenty things to twenty people. Having wit- 
 nessed the action of pole on pole through the 
 lr, you will perhaps try whether tbe mng- 
 pctic power is not to be screened off. You 
 use plates of glass, wood, slate, pasteboaid, 
 or gutta-percha, but find them all pervious to 
 this wondrous force. One magnetic pole nets 
 upon another through these bodies as if they 
 were not present. And should you become 
 a patentee for the regulation of fillips' com- 
 passes, you will not fall, as some projectors 
 have done, into the error of screening off the 
 magnetism of the ship by the inter position of 
 auch substances. 
 
 If you wish to tench a class you must con- 
 trive that the effects which you have thus far 
 witnessed for yourself shall be witnessed by 
 twenty or thirty pupils. And here your pri- 
 vate ingenuity must come into play. You 
 will attach bits of paper to your needles, so 
 as to render their movements visible ot a dis- 
 tance, denoting the north and south poles by 
 different colors, say gieen and red. \ou 
 may also improve upon your darning-needle. 
 Take a strip of sheet-steel, the rib of a lady's 
 Hays will answer, heat it to vivid redness and 
 plunge it into cold water. It is thereby hard- 
 ened rendered, in tact, almost as brittle as 
 glass. Six inches of this, magnetized in the 
 manner of the darning-needle, will be better 
 aole to carry your paper indexes. Having 
 aeriireil such a strip, you proceed thus : 
 
 Magnetize a small sewing-needle and deter- 
 mine its [>oles ; or, break half an inch or an 
 inch off your magnetized darning-needle, and 
 suspend it by a tine silk fibre. The sewing- 
 needle or the fiagment of the darning-needle 
 is now to be used as a test-needle to examine 
 the distribution of the magnetism in your 
 strip of steel. Hold the strip upright in your 
 left hand, and cause the test-needle to ap- 
 proach the lower end of your strip ; one end 
 is attracted the other ie repelled. Raise your 
 
 needle along the strip ; its oscillations, which 
 at first weie quick, become slower ; opposite 
 the middle of the strip they cease enli ely ; 
 neither end of the needle is attracted ; above 
 the middle the test-needle turns suddenly 
 round, its other end being now attiacted. Go 
 through the experiment thoroughly ; yen 
 thus learn that the entire lower half cf'the 
 strip Attracts one end of the needle, while the 
 entire upper half attracts the opposite end. 
 Supposing the north end of your little reed-lc 
 to be that attracted below, you infer that tho 
 entire Ijwer half of your magnetized strip 
 exhibits south magnetism, while the enliio 
 upper half exhibits north magnetism. So 
 far, then, you have determined the distribu- 
 tion of magnetism in your strip of steel. 
 
 You look HI this fact, you think of it ; in 
 its suggest iveness the value of the i xpc-i imeut 
 chiefly consists. The thought arises, " What 
 will occur if 1 break my ship of sled ncm&s 
 iu the middle ? Shall 1 obtain two magnet*,, 
 each possessing a single pole ?" Try the ex- 
 periment ; break your sliip of steel, and le&t 
 each half as you tested the whole. The mere- 
 presentation of its two ends iu succession tu. 
 your test-needle suffices to show you that 
 you have not a magnet with a single pole,, 
 that each half possesses two poles with aneiK 
 tral point between them. And if you again? 
 break the half into two other halves, you will 
 find that each quaiter of the original (strip ex.- 
 hibits precisely Ihe sanio magnetic dlsliibu-- 
 lion as the strip itself. You limy continue th* 
 breaking process ; no matter how small your 
 fragment may be, it still possesses two op-. 
 posite poles a"nd a. neutral point between them. 
 Well, your hand ceases to break vUieie hi cak- 
 ing becomes a mechanical impossibility : but 
 does the mind stop there ? No : you follow 
 the breaking process in idea when you can no 
 longer icalize it in fact ; your thoughts wan- 
 der amid the very atoms of your steel, and 
 you conclude that tach atom is a magnet, 
 and that the force exerted by the strip of steel 
 is the mere summation or lesullant of the 
 forces of its ultimate particles. 
 
 Here, then, is an exhibition of power which 
 we can call forth cr cause to disappear at 
 pleasure. We magnetize onrstiip ot steel by 
 drawing it along the pole of a magnet ; we 
 can demagnetize it, or reverse its magnetism, 
 by properly drawing it along the same pole- 
 in the opposite direction. What, then, i- the 
 real nature of this wondrous change ? What 
 is it that takes place among the atoms of the 
 steel when thefcubstance is magnetized ? The- 
 question leads us beyond the legion of sense, 
 and into that of imagination. This faculty, 
 indeed, is the divining iod of the man of sci- 
 ence. Not, however, an imagination which 
 catches its creations fn,rn the air. but one in- 
 formed and inspired by fj'Cts, caj able </f seiz- 
 ing fiimly on a physical image as a principle, 
 of discerning its consequences, and of ('evising 
 means whereby these t\ recasts of thought may 
 be brought to an experimental test. If Mich 
 a principle be adequate to account for all the 
 phenomena, if from an assumed cause the ob- 
 served facts necessarily follow, wecail the as 
 
33S 
 
 LESSONS IN ELECTRICITY. 
 
 sumption a theory, and, once possessing it,we 
 can not only revive at pleasure facts already 
 known, but we can predict ot tiers which we 
 luwe never seen. Thus, tnen, in the prose- 
 cut ir/n of physical science, our powers of ob 
 serration, memory, imagination, and in- 
 teren-e, are nil drawn upon. We observe 
 Jncts and i-tore them tip ; imagination breed 6 ' 
 upon these memoiies, end by the aid of 
 leasou tries to discern their interdependence. 
 The theoretic principle flashes, or slovly 
 dawns upon the mind, aud then the deductive 
 faculty interposes to carry out the principle 
 to its logical consequences, A perfect theory 
 gives dominion over natural facts ; and even 
 an assumption which can only partially stand 
 the test of a comparison with facts, may be 
 of eminent 11*0 in enabling us tq connect and 
 classify groups of phenomena. The theory 
 of magnetic fluids is of this latter character, 
 and with it we must now make ourselves 
 familiar. 
 
 With the view of stamping the thing more 
 firmly on your minds, 1 will make use of a 
 strong and vivid image. In optics, red aud 
 green are called complementary colors ; their 
 mixture produces white. Now I ask you to 
 | imagine each of these colors to possess a self- 
 \ repulsive power ; that red repels red, and 
 * that greeti repels green ; but that red attracts 
 green and green attracts red, the attraction of 
 5 the dissimilar colors being equal to the repul- 
 sion of the similar ones. Imagine the two 
 colors mixed so as to produce white, and sup- 
 pose two strips of wood painted with this 
 white ; what will be their action upon each 
 othei ? Suspend one of them freely as we 
 suspended our darning-needle, and bring the 
 other near it; what will occur? The red 
 component of the strip you hold in your hand 
 will *epel the red component of your sus- 
 pended strip, but then it will attract the 
 green ; and the forces being equal they neu- 
 tralize each other. In fact, the least reflec- 
 tion shows you that the strips will be as in- 
 different to each other as two unmagnetized 
 darning-needles would be under the same cir- 
 cumstances. 
 
 But suppose, instead of mixing the colors, 
 w*i painted one half of each strip from centre 
 to end red, and the other half green, it is per- 
 fectly manifest that the two strips would now 
 behave toward each other exactly as our two 
 magnetized darning - needles the red end 
 would repel the red and attract the green, 
 the green would repel the green and attract 
 the red ; so that, assuming tw r o colors thus 
 related to each other, we could by their mix- 
 ture produce the neutrality of an unmag- 
 netized body, while by their separation we 
 could produce the duality of action of mag- 
 netized bodies. 
 
 But you have already anticipated a defect 
 in my conception ; for if we break one of our 
 strips of wood in the middle we have one half 
 entirely red and the other entirely green, and 
 with these it would be impossible to imitate 
 the action of our broken magnet. How, then, 
 must we modify our conception ? We must 
 evidently suppose each atom of wood painted 
 
 green on one face and red on the opposite? 
 one. If this were done the resultant action 
 of all the atoms would exactly resemble tho 
 action of a magnet. Here, also, if the two 
 opposite colois of each atom could be caused, 
 to mix so as to produce white, we should 
 have, as before, pei feet neutrality. 
 
 Substitute in your minds far these two self 
 repeilant anl mutually attractive colors two 
 invisible self-repellant and mutually attrac- 
 tive fluids, which in ordinary steel are mixed 
 to form a neutral compound, but which tho 
 act of magnetization separnl.es from each 
 other, placing the opposite fluids on the op- 
 posite faces of each atom, and you have a 
 perfectly distinct conception of the celebrated 
 theory of magnetic fluids. The strength of 
 the magnetism excited is supposed to be pro- 
 portional to the quantity of neutral fluid de- 
 composed. According to this theory nothing 
 is actually transferred from the exciting mag 
 net to the excited steel. The act of mag- 
 netization consists in the forcible separation 
 of two powers which existed in the steel be- 
 fore it was magnetizefl, but which then neu- 
 tralized each other by their coalescence. And 
 i f you lest your magnet after it has excited a 
 hundred pieces of steel, you will find that it 
 has lost no force no more, indeed, th>m I 
 should lose had my words such a magnetic in- 
 fluence on your minds as to excite m them p. 
 strong resolve to study natural philosophy. 
 I should, in fact, be the gainer by my own 
 utterance and by the reaction of your 
 strength ; and so also the magnet is the gainer 
 by the reaction of the body which it mag- 
 netizes. 
 
 Look now to your excited piece of steel ; 
 figure each atom to your minds with its op- 
 posed fluids spread over its opposite faces. 
 How can this state of things be permanent ? 
 The fluids, by hypothesis, attract each other : 
 what, then, keeps them apart ? Why do they 
 not instantly rush together across the equator 
 of the atom, aud thus neutralize each other? 
 To meet this question, philosophers have 
 been obliged to infertile existence of a special 
 force which holds the fluids asunder. They 
 call it coercive force ; and it is found that those 
 kinds of steel which offer most resistance to 
 being magnetized, which require the greatest 
 amount of coercion to tear their fluids asunder, 
 are the very ones which offer the greatest re- 
 sistance to the reunion of the fluids after they 
 have been once separated. Such kinds of 
 steel are most suited to the formation of 
 permanent magnets. It is manifest, indeed, 
 that without coercive force a permanent mag- 
 net would not be at all possible. 
 
 You have not forgotten that, previous to 
 magnetizing your darning-needle, botfi its 
 ends were attracted by your magnet ; and 
 that both ends of your bit of iron wire were 
 acted upon in the same way. Probably also 
 long before this you will have dipped the end 
 of your magnet among iron filings, and ob- 
 served how they cling to it, or into a nail- 
 box, and found how it drags the nails after 
 it. I know very well that Vf you are not the 
 slaves of routine, you will have by this time 
 
LESSONS IN ELECTRICITY. 
 
 380 
 
 Jone many things that I have not told you to 
 do, and thus multiplied your experience be- 
 yond what 1 have indicated. You ate almost 
 bure to have caused a bit of iron to hang from 
 tkt end of your magnet, and you hare prob- 
 ably succeeded in causing a second piece to 
 attach itself to the first, a third to the second ; 
 until finally the force lias become too feeble 
 to bear the weight of more. If you have 
 operated with nails, you may have observed 
 that the points and edges hold together with 
 the greatest tenacity ; and that u bit of iron 
 clings more firmly to the corner of your raag- 
 net than to one of its flat surfaces. In short, 
 you will, in all likelihood, have enriched your 
 experience in many ways without any special 
 direction from me. 
 
 Well, the magnet attracts the nail, and that 
 nail attracts a second one. This proves tiiat 
 the nail in contact willi the magnet has had 
 the magnetic quality developed m it by that 
 contact. If it be withdrawn from the mag- 
 net, its power to attract its fellow-nail ceases. 
 Contact, however, is not necessary. A sheet 
 of glass or paper, or a space of air, may exist 
 between the magnet and the nail ; the latter 
 is still magnetized, though uot so forcibly as 
 when in actual contact. The nail then pre- 
 sented to the magnet is itself a temporary 
 magnet. That end which is turned toward 
 the 'magnetic pole has the opposite magnetism 
 of the pole which excites it ; the end most 
 remote from the pole has the same magnet- 
 ism as the pole itself, and between the two 
 poles the nail, like the magnet, possesses a 
 magnetic equator. 
 
 Conversant a.'? you now are with the theory 
 of magnetic fluids, you have already, I doubt 
 not, anticipated me in imagining the exact 
 condition of the iron under the influence of 
 the magnet. You picture the iron as possess- 
 ing the neutral fluid in abundance ; you pic- 
 ture the magnetic pole, when brought near, 
 decomposing the fluid ; repelling the fluid of 
 a like kind with itself, and attracting the un- 
 like fluid ; thus exciting in the parts of the 
 iron nearest to itself the opposite polarity. 
 But the iron is incapable of becoming a per- 
 manent magnet. It only shows its virtue as 
 long as the magnet acts upon it. What, then, 
 does the iron lack which the sluel possesses? 
 It lacks coercive force. Its fluids are sepa- 
 rated with ease, but, once the separating 
 cause is removed, they flow together again, 
 and neutrality is restored. Y our imagination 
 must be quite nimble in picturing these 
 changes. You must be able to see the fluids 
 dividing and reuniting according as the mag- 
 net is brought near or withdrawn. Fixing a 
 definite pole in your imagination, you must 
 picture the precise arrangement of the two 
 fluids with reference to this pole. And you 
 nust not only be well drilled in the use of 
 this mental imagery yourself, but you must 
 bo- able fo arouee the same pictures in the 
 minds of your pupils, and satisfy yourself 
 that they possess this power of placing iiciu- 
 ally before themselves magnets and iron in 
 various positions, and describing the exact 
 magnetic slate of the iron, in each j/ailicultir 
 
 case. The mere facts of magnetism will have 
 their interest immensely augmented by an 
 acquaintance with those hidden principles 
 whereon the facts depend. Still, while you 
 use this theory of magnetic fluids to track out 
 the phenomena and link them together, bo 
 sure to tell your pupils that it is to be regarded 
 as a symbol merely a symbol, moreover, 
 which is incompetent to cover all the facts,* 
 but which does good practical service while 
 we arc waiting for the actual truth. 
 
 This state of excitement into which the soft 
 iron is thrown by the influence of the magnet, 
 is sometimes called "magnetization by in- 
 fluence." More commonly, however, the 
 magnetism is said to bo "induced" in the 
 soft iron, and hence this way of magnetizing 
 is called " magnetic induction." Now, there 
 is nothing theoretically perfect in Nature : 
 there is no iron so soft as not to possess a 
 certain amount of coercive force, and no steel 
 so hard as not to be capable, in some degree, 
 of magnetic induction. The quality of steel 
 is in some measure possessed by iron, and the 
 quality of iron is shared in some degree by 
 steel. It is in virtue of this latter fact that the 
 unmagnetiztd darning needle was attracted 
 in your first experiment ; and from this you 
 may at once deduce the consequence that, 
 after the steel has been magnetized, the ;e- 
 pulsive action of a magnet must be always 
 less than its attractive action. For the re- 
 pulsion is opposed by the inductive action of 
 the magnet on the steel, while the attraction 
 is assisted by the same inductive action. 
 Make this clear to your miuds, and verify it 
 by your experiments. In sume cases you can 
 actually make the attraction due to the tem- 
 porary magnetism overbalance the repulsion 
 due to the permanent magnetism, and thus 
 cause two poles of the Fame kind apparently 
 to attract each other. When, however, good 
 hard magnets act on each other from a suffi- 
 cient distance, the inductive action practi- 
 cally vanishes, and the repulsion of like poles 
 is sensibly equal to the- attraction of unlike 
 ones. 
 
 I dwell thus longon elementary principles, 
 because they are of the first importance, and 
 it is the temptation of this age of unhealthy 
 cramming to neglect them. Now follow me 
 a little further. In examining the distribu- 
 tion of magnetism in your strip of steel, you 
 raised the needle slowly from bottom to top, 
 and found what we called a neutral point at 
 the centre. Now does the magntt really ex- 
 ert no influence on the pole presented to its 
 centre ? Let us pee. 
 
 Let S N, Fig. 1, be your magnet, and let i 
 n represent a particle of north magnetism 
 placed exactly opposite the middJe of the 
 magnet. Of course this is an imaginary case, 
 as you can never in reality thus detach your 
 north magnetism from its neighbor. What is 
 
 * This theory breaks down when applied to diamag- 
 netic bodies, which are repelled by magnets. Like 
 soft iron, fnch bodies nre thrown into a state of tem- 
 porary excitement in virtue of which they are repelled, 
 bur any attempt 10 explain wirh. a repulsion by the de- 
 composition of a fluid -will demonstrate its own 
 futility. 
 
S40 
 
 LESSONS IN ELECTRICITY. 
 
 the action of the two poles of the magnet 
 on n ? Your reply will of course be that the 
 pole S attracts n while the pole N repels it. 
 Let the magnitude and direction of the at- 
 traction be expressed by the line n m, and the 
 magnitude and direction of the repulsion by 
 the line n o. Now the particle n being equally 
 distant from S and N, the line no, expressing 
 the repulsion, will be equal to m n, which 
 expresses the attraction, and the particle n t 
 acted upon by two such force-s, must evi- 
 dently move in the direction^? n, exactly mid- 
 way between in n and n o. Hence you see 
 that, although there is no tendency of the 
 particle n to move toward the magnetic 
 equator, there is a tendency on its part to 
 move parallel to the magnet. If instead of a 
 particle of north magnetism we placed a par- 
 ticle of south magnetism opposite to the mag- 
 
 FIG. 1. 
 
 netic equator, it would evidently be urged 
 along the line n q ; and if instead of two sep- 
 arate particles of magnetism we place H little 
 magnetic needle, containing both north and 
 south magnetism, opposite the magnetic 
 equator, its south pole being urged along n q, 
 and its north along n p, the little needle will 
 be compelled to set itself parallel to the mag- 
 net S N. Make the experiment, and satisfy 
 yourselves that this is the case. 
 
 Substitute for your magnetic needle a bit 
 of iron wire, devoid of permanent magnetism, 
 and it will set itself exactly as the needle 
 does. Acted upon by the magnet, the wire, 
 as you know, becomes a magnet and behaves 
 as such ; it will, of course, turn its north pole 
 to ward p, and south pole toward q, just like 
 the needle. 
 
 But supposing you shift the position of 
 your particle of north magnetism, and bring 
 it nearer to one end of your magnet, than to 
 the other, the forces acting on the particle 
 are no longer equal ; the nearest pole of the 
 magnet will act more powerfully on the par- 
 
 Fio. 2. 
 
 tide than the more distant one. Let S N, 
 Fig. 2, be the magnet and n the particle of 
 north magnetism in its new position. Well, 
 it is repelled by N, and attracted by S. Let 
 the repulsion be represented in magnitude 
 and direction by the line n o, and the attrac- 
 tion by the shorter line n in. The resultant 
 of these two forces will bo found by complet- 
 ing the parallelogram m n o p, and drawing 
 its diagonal n p. Along np, Ihrn, a particle 
 of north magnetism would be urged by the 
 simultaneous action of S and N. Substitut- 
 ing a particle of south magnetism for n, the 
 same reasoning would lead to the conclusion 
 that the particle would be urged along n q, 
 and if we place at n a short magnetic needle, 
 its north pole will be urged along n p, its 
 south pole along n q, 9nd"the onlv ^osition 
 possible to the needle, thus acted on, is along 
 the line p q, which, as you see, is no longer 
 parallel to the magnet. Verify this by actual 
 experiment. 
 
 In this way we might go round the entire 
 magnet, and considering its two poles as two 
 centres from which the force emanates, we 
 could, in accordance with ordinary mechani- 
 cal principles, assign a definite direction to 
 the magnetic needle at every particular place. 
 And substituting, as before, a bit of iron 
 wire for the magnetic needle, the positions of 
 both will be the same. 
 
 Now, I think, without further preface, you 
 will be able to comprehend for yourselves, 
 and explain to others, one of the most in- 
 teresting effects in the whole domain of mag- 
 netism. Iron filings you know are particles 
 of iron, irregular in shape, being longer in 
 some directions than in others. For the pres- 
 ent experiment, moreover, instead of the iroo 
 filings, very small scraps of thin iron wire 
 might be employed. I place a sheet of paper 
 over the magnet ; it is all the better if the 
 paper be stretched on a wooden frame, as 
 this enables us to keep it quite level. I scat- 
 ter the filings, or the scraps of wire, from a 
 sieve upon the paper, and tap the latter gently, 
 so as to liberate the particles for a moment 
 from its friction. The magnet acts on the fil- 
 ings through the paper, and see how it 
 arranges them ! They embrace the magnet 
 in u series of beautiful curves, which are 
 technically called magnetic curves, or lines 
 of magnetic force. t)oes the meaning of 
 these lines yet flash upon you? Set your 
 magnetic needle or your suspended bit of 
 wire at any point of one of the curves, and 
 you will find the direction of the needle or of 
 the wire to be exactly that of the particle cf 
 iron, or of the magnetic curve at the point. 
 Go round and round the magnet ; the direc- 
 tion of your needle always coincides with the 
 direction of the curve on which it is placed. 
 These, then, are the lines along which a par- 
 ticle of south magnetism, if you could detach 
 it, would move to the north pole, and a bit 
 of north magnetism to the south pole ; they 
 are the lines along which the decomposition 
 of the neutral fluFd takes place, ancf in tho 
 case of the magnetic needle, one of its poles 
 being urged in one direction, and the other 
 
LESSONS IN ELECTRICITY. 
 
 341 
 
 pole in the opposite direction, the needle must 
 necessarily set itself as a tanffenttv the curve. 
 I \vill not seek to simplify this subject fur- 
 ther. If there be anything obscure or con- 
 fused or incomplete in my statement, you 
 ought now, by patient thought, to be able 
 to clear away the obscurity, to reduce the 
 confusion to order, and to supply what is 
 needed to render the explanation complete. 
 Do not quit the subject until you thoroughly 
 understand it ; and if you are able to look 
 with your mind's eye at the play of forces 
 around a magnet," and see distinctly the 
 operation of those forces in the production of 
 the magnetic curves, the time which we have 
 spent together has not been spent in vain. 
 
 In this thorough manner we must master 
 our materials, reason upon them, and, by de- 
 termined study, attain to clearness of concep- 
 tion. Facts thus dealt with exercise an ex- 
 pansive force upon the boundaries of thought; 
 they widen the mind to generalization. We 
 soon recognize a brotherhood between the 
 larger phenomena cf Nature and the minute 
 effects which we have observed in our private 
 chambers. Why, we inquire, does the mag- 
 netic needle set north and south ? Evidently 
 it is compelled to do sj by tho earth ; the 
 great globe which we inherit is itself a mag- 
 net. Let us learn a lit tie more about it. By 
 means of a bit of wax or otherwise, attach 
 your silk fibre to your magnetic needle by a 
 single point at its middle, the needle will thus 
 be uuinterfered with by the paper loop, and 
 will enjoy to some extent a power of dipping 
 its point or its eye below the horizon. Lay 
 your magnet on a table, and hold the needle 
 over the equator of the magnet. The needle 
 sets horizontal. Move it toward the north 
 end of the magnet ; the south end of the 
 needle diijs, the dip augmenting as you ap- 
 proach the nortli pole, over which the needle 
 tf free to move, will set itself exactly vertical. 
 Move it back to the centre, it resumes its 
 konzontality ; pass it on toward the south 
 pole, its north end now dips, and directly 
 over the south pole the needle becomes ver- 
 tical, its north end being now turned down- 
 ward. Thus we learn that on the one side 
 of the magnetic equator the north end of the 
 needle dips ; on the other side the south end 
 dips, the dip varying from nothing to ninety 
 degrees. If we go to the equatorial regions 
 of the earth with a suitably suspended needle, 
 AVC shall find there the position of the needle 
 horizontal. If we sail north, one end of the 
 needle dips; if we sail south, the opposite 
 end dips ; and over the north or south terres- 
 trial magnetic pole the needle sets vertical. 
 The south magnetic pole has not yet been 
 found,, but Sir James Ross discovered the 
 north magnetic pole on the 1st of June, 1881. 
 In this manner we establish a complete par- 
 allelism between the action of the earth and, 
 that of an ordinary magnet. 
 
 The terrestrial magnetic poles do not coin- 
 cide with the geographical ones ; nor does tho 
 earth's magnetic equator quite coincide with 
 the geographical emiator. The direction of 
 the magnetic neceiltt in London, which is 
 
 called the magnetic meridian, incloses an an- 
 gle of 24 degrees with the 1n:e astronomical 
 meridian, this angle being called the declina- 
 tion of the needle f<T London. The north 
 pole of the needle now lirs to the west of the 
 true meridian ; the declination is westerly. 
 In the year 1GGO, however, the declination 
 was nothing, while before that lirnu it was 
 casteily. All this proves that the earth's 
 magnetic constituents are gradually changing 
 iheir distribution. This change is very 
 slow: it is technically called \l\esecularchange, 
 and the observation of it has notj'et extended 
 over a sufficient period of time to enable us 
 to trucks, even approximately, at its laws. 
 
 Having thus discovered, to some extent, 
 the secret of the earth's power, we can turn 
 it to account. 1 hold in my hand a poker 
 formed of good soft iron ; it is now in the line 
 of dip, a tangent, in fact, to the earth's line of 
 magnetic force. The earth, acting as a mag- 
 net, is at this moment constraining the two 
 fluids of the poker to separate, making th 
 lower end of the poker a north pole, and th 
 upper end a south pole. Matk the experi- 
 ment : I hold the knob uppermost, and it at- 
 tracts the north end of a magnetic needle. 1 
 now reverse the poker, bringing its knob un- 
 dermost ; the knob is now a north pole and 
 attracts the south end of a magnetic needle. 
 Get such a poker and carefully repeat this 
 experiment ; satisfy yourselves that the fluids 
 Biiift their position according to the manner 
 in which the poker is presented to the caith. 
 It has already been stated that the softest 
 iron possesses a certain amount of coerciyo 
 force. The earth, at this moment, finds in 
 this force an antagonist which opposes the 
 full decomposition of the neutral fluid. The 
 component fluids may be figured as meeting 
 an amount of friction, or possessing an 
 amount of adhesion, which prevents them 
 from gliding over the atoms of the poker. 
 Can we assist the earth in this case ? If w 
 wish to remove the residue of a powder from 
 the interior surface of a glass to which the 
 powder clings, we invert the glass, tap it, 
 loosen the hold of the powder, and thus en- 
 able the force of gravity to pull it down. 80 
 also by tapping the end of the poker we loosen 
 the adhesion of the fluid to the atoms and en- 
 able the earth to pull them apart. But what 
 is the consequence ? The portion of fluid 
 which has been thus forcibly dragged over 
 the atoms refuses to return when the poker 
 has been removed from the line of dip ; tho 
 iron, as you see, has become a permanent 
 magnet. By reversing its position and tap- 
 ping it again we reverse its magnetism. A 
 thoughtful and competent teacher will well 
 know how to place these rcmarkabl' 1 facts 
 before his pupils in a manner which will ex- 
 cite their interest ; he will know, and if not, 
 will try to learn, how, by the use of sensiblo 
 images, more or less gross, to give those he 
 teaches definite conceptions, purifying these 
 conceptions more and more as the minds of 
 his pupils become more capable of abstraction. 
 He will cause his logic to run like a line of 
 light through these images, and by thus act- 
 
LESSONS IN ELECTOICITT. 
 
 Ing he will cause his boys to march at his 
 Bide with a profit and a joy, which the mere 
 exhibition of facts without principles, or the 
 appeal to the bodily senses and the power of 
 memory alone, could never inspire. 
 
 As an expansion of the note at page 339 the 
 following extract may find a place here : 
 
 4 ' It is well known that a voltaic current 
 exerts an attractive force upon a second cur- 
 rent, flowing in the same direction ; and that 
 when the directions are opposed to each other 
 the force exerted is a repulsive one. By coil- 
 ing wires into spirals. Ampere was enabled 
 to make them produce all the phenomena of 
 attraction and repulsion exhibited by mag- 
 nets, and from this it was but a step to his 
 celebrated theory of molecular currents. He 
 supposed the molecules of a magnetic body to 
 be surrounded by such currents, which, how- 
 ever, in the natural state of the body mutually 
 neutralized each other, on account of their 
 confused grouping. The act of magnetization 
 he supposed to consist in setting these mole- 
 cular currents parallel to each other ; and, 
 starting from this principle, he reduced all the 
 phenomena of magnetism to the mutual action 
 of ecletric currents. 
 
 "If we reflect upon the experiments re- 
 corded in the foregoing pages from first to 
 last, we can hardly fail to be convinced that 
 diamagnetic bodies operated on by magnetic 
 forces possess a polarity * the same in kind 
 ns but the reverse in direction of, that ac- 
 quired by magnetic bodies.' But, if this be 
 the case, how are we to conceive the physical 
 mechanism of this polarity ? According to 
 Coulcoin'j's and Poisson's theory, the act of 
 magnetization consists in the decomposition 
 of a neutral magnetic fluid ; the north pole of 
 a magnet, for example, possesses an attraction 
 for the south fluid of a piece of soft iron sub- 
 
 mitted to its influence, draws the said fluid 
 toward it, and with it the material particles 
 with which the fluid is associated. To ac- 
 count for diamagnetic phenomena this theory 
 seems to fail altogether ; according to it, in 
 deed, the oft used phrase, 'a north pole ex- 
 citing a north pole, and a south pole a south 
 pole/ involves a contradiction. For if the 
 north fluid be supposed to be attracted toward 
 the influencing north pole, it b absurd to sup- 
 pose that its presence there could produce re- 
 pulsion. The theory of Ampere is equally at 
 a loss to explain diamagnetic action ; for if 
 we suppose the pai tides of bismuth sur 
 rounded by molecular cui rents, then, accoid- 
 ing to all that is known of electro-dynamic 
 laws, these currents would set themselves 
 parallel to, and in the same direction as those 
 of the magnet, and hence attraction, and not 
 repulsion, would be the icsult. The fact, 
 however, of this not being the case proves 
 that these molecular curients are not the 
 mechanism by which diamagnetic induction 
 is effected. The consciousness of this, I 
 doubt not, drove M. Weber to the assumption 
 that the phenomena of diamagnetism are pro- 
 duced by molecular currents, not directed, but 
 actually excited in the bismuth by the magnet. 
 Such induced currents would, according to 
 known laws, have a direct ion opposed to those 
 of the inducing magnet, and hence would 
 produce the phenomena of repulsion. To 
 carry out tb" assumption here made. M. 
 Weber is obliged to suppose that the mole- 
 cules of diamagnetic bodies are surrounded 
 by channels, in which the induced molecular 
 currents, once excited, continue to flow with- 
 out resistance." Diam agnetism and Magne- 
 crystalhc Action, pp. 13G, 137. 
 
 THE END. 
 
 CONTENTS. 
 
 .fttroductton 288 
 
 JlUtoric Notes 288 
 
 T>\-3 Art of Experiment 289 
 
 KUiCtJ-ic Attractions 290 
 
 I)is3>v-ry* of Conduction and Insulation 292 
 
 T:i3 Electroscope &-3 
 
 Electric an,l Non-Electrics 21)5 
 
 Elactri-j R-pulsions 208 
 
 Ftmlirnsatal Law of Electric Action 297 
 
 Double or " Polar " Character of the Electric 
 
 Force 290 
 
 What is Electricity? 301 
 
 Electric Induction 302 
 
 The Electropho*-us 3"7 
 
 Action of Points and Flames 308 
 
 The Electrical Machine 309 
 
 The Leyden Jar 314 
 
 Franklin's Cascade Battery 317 
 
 Leyilen Jars of the Simplest Form 318 
 
 Itrnition by the Electric Spark 320 
 
 Duration of the Electric Spark 3'J3 
 
 Electric Light in Vacuo 324 
 
 Lichtenberg's Figures 326 
 
 Surface Compared with Mass 3-26 
 
 Physiological Effects of the Electrical Discharge 328 
 
 Atmospheric Electricity 3"<i8 
 
 The Returning Stroke 330 
 
 The Leyden Battery 888 
 
 Appendix: An Elementary Lecture on Mag- 
 netism 334 
 
SIX LECTURES ON LIGHT. 
 
 BY 
 
 JOHN TYNDALL. 
 
LECTURES ON LIGHT. 
 
 CONTENTS: 
 
 LECTURE I. 
 Introduction, - -,---- 2 
 
 LECTURE II. 
 Origin of Physical Theories, - 8 
 
 LECTURE III. 
 Relation of Theories to Experience, - - 18 
 
 LECTURE IV. 
 Chromatic Phenomena produced by Crystals, - - 27 
 
 LECTURE V. 
 Range of Vision and Range of Radiation. - - 34 
 
 LECTURE VI. 
 Spectrum Analyses, - - 41 
 
SIX LECTURES ON LIGHT. 
 
 BY Prof. JOHN TYNDALL, F.R.S. 
 
 PREFACE. 
 
 MY eminent friend, Prof. Joseph Henry, 
 of Washington, did me the honor of taking 
 these lectures under .his personal direction, 
 and of arranging the times and places at 
 which they were to be -delivered. 
 
 Deeming that my home-dud .s could not, 
 with propriety, be suspended for a longer 
 period, I did not, at the outset, expect to be 
 able to prolong my visit to the United 
 States beyond the end of 1872. 
 
 Thus limited as to time, Prof. Henry began 
 in the North, and, proceeding southwards, 
 arranged for the successive delivery of the 
 lectures in Boston, New York, Philadelphia, 
 Baltimore, and Washington. 
 
 By this arrangement, which circumstances 
 at the time rendered unavoidable, the lec- 
 tures in New York were rendered coincident 
 with the period of the presidential election. 
 This was deemed unsatisfactory, and when 
 fhe fact was represented to me I it once of- 
 fered to extend the time of my visit so as to 
 'make the lectures in New Ycrk succeed 
 those in Washington. The proposition was 
 cordially accepted by my friends. 
 
 To me personally this modified arrange- 
 ment has proved in the highest degree satis- 
 factory. It gave mt a much-needed holiday 
 at Niagara Falls ; it, moreover, rendered the 
 successive stages of my work a kind oi grow'/i, 
 which reached its most impressive develop 
 scent in New York and Brooklyn. 
 
 In every city that I have visited, my recep- 
 tion has been that of a friend ; and, now 
 that my visit has become virtually a thing of 
 the past, I can look back upon it with unqual- 
 ified pleasure. It is a memory without a 
 stain an experience of deep and genuine 
 kindness on the part of the American people 
 never, on my part, to be forgotten. 
 
 This relates to what may be called the pos- 
 itive side of my visit to the circumstances 
 attending the work actually done. My only 
 drawback relates to work undone; for 1 carry 
 home with me the consciousness of having 
 been unable to respond to the invitations of 
 the great cities of the West ; thus, I fear, 
 causing, in many cases, disappointment. 
 Would that this could have been averted ! 
 But the character of the lectures, and the 
 weight of instrumental appliances which they 
 involved, entailed loss of time and heavy 
 labor. The need of rest alone would be a 
 sufficient admonition to me to pause here ; 
 but, besides this, each successive mail from 
 London brings me intelligence of work sus- 
 pended and duties postponed through my 
 absence. These are the considerations 
 which prevent me from responding, with a 
 warmth commensurate with their own, to 
 the wishes of my friends in the West. 
 
 On quitting England I had no intention 
 of publishing these lectures, wnd, except a 
 fragment or two, not a line of them was writtes 
 
SIX LECTURES ON LIGHT. 
 
 when 1 reached this city. They have been 
 begun, continued, and ended in New York, 
 and bear bniy too evident marks of the rapid- 
 ity of their production. I thought it, how- 
 ever, due. bo;h to those who heard them with 
 such marked attention, and to those who wish- 
 ed to hear them, but were unable to do so, to 
 leave t.:em behind me in an authentic form. 
 The execution of this work has cut me off 
 from many social pleasures ; it has also pre- 
 vented me from makingmyself acquainted with 
 institutions in the working of which I feel a 
 deep interest. But human power is finite, 
 and mine has been expended in the way which 
 I deemed most agreeable, not to my more 
 intimate friends, but to the people of the 
 United States. 
 
 In the opening lecture are mentioned the 
 names of gentlemen to whom I am under 
 lasting obligations for their friendly and often 
 laborious aid. The list might readily be ex- 
 tended, for in every city I have visited willing 
 
 helpers were at hand. I must not, however, 
 omit the name of Mr. Rhets, Professor 
 Plenry's private secretary, who, not only in 
 Washington, but in Boston, gave me most 
 important assistance. To the trustees of the 
 Cooper Institute my acknowledgments are 
 due ; also to the directors of the Mercantile 
 Library at Brooklyn. I would add to these a 
 brief but grateful reference to my high-miuded 
 friend and kinsman, General Hector Tyn- 
 dale, for his long-continued care of me, and for 
 the thoughtful tenderness by which he and his 
 family softened, both to me and to the parents 
 of the>vuth, the pain occasioned by the death 
 of my junior assistant in Philadelphia. 
 
 Finally, I have to mention with warm com- 
 mendation the integrity, ability, and devo- 
 tion, with which, from first to last, I have 
 been aided by my principal assistant, Mr. 
 John Cottrell. 
 
 NEW YORK, February, 1873. 
 
 LECTURE I. 
 
 INTRODUCTORY : Uses of Experiment : Early Scien- 
 tific Notions: Sciences of Observation: Knowl- 
 edge of the Ancients Regarding Light : Nature 
 judged from Theory defective: Detects of the 
 Eye: Our Instruments: Rectilineal Propagation 
 of Light : Law of Incidence and Reflection : 
 Sterility of the Middle Ages: Refraction: Dis- 
 covery of Snell : Descartes and the Rainbow : 
 Newton r s Experiments on the Composition of 
 Solar Light : His Mistake as regards Achroma- 
 tism : Synthesis ot White Li<ht : Yellow and 
 lUut: Lights proved to produce White by their 
 Mixture: Colors of Natural Bodies: Absorption: 
 Mixture of Pigments contrasted with Mixture of 
 LLhts. 
 
 SOME twelve years ago I published, in 
 England, a little book entitled the " Glaciers 
 of the Alps," and, a couple of years subse- 
 quently, a second volume, entitled " Heat as 
 a Mode of Motion." These volumes were 
 followed by others, written with equal plain- 
 ness, and with a similar aim, that aim being 
 to develop and deepen sympathy between 
 science and the world outside of science. I 
 agreed with thoughtful men* who deemed 
 it good for neither world to be isolated from 
 the other, or unsympathetic towards the 
 other, and, to lessen this isolation, at least in 
 one department of science, I swerved aside 
 from those original researches which had pre- 
 viously been the pursuit and pleasure of my 
 life. 
 
 These books were, for the most part, re- 
 published by the Messrs. Appleton, under 
 
 *Among whom may be mentioned, specially, the 
 l:e Sir Edmund Head, Bart. 
 
 the auspices of a man who is untiring in his 
 efforts to diffuse sound scientific knowledge 
 among the people of this country; whose 
 energy, ability, and single-mindedness, in the 
 prosecution of an arduous task, have won for 
 him the sympathy and support of many of us 
 in "the old country." 1 allude to Professor 
 Youmans, of this city. Quite as rapidly as 
 in England, the aim of these works was un- 
 derstood and appreciated in the United 
 States, and they brought me from this side 
 of the Atlantic innumerable evidences of 
 good-will. Year after year, invitations 
 reached me * to visit America, and last year 
 I was honored with a request so cordial, and 
 signed by five-and-twenty names so distin- 
 guished in science, in literature, and in ad- 
 ministrative position, that I at once resolved 
 to respond to it by braving, not only the dis- 
 quieting oscillations of the Atlantic, but the 
 far more disquieting ordeal of appearing in 
 person before the people of the United 
 States. , 
 
 This request, conveyed to me by my ac- 
 complished friend, Professor Lesley, of Phil- 
 adelphia, and preceded by a letter of the 
 same purport from your scientific Nestor, 
 Professor Joseph Henry, of Washington, de- 
 sired that I would lecture in some of the 
 principal cities of the Union. This I agreed 
 to do, though much in the dark as to what 
 foim such lectures ought to to take. In 
 
 * One of the earliest came from Mr. John Amory 
 Lowell, of Boston. 
 
SIX LECTURES ON LIGHT 
 
 answer to my inquiries, however, I was given 
 to understand (by Professor Youmans princi- 
 pally) that a course of experimental lectures 
 would materially promote scientific education 
 in this country, and I at once resolved to 
 meet this desire, as far as my time allowed. 
 
 Experiments have two uses a use in dis- 
 covery, and a use in tuition. They are the 
 investigator's language addressed to Nature, 
 to which she sends intelligible replies. These 
 replies, however, are, for the most part, at 
 first too feeble for the public ear ; for the in- 
 vestigator cares little for the loudness of Na- 
 ture's voice if he can only unravel its meaning. 
 But after the discoverer comes the teacher, 
 whose function it is so to exalt and modify the 
 resu ts of the discoverer as to render them fit 
 for public presentation. This secondary 
 function I shall endeavor, in the present in- 
 stance, to fulfil. 
 
 I propose to take a single department of 
 natural philosophy, and illustrate, by means 
 of it, the growth of scientific knowledge under 
 the guidance of experiment. I wish, in this 
 ii st lecture, to make you acquainted with cer- 
 
 single increment is made good by the indefi- 
 nite number of su-ch increments, summed up 
 in what may be regarded as practically infinite 
 time. 
 
 We will not now go back to man's first 
 intellectual gropings ; much less shall we 
 enter upon the thorny discussion as to how 
 the groping man arose. We will take him at 
 a certain stage of his development, when, fay 
 evolution or sudden endowment, he became 
 possessed of the apparatus of thought and 
 the power of using it. For a time and that 
 historically a long one he was limited to 
 mere observation, accepting what Nature of- 
 fered, and confining intellectual action to it. 
 The apparent motions of sun and stars first 
 drew towards them the questionings of the in- 
 tellect, and accordingly astronomy was the 
 first science developed. Slowly, and with difE.- 
 culty, the notion of natural forces took root 
 in the mind, the seedling of this notion being 
 the actual observation of electric and mag- 
 netic attractions. Slowly, and with difficulty, 
 the science of mechanics had to grow out of 
 this notion ; and slowly at last came the full 
 
 tain elementary phenomena ; then to point application of mechanical principles to the 
 out to you how those theoretic principles by motions of the heavenly bodies. We trace 
 which phenomena are explained, take rooi, I the progress of astronomy through Hip- 
 and flourish in the human mind, and after- J parchus and Ptolemy; and, after a long halt, 
 wards to apply these principles to the whole through Copernicus, Galileo, Tycho Bratue, 
 
 and Kepler; w ile, from the high table-land 
 of thought raised by these men, Newton-. 
 
 tx dy of knowledge covered by the lectures. 
 
 The science of optics lends itself to this mode 
 
 of reatment, and on it, therefore, I propose Lshoots upward like a peak, overlooking all 
 
 to draw for the materials of the present course. I others from his dominant elevation. 
 
 It will be best to begin with the few simple I But other objects than the motions of the 
 
 facts regarding light which were known to the stars attracted the attention of the ancient 
 
 ancients, and to pass from them in historic 
 gradation to the more abstruse discoveries of 
 modern times. 
 
 All our notions of Nature, however exalted 
 
 world. Light was a familiar phenomenon, 
 and from the earliest times we find men's 
 rr.inds busy with the attempt to render some 
 account of it. But, without experiment, 
 
 or however grotesque, have some foundation ; which belongs to a later stage of scientific 
 
 development, little progress could be madein 
 this subject. The ancients, accordingly, 
 
 in experience. The notion of personal voli- 
 tion in Nature had this basis. In the fury 
 and the serenity of natural phenomena the 
 savage saw the transcript of his own varying 
 moods, and he accordingly ascribed these 
 phenomena to beings of like passions with 
 himself, but vastly transcending him in power. 
 Thus the notion of causality the assumption 
 that natural things did not com 2 of themselves, 
 
 were far less successful in dealing with light 
 than in dealing with solar and stellar mo- 
 tions. Still, they did make some progress. 
 They satisfied themselves that light moved 
 in straight lines; they knew, also, that these 
 lines or rays of light were reflected from pol- 
 ished surfaces, and that the angle of inei- 
 
 but had unseen antecedents lay at the root \ dence was equal to the angle of reflection, 
 of even the savage's interpretation of Nature. ! These two results of ancient scientific curios- 
 Out of this bias of the human mind to seek j ity constitute the starting-point of our pres- 
 for the antecedents of phenomena all science ' ent course of lectures. 
 
 But, in the first place, it may be useful to 
 say a few words regarding the source of light' 
 to be employed in our experiments. The, 
 
 has sprung. 
 
 The development of man, indeed, is ulti- 
 mately due to his interaction with Nature. 
 Natural phenomena arrest his attention and 
 excite his questionings, the intellectual activity 
 
 rusting of iron is, to all intents and purposes, 
 the slow burning of iron. It develops heat, 
 
 thus provoked reacting on the intellect itself, j and, if the heat be preserved, a high temper- 
 and adding to its strength. The quantity of j ature may be thus attained. The destruc- 
 power added by any single effort of the in- tion of the first Atlantic cable was probably 
 tellect may be indefinitely small ; but the in- due to heat developed in this way. Other 
 tegration of innumerable increments of this J metals are still more combustible than iron, 
 kind has raised intellectual power from its j You may light strips of zinc in a candle- 
 rudiments to the magnitude it possesses to- flame, and cause them to burn almost like 
 day. In fact, the indefinite smallness of the strips of paper. But, besides combustion in 
 
SIX LECTURES ON LIGHT. 
 
 the air, we may also have combustion it* a 
 liquid. Watr, for example, contains a store 
 of oxygen which may unite with and consume 
 a metal immersed in it. It is from this kind 
 of combustion that we are to derive the heat 
 and light employed in the present course. 
 
 Their generation merits a moment's atten- 
 tion. Before you is an instrument a small 
 voltaic battery in which zinc is immersed in 
 ! a suitable liquid. Matters are so arranged 
 that an attraction is set up between the metal 
 and the oxygen, actual union, however, being 
 in- the first instance avoided. Uniting the 
 two ends of the battery by a thick wire, the 
 attraction is satisfied, the oxygen unites with 
 the metal, the zinc is consumed, and heat, as 
 usual, is the result of the combustion. A 
 power, which, for want of a better name, we 
 call an electric current, passes at the same 
 time through the wire. 
 
 Cutting the thick wire in two, I unite the 
 severed ends by a thin one. It glows with a 
 white heat. Whence comes that heat ? The 
 question is well worthy of an answer. Sup- 
 pose in the first instance, when the thick wire 
 
 to form the image. It H not sharp, but sur, 
 rounded by a halo whic'i nearly obliterates it. 
 This arises from an imperfection of the lens, 
 called its spJierical aberration, due to the fact 
 that the circumferential and central rays have 
 not the same focus. The human eye labors 
 under a similar defect, and, when you looked 
 at the naked light from fifty cells, the blur of 
 light upon the retfna was sufficient to destroy 
 the definition of th<; retinal image of the car- 
 bons. A long list of indictments might in- 
 deed be brought against the eye its opacity, 
 its want of symmetry, frs lack of achroma- 
 tism, its absolute blindness, in pa;t. All 
 these taken together caused an eminent Ger- 
 man philosopher to say that, if any optician 
 sent him an instrument so full of defects, he 
 would send it back to him with the severesf 
 censure. But the eye is not to be judged 
 from the standpoint of theory. As a practi' 
 cal instrument, and taking the adjustments by 
 which its defects are neutralized into account, 
 it must ever remain a marvel to the reflecting 
 mind 
 
 Tae ancients, as I have said, were aware o/ 
 
 was employed, that we had permitted the ac- | the rectilineal propagation of light. They 
 
 lion to continue until 100 grains of zinc were 
 consumed, the amount of heat generated in 
 the battery would be capable of accurrte nu- 
 merical expression. Let the action now con- 
 tinue, with this thin wire glowing, until 100 
 
 knew that an opaque body, placed between 
 the eye and a point of light, intecepted the 
 light of the point. Possibly the terms " ray " 
 and "beam" may have been suggested by 
 those straight spokes of light which, in c^r 
 
 grains of zinc are consumed. Will the amount j tain states of the atmosphere, dart from thi 
 
 t-f heat generated in the battery be the same 
 as before ? No, it will b>i less by the precise 
 amount generated in the thin wire outside the 
 battery. In fact, by adding the internal heat 
 to the external, we obtain for the combustion 
 of 100 grains of zinc a total which never va- 
 ries. By this arrangement, then, we are able 
 to burn our zinc at one place, and to exhibit 
 the heat and light of its combustion at a dis- 
 tant place. In New York, for example, we 
 have our grate and fuel ; but the heat and 
 light of our fire may be made to appear at 
 San Francisco. 
 
 I now remove the thin wire and attach to 
 the severed ends of the thick one two thin 
 rods of coke. On bringing the rods together 
 we obtain a small star of light. Now, the 
 light to be employed in our lectures is a sim- 
 ple exaggeration of this star. Instead of 
 being produced by ten cells, it is produced by 
 fifty. Placed in a suitable camera, provided 
 with a suitable lens, this light will give us all 
 the beams necessary for our experiments. 
 
 And here, in passing, let me refer to the 
 
 sun at his rising and his setting. The recti-V 
 lineal propagation of light may be illustrated 
 at home in th : s way: Make a small hole in a 
 closed window-shutter, before which stands a 
 house or a tree, and place within the dark- 
 ened room a white screen at some distance 
 from the orifice. Every straight ray proceed- 
 ing from the house or tree stamps its color 
 upon the screen, and the sum of all the rays 
 forms an image of the object. But, as the 
 rays cross each other at the orifice, the image 
 is inverted. Here we may illustrate the sub- 
 ject thus: In front of our camera is a large 
 opening, closed at present by a sheet of tin- 
 foil. Pricking by means of a common sew- 
 ing-needle a small aperture in the tin-foil, an 
 inverted image of the carbon-points starts 
 forth upon the screen. A dozen apertures 
 will give a dozen images, a hundred a hun- 
 dred, a thousand a thousand. But. as the 
 apertures come closer to each other, that is to 
 say, as the tin-foil between the apertures van- 
 ishes, the images overlap more and more. 
 Removing the tin-foil altogether, the screen 
 
 common delusion that the works of Nature, I becomes uniformly illuminated Hence the 
 the human eye included, are theoretically per- | light upon the screen may be regarded as the 
 
 overlapping of innumerable images of the 
 carbon-points. In like manner the light 
 upon every white wall on a cloudless day 
 may be regarded as produced by the super- 
 position of innumerable images of the sun. 
 
 The law that the angle of incidence is 
 equal to the angle of reflection is illustrated 
 
 The degree of perfection of any organ 
 is determined by what it has to do. Looking 
 at the dazzling light from our large battery, 
 you see a globe of light, but entirely fail to 
 see the shape of the coke-points whence the 
 light issues. The cause may be thus made 
 clear : On the screen before you is projected 
 
 an. image of the carbon-points, the whole of j in this simple way: A straight lath is placed 
 the lens in front of the camera being employed I as an index perpendicular to a small looking- 
 
SIX LECTURES ON LIGHT. 
 
 glass capable of rotation. A beam of light 
 is received upon the glass and reflected back 
 upon the line of its incidence. Though the 
 incident and the reflected beams pass in 
 opposite directions, they do not jostle or dis- 
 place each other. The index being turned, 
 the mirror turns along with it, and at each 
 side of the index the incident and the 
 reflected beams are seen tracking themselves 
 through the dust of the room. The mere 
 inspection of the two angles enclosed be- 
 tween the index and the two beams suffices 
 to show their equality. The same simple 
 apparatus enables us to illustrate a law of 
 great practical importance, name y, that, 
 when a mirror rotates, the angular velocity 
 of a beam reflected from it is twic^ that of 
 the reflecting mirror. One experiment will 
 make this pla n to you. The mirror is 
 now vertical, and both the incident and the 
 reflected beams are horizontal. Turning the 
 mirror through an angle of 45 the reflected 
 beam is vertical ; that is to say, it has moved 
 9O r , or through twice the angle of the mirror. 
 
 One of the problems of science, on which 
 scientific progress mainly depends, is to help 
 the senses of man by carrying them into re- 
 gions which could never be attained without 
 such help. Thus we arm the eye with the 
 telescope when we want to sound the depths 
 of space, and with the miscroscope when we 
 want to explore motion and structure in their 
 infinitesimal dimensions. Now, this law of 
 angular reflection, coupled with the fact that 
 a beam of light possesses no weight, gives us 
 the means of magnifying small motions to an 
 extraordinary degree. Thus, by attaching 
 mirrors to his suspended magnets, and by 
 wa ching the images of scales reflected from 
 the mirrors, the celebrated Gauss was able to 
 detect the slightest thrill or variation on the 
 part of the earth's magnetic force. The mi- 
 nute elongation of a bar of metal by the mere 
 warmth of the hand may be so magnified by 
 this method as to cause the index-beam to 
 move from the ceiling to the floor of this 
 room. The elongation of a bar of iron when 
 it is magnetized may be thus demonstrated. 
 By a similar arrangement the feeble attrac- 
 tions and repulsions of the diamagnetic force 
 have been made manifest; while in Sir William 
 Thompson's reflecting galvanometer the prin- 
 ciple receives one of its latest applications. 
 
 For more than 1,000 years no step was 
 taken in optics beyond this law of reflection. 
 The men of the Middle Ages, in fact, endeav- 
 ored on the o ,e hand to develop the laws of 
 the universe out of their own consciousness, 
 while many of them were so occupied with 
 the concerns of a future world that they 
 looked with a lofty scorn on all things pertain- 
 ing to this one. Notwithstanding its demon- 
 strated failure during 1,500 years of trial, 
 there are still men among us who think the 
 riddle of the universe is to be solved by this 
 appeal to consciousness. And, like most 
 people who support a delusion, they maintain 
 
 theirs warmly, and show scant respect for 
 those who dissent from their views.* As re- 
 gards the refraction of light, the course of 
 real inquiry was resumed in noo by an Ara- 
 bian philosopher named Alhazen. Then it 
 was taken up in succession by Roger Bacon, 
 Vitellio, and Kepler. One of the most im- 
 portant occupations of science is the deter- 
 mination, by precise measurements, of the 
 I quantitative relations of phenomena. The 
 value of such measurements depends upon the 
 skill and conscientiousness of the man who 
 makes them. Vitellio appears to have been 
 both skilful and conscientious, while Kepler's 
 nabit was to rummage through the ob.-erva- 
 tions of his predecessors, look at them in ail 
 lights, and thus distill from them the princi- 
 ples which united them. He had done this 
 with the astronomical measurements of 
 Tycho Brahe, and had extracted from them 
 the celebrated " laws of Kepler." He did k 
 also with the measurements of Vitellio. ^3ut 
 in the case of refraction he was not success- 
 ful. The principle, though a simple one, es- 
 caped him. It was firs discovered by \Ville- 
 brod Snell, about the year 1621. 
 
 Less with the view of dwelling 1 upon the 
 phenomenon itself than of introducing it to 
 you in a form which will render intelligible 
 the play of theoretic thought in Newton's 
 mind, I will show you the fact of refraction. 
 The dust of the air and the turbidity of a liquid 
 may here be turned to account. A shallow 
 circula- vessel with a glass face, half rilled 
 with water, rendered barely turbid by the 
 precipitation of a little mastic, is placed upon 
 its edge with its glass face vertical. Through 
 a slit in the hoop surrounding the vessel a 
 beam of light is admitted. It impinges upon 
 the water, enters it, and tracks itself through 
 the liquid in a sharp, bright band. Meanwhile 
 the beam passes unseen through the air above 
 the water, for the air is not competent to 
 scatter the light. A puff of tobacco smoke 
 into this space at once reveals the track of the 
 incident-beam. If the incidence be vertical, 
 the beam is unrefracted. If oblique, its re- 
 fraction at the common surface of. air and 
 water is rendered clearly visible. It is also 
 seen that reflection accompanies refraction, 
 the beam dividing itself at the point of inci- 
 dence into a refracted and a reflected portion. 
 The law by which Snell connected together 
 all the measurements executed up to his time, 
 is this : Let A B C D represent the outline 
 of our circular vessel (Fig. i), A C being the 
 water-line. When the beam is incident along 
 B E, which is perpendicular to A C, there is 
 no refraction. When it is incident along m 
 E, there is refraction : it is bent at E and 
 strikes the circle at n. When it is incident 
 
 * Schelling thus expresses his contempt for experi- 
 mental knowledge : " Newton's Optics is the greatest 
 illustration of a whole structure of fallacies, which in 
 all its parts is founded on observation and experi- 
 ment." There are some small imitators of Schelling 
 still in Germany. 
 
SIX LECTURES ON LIGHT. 
 
 along m r E, there is also refraction at E, the 
 beam striking the point n f . From the ends 
 of the incident beams, let the perpendiculars 
 111 o, m' o / ba drawn upon 1) D, and from the 
 ends of the refr-cted beams let the perpen- 
 diculars p n,p' n f be also drawn. Measure 
 the lengths of o in and of p n. and divide the 
 
 one by the ether. You obtain a certain quo- 
 tient. In like manner divide m' o f by the 
 corresponding perpendicular p r ;/; you ob- 
 tain in each case the same quotient. Snell, in 
 fact, found this quotient to be a constant 
 quantity for each particular substance, though 
 ic varied in amount from substance to sub- 
 st.mce He called the quotient the index of 
 refraction. 
 
 This law is one of the corner-stones of 
 optical science, and its applications to-day 
 are million-fold. Immediately after its dis- 
 covery, Descartes applied it to the explana- 
 tion of the rainbow. The bow :s seen when 
 tbe back is turned to the sun. Draw a 
 straight line through the spectator's eye and 
 the sun, the bow is always seen at the same 
 angular distance from this line. This was 
 the great difficulty. Why should the bow be 
 always and at all its parts, forty-one degrees 
 from this line ? Taking a pen and calculat- 
 
 certain that he did net enunciate the true 
 law. This was reserved for Newton, who 
 went to work in this way: Through the closed 
 window-shutter of a room he pieiced an ori- 
 fice, and allowed a thin sunbeam to pass 
 through it. The beam stamped a round 
 image of the sun on the opposite white wall 
 of the room. In the path of this beam New- 
 ton placed a prism, expecting to see the beam 
 refracted, but also expecting to see the image 
 of the sun, af'cr refi action, round; to his 
 astonishment, it was drawn out to an image 
 whose length was five times its breadth; and 
 this image was divided into bands of differ- 
 ent colors. Newton saw immediately that 
 solar light was composite, not simple. His 
 image revealed to him the fact that some con- 
 stituents of the solar light were more deflect- 
 ed by the prism than others, and he conclud- 
 ed, therefore, that white solar light was a 
 mixture of lights of different colors and of 
 different degrees of rcfrangibility. 
 
 Let us reproduce this celebrated experi- 
 ment. On tne screen is now stamped a lu- 
 minous disk, which may stand for Newton's 
 image of the sun. Causing the beam wl ich 
 produces the disk to pass through a pri-rn, 
 we obtain Newton's elongated colored image, 
 which he called a spectrum. Newton divided 
 the spectrum into seven parts red, orange, 
 yellow, green, blue, indigo, violet which 
 are commonly called the seven primary or 
 prismatic colors. This drawing out of the 
 white light into its constituent colors is called 
 dispersion. 
 
 This was the first analysis of solar light bv 
 Newton ; but the scientific mind is fond of 
 verification, and never neglects it where it is 
 possible. It is this stern conscientiousness in 
 testing its conclusions that gives adamantine 
 trength to science, and renders all assaults 
 on it unavailing. Newton completed his 
 proof by synthesis in this way : The spec- 
 rum now before you is produced by a glass 
 prism. Causing the decomposed beam to 
 
 ing the track of every ray through a rain- pass through a second simitar prism, but so 
 drop, Descartes found that, at one particular placed that the colors are refracted back and 
 
 angle, the rays emerged from the drop almost 
 parallel to each other; being t :us enabled to 
 preserve their intensity through long atmos- 
 pheric distances; at all other angles the rays 
 quitted the drop divergent, and through this 
 divergence became so enfeebled as to be 
 practically ;ost to the eye. The particular 
 angle here referred to was the ioregoing 
 angle of forty-one degrees, which observa- 
 tion had proved to be invariably that of the 
 
 rainbow. 
 But in 
 
 the rainbow a new phenomenon 
 
 was introduced the phenomenon of color. 
 And here we arrive at one of those points in 
 the history of science, when men's labors so 
 intermingle, that it is difficult to assign to 
 each worker his precise meed cf honor. Des- 
 cartes was at the threshold of the discovery 
 of the composition of solar light. But he 
 failed to attain perfect clearness, and it is 
 
 reblended, the perfectly white image of the 
 slit is restored. Here, then, refraction and 
 dispersion are simultaneously abolished. Are 
 they always so ? Can we have the one with- 
 out the other? It was Newton's conclusion 
 that we could not. Here he erred, and his 
 error, which he maintained to the end of his 
 life, retarded the progress of optical discovery. 
 Dolland subsequently proved that, by com 
 bining two different kinds of glass, the color 
 could be extinguished, still leaving a rcsidui 
 of refraction, and he employed this residue 
 in the construction of achromatic lenses - 
 lenses which yield no color which Newion 
 thought an impossibility. By setting a water 
 prism water contained in a wedge-shaped 
 vessel with glass sides in opposition to a 
 prism of glass, this point can be illustrated 
 before you. We have first the position of the 
 unrefracted beam marked upon the screen ; 
 
SIX LECTURES ON LIGHT, 
 
 then we produce the water-spectrum ; finally, 
 by introducing a flint glass prism, we refract 
 the beam back, until the color cisappears. 
 The image of the slit is now white ; but you 
 see that, though the dispersion is abolished, 
 the refraction is not. 
 
 This is the place to illustrate another point 
 bearing upon the instrumental means em- 
 ployed in these lectures. Note the position 
 ot the water-spectrum upon the screen. Alter- 
 ing, in no particular, the wedge-shaped ves- 
 sel, but simply substituting for the water the 
 transparent bisulphide of carbon, you notice 
 how much higher the beam is thrown, and 
 how much richer is the display of color. 
 This will explain to you the use of this sub- 
 stance in our subsequent experiments. 
 
 The synthesis of white light may be 
 effected in three ways, which are now worthy 
 of special attention: Here, in the first in- 
 stance, we have a rich spectrum produced by 
 a prism of bisulphide of carbon. One face 
 of the prism is protected by a diaphragm 
 with a longitudinal slit, through which the 
 beam passes into the prism. It emerges de- 
 composed at the other side. I permit the 
 colors to pass through a cylindrical lens, 
 which so squeezes them together as to pro- 
 duce upon the screen a sharply-defined rect- 
 angular image of the longitudinal slit. In 
 that image the colors are re-blended, and you 
 see it perfectly white. Between the prism 
 and the cylindrical lens may be seen the 
 colors tracking themselves through the dust 
 Df the room. Cutting off the more refrangi- 
 ble fringe by u card, the rectangle is seen 
 red ; cutting off the less refrangible fringe, 
 the rectangle is seen blue. By means of a 
 thin glass prism, I deflect one portion of the 
 colors, and leave the residual portion. On 
 the screen are now two colored rectangles 
 produced in this way. These are comple- 
 mentary colors colors which, by their union, 
 produce white. Note that, by judicious 
 management, one of these colors is icndered 
 yellow, and the other blue. I withdraw the 
 thin prism ; yellow falls upon blue, and we 
 have white as the result of their union. On 
 our way, we thus abolish the fallacy first 
 exposed by Helmholtz, that the mixture of 
 blue and yellow lights produces green. 
 
 Again, restoring the circular aperture, we 
 obtain once more a spectrum like that of 
 Newton. By means of a lens, we gather up 
 these colors, and build them together not to 
 an image of the aperture, but to an image of 
 the carbon points themselves. Finally, in 
 virtue of the persistence of impressions upon 
 the retina, by means of a rotating disk, on 
 which are spread in sectors the colors of the 
 spectrum, we blend together the prismatic 
 colors in the eye itself, and thus produce the 
 impression of whiteness. 
 
 Having unravelled the interwoven con- 
 stituents of white light, we have next to 
 inquire, What part the constitution s:> 
 revealed enables this agent to play in Nature ? 
 
 To it we owe all the phenomena of color; 
 and yet not to it alone, for there must be a 
 certain relationship between the ultimate par- 
 ticles of natural bodies and light to enable 
 them to extract from it the luxuries of color. 
 But the function of natural bodies is here 
 selective, not creative. There is no color gen- 
 erated by any natural body whatever Natural 
 bodies have showered upon them, in the 
 white light of the sun, the sum total of all 
 possible colors, and their action is limited to 
 the sifting of that total, the appropriating 
 from it of the colors which really belong to 
 them, and the rejecting of those which do 
 not. It will fix this subject in your minds if 
 I say that it is the portion of light which 
 they reject, and not that which belongs to 
 them, that gives bodies their colors. 
 
 Let us begin our experimental inquiries here 
 by asking, \Vhatisthemeaningof blackness? 
 Pass a black ribbon in succession through the 
 colors of the spectium ; it quenches all. 
 This is the meaning of blackness it is the 
 result of the absorption of all the constituents 
 of solar light. Pass a red r'.bbon through the^ 
 spectrum. In the red light the ribbon is a 
 vivid red. "Why ? Because the light that 
 enters the ribbon is not quenched or absorbed, 
 but sent back to the eye. Place the same rib. 
 bon in the green or blue of the spectrum ; \fe 
 is black as jet. It. absorbs the green ai/4. 
 blue light, and leaves the s pace on which they 
 fall a space of intense darkness. Place a. 
 green ribbon in the green of the spectrum. 
 It shines vividly with its proper color ; transfei;- 
 it to the red, it is black as jet. Here it ab- 
 sorbs all the light that falls upon it, and offers , 
 mere darkness to the eye. Whert white light 
 is employed, the red sifts it by quenching the 
 green, and the green sifts it by quenching the 
 red, both exhibiting the residual color. Thus 
 the process through which natural bodies ac- 
 quire their colors is a negative one. The 
 colors are produced by subtraction, not by 
 addition. This red glass is , red because it 
 destroys all the more refrangible rays of the 
 spectrum. This blue liquid is blu,e^ because it, 
 destroys all the less refrangible rays. Both 
 together are opaque because the light trans- 
 mitted by the one is quenched by the other. 
 In this way by the union of two transparent, 
 substances we obtain a combination as dark 
 as pitch to solar li^ht. This other liquid, 
 finally is purple because it destroys the gree.i 
 and the yellow, and allows the terminal colors . 
 of the Lpectrum to pass unimpeded. From 
 thp blending of the blue and the red this gor- 
 geous color is produced. 
 
 These experiments prepare us for the fur-, 
 ther consideration of a point already adverted 
 to, and regarding which error has found cur- 
 rency for ages. You will find it stated in 
 books that blue and yellow lights mixed to- 
 gether produce green. But blue and yellow 
 nave been just proved to be complementary 
 colors, producing white by their mixture. 
 The mixture of blue and yellow pigments un- 
 
SIX LECTURES ON LIGHT. 
 
 doubtedly produces green, but the mixture of 
 pigments is totally different from the mixture 
 of lights. Helmholtz, who first proved yel- 
 low and blue to complementary ..colors, has 
 revealed the cause of the green in the case of 
 the pigments. No natural color is pure. A 
 blue liquid or a blue powder permits not only 
 the blue to pass through it, but a portion of 
 the adjacent green. A yellow powder is 
 transparent not only to the yellow light, but 
 also in part transparent to the adjacent green. 
 Now, when blue and yellow are mixed to- 
 gether, the blue cuts off the yellow, the orange, 
 and the red ; the yellow, on the other hand, 
 cuts off the violet, the indigo, and the blue. 
 Green is the only color to which both are 
 transparent, and the consequence is that, 
 when white light falls upon a mixture of yel- 
 low and blue powders, the green alone is sent 
 back to the eye. I have already shown you 
 that the fine blue ammonia-sulphate of copper 
 transmits a large portion of green, while cut- 
 ting off all the less refrangible light. A yel- 
 low solution of picric acid also allows the 
 green to pass, but quenches all the more re- 
 frangible light. What must occur when we 
 send a beam through both liquids ? The 
 green band of the spectrum alone remains 
 upon the screen. 
 
 This question of absorption is one of the 
 most subtle and difficult in molecular physics. 
 Vvc are not yet in a condition to grapple with 
 : but we shall be by-and-by. Meanwhile, 
 "'we may profitably glance back on the web of 
 jwtlations which these experiments reveal to 
 its. We have, in the first place, in solar 
 light an agent of exceeding complexity, com- 
 posed of innumerable constituents, refrangi- 
 ble in different degrees. We find, secondly, 
 tike atoms and molecules of bodies gifted 
 writ la the power of sifting solar light in the 
 most various ways, and producing by this 
 ifting the colors observed in nature and art. 
 To do this they must possess a molecular j 
 'Structure commensurate in complexity with j 
 that of light itself. Thirdly, we have :he j 
 .human eye and brain so organized as to be 
 able to take in and distinguish the multitude 
 Oif impressions thus generated. Thus, the 
 light .at starting is complex; to sift and select 
 ic as they do natural bodies must be complex. 
 Fiaally, to take in the impressions thus gen- 
 vcjated, the human eye and brain must be 
 iiighly complex. Whence this triple com 
 p.txiy? If what are called material pur- 
 pios^s were the only end to be served, a much 
 simpler mechanism would be sufficient. But, 
 instead of simplicity instead of the princi- 
 ple of parsimony we have prodigality of re- 
 lation and adaptation, and this apparently 
 for the sole purpose of enabling us to see 
 things robed in the splendor of color. Would 
 it not seem that Nature harbored the inten- 
 tion <<if educating us for other enjoyments 
 than those derivable from meat and drink ? 
 At all events, whatever Nature meant and 
 k would, be. mere ..pres. mption to dogmatize 
 
 as to what she meant we find ourselves here 
 as the issue and upshot of her operations, en- 
 dowed with capacities to enjov not only the 
 materially useful, but endowed with others of 
 indefinite scope and application, which deal 
 alone with the beautiful and the true. 
 
 LECTURE II. 
 
 Origin of Physical Theories : Scope of the Imagina- 
 tion : Newton and the Emission Theory: Verifica- 
 tion of Physical Theories: The Luminiferous ; 
 Ether: Wave-Theory of Light: Thomas Young: 
 Fresnel and Arago : Conceptions of Wave-Motion: 
 Interference of Wavts; Constitution of Sound- 
 Waves: Analogies of Sound and Light: Illustra- 
 tions of Wave-Motion: Interference of Sound- 
 Waves : Optical Illustrations: Pitch and Color: 
 Lengths of the Waves of Light and Kates of 
 Vibration of the Ether- Particles: Interference of 
 Light : Phenomena which first suggested the Un- 
 dulatory Theory: Hooke and the .Colors of Thin 
 Plates: The Soip-Bubble: Newton's Rings: 
 Theory of lv Fits: " Its Explanation of the Kings : 
 Overthrow of the Tneory : Colors of Mother-uf- 
 Pearl. 
 
 WE might vary and extend our experi- 
 ments on light indefinitely, and they cer- 
 tainly would prove us to possess a wonderful 
 mastery over the phenomena. But the vts- 
 ture of the agent only would thus be re- 
 vealed, not the agent itself. The human 
 mind, however, is so constituted and so edu- 
 cated as regards natural things, that it can 
 never rest satisfied with this outward view of 
 them. Brightness and freshness take pos- 
 session of the mind when it is crossed by the 
 light of principles, which show the facts of 
 Nature to be organically connected. 
 
 Let us, then, inquire what this thing is 
 that we have been generating, reflecting, re- 
 fracting, and analyzing. 
 
 In doing this, we shall learn that the life 
 of the experimental philosopher is twolold. 
 He lives, in his vocation, a life of the senses, 
 using his hands, eyes, and ears in his experi- 
 ments, but such a question as that now before 
 us carries him beyond the margin of the 
 senses. He cannot consider, much less an- 
 swer, the question, "What is light?" with- 
 out transporting himself to a world which 
 undelies the sensible one, and out of wnich, 
 in accordance with rigid law, all optical phe- 
 nomena spring. To realize this subsensible 
 world, if 1 may use the term, the mit..d must 
 possess a certain pictorial power. It has to 
 visualize the invisible. It must be able to 
 form definite images of the things which that 
 subsensib'e world contains ; and to say that, 
 if such or such a state of things exist in that 
 world, then the phenomena which appear in 
 ours must, of necessity, grow out of this 
 state of things. If the picture be correct, 
 the phenomena are accounted for ; a physical 
 theory has been enunciated which unites and 
 explains them all. 
 
 This conception of physical theory implies, 
 as you perceive, the exercise of the imagina- 
 tion. Do not be afraid of this word, which 
 seems to rentier so many respectable people, 
 
SIX LECTURES ON LIGHT. 
 
 both m the ranks of science and out of them, of elastic collision. The fact of optical re- 
 
 uncomfortable. That men in the ranks of 
 science should feel thus is, I think, a proof 
 
 flection certainly occurred as if light consist- 
 ed of elastic particles, and this was Newton's 
 
 that they have suffered themselves to be mis- ( sole justification for introducing them, 
 led by the popular definition of a great ( But this is not all. In another important 
 faculty instead of observing its operation in | particular, also. Newton's conceptions re- 
 their own minds. Without imagination we garding the nature of light were influenced 
 
 by his previous knowledge. He had been 
 working at the phenomena of eravitation. 
 and had made himself at home amid the oper- 
 ations of this universal power. Perhaps hrs 
 mind at this time was too freshly and too 
 deeply imbued with these notions to permit 
 of his forming an unfettered judgment re- 
 
 imagmation 
 
 cannot take a step beyond the bourne of the 
 mere animal world, perhaps not even to the 
 edge of this. But, in speaking thus of 
 imagination, I do not mean a riotous power 
 which deals capriciously with facts, but a 
 well-ordered and disciplined power, whose 
 sole function is to form conceptions which 
 the intellect imperatively demands. Imagina- 
 tion thus exercised never really severs itself 
 from the world of fact. This is the store- 
 house from which all its pictures are drawn ; 
 and the magic of its art consists, not in 
 creating things anew, but in so changing the 
 magnitude, position, and other relations of 
 sensible things, as to render them fit for the 
 Requirements of the intellect in the subsen- 
 uible world.* 
 
 I will take, as an illustration of this sub- 
 ject, the case of Newton. Before he began 
 to deal with light, he was intimately ac- 
 quainted with the laws of elastic collision, 
 which all of you have seen more or less per- 
 fectly illustrated on a billiard-table. As re- 
 gards the collision of sens : ble masses, New- 
 ton knew the angle of incidence to be equal 
 to the angle of reflection, and he also knew 
 that experiment, as shown in our last lecture, 
 had established the same law with regard to 
 light. He thus found in his previous knowl- 
 edge the mateiial for theoretic images. He 
 had only to change the magnitude of concep- 
 tions already in his mind to arrive at the 
 Emission Theory of Light. He supposed 
 light to consist of elastic particles of incon- 
 ceivable minuteness shot out with inconceiv- 
 able rapidity by luminous bodies. Such par- 
 ticles impinging upon smooth surfaces were 
 reflected in accordance with the ordinary law 
 
 * The following charming extract, bearing upon 
 this point, was discovered and written out for me by 
 my friend, Dr. Bence Jones, Hon. Secretary to the 
 Royal Institution 
 
 " In every kind of magnitude there is a degree or 
 'sort to which our sense is proportioned, the percep- 
 tion and knowledge cf which is of the greatest use to 
 mankind. The same is the groundwork of philoso- 
 phy : for, though all sorts and degrees are equally 
 the object of philosophical speculation, yet it is from 
 those which are proportioned to sense that a philoso- 
 pher must set out in his inquiries, ascending or de- 
 scending afterwards, as his pursuits may require. He 
 
 garding the nature of light. Be that 
 as it may, Newton saw in refraction 
 the action of an attractive force exerted 
 on the light-particles. He carried his 
 conception out with the most severe con- 
 sistency. Dropping vertically downwards 
 towards the earth's surface, the motion of a 
 body is accelerated as it approaches the earth. 
 Dropping in the same manner downwards on 
 a horizontal surface, say through air on glass 
 or water, the velocity of the light-particles, 
 when they come close to th2 surface, was, 
 according to Newton, also accelerated. Ap- 
 proaching such a surface obliquely, he sup- 
 posed the particles, when close to it, to be 
 drawn down upon it, as a projectile is 
 drawn by gravity to the surface of the earth. 
 This deflection was, according to Newton, 
 the refraction seen in our last lecture. Final- 
 ly, it was supposed that differences of color 
 might be due to differences in the sizes of the 
 particles. This was the physical theory of 
 light enunciated and defended by Newton; 
 and you will observe that it simply consists 
 in the transference of conceptions born in the 
 world of the senses to a substnsible world. 
 
 But, though the region of physical theory 
 lies thus behind the world of senses, the veri- 
 fications of theory occur in that world. Lay- 
 ing the theoretic conception at the root of 
 matters, we determine by rigid deduction 
 what are the phenomena which must of neces- 
 sity grow out of this root. If the phenomena 
 thus deduced agree with those of the actual 
 world, it is a presumption in favor of the 
 theory. If as new classes of phenomena arise 
 they also are found to harmonize with theo- 
 retic deduction, the presumption becomes still 
 stronger. If, finally, the theory confers pro- 
 phetic vision upon the investigator, enabling 
 him to predict the existence of phenomena 
 which have never yet been seen, and if those 
 
 does well indeed to take his views from many points | predictions be found on trial to be rigidly cor- 
 rect, the persuasion of the truth of the theory 
 becomes overpowering. Thus working back- 
 wards from a limited number of phenomena, 
 genius, by its own expansive force, reaches a 
 conception which covers all the phenomena. 
 There is no more wonderful performance of 
 the intellect than this. And we can render 
 no account of ii. Like the scriptural gift of 
 the Spirit, no man can tell whence it cometh. 
 The passage from fact to principle is some- 
 
 of sight, and supply the defects of sense by a well- 
 regulated imagination ; nor is he to be confined by 
 any limit in space or time ; but, as his knowledge of 
 Nature is founded on the observation of sensible 
 things, he must begin with these, and must often re- 
 turn to them to examine his progress by them. 
 Here is his secure hold ; and as he sets out from 
 thence, so if he likewise trace not often his steps 
 backwards with caution, he will be m hazard of losing 
 ki way in the labyrinths of Nature." (Maclaurin : 
 /in Account of Sir I. Neivtoris Philosophical Dis- 
 coveries. Written 1728 ; sttcond edition, 1750 ; pp. 
 
10 
 
 SIX LECTURES ON LIGHT. 
 
 times slow, sometimes rapid, and at all times 
 a source of intellectual joy. When rapid, the 
 pleasure is concentrated and becomes a kind 
 of ecstasy or intoxication. To any one who 
 has experienced this pleasure, even in a mod- 
 erate degree, the action of Archimedes when 
 he quitted the bath, and ran naked, crying 
 " Eureka! " through the streets of Syracuse, 
 becomes intelligible. 
 
 How, then, did it fare with the theory cf 
 Newton, when the deductions from it were 
 brought face to face with natural phenomena ? 
 To the mind's eye, Newton's elastic particles 
 present themselves like particles of sensible 
 magnitude. The same reasoning applies to 
 both ; the same experimental checks exist for 
 both. Tested by experiment, then, Newton's 
 theory was found competent to explain many 
 facts, and with transcendent ingenuity its 
 author sought to make it account for all. He 
 so far succeeded, that men so celebrated as 
 Laplace and Malus, who lived till 1812, and 
 13iot and Brewster, who lived till our own 
 lime, were found among his disciples. 
 
 Still, even at an early period of the existence 
 of the Emission Theory, one or two great 
 names were found recording a protest against 
 it ; and they furnish another illustration of 
 the law that, in forming theories, the scientific 
 imagination must draw its materials from the 
 world of fact and experience. It was known 
 long ago that ; ound is conveyed in waves or 
 puses through the air ; and no sooner was this 
 truth well housed in the mind than it was trans- 
 formed into a theoretic conception. It was 
 supposed that light, like sound, might also be 
 the product of wave-motion. But what, in 
 this case, could be the material forming the 
 waves ? For the waves of sound we have the 
 air cf our atmospheie ; but the stretch of im- 
 agination which filled all space with a luminif- 
 erous ether trembling with the waves of light 
 was so bold as to shock cautious minds. In 
 one of my latest conversations with Sir David 
 Brewster he said to me that his chief objection 
 to the undulatory theory of light was that he 
 could not think the Creator guilty of so clumsy 
 a contrivance as the filling of space with ether 
 in order to produce light. This, I may say, 
 is very dangerous ground, and the quarrel of 
 science with Sir David, on this point, as with 
 many other persons on other points, is, that 
 they profess to know too much about the 
 mind of the Creator. 
 
 This conception of an ether was advocated 
 and indeed applied to various phenomena of 
 optics by the celebrated astronomer, Huy- 
 gh^ns. It was espoused and defended by the 
 celebrated mathematician, Euler. They were, 
 however, opposed by Newton, whose authority 
 at -lie time bore them down. Or shall I say 
 it was authority merely ? Not quite so. 
 Newton's preponderance was in some degree 
 due to the fact that, though IIuyghe::s and 
 Euler were right in the main, t..ey did not 
 oossess sufficient data to prove themselves 
 right. No human authority, however high, 
 
 can maintain itself against the voice of Nature 
 speaking through experiment. But the voice 
 of Nature may be an uncertain voice, through 
 the scantiness of data. This was the case at 
 the period now referred to, and at such a pe- 
 riod by the authority of Newton all antago- 
 nists were naturally overborne. 
 
 Still, this great Emission Theory, which 
 held its ground so long, resembled one of 
 those circles which, according to your coun- 
 tryman Emerson, the force of genius periodi- 
 cally draws round the operations of the in- 
 tellect, but which are eventually broken 
 through by p es^ure from behind. In the 
 year 1773 was born, at Milverton, in Somer- 
 setshire, one of the most remarkable men that 
 England ever produced. He was educated 
 for the profession of a physician, but was too 
 strong to be tied down to professional routine. 
 He devoted himself to the study of natural 
 philosophy, and became in all its departments 
 a master. He was also a master of letters. 
 Languages, ancient and modern, were housed 
 within his brain, and, to use the words of his 
 epitaph, "he first penetrated the obscurity 
 which had veiled for ages the hieroglyphics of 
 Egypt." It fell to the lot of this man to dis- 
 cover facts in optics which Newton's theory 
 was incompetent to explain, and his mind 
 roamed i* search of a sufficient theory. He 
 had made himself acquainted with all the 
 phenomena of wave-moti ,n ; with all the 
 phenomena of sound ; working successfully 
 in this domain as an original discoverer. 
 Thus informed and disciplined, he was pre- 
 pared to detect any resemblance which might 
 reveal itself between the phenomena of light 
 and those of wave-motion. Such resem- 
 blances he did detect ; and, spurred on by 
 the discovery, he pursued his speculations and 
 his experiments, until he finally succeeded in 
 placing on an immovable basis the Undulatory 
 Theory of Light. 
 
 The founder of this great theory "yas 
 Thomas Young, a name, perhaps, unfamiliar 
 to many of you. Permit me, by a kind of 
 geometrical construction which I once em- 
 ployed in London, to give you a notion of the 
 magnitude of this man. Let Newton stand 
 erect in his age, and Young in his. Draw a 
 straight line from Newton to Youn^,, which 
 shall form a tangent to the heads of both. 
 This line would slope downwards from New- 
 ton to Young, because Newton was certainly 
 the taller man of the two. But the slope 
 would not be steep, for the difference of stat- 
 ure was not excessive. The line would form 
 what engineers call a gentle gradient from 
 Newton to Young. Place underneath this 
 line the biggest man born in the interval 
 between both. Pie would not, in my opinion, 
 reach the line ; for if he did he would be 
 taller intellectually than Young, and there 
 was, I believe, none taller. But I do not 
 want you to rest on English estimates of 
 Young; the German, Helmholtz, a kindred 
 jenius, thus speaks of him : " His was one 
 
SIX LECTURES ON LIGHT. 
 
 of the most profound minds that the world j which ?.t any moment constitute the wave, 
 
 Stand upon the sea-shore and observe ihe 
 
 has ever seen ; but he had the misfortune to 
 be too much in advance of his age. He ex- 
 cited the wonder of his contemporaries, who, 
 however, were unab'e to follow him to the 
 heights at which his daring intellect was 
 
 accustomed to soar. His most important j that every particle of water along the front 
 ide s lay, therefore, buried and forgotten in of the wave is in the act of rising, while 
 'the folios of the Royal Society, until a new every particle along its back is in the act of 
 generation gradually and painfully made t^e | sinking. The particles in front reach in suc- 
 same discoveries, and proved the exactness j cession the crest of the wave, and as soon as 
 
 advancing rollers before they are distorted by 
 the friction of the bottom. Every wave has 
 a back and a front, and, if you clearly seiae 
 the image of the moving wave, you will see 
 
 of his assertions and the truth of his demon- 
 strations." 
 
 It is quite true, as Helmholtz says, that 
 Ar oung was in advance of his age ; but some- 
 thing is to be added which illustrates the 
 responsibility of our public writers. Foi 
 twenty years this man of genius was quenched 
 hidden from the appreciative intellect of 
 his countrymen deemed in fact a creamer, 
 liirough the vigorous audacity of a writer 
 who had then possession of the public ear, 
 and who in the Edinburgh Review pourtd 
 ridicule upon Young and his speculations. 
 To the celebrated Frenchmen, Fresnel and 
 Arago, lie was first indebted for the restitu- 
 tion of hu rights, for they, especially Fresnel, 
 remade independently, as Helmholtz says, 
 and vastly extended l.is discoveries. To the 
 
 the crest is passed they begin to fall. They 
 then reac i the furrow or sinus of the wave, 
 and can sink no farther. Immediately after- 
 wards they become the front of the succeed- 
 ing wave, rise again until they reach the 
 crest, and then sink as before. Thus, while 
 the waves pass onward horizontally, the 
 individual particles are simply lifted up and 
 down vertically. Observe a sea-fow], or, if 
 you are a swimmer, abandon yourself to the 
 action cf the waves ; you are not carried for- 
 ward, but simply rocked up and down. The 
 propagation of a wave is the propagation of 
 a form, and not the transference of the sub- 
 stance which constitutes the wave. 
 
 The length of the wave is the d stance 
 from crest to crest, while the distance through 
 which the individual particles oscillate is 
 
 students cf his works Young has long since j called the amplitude of the oscillation. You 
 appeared in his true light, but these twenty will notice that in this description the parti- 
 cles of water are made to vibrate across the 
 line of propagation.* 
 
 blank years pushed him from the public 
 mind, which became in turn filled with the 
 fame of Young's colleague at the Royal In 
 stitution, Davy, and afterwards with the 
 fame of Faraday. Carlyle refers to the re- 
 mark of Novalis, that a man's self-trust is 
 enormously increased the moment he finds 
 that others believe him. If the opposite 
 remark be true if it be a fact that public 
 disbelief weakens a man's force there is no 
 calculating the amount of damage these 
 twenty years of neglect may have done to 
 Young's productiveness as an investigator. 
 It remains to be s:ated thai his assailant was 
 Mr. Henry Brougham, afterwards Lord 
 Chancellor of England. 
 
 Our hardest woik is now before us. And, 
 as I have often had occasion to notice that 
 capacity for hard work depends in a great 
 measure on the antecedent winding up of the 
 will and determination, I would call upon 
 you to gird up your loins for our coming 
 labors. If we succeed in climbing the hill 
 which faces us to-night, our future efforts 
 will be comparatively light. 
 
 In the earliest writings of the ancients we 
 find the notion that sound is conveyed by the 
 air. Aristotle gives expression to this no- 
 tion, and the great architect Vitruvius com- 
 pares the waves of sound to waves of water. 
 But the real mechanism of wave-motion was i 
 hidden from the ancitnts, and indeed was 
 not mad- clear until the time of Newton. 
 The a ntral difficulty of the subject was, to 
 distinguish between the motion of the wave 
 itself and the motion of the particles 
 
 And now we have to take a step forward, 
 and it is the most important, step of ail. You 
 can picture two serie: of waves proceeding 
 from different origins through the same 
 water. When, for example, you throw two 
 stones into still water, the ring- waves pro- 
 ceeding from the two centres of disturbance 
 intersect each other. Now, no mailer how 
 numerous these waves may be. the law holds 
 good that the mo lion of every particle of the 
 water is the algebraic sum of a.l the motions 
 imparted to it. If crest coincide with crest, 
 the wave is lifted to a double height; if fur- 
 row coincide with crest, the motions are in 
 opposition, and thei. sum ij zero. We have 
 then still water, which we shall learn pres- 
 ently corresponds to what we call darkness in 
 
 reference to our present subjs 
 
 This action 
 
 of wave upon wave is technically called in- 
 terference, a term to be remembered. 
 
 Thomas Young's fundamental discovery in 
 optics was that the principle of Interference 
 applied to light. Long prior to his timCj an 
 Italian philosopher. Giimaldi, had stated 
 that, under certain circumstances, two thin 
 beams of light, each of which, acting singly, 
 produced a luminous spot upon a wnite wall, 
 when caused to act together, partially 
 
 * I do not wish to encumber the conception here 
 with the details of the motion, but I inay draw atten- 
 tion to the beautiful model of Professor Lyinao, 
 wherein waves are shown 10 be produc.ed by tho cir- 
 cular motion if the -particles. This, as uroved by 
 the brothers Weber, is the real motley .: :~e case uf 
 water-waves. 
 
12 
 
 SIX LECTURES ON LIGHT. 
 
 quenched each other and darkened the spot. 
 This was a statement of fundamental signifi- 
 cance, but it required the discoveries and the 
 genius of Young to give it meaning. How 
 he did so, I will now try to make clear to 
 you. You know that air is compressible ; 
 that by pressure it can be rendered more 
 dense, and that by dilatation it can be ren- 
 dered more rare. Properly agitated, a tun- 
 ing-fork now sounds in a manner audible to 
 you all, and most of you know that the air 
 through which the sound is passing is par- 
 celled Out into spaces in which the air is con- 
 densed, followed by other spaces in which 
 the air is rarefied. These condensations and 
 rarefactions constitute what we call waves of 
 sound. You can imagine the air of a room 
 traversed by u series of such waves, and you 
 can imagine a second series sent through the 
 same air, and so related to the first that con- 
 densation coincides with condensation and 
 rarefaction with rarefaction. The conse- 
 quence of this coincidence would be a louder 
 sound than that produced by either system of 
 waves taken singly. But you can also ima- 
 gine a state of things where the condensa- 
 tions of the one system fall upon the rarefac- 
 tions of the other system. In this case th: 
 two systems would completely neutralize each 
 other. Each of them, taken singly, produces 
 sound; both of them, taken together, pro- 
 duce no sound. Thus, by adding sound to 
 sound we produce silence, as Grimaldi in his 
 experiment produced darkness by adding 
 light to light. 
 
 The analogy between sound and light here 
 at once flashes upon the mind. Young gen 
 eralized this observation. He discovered a 
 multitude of similar cases, and determined 
 their precise conditions. On the assumption 
 that light was wave-motion, all his experi- 
 ments on interference were explained ; on the 
 assumption that light wai flying particles, 
 nothing was explained. In the time of Huy- 
 ghens and Euler a medium had been assumed 
 tor the transmission of the waves of light ; 
 but Newton raised the objection that, if light 
 consisted of the waves of such a medium, ' 
 shadows could not exist. The waves, he 
 contended, would bend round opaque bodies 
 and produce the motion of light behind them, 
 as sound turns a corner, or as waves of water 
 wash round a rock. It was proved that the 
 bending round referred to by Newton actually 
 occurs, but that the inflected waves abolish 
 each other by their mutual interference. 
 Young also discerned a fundamental differ- 
 ence between ti'.e waves of light and those of 
 sound. Could you see the air through which 
 sound-waves are passing, you would observe 
 every individual particle of air oscillating to 
 fcnd fro in the direction of propagation. 
 Could you see the ether, you would also find 
 every individual particle making a small ex- 
 cursion to and fro, but here the motion, like 
 that assigned to the water-particles above re- 
 ferred to, would be across the line of propa- 
 
 gation. The vibrations of the air are longi- 
 tudinal, the vibrations of the ether are trans- 
 versal. 
 
 It is my desire that you should realize with 
 clearness the character of wave-motion, both 
 in ether and in air. And, with this view. I 
 bring bcfcre you an experiment wherein the 
 air-particles are represented by small spots of 
 light. They are parts of a spiral, drawn upon 
 a circle of blackened glass, and, when the cir- 
 cle rotates, the spots move in successive pulses 
 over the screen. You have here clearly set 
 before you how the pulses travel incessantly 
 forward, while the particles that compose 
 them perform oscillations to and fro. This 
 is the picture of a sound-wave, in which the 
 vibrations are longitudinal, By another glass 
 wheel, we produce an image of a trans- 
 verse wave, and here we observe the waves 
 travelling in succession over the screen, while 
 each individual spot of light performs an ex- 
 cursion to and fro across the line of propaga- 
 tion. 
 
 Notice what follows when the glass wheel 
 is turned very quickly. Objectively consid- 
 ered, the transverse waves propagate them- 
 selves as before, but subjectively the effect is 
 totally changed. Because of the retention of 
 impressions upon the retina, the spots of light 
 simply describe a series of parallel luminous 
 lines upon the screen, the length of these 
 lines marking the amplitude of the vibration. 
 The impression of wave-motion has totally 
 disappeared. 
 
 The most familiar illustration of the inter- 
 ference of sound-waves is furnished by the 
 beats produced by two musical sounds slightly 
 out of unison. These two tuning-forks are 
 now in perfect unison, and when they are 
 agitated together the two sounds flow without 
 roughness, as if they were but one. But, by 
 attaching to one of the forks a two-cent piece, 
 we cause it to vibrate a little more slowly 
 than its neighbor. Suppose that one of them 
 performs 101 vibrations in the time required 
 by the other to perform 100, and suppose 
 that at starting the condensations and rare- 
 factions of both forks coincide. At the loist 
 vibration of the quickest fork they will again 
 coincide, the quicker fork at this point hav- 
 ing gained one whole vibration, or one whole 
 wave upon the other. But a little reflection 
 will make it clear that, at the 5oth vibration, 
 the two forks are in opposition; here the one 
 tends to produce a condensation where the 
 other tends to produce a rarefaction; by the 
 united action of the two forks, therefore, the 
 sound is quenched, and we have a pause of 
 silence. This occurs where one fork has 
 gained half a tuave-/cngl/i upon the other. 
 At the loist vibration we have again coinci 
 dence, and, therefore, augmented sound; at 
 the isoth vibration we have again a quench- 
 ing of the sound. Here the one fork is three 
 half-waves in advance of the other In gen- 
 eral terms, the waves conspire when the one 
 series is an even number of half-wave lengths, 
 
SIX LECTURES ON LIGHT. 
 
 13 
 
 and they are destroyed when the one series is band of light gradually shortening as th 
 
 an odd number of half-wave lengths in ad- tion subsides, until, when the motion ceases, 
 
 vance of the other. With two forks so cir- we hare our luminous disk restored. Weight- 
 
 cumstanced, we obtain those intermittent ing one of the forks as we did before, with a 
 
 shocks of sound separated by pauses of si- 
 lence, to which we give the name of beats. 
 
 I new wish to show you what may be 
 called the optical expression of those beats. 
 Attached to a large tuning-fork, F (Fig. 2), 
 is a small mirror, which shares the vibrations 
 of the fork, and on to the mirror is thrown a 
 thin beam of light, which shares the vibra- 
 tions of the mirror. The beam reflected 
 from the fork is received upon a piece of 
 looking-glass, and thrown back upon the 
 screen, where it stamps itself as a small lu- 
 minous disk. The agitation of the fork by a 
 
 two-cent piece, sometimes the fork; conspire, 
 and then you have the band of light drawn 
 out to its maximum length ; sometimes the? 
 oppose each other, and then you have tha 
 band of light diminished to a circle. Thus, 
 the beats which address the ear express them- 
 selves optically as the alternate elongation and 
 shortening of the band of light. If I move 
 the mirror of this second fork, you have a 
 sinuous line, as before ; but the sinuosities 
 are sometimes deep, and sometimes they al- 
 most disappear, as in Fig. 3, thus expressing 
 the alternate increase and diminution of the 
 
 violin-bow converts that disk into a band of sound, the intensity of which is expressed by 
 light, and if yoi simp:y move your heads to j the depth of the sinuosities. To Lissajous ws 
 
 and fro you cause the image of the band to 
 sweep over the retina, drawing it out to a sin- 
 uous line, thus proving the periodic character 
 of the motion which produces it. By a sweep 
 of the looking-glass, we can also cover the 
 screen from side to side by a luminous scroll, 
 m n, Fig. 2, the depth of the sinuosities indi- 
 cating the amplitude of the vibration. 
 
 Instead of receiving the beam reflected from 
 the fork on a piece of looking-glass, we now 
 receive it upon a second mirror attached t:> a 
 second fork, and cast by it upon the screer. 
 Both forks now act in combination upon the 
 beam. The disk is drawn out, as before, the 
 
 ou e this mode of illustration. 
 
 The pitch of a sound is wholl / ^e 
 by the rapidity of the vibration, ay tLie ,nten- 
 sity'is by the amplitude. The us*: of pitch 
 with the rapidity of the impulses may be illus- 
 trated by the syren, which consists of a per- 
 forated disk rotating over a cylinder into 
 which air is forced, and the end of which is 
 also perforated. When the perforations o 
 the disk coincide with those of the cylinder, a 
 puff escapes ; and, when the puffs succeed 
 each other with sufficient rapidity, the im- 
 | pressions upon the auditory nerve link them- 
 selves together to a continuous musical note. 
 | The more r"apid the rotation of the disk the 
 j quicker is the succession of the impulses, and 
 ' the higher the pitch of the note. Indeed, by 
 
SIX LECTURES ON LIGHT. 
 
 ireans cf the syren the number of vibrations 
 due to any musical no'e, whether it be that of 
 an instrument, of the human voice, or of a 
 flying insect, may be accurately determined. 
 
 In front of our lamp now stands a very 
 homely instrument, S, Fi<?-. 4, of this charac- 
 ter. The perforated disk is turned by a 
 wheel and band, and, when the two sets of 
 perforations coincide, a series of spots of 
 light, sharply denned by the lens L, ranged 
 on the circumference of a circle, is seen upon 
 the screen. On slowly turning the disk, a 
 flicker is produced by the alternate stoppage 
 and transmission of the light. At the same 
 time air is urged into the syren, and you hear 
 a fluttering sound corresponding to the flick- 
 ering light. But, by augmenting the rapid- 
 ity of rotation, the light, though intercepted 
 as before, appears perfectly steady, through 
 the persistence of impressions upon the 
 retina ; and, about the time when the optical 
 impression becomes continuous, the auditory 
 impression becomes equally so ; the puffs 
 from the syren linking themselves then to- 
 gether to a continuous musical note, which 
 rises in pitch with the rapidity of the rota- 
 lion. A movement of the head causes the 
 image of the spots to sweep over the retina, 
 producing beaded lines : the same effect is 
 produced upon our screen by the sweep of a 
 looking glass which has received the thin 
 beams from the syren. 
 
 In the undulatory theory, what pitch is to 
 the ear, cclor is to the eye. Though never 
 seen, the lengths of the waves of light have 
 been determined. Their existence is proved 
 .by their effects, and from their effects also 
 their lengths may be accurately deduced. 
 This may, moreover, be done in many ways, 
 ami, when the different determinations are 
 
 compared, the strictest harmony is found to 
 exist between them. The shortest waves- of 
 the visible spectrum are those of the extreme 
 violet ; the longest, those of the extreme 
 red ; while the other colors are of intermedi- 
 ate pitch or wave-length. The length of a 
 wave of the extreme red is such that it would 
 require 36,918 of them placed end to end to 
 cover one inch, while 64,631 of the extreme 
 violet waves would be required to span the 
 same distance. 
 
 Now, the velocity of light, in round num- 
 bers, is 190,000 miles per second. Reducing 
 this to inches, and multiplying the number 
 ihus found by 36,918, we obtain the number 
 of waves of the extreme red in 190,000 miles. 
 All these "waves enter the eye, and hit the 
 retina fit the back of the eye in one second. 
 The number of shocks per second necessary 
 to the production of the impression of red is, 
 therefore, four hundred and fifty-one millions 
 of millions. In a similar manner, it may be 
 found that the number of shocks correspond- 
 ing to the impression of violet is seven hun- 
 dred and eighty-nine millions of millions. 
 All space is rilled with matter oscillating at 
 such rates. From every star waves of these 
 dimensions move with the velocity of light 
 like spherical shells outwards. And in the 
 ether, just as in the water, the motion of 
 cveiy particle ij the algebraic sum cf all the 
 separate motions imparted to it. Still, one 
 motion does not blot the other out ; or, if 
 extinction occur at one point, it is atoned for 
 at some other point. Every star declares by 
 its light its undamaged individuality, as if it 
 alone had sent its thrills through space. 
 
 The principle of interference applies to the 
 waves of light as it does to the waves of 
 water and the waves of sound. And the 
 condi ions of interference are the same in all 
 three. If two series of light-waves of the 
 same length start at the same moment from 
 a common origin, crest coincides with crest, 
 sinus with sinus, and the two systems blend 
 together to a single system of double ampli- 
 tude. If both series start at the same mo- 
 ment, one of them being, at starting, a whole 
 wave-length in advance of the other, they 
 also add themselves together, and we have 
 an augmented luminous effect. Just as in 
 the case of sound, the same occurs when the 
 one system of waves is any even number of 
 semi-undulations in advance of the other. 
 But if the one system be half a wave-length, 
 or any odd number of half wave-lengths in 
 advance, then the crests of the one fail upon 
 the sinuses of the other ; the one system, in 
 fact, tends to lift the particles of ether at the 
 precise places where the other tends to depress 
 them ; hence, through their joint action the 
 ether remains perfectly stiil This stillness 
 of the ether is what we call darkness, which 
 corresponds, as already stated, with a dead 
 level in the case of water. 
 
 It was said in our first lecture, with refer- 
 ence to the colors produced by absorption, 
 
SIX LECTURES ON LIGHT. 
 
 that the function of natural bodies is selec- 
 tive, not creative ; that they extinguish cer- 
 tain constituents of the \vhite solar light, and 
 appear in the colors of the unextinguished 
 iight. It must atonce flash upon your minds 
 that, inasmuch as we have in interference an 
 agency by which light may be self-extin- 
 quished, we may have in it the conditions 
 for the production of color. But this would 
 imply that certain constituents are quenched 
 by interference, while others are permitted to 
 remain. This is the fact ; and it is entirely 
 due to the difference in the lengths of the 
 waves of light. 
 
 The subject is most easily illustrated by the 
 class of phenomena which (irst suggested the 
 undulatory theory to the mind of Hooke. 
 These are the colors of thin films of all kinds, 
 which are known as the colors of thin plates. 
 In this relation no object in the world pos- 
 sesses a deeper scientific interest than a com- 
 mon soap-bubble. And here let me say 
 emerges one of the difficulties which the stu- 
 dent of pure science encounters in the pres- 
 ence of "practical" communities like those 
 of America and England ; it is not to be ex- 
 pected that such communities can entertain 
 any profound sympathy with labors which 
 seem so far removed from the domain of 
 practice as many of the labors of the man of 
 science are. Imagine Dr. Draper spending 
 his days in blowing soap-bubbles and in 
 studying their colors ! Would you show him 
 the necessary patience, or grant him the nec- 
 essary support ? And yet, be it remembered, 
 it was thus that Newton spe::t a large portion 
 of his time ; and that on such experiments 
 has been founded a theory, the issues of 
 which are incalculable. I see no other way 
 for you laymen than to trust the scientific man 
 with the choice of his inquiries ; he stands 
 before the tribunal of his peers, and by their 
 verdict on his labors you ought to abide. 
 
 Whence, then, are derived the colors of the 
 soap-bubble ? Imagine abeam of white l : ght 
 impinging on the bubble. When it reaches 
 the first surface of the film, a known fraction 
 of the light is reflected back. But a large 
 portion of the beam enters the film, reaches 
 its second surface, and is again in part re- 
 flected. The waves from the second surface 
 thus turn back and hotly pursue the waves 
 from the first surface. And, if the thickness 
 of the film be such as to cause the necessary 
 retardation, the two systems of waves inter- 
 fere with each other, producing augmented 
 or diminished light, as the case may be. But, 
 inasmuch as the waves of light are of different 
 lengths, it is plain that, to produce self-ex- 
 tinction in the case of the longer waves, a 
 greater thickness of film is necessary than in 
 the case of the shorter ones. Different colors, 
 therefore, appear at different thicknesses of 
 the film. 
 
 Take with you a little bottle of spirit of 
 turpentine, and pour it into one of the ponds 
 in the Central Park. You will then see the 
 
 flashing of those colors over the surface of 
 the wa^er. On a small scale we produce them 
 thus : A common tea-tray is filled with water, 
 beneath the surface of which dips the end of 
 a pipette. A beam of light falls upon the 
 water, and is reflected by it to the screen. 
 Spirit of turpentine is poured into the pipette; 
 it descends, issues from the end in minute 
 drops, which lise in - uccession to the surface. 
 i On reaching it, each drop spreads suddenly 
 j out as a film, and glowing colors immediately 
 j flash forth upon the screen. The colors 
 change as the thickness of the film changes 
 by evaporation. They are also arranged in 
 zones in consequence of the gradual diminu- 
 tion of thickness from the centre outwards. 
 
 Any film whatever will produce these colors. 
 The film of air between two plates of window- 
 glass, squeezed together, exhibits rich fringes 
 of color. Nor is even air necessary ; the 
 mere rupture of optical continuity suffices. 
 Smite with an axe the black, transparent ice 
 black, because it is transparent and of great 
 depth under the moraine of a glacier ; you 
 readily produce in the interior flaws which no 
 air can reach, and from these flaws the colors 
 of thin plates sometimes break like fire. The 
 colors are commonly seen in flawed crystals ; 
 they are also formed by the film of oxide 
 which collects upon molten lead. It is the 
 colors of thin plates that guide the tempering 
 of steel. But the origin of most historic in- 
 terest is, ns already stated, the soap-bubble. 
 With one of those mixtures employed by the 
 eminent blind philosopher Plateau in his re- 
 searches on the cohesion figures of thin films, 
 \\e obtain in still airabv.bblc twelve or fifteen 
 inches in diameter. You -may look at the 
 bubble itself, or you may look at its projec- 
 tion upon the screen, rich colors arranged in 
 zones are, in both cases, exhibited. Render- 
 ing the beam parallel, and permitting it to 
 impinge upon the sides, bottom, and top of 
 the bubble, gorgeous fans of color overspread 
 the screen, which rotate as the beam is carried 
 round the circumference of the bubble. By 
 this experiment the internal motions of the 
 film are also strikingly displayed. 
 
 Newton sought to measure the thickness of 
 the bubble corresponding to each of these 
 colors ; in fact, he sought to determine gen- 
 erally the relation of color to thickness. His 
 first care was to obtain a film of variable and 
 calculable depth. On a plano-convex glass 
 lens of very feeble curvature he laid a plate of 
 glass with a plane surface, thus obtaining a 
 him of air of gradually increasing depth from 
 the point of contact outwards. On looking at 
 the film in monochromatic light he saw su:- 
 roundingthe place of contact a series of bright 
 rings separated from each other by dark ones, 
 and becoming more closely packed together as 
 the distance from the point of conta t aug- 
 mented. When he employed red light, his 
 rings had certain diameters ; when he em- 
 ployed blue light, the diameters were less. 
 Causing his glasses to pass through the spev/ 
 
SIX LECTURES ON LIGHT- 
 
 trurn from red to blue, the rings contracted ; 
 when the passage was from blue to red, the 
 rings expanded. When white light fell upon 
 the glasses, inasmuch as the colors were not 
 superposed, a series of iris-colored circles were 
 obtained. They became paler as the film be- 
 came thicker, until finally ihe colors became 
 so intimately reblended as to produce white 
 light. A magnified image of Newton's rings 
 is now before you, and, by employing in suc- 
 cession red, blue, and white light, we obtain 
 all the effects observed by Newton. 
 
 He compared the tints thus obtained v ith 
 the tints of the sda--bubble, and he calcu- 
 lated the corresponding thickness. How he 
 did this may be thus made plain to you : 
 Suppose the' water of the ocean to be abso- 
 lutely smooth ; it would then accurately repre- 
 sent the earth's curved surface. Let a per- 
 fectly horizontal plane touch the surface at 
 any point. Knowing the earth's diameter, 
 any engineer or mathematician in this room 
 could tell you how far the sea's surface will 
 lie below this plane, at the distance of a yard, 
 ten yards, a hundred yards, or a thousand 
 yards from the point of contact of the plane 
 and the sea. It is common, indeed, in lev- 
 elling operations, to a'. low for the curvature 
 of the earth. Newton's calculation was pre- 
 cisely similar. His plane glass was a tan- 
 gent to his curved cne From its refractive 
 index and focal distance he determined the 
 diameter of the sphere of which his curved 
 glass formed a segment, he measured the 
 distances of his rings from the place of con- 
 tact, and he calculated the depth between the 
 tangent plane and the curved surface, exactly 
 as the engineer would calculate the distance 
 between his tangent plane and the surface of 
 the sea. The wonder is, that, where such 
 infinitesimal distances arc involved, Newton, 
 with the means at his disposal, could have 
 worked with such marvellous exactitude. 
 
 To account for these rings was the great- 
 est difficulty that Newton ever encoun- 
 tered. He quite appreciated the difficulty. 
 Over his eagle-eye there was no film no 
 vagueness in his conceptions. At the very 
 outset his theory was confronted by the ques- 
 tion, "Why, when a beam of light is incident 
 on a transparent body, are some of the light- 
 particles reflected and some transmitted ? Is 
 it that there are two kinds of particles, the 
 one specially fitted for transmission and the 
 other for reflection? This cannot be the 
 reason ; for, if we allow a beam of light 
 which has been reflected from one piece of 
 glass to fall upon another, it, as a general 
 rule, is also divided into a reflected and a 
 transmitted portion. Thus the particles once 
 reflected are not always reflected, nor are the 
 particles once transmitted always transmitted. 
 Newton saw all this ; he knew he had to ex 
 plain why it is that the self-same particle is 
 at one moment reflected and at the next mo- 
 ment transmitted. It could only be through 
 change in the condition of the particle 
 
 itself. The self-same particle, he affirmed, 
 was affected by " fits" of easy transmission 
 and reflection. 
 
 If you are willing to follow me while I un- 
 ravel this theory of fits, the most subtle, per- 
 haps, that ever entered the human mind, the 
 intellectual discipline will repay you for the 
 necessary effort of attention. Newton was 
 chary of stating what he considered to be the 
 cause of the fits, but there cannot be a doubt 
 that his mind rested on a mechanical cause. 
 Nor can there be a doubt that, as in all 
 attempts at theorizing, he was compelled to 
 fall back upon experience for the materials of 
 his theory. His course of observation anqf 
 of thought may have been this : From a 
 magnet he might obtain the notion of at- 
 tracted and repelled poles. What more 
 natural than that he should endow his light- 
 particles with such poles ? Turning their 
 attracted poles towards a transparent sub- 
 stance, the particles would be sucked in and 
 transmitted ; turning their repelled poles, 
 they would be driven away or reflected. 
 Thus, by the ascription of poles, the trans- 
 mission and reflection of the self-same parti- 
 cle at different times might be accounted for. 
 Regard these rings of Newton as seen in 
 pure red light : they are alternately bright 
 and dark. The film of air corresponding to 
 the outermost of them is not thicker than an 
 ordinary soap-bubble, and it becomes thinner 
 on approaching the centre ; still Newton, as 
 I have said, measured the thickness corre- 
 sponding to every ring and showed the differ- 
 ence of thickness between ring and ring. 
 Now, mark the result. For the sake of con- 
 venience, let us call the thickness of the film 
 of air corresponding to the first dark ring d, 
 then Newton found the distance correspond- 
 ing to the second dark ring' 2 d; the thick 
 ness corresponding to the third dark ring: 3 d; 
 the thickness corresponding to the tenth dark 
 ring 10 d, and so on. Surely there must be 
 some hidden meaning in this little distanced, 
 which turns up so constantly ? One can im- 
 agine the intense interest with which Newton 
 pondered its meaning. Observe the probably 
 outcome of his thought. He had endowed 
 his light-particles with poles, but now he is 
 forced to introduce the no'. ion of periodic re- 
 currence. How was this to be done ? By 
 supposing the light -particles animated, not 
 only with a motion of translation, but 
 also with a motion of rotation. Newton's 
 astronomical knowledge would render all such 
 conceptions familiar to him. The earth has 
 such a motion. In the time occupied in pass- 
 ing over a million and a half of miles of its 
 orbit that is in twenty four hours our 
 planet performs a complete rotation, and, in 
 the time required to pass over the distance d, 
 Newton's light-particle must be supposed to 
 perform a complete rotation. True, the 
 light-particle is smaller than the planet and 
 the distance d, instead of being a million and 
 a half of miles, is a little over the ninety- 
 
SIX LECTURES ON LIGHT/ 
 
 ir 
 
 thousandth of an inch. But the two con- 
 ceptions are, in point of intellectual quality, 
 identical. 
 
 N Imagine, then, a particle entering the film 
 of air where it possesses this precise thick- 
 ness. To enter the film, its attracted end 
 must be presented. Within the film it is able 
 to turn once completely round ; at the other 
 side of the film its attracted pole will be again 
 presented ; it will, therefore, enter the glass 
 at the opposite side of the film and be lost to 
 the eye. All round the place of contact, 
 wherever the film possesses this precise thick- 
 ness, the light will equally disappear we 
 shall have a ring of darkness. 
 
 And now observe how well this conception 
 falls in with the law of proportionality dis- 
 covered by Newton. When the thickness of 
 the film is 2 </, the particle has time to per- 
 form two complete somersaults within the 
 film ; when the thickness is 3 d, three com- 
 plete somersaults ; when 10 d, ten complete 
 somersaults are performed. It is manifest 
 that in each of these cases, on arriving at the 
 second surface of the film, the attracted pole 
 of the particle will be presented. It will, 
 therefore, be transmitted, and, because no 
 light is sent to the eye, we shall have a ring 
 of darkness at each of these placss. 
 
 The bright rings follow immediately from 
 the same conception. They occur between 
 the dark rings, the thicknesses to which they 
 correspond being also intermediate between 
 those of the dark ones. Take the case of the 
 first bright ring. The thickness of the film 
 is l /t d; in this interval the rotating particle 
 can perform only half a rotation. When, 
 therefore, it reaches t.'.e second surface of the 
 film, its repelled pole is presented ; it is, 
 therefore, driven back and reaches the eye. 
 At all distances round the centre correspond- 
 r~.g to this thickness the same effect is pro- 
 duced, and the consequence is a ring of 
 brightness. The other bright rings are sim- 
 ilarly accounted for. At the second one, 
 where the thickness is i^ d, a rotation and 
 a half is performed ; at the third, two rota- 
 tions and a half ; and at each of these places 
 the particles present their repelled poles to 
 the lower surface of the film. They are there- 
 fore sent back to the eye, producing the im- 
 pression of brightness. Here, then, we havo 
 unravelled the most subtle application that 
 Newton ever made of the Emission Theory. 
 
 It has been stated in the early part of this 
 lecture, that the Emission Theory assigned a 
 greater velocity to light in glass and water, 
 than in air or stellar space. Here it was at 
 direct issue with the theory of undulation, 
 v/hich makes the velocity in air or stellar 
 space less than in glass or water. By an ex- 
 periment proposed by Arago, and executed 
 with consummate skill by Foucault and 
 Fizeau, this question was b -ought to a crucial 
 test, and decided in favor of the theory of 
 i.mdulation. In the present instance also the 
 two theories are at variance. Newton as- 
 
 sumed that the action which produces the al- 
 ternate bright and dark rings took place at a 
 single surface ; that is, the second surface of 
 the film. The undulatory theory affirms 
 that the rings are caused by the interference 
 of waves reflected from both surfaces. This 
 also has been demonstrated by experiment. 
 By proper devices we may abolish reflection 
 from one of the surfaces of the film, ancj 
 when this is done the rings vanish altogether. 
 
 Rings of feeble intensity are also formed by 
 transmitted light. These are referred by the, 
 undulatcry theory to the interference of 
 waves which have passed directly through the 
 film, with others which have suffered two re- 
 flections within the film. They are thus com- 
 pletely accounted for. 
 
 Newton, by the foregoing exceedingly 
 subtle assumption, vaulted over the difficulty 
 presented by the colors of thin plates. And, 
 as further difficulties in process of time thick- 
 ened round the theory, his disciples tried to 
 sustain it with an ingenuity worthy of their 
 master. The new difficulties were not an- 
 ticipated by the theory, but were met by new 
 assumptions, until at length the Emission 
 Theory became what a distinguished writer 
 calls a " mob of hypotheses." In the pres- 
 ence of the phenomena of interference, the 
 theory finally broke down, while the whole of 
 these phenomena lie, as it were, latent in the 
 theory of undulation. Newton's " fits," for 
 example, are immediately translatable into 
 the lengths of the ether-waves. We have 
 the observed periodic recurrence as the thick- 
 ness varies so as to produce a retardation of 
 an odd or even number of semi-undulations.* 
 
 Numerous other colors are due to interfer- 
 ence. Fine scratches drawn upon glass ot 
 polished metal reflect the waves of light from 
 their sides; and some, being reflected from 
 opposite sides of the same furrow, interfere 
 with and quench each other. But the ob- 
 liquity of reflection which extinguishes the 
 shorter waves does not extinguish the longer 
 ones, hence the phenomena of color. These 
 are called the colors of striated surfaces. 
 They are well illustrated by mother-of-pearl. 
 This shell is composed of exceedingly thin 
 layers, which, when cut across by the polish- 
 ing of the shell, expose their edges and fur- 
 nish the necessary small and regular grooves. 
 The most conclusive proof that the colors ar- 
 due to the mechanical state of tr.e surface i -. 
 to be found in the fact, established by Brew 
 ter, that, by stamping; the shell carefully 
 
 * In the explanation of Newton's rings, something 
 besides thickness is to be taken into account. In the 
 case of the first surface of the film of air, the waves 
 pass from a denser to a r?rer medium, while in the 
 case of the second surface, the waves pass from a 
 rarer to a denser medium. This difference at the 
 two reflecting surfaces can be proved to be equivalent 
 to the addition of Jialf a wave-length to the thick- 
 ness of the film. To the absolute thickness, as de- 
 ermined by Newton, half a wave-length is in each 
 :ase to be added. When this is done, the dark and 
 Bright rings follow each other in exact accordance 
 with the law of interference already enunciated. 
 
SIX LECTURES ON LIGHT. 
 
 upon black sealing-wax, we transfer the 
 grooves, and produce upon the wax the colors 
 of *nother-of-pearl. 
 
 LECTURE III. 
 
 Relation of Theories to Experience : Origin of the 
 Notion of the Attraction of Gravitation : Notion 
 of Polarity, how generated : Atomic Polarity : 
 Structural Arrangements due to Polarity : Archi- 
 tecture of Crystals considered as an 'Introduction to 
 their Action upon Light: Notion of Atomic Po- 
 larity applied to Crystalline Structure : Experi- 
 mental Illustrations: Crystallization of Water: 
 Expansion by Heat and by Cold : Deportment of 
 Water considered and explained: Molecular Ac- 
 tion illustrated by a Model: Force of Solidifica- 
 tion : Bearings of Crystallization on Optical Phe- 
 nomena : Refraction : Double Refraction : Po- 
 larization : Action of Tourmaline: Character of 
 the Beams emergent from Iceland Spar : Polariza- 
 tion by ordinary Refraction and Reflection : De- 
 polarization. 
 
 IN our last lecture we sought to familiarize 
 Our minds with the characteristics of wave- 
 motion. We drew a clear distinction between 
 the motio i of the wave itself and the motion 
 of its constituent particles. Passing through 
 water-waves and air-waves, we prepared our 
 mi ds for the conception of light-waves prop- 
 agated through the luminiferous ether. The 
 analogy of sound will fix the whole mechan- 
 ism in your minds. Here we have a vibrat- 
 ing body which originates the wave motion, 
 we have, in the air, a vehicle which conveys 
 it, and we have the auditory nerve which re- 
 ceives the impressions of the sonorous waves. 
 In the case of light we have in the vibrating 
 atoms of the luminous body the originators of 
 the wave-motion, we have in the ether its 
 vehicle, while the optic nerve receives the im- 
 pression of the luminiferous waves. We 
 learned, also, that color is the analogue of 
 pitch, that the rapidity of atomic vibration 
 augmented, and the length of the ether-waves 
 decreased, in passing from the red to the blue 
 end of the spectrum. The fruitful principle 
 of interference we also found applicable to 
 the phenomena of light ; and we learned that, 
 in consequence of the different lengths of the 
 ether- waves, they were extinguished by dif- 
 ferent thicknesses of a transparent film, the 
 particular thickness which quenched one color 
 glowing, therefore, with the complementary 
 one. Thus the colors of thin plates were ac- 
 counted for. 
 
 But one of the objects of our last lecture, 
 and that not the least important, was to illus- 
 trate the manner in which scientific theories 
 are iormed. They, in the first place, take 
 their rise in the desire of the mind to pene- 
 trate to the sources of phenomena. This de- 
 sire has long been a part of human nature. 
 It yrompted Caesar to say that he would ex- 
 change his victories for a rlirnpse of the 
 sources of the Nile ; it may be seen working 
 in Lucretius ; it impels Darwin to those dar- 
 ing speculations which of late years have so 
 agitated the public mind. We have learned ' 
 that in framing theories the imagination does 
 
 not create, but that it expands, diminishes, 
 moulds, and refines, as the case may be, 
 mater als derived from the world of fact and 
 observation. 
 
 This is more evidently the case in a theory 
 like that of light, where the motions of a sub- 
 sensible medium, the ether, are presented to 
 the mind. But no theory escapes the condi- 
 tion. Newton took care not to encumber 
 gravitation with unnecessary physical concep- 
 tions ; but we have reason to kno.v that he 
 indulged in them, though he did not connect 
 them with his theory. But even the theory 
 as it stands did not enter the mind ai' a reve- 
 lation dissevered from the world of experi- 
 ence. The germ of the conception that the 
 sun and planets are held together by a force 
 of attraction is to be found in the fact that a 
 magnet had been previously seen to attract 
 iron. The notion of matter attracting matter 
 came thus from without, not from within. In 
 our present lecture the magnetic force must 
 serve us till further ; but here we must master 
 its elementary phenomena. 
 
 The general facts of magnetism are most 
 simply illustrated by a magnetized bar of 
 steel, commonly called a bar magnet. Placing 
 such a magnet uyrfurht upon a table, and 
 bringing a magne, ieedle near its bottom, 
 one end of the rtf"r*.>K promptly retreats from 
 the magnet, v"> the other as promptly 
 approaches. ' aeedle is held quivering 
 there by some ^risible influence exerted 
 upon it. Raising the needle along the mag- 
 net, but still avoiding contact the rapidity 
 of its oscillations decreases, because the force 
 acting upon it becomes weaker. At the 
 centre the oscillations cease. Above the 
 centre, the end of the needle which had been 
 previously drawn towards the magnet re- 
 treats, and the opposite end approaches. As 
 we ascend higher, the oscillations become 
 mere violent, because the force becomes 
 stronger. At the upper end of the magnet, 
 
 FIG. 5. 
 
 as at the lower, the force reaches a maximum, 
 t>ut all the lower half of the magnet, from 
 E to S (Fig. 5), attracts one end of the 
 
SIX LECTURES ON LIGHT. 
 
 19 
 
 needle, while all the upper half, from E to 
 N, attracts the opposite end. This double- 
 ness of the magnetic force is called polarity, 
 and the points near the ends of the magnet in 
 which the forces seem concentrated are called 
 its poles. 
 
 What, then, will occur if we break this 
 magnet in two at the cent:e E > Will each 
 of the separate halves act as it did when it 
 formed part of the whole magnet? No; 
 each half is in itself a perfect magnet, pos 
 sessing two poles. This may be proved by 
 breaking something of less value than the 
 magnet the steel of a lady's stays, for ex- 
 t xample, hardened and magnetized. It acts 
 like the magnet. When broken, each half 
 acts like the whole ; and when these parts 
 are again broken, we have still the perfect 
 magnet, possessing, as in the first instance, 
 two poles. Push your breaking to its utmost 
 limit ; you will be driven to prolong your 
 
 rection of the needle, and no other. A needle 
 of iron will answer as well as the magnetic 
 needle ; for the need e of iron is magnetized 
 by the magnet, and acts exactly like a needle 
 independently magnetized. 
 
 If we place two or more needles of iron 
 near the magnet, the action becomes more 
 complex, for the the iron needles are not only;' 
 acted on by the magnet, but they act upon 
 each other. And if we pass to smaller masses 
 of iron to iron filings, for example we find 
 that they act substantially as the needles, ar- 
 ranging themselves in definite forms, in obe- 
 dience to the magnetic action. 
 
 Placing a sheet of paper or glass over this 
 bar magnet and showering iron filings upon 
 the paper, I notice a tendency of the filings 
 to arrange themselves in determinate lines. 
 They cannot freely follow this tendency, for 
 they are hampered by the friction against che 
 paper, They are helped by tapping the 
 
 FIG. 6. 
 
 K is the nozzle of the lamp ; M a plane mirror, reflecting: the beam upwards. At P, the magnets and 
 iron filings are placed ; L is a lens which forms an image of the magnets and filings ; and R is a total' 
 ly- reflecting prism which casts the image, G, upon the screen. 
 
 vision beyond that limit, and to contemplate 
 this thing that we call magnetic polarity as 
 resident in t)ie ^lltimate particles of the mag- 
 net. Each atom is endowed with this polar 
 force. 
 
 Like all other forces, this force of magnet- 
 ism is amenable to mechanical laws ; and 
 knowing the direction and magnitude of the 
 force, we can predict its action. Placing a 
 small magnetic needle near a bar magnet, it 
 takes up a determinate position. That posi- 
 tion might be deduced theoretically from the 
 mutual action of the poles. Moving the 
 needle round the magnet, for each point of 
 the surrounding space there is a definite di- 
 
 paper: each tap releases them for a moment j. 
 and enables them to follow their bias. Bu 
 this is an experiment which can only be seen 
 by myself. To enable you to see it, I take a 
 pair of small magnets and by a simple optical* 
 arrangement throw the images of the mag- 
 net i upon the screen. Scattering iron filings 
 over the glass plate to which the small magnets 
 are attached, and tapping the |>late, you see 
 the arrangement of the iron filings in those 
 magnetic curves which have been so long 
 familiar to scientific men.* 
 
 *Very beautiful specimens of these curves have 
 been recently obtained, and fixed, by Prof. Mayer, 
 of Hoboken. 
 
20 
 
 SIX LECTURES ON LIGHT. 
 
 The aspect of these curves so fascinated 
 Faraday that the greater portion of his intel- 
 lectual life was devoted to pondering ove 
 them. He invested the space through which 
 they run with a kind of -materiality ; and th 
 probability is, that the progress of science b) 
 connecting the phenomena of magnetism 
 with the luminiferous ether, will prove these 
 " lines of force," as Faraday loved to cal 
 the magnetic curves, to represent a condition 
 of this mysterious substratum of all radianl 
 action. 
 
 But it is not with the magnetic curves, as 
 such, that I now wish to occupy your atten- 
 tion ; it is their relationship to theoretic con- 
 ceptions that we have now to consider. By 
 the action of the bar magnet upon the needle 
 we obtain the notion of a polar forcf ; by the 
 breaking of the strip of magnetized steel, we 
 attain die notion that polarity can attach 
 itself to the ultimate particles of matter. The 
 experiment with the iron filings introduces a 
 new idea into the mind ; the idea, namely, of 
 structural arrangement. Every pair of filings 
 possesses four poles, two of which arc attrac- 
 tive and two repulsive. The attractive poles 
 approach, the repulsive poles retreat ; the 
 consequence being a certain definite arrange- 
 ment of the particles with reference to each 
 other. 
 
 Now, this idea of structure, as produced 
 by polar force, opens a way for the intellect 
 into an entirely new region, and the reason you 
 are asked to accompany me into this region 
 is, that our next inquiry relates to the action 
 of crystals upon light. Before I speak of 
 this action, I wish y^u to realize the process 
 of crystalline architecture. Look then into a 
 granite quarry, and spend a few minutes in 
 examining the rock. It is not of perfectly 
 uniform texture. It is rather an agglomera- 
 tion of pieces, which, on examination, pre- 
 sent curiously-defined forms. You have there 
 what mineralogists call quartz, you have 
 felspar, you have mica. In a mineralogical 
 cabinet, where these substances are preserve.! 
 separately, you will obtain some notion of 
 their forms. You will see there, also, speci- 
 mens of beryl, topaz, emerald, tourmaline, 
 heavy spar, fluor-spar, Iceland spar possibly 
 a full-formed diamond, as it quitted the hand 
 of .Nature, not yet having got into the hands 
 of the lapidary. These crystals, you will ob- 
 serve, are put together according to law ; 
 they are not chance productions ; and, if 
 you care to examine them moro minutely, 
 you will find their architecture capable of 
 being to some extent revealed. They split 
 in certain directions before a knife-edge, ex- 
 P" sing smooth and shining surfaces, which 
 are called planes of cleavage ; and by follow- 
 ing these planes you sometimes reach an in 
 ternal form, disguised beneath the external 
 form of the crystal. Ponder these beautiful 
 edifices of a hidden builder. You cannot 
 help asking yourself how they were built ; 
 and familiar as you now are with the notion 
 
 of a polar force, and the ability of that force 
 to produce structural arrangement, your in- 
 evitable answer will be, that those crystals 
 are built by the play of polar forces with 
 which their ultimate molecules are endowed. 
 In virtue of these forces, atom lays itself to 
 atom in a perfectly definite way, the final 
 visible form of the crystal depending upon 
 this play of its molecules. 
 
 Everywhere in Nature we observe this 
 tendency to run into definite forms, and 
 nothing is easier than to give scope to this 
 tendency toy artificial arrangements. Dis- 
 solve nitre in water, and allow the water 
 slowly to evaporate; the nitre remains, and 
 the solution soon becomes so concentrated 
 that the liquid form can no longer be pre- 
 served. The nitre-molecules approach each 
 other, and come at length within the range 
 of their polar forces. They arrange them- 
 selves in obedience to these forces, a minute 
 crystal of nitre being at first produced. On 
 this crystal the molecules continue to deposit 
 themselves from the surrounding liquid. The 
 crystal grows, and finally we have large 
 prisms of nitre, each of a perfectly definite 
 hape. Alum crystallizes with the utmost 
 ease in this fash. on. The resultant crystal 
 s, however, different in shape from that Ot 
 nitre, because the poles of the molecules are 
 differently disposed; and, if they be only 
 nursed with proper care, crystals of these 
 substances may be caused to grow to a great 
 ize. 
 
 The condition of perfect crystallization is, 
 that thi crystallizing force shall act with de- 
 ibera^ion. There should be no hurry in its 
 operation; but every molecule ought to be 
 Dermitted, without disturbance from its neigh- 
 >ors, to exercise its own molecular lights. If 
 he crystallization be too sudden, the regu- 
 arity disappears. Water may be saturated 
 vith sulphate of soda, dissolved when the 
 vater is hot, and afterward permitted to cool. 
 When cold, the solution, is supersaturated ; 
 hat is to say, more solid matter is contained 
 n it than corresponds to its temperature. 
 Still the molecules show no signs of building 
 hemselves together. This is a very remaik- 
 able, though a very common fact. The 
 molecules in the centre of the liquid are so 
 lampered by the action of their neighbors 
 hat freedom to fohow their own tendencies 
 s denied to them. Fix your mind's eye upon 
 a molecule within the mass. It wishes to 
 unite with its neighbor to the right but it 
 vishes equally to unite with its neighbor to 
 he left ; the one tendency neutralizes the 
 >ther, ana it unites with neither. We have 
 nere, in fact, translated into molecular action 
 he well-known suspension of animal volition 
 jroduced by two equally inviting bundles of 
 lay. But, if a crystal of sulphate of soda be 
 Iropped into the solution, the molecular in- 
 lecision ceases On the crystal the adjacent 
 molecules will immediately precipitate them- 
 selves; on these again others will be precipi- 
 
SIX LECTURES ON LIGHT. 
 
 21 
 
 ) and this act of precipitation will con- 
 tinue from the top of the flask to the bottom, 
 until the solution has, as far as possible, as- 
 sumed the solid form. The crystals here 
 formed are small, and confusedly arranged. 
 The process has been too hasty to a imit of 
 the pure and orderly action of the crystalliz- 
 ing lorce. It typities the state of a nation in 
 which natural and healthy change is resisted, 
 until society becomes, as it were, supersatu- 
 rated with the desire lor change, the change 
 being effected through confusion and revolu- 
 tion, which a wise foresight might have 
 avoided. 
 
 Let me illustrate the action of crystallizing 
 force by two examples of it : Nitre might be 
 employed, but another well-known substance 
 enables me to make the experiment in a bet- 
 ter form. The substance is common sal- 
 ammoniac, or chloride of ammonium, dis- 
 solved in water. Cleansing perfectly a glass 
 plate, the solution of the chloride is poured 
 over the glass, to which, when the plate is set 
 on edge, a thin film of the liquid adheres. 
 Warming the glass slightly, evaporation is 
 promoted ; the plate is then placed in a solar 
 microscope, and an image of the film is thrown 
 upon a white screen. The warmth of the il- 
 luminating beam adds itself to that already 
 impa- ted to the glass plate, so that after a 
 moment or two the film can no longer exist in 
 the liquid condition. Molecule then closes 
 with molecule, and you have a most impres- 
 sive display of crystallizing energy overspread- 
 ing the whole screen. You may produce 
 something similar if you breathe upon the 
 frost ferns which overspread your window- 
 panes in winter, and then observe through a 
 lens the subsequent recongelation of the film. 
 
 Here the crystallizing force is hampered by 
 the adhesion of the film to the glasys ; never- 
 theless, the play of power is strikingly beau- 
 tiful. Sometimes the crystals start from the 
 edge of the film and run through it from that 
 edge, for, the crystallization being once 
 started, the molecules throw themselves by 
 preference on the crystals already formed. 
 Sometimes the crystals start from definite 
 nuclei in the centre of the film ; every small 
 crystailins particle which rests in the film fur- 
 nishes a starting-point. Throughout the pro- 
 cess you notice one feature which is perfectly 
 unalterable, and that is, angular magnitude. 
 The spiculce branch from the trunk, and from 
 these branches others shoot ; but the angles 
 enclosed by the spiculre are unalterable. In 
 like manner you may find alum-crystals, 
 quartz-crystals, and all other crystals, dis- 
 torted in shape. They are thus far at the 
 mercy of the accidents of crystallization ; but 
 in one particular they assert their superiority 
 over ail such accidents angttlar magnitude 
 is always rigidly preserved. 
 
 My second example of the action of crys- 
 tallizing force is this: ! y sending a voltaic 
 current through a liquid, you know that we 
 decompose the liquid, and if it contains a 
 
 metal, we liberate this metal by the electro- 
 lysis. This small cell contains a solution of 
 acetate of lead, and this substance is chosen 
 because lead lends itself freely to this crys- 
 tallizing power. Into the cell dip two very 
 thin platLum wires, and these are connected 
 by other wires with a small voltaic battery. 
 On sending the voltaic current through the 
 solution, the 1 ad will be sl.wly severed from 
 the atoms with which it is now combined; it 
 will be liberated upon one of the wires, and 
 at the moment of its liberation it will obey 
 the polar forces of its atoms, and produce 
 crystalline forms of exquisite beauty. They 
 are now before you, sprouting like ferns 
 from the wire, appearing indeed like vegeta- 
 ble growths rendered so rapid as to be plain- 
 ly visible to the naked eye. On reversing the 
 current, these wonderful lead-fror ds will dis- 
 solve, while from the other wire filaments of 
 lead dart through the liquid. In a moment 1 
 or two the growth of the lead-trees recom- 
 mences, but they now cover the other wire. 
 In the process of crystallization, Nature first 
 reveals herself as a builder. Where do her 
 operations stop ? Does she continue, by the 
 play of the same forces, to form the vegeta- 
 ble, and afterwards the animal ? Whatever 
 the answer to these questions may be, trust 
 me that the notions of the coming genera- 
 lions regarding this mysterious thing, which 
 some have called "brute matter," will be 
 very different from those of the generations 
 past. 
 
 There is hardly a more beautiful and in- 
 structive example of this play of molecular 
 force ti an that furnished by the case of water. 
 You have seen the exquisite fern-like forms 
 produced by the crystallization of a film of 
 water on a cold window pane. You have 
 also probably noticed the beautiful rosettes 
 tied together by the crystallizing force during 
 the descent of a snow-shower on a very calm 
 day. The slopes and summits of the Alps 
 are loaded in winter with these blossoms of 
 the frost. They vary infinitely in detail of 
 beauty, but the same angular magnitude is 
 preserved throughout. An inflexible power 
 binds spears and spiculce to the angle of 60 
 degrees. The common ice of our lakes ii 
 also ruled ki its deposition by the same angle. 
 You may sometimes see in freezing water 
 small crystals of stellar shapes, each star con- 
 sisting of six rays, with this angle of 60 be- 
 tween every two of them. 'i his structure 
 may be revealed in ordinary ice. In a su<fc- 
 j beam, or, failing that, in our electric beam, 
 I we have an instrument delicate enough to 
 I unlock the frozen molecules without disturb- 
 | ing the oider of their architecture. Cutting 
 I from clear, sound, regularly-frozen ice a sl-tb 
 i parallel to the planes of freezing, and send- 
 I ing a sunbeam through such a slab, it lique- 
 j fies internally at special points, round each 
 ! point a six-petalled liquid flower of exquisite 
 I beauty being formed. Crowds of such flow?' 
 ' ers are thus produced. 
 
22 
 
 SIX LECTURES ON LIGHT. 
 
 A moment's further devotion to the crys- 
 tallization of water will be well repaid ; for 
 the sum of qualities which renders ;his sub- 
 stance fitted to play its part in Nature may 
 well excite wonder and stimulate thought. 
 Like almost all other substances, water is ex- 
 panded by heat and contracted by cold. Let 
 this expansion and contraction be first illus- 
 trated : 
 
 A small fla 1: is filled with colored water, 
 and stopped with a cork. Through the cork 
 passes a glass tube water-tight, the liquid 
 standing at a Certain height (/', Fig. 7) in the 
 tube. The flask and its tube resemble the 
 bulb and stem of a thermometer. Applying 
 the heat of a spirit-lamp, the water rises in 
 the tube, and finally trickles over the top (/). 
 Expansion by heat is thus illustrated. 
 
 the definite temperature of 39 Fahr. Crys- 
 tallization has virtually here commenced, the 
 molecules preparing themselves for the subse- 
 quent act of solidification which occurs at 
 32, and in which the expansion suddenly 
 culminates. In virtue of this expansion, 
 ice, as you know, is lighter than water in the 
 proportion of 8 to 9.* 
 
 It is my desire, in these lectures, to lead 
 you as closely a* possible to trie limits 
 hitherto attained by scientific thought, and, 
 in pursuance rf this desire, I have now to 
 invite your attention to a molecular problem 
 of great interest, but of great complexity. I 
 wish you to obtain sue i an insight of the 
 molecular world as shall give the intellect 
 satisfaction when reflecting on the deport- 
 n:ent of water before and during the act of 
 
 FIG. 7. 
 
 Projection of experiment : E is the nozzle of the lamp, L a converging lens, and / i the image of the liquid 
 column. 
 
 Removing the lamp and piling a freezing 
 mixture in the vessel (B) round the flask, the 
 liquid column falls, thus - showing the con- 
 traction of the water by the cold. But let 
 the- freezing mixture continue to act : the 
 tailing of the column continues to a certain 
 point ; it then ceases. The top of tne col- 
 umn remains stationary for son-e seconds, 
 and afterwards begins to rise. The contrac- 
 tion has ceased, and expansion by cold sets in. 
 Let the expansion continue till the liquid 
 trickles a second time over the top of the 
 tube. The freezing mixture has here pro- 
 duced to all appearance the same effect as the 
 flame. In the case of water, contraction by 
 cold ceases and expansion by cold sets in at 
 
 crystallization. Consider, then, the ideal 
 case of a number of magnets deprived of 
 
 * In a little volume entitled " Forms of Water," 
 I have mentioned that cold iron floats upon molten 
 iron. In company with my friend Sir William Arm- 
 strong, I had repeated opportunities of witnessing 
 this fact in his works at Elswick, in 1863. Faraday, 
 I remember, spoke to me subsequently of the com- 
 pleteness of iron castings as probably due to the 
 swelling of the metal on solidification. Beyond this, 
 I have giv^n the subject no special attention, and I 
 know that many intelligent iron-founders doubt the 
 fact of expansion. It is quite possible that the solid 
 floats because it is not wetted by the molten iron, its 
 volume being virtually augmented by capillary re- 
 pulsion. Certain flies walk freely upon water in vir- 
 tue of an action of this kind. With bismuth, how- 
 ever, it is easy to burst iron bottles by the force of 
 solidification. 
 
SIX LECTURES ON LIGHT. 
 
 23 
 
 weight, but retaining thtir polar forces. If 
 we had a liquid of the specific gravity of 
 steel, we might, by making the magnets 
 float in it, realize this state of things, for in 
 such a liquid the magnets would neither sink 
 nor swim. Now, the principle of gravitation 
 is .that every particle of matter attracts every 
 other particle with a force varying as the in- 
 inverse square of the distance. In virtue of 
 the attraction of gravity, then, the magnets, 
 if perfectly free to move, would slowly ap- 
 proach each other. 
 
 But besides the unjsolar force of gravity, 
 which belongs to matter in general, the mag- 
 nets are endowed with the polar force of 
 magnetism. For a time, however, the polar 
 forces do not sensibly come into play. In 
 this condition the magnets resemble our water 
 molecules at the temperature say of 50". 
 
 E 
 
 ; with the force of contraction until the freezing 
 j temperature is attained. Here the polar 
 | forces suddenly and finally gain the victory. 
 { The molecules close up and form solid crys- 
 I tals, a considerable augmentation of volume 
 | being the immediate consequence. 
 
 \Ve can still further satisfy the intellect by 
 I showing that these conceptions can be real- 
 ized by a model. The molecule of water is 
 composed of two atoms of hydrogen, united 
 to one of oxygen. We may assume the mole- 
 cule built up of these atoms to be pyramidal. 
 Suppose the triangles in Fig. 8 to be drawn 
 touching the sides of the molecule, and the 
 disposition of the polar forces to be that indi- 
 cated by the letters ; the points marked A 
 being attractive, and those marked R repel- 
 lent. In virtue of the general attraction of 
 the molecules, let them be drawn towards the 
 
 But the magnets come at length sufiici"Tily 
 near each other to enable their poles to tiler- 
 act. From this point the action cease* to be 
 a general attraction of the masses. An at- 
 traction of special points of the masses and a 
 repulsion of other points now come into play; 
 and it is easy to see that the rearrangement 
 of the magnets consequent upon the intro- 
 duction of these new forces may be suth as 
 to require a greater amount of room. This, 
 I take it, is the case with our water-mole- 
 cules. Like the magnets, they approach each 
 other as wholes ^ until the temperature ^9 is 
 reached. Previous to this temperature, 
 doubtless, the polar forces had begun to act, 
 and at this temperature their action exactly 
 balances the contraction due to cold. At 
 lower temperatures the polar forces predomi- 
 nate. But they carry on a gradual struggle 
 
 positions marked by the full lines, and then 
 suppose the polar attractions and repulsions 
 to act. A will turn towards A', and R will 
 retreat from R x . The molecules will be caused 
 lo lotate, their final positions being that shown 
 by the dotted lines. But the circle surround 
 ing the latter is larger than that surrounding 
 the full lines, which shows that the molecules 
 in their new positions require more room. In 
 this v.ay we obtain an image of the molecular 
 mechai ism active in the case of water. The 
 demand for more room is made with an energy 
 sufficient to overcome all ordinary resistances. 
 Your lead pipes yield readily to this power; but 
 iron does the same, and bomb-shells, as you 
 know, can be burst by the freezing of water. 
 Thick iron bottles filled with water and placed 
 in a freezing mixture are shivered into frag- 
 ments by the resistless vigor of molecular force. 
 
24 
 
 SIX LECTURES ON LIGHT. 
 
 We have now to exhibit the bearings of 
 crystallization upon optical phenomena. Ac- 
 cording to the undulatory theory, the velocity 
 of light in water and glass is less than in 
 air. Consider, then, a small portion of a 
 wave issuing from a point of light so distant 
 that the portion may be regarded as practi- 
 cally straight. Moving vertically downwards, 
 and impinging on an horizontal surface of 
 glass, the wave would go through the glass 
 without change of direction. But, as the 
 velocity in glass is less than the velocity in 
 air, the wave would be retarded on passing 
 into the denser medium. 
 
 But suppose the wave, before reaching the 
 glass, to be oblique to the surface ; that end 
 of the wave which first reaches the glass will 
 be the first retarded, the other portions as 
 they enter the glass being retarded in succes- 
 sion. This retardation of the one end of the 
 wave causes it to swing round and change its 
 front, so that when the wave has fully entered 
 the glass its course is oblique to its original di- 
 rection. According to the undulatory theory, 
 light is thus refracted. 
 
 In water, fcr example, there is nothing in 
 the grouping of the molecules to interfere 
 with the perfect homogeneity of the ether ; 
 but, when water crystallizes to ice, the case 
 is different. In a plate of ice the elasticity 
 of the ether in a direction perpendicular to 
 the surface of freezing is different from what 
 it is parallel to the surface of freezing ; ice is. 
 therefore, a double refracting substance. 
 Double refraction is displayed in a particu- 
 larly impressive manner by Iceland spar, 
 which is crystallized carbonate of lime. The 
 difference of ethereal density in two direc- 
 tions in this crystal is very great, the separa- 
 tion oi the beam into the two halves being, 
 therefore, particularly striking. 
 
 Before you is now projected an image of our 
 carbon-points. Introducing the spar, the beam 
 which builds the image is permitted to pass 
 through it; instantly you have the single image 
 divided into two. Projecting an image ot the 
 aperature through which the light issues from 
 the electric lamp, and introducing the spar, 
 two luminous ti k;, insteadof one, appear 
 immediately upon the screen. (See Fig. Q.X 
 
 FIG. 9. 
 
 The two elements of rapidity of propaga- 
 tion, both of sound and light, in any sub- 
 stance whatever, are elasticity and density, 
 and the enormous velocity of light is attain- 
 able because the ether is at the same time of 
 infinitesimal density and of enormous elas- 
 ticity. It surrounds the atoms of all bodies, 
 but seems to be so acted upon by them that 
 its density is increased without a proportionate 
 increase of elasticity ; this would account for 
 the diminished velocity of light in refracting 
 bodies. In virtue of the crystalline archi- 
 ' lecture that we have been considering, the 
 ether in many crystals possesses different j 
 densities in different directions ; and the con- 
 sequence is, that some of these media trans- 
 mit light with two different velocities. Now, 
 refraction depends wholly upon the change 
 of velocity on entering the refracting medium ; 
 and is greatest where the change of volicity 
 is greatest. Hence, as, in many crystals, we 
 have two different velocities, we have also 
 two different refractions, a beam of light being 
 divided by such crystals into two. This ef- 
 fect Uattad<tifai& refraction. 
 
 The two beams into which the spar divides 
 the single incident-beam do not behave alike. 
 One of them obeys the ordinary law of re- 
 fraction discovered by Snell, and this is 
 called the ordinary ray. The other does not 
 obey the ordinary law. Its index of refrac- 
 tion, for example, is not constant, nor do the 
 incident and refracted rays always lie in the 
 same plane. It is, therefore, called the ex- 
 traordinary ray. Pour water, and bisulphide 
 of carbon into two cups of the same depth ; 
 looked at through the liquid, the cup that con- 
 tains the more strongly-refracting liquid will 
 appear shallower than the other. Place a 
 piece cf Iceland spar over a dot of ink ; two 
 dots are seen, but one appears nearer than the 
 other. The nearest dot belongs to the most, 
 strongly-refracted ray, which in this case is 
 ths ordinary ray. Turn the spar round, and 
 the extraordinary image of the spot rotates 
 roi:nd the ordinary one. 
 
 The double refraction of Iceland spar was 
 first treated in a work published by Erasmus 
 Bartholimus, in 1669. The celebrated Huy- 
 ghens sought to account for the phenomenon 
 
SIX LECTURES ON LIGHT. 
 
 on the principles of the wave theory, and he 
 succeeded in doirg so. He made highly im- 
 portant observations on the distinctive charac- 
 ter of the two beams transmitted by the spar. 
 Newton, reflecting on the observations of 
 Huyghens, came to the conclusion that each 
 of the beams had two sides ; and from the 
 analogy of this two sidedness with the two 
 tndedness of a magnet, wherein consists its 
 polarity, the two beams came subsequently to 
 be described as polarized. 
 
 We shall study this subject of \b& polariza- 
 tion of light with great ease and profit by 
 means of a crystal of tourmaline. But let us 
 start with a clear conception of an ordinary 
 beam of light. It has been already explained 
 that the vibrations of the individual ether- 
 particles are executed across the line of prop- 
 agation. In the case of ordinary light we 
 are to figure the ether particles as vibrating 
 in all directions, or azimuths, as it is some- 
 times expressed, across this line. 
 
 Now, in a plate of tourmaline cut parallel 
 to the axis of the crystal, the beam of incident 
 light is divided into two, the one vibrating 
 parallel to the axis of the crystal, the other at 
 right angles to the axis. The grouping of 
 the molecules, and of the ether associated 
 with the molecules, reduces all the vibrations 
 incident upon the crystal to these two direc- 
 tions. One of these beams, namely that one 
 whose vibrations are perpendicular to the 
 axis, is quenched with exceeding rapidity by 
 the tourma ine, so '.hat, after having passed 
 through a very small thickness of the crystal, 
 the light emerges with all its vibrations re- 
 duced to a single plane. In this condition it 
 is what we call a beam of plane polarized 
 light. 
 
 A moment's reflection will show, if what 
 has been stated be correct, that, on placing 
 a second plate of tourmaline with its axis 
 parallel to the first, the light will pass through 
 both ; but that, if the axes be crossed, the 
 
 FIG. 10. 
 
 light that passes through the one plate will 
 be quenched by the other, a total interception 
 
 of the light being the consequence. The 
 image of a plate of tourmaline, 1 1 (Fig. 10), 
 is now before you. I place parallel to it 
 another plate, t' t f : the green of the crystal 
 is a little deepened, nothing more. By means 
 of an endless screw, I now turn one of the 
 crystals gradually round ; as long as the two 
 plates are 'oblique to each other, a certain 
 portion of light gets through ; but, when they 
 are at right angles to each other, the space 
 common to both is a space of darkness, as 
 shown in Fig. n. 
 
 Let us return to a single plate ; and let me 
 say that it is on the green light transmitted 
 by the tourmaline that you are to fix your at- 
 tention. We have now to illustrate the two- 
 sidedness of that green light. The light sur- 
 rounding the green image being ordinary 
 light, is reflected by a plane glass mirror in i 
 all directions ; the green light, on the con- 
 trary, is not so reflected. The image of the 
 tourmaline is now horizontal ; reflected up- 
 wards, it is still green ; reflected sideways, 
 the image is reduced to blackness, because of 
 the incompetency of the green 1'ght to be re- 
 flected in this direction. Making the plate 
 of tourmaline vertical and reflecting it as 
 before, in the upper image the light is 
 quenched ; in the side image you have now 
 the green. Picture the thing clearly. In 
 the one case the mirror receives the impact 
 ot the edges of the waves, ana the green light 
 is quenched. In the other case the sides of 
 the waves strike the mirror, and t e green 
 light is reflected. To render the extinction 
 complete, the light must be received upon 
 the mirror at a special angle. What this 
 angle is we shall learn presently. 
 
 The quality of two-sidedness conferred 
 upon light by crystals may also be conferred 
 upon it by ordinary reflection. Malus made 
 this discovery in 1808, while looking through 
 Iceland spar at the light of the sun reflected 
 from the windows of the Luxembourg palace 
 in Paris. I receive upon a plate of window- 
 glass the beam from our lamp ; a great por- 
 tion of the light reflected from the glass is 
 polarized ; the vibrations of this reflected 
 beam are executed, for the most part, paral- 
 lel to the surface of the glass, and, if the 
 glass be held so that the beam shall make an 
 angle of 58 with the perpendicular to the 
 glass, the whole of the reflected beam is polar- 
 ized. It was at this angle that the image 
 of the tourmaline was completely quenched 
 in our former experiments. It is called the 
 polarizing angle. 
 
 And now let us try to make substantially 
 the experiment of Malus. I receive the beam 
 from the lamp upon this plate of glass and 
 reflect it through the spar. Instead of two 
 images, you see but one. So that the light, 
 when polarized, as it now is, can only get 
 through the spar in one direction, and conse- 
 quently produce <but one image. Why is 
 this ? In the Iceland spar, as in the tour- 
 maline, all the vibrations of the ordinary light 
 
26 
 
 SIX LECTURES ON LIGHT. 
 
 are reduced to two planes at right angles to 
 each other ; but, unlike the tourmaline, both 
 beams are transmitted with equal facility by 
 the spar. The two beams, in short, emerg- 
 ent from the spar are polarized, their direc- 
 tions of vibration being at right angles to 
 each other. When, therefore, the light was 
 Dolarized by reflection, the direction of vibra- 
 tion in the spar which corresponded to the 
 
 conclude? That the green light will be 
 transmitted along the latter, which is parallel 
 to the tourmaline, a:id not along the former, 
 which is perpendicular to it. Hence we may 
 infer that one image of the tourmaline will 
 show the ordinary green light cf the crystal, 
 while the other image will be black. Let us 
 test our reasoning by experiment : it is veri- 
 fied to the letter. (Fig. 13.) 
 
 FIG 
 
 direction of vibration of the pokrized beam 
 transmitted it, and that direction only. But 
 one image, therefore, was possible under the 
 conditions. 
 
 And now you have it in your power to 
 check many of my statements, and you will 
 observe that such logic as connect our experi- 
 ments is simply a transcript of the logic of 
 Nature. On the screen before you arc the 
 
 Let us push our test still further. By 
 means of an endless screw, the crystal can 
 l*e turned ninety degrees round. The black 
 image, as I turn, becomes gradually brighter 
 and the bright one gradually darker; at an 
 angle cf forty- five degrees both images are 
 equally bright (Fig. 13); while, when ninety 
 degrees have been obtained, the axis cf the 
 crystal being then vertical, the briglu and 
 
 FIG 
 
 two disks of light produced by the double re- 
 fraction of the spar. They are, as you 
 know, two images of the aperture through 
 which the light issues from the camera. 
 Placing the tourmaline in front of the aper- 
 ture, two images of the crystal will be ob- 
 tained ; but now let us reason out what is to 
 be expected from this experiment. The light 
 emergent from the tourmaline is polarized. 
 
 black ftnages have changed places. (Fig. 
 I4-) 
 
 Given two beams transmitted through Ice- 
 land spar, it is perfectly manifest that we 
 have it in our power to determine instantly, 
 by means of a plate of tourmaline, the direc- 
 tions in which the ether-particles vibrate in 
 the two beams. I might place the double- 
 refracting spar in any position whatever. A 
 
 FIG. 14. 
 
 Placing the crystal with its axis horizontal, 
 the vibrations of the transmitteu light will be 
 horizontal. Now the spar, as already stated, 
 has two perpendicular directions ot vibration, 
 one of which, at the present moment, is ver- 
 tical, the other horizontal. "What are we to 
 
 minute's trial with the tourmaline would 
 enable you to determine the position which 
 yields a black and a bright image, and from 
 these you would at once infer the directions 
 of vibration. 
 
 Further, the two beams from the spar 
 
SIX LECTURES ON LIGHT. 
 
 27 
 
 being thus polarized, if they be suitably re- 
 ceived upon a plate of glass at the polarizing 
 angle, one of them will be reflected, the 
 other not. This is the conclusion of reason 
 from our previous knowledge; but you ob- 
 serve that reason is justified by experiment. 
 (Figs. 15 and 16.) 
 
 I have said that the whole of the beam re- 
 flected from glass at the polarizing angle is 
 polarized; a word must now be added regard- 
 ing the larger portion of the light transmitted 
 by the glass. The transmitted beam contains 
 a quantity of polarized light equal to that of 
 tie reflected beam; but this quantity is only 
 a fi action of the whole transmitted light. By 
 taking two plates of glass instead cf one, we 
 
 (B is the birefracting spar, dividing the incident 
 li^fit into the two beams, o and e. G is the mirror). 
 Tne beam is here reflected laterally. When the re- 
 flection is ufitvards^ the other beam is reflected, as 
 shown in Fig. 16. 
 
 augment the quantity of the transmitted polar- 
 ized light; and, by taking a bundle of plates, we 
 
 so increase the quantity as to render the trans- 
 mitted beam, for all practical purposes, per- 
 fectly polarized. Indeed, bundles of glass 
 plates are often cmp'oyed as a means of fur- 
 nishing polarized light. 
 
 One word more. When the tourmalines 
 are crossed, the space where they cross each 
 other is black. But we have seen that the 
 least obliquity on the part of the crystals per- 
 mits light to get through both. Now sup- 
 I ose, when the two plates are crossed, that 
 we interpose a third plate of tourmaline be- 
 tween them, wiih its axis oblique to both. A 
 portion of the light transmitted by the first 
 plate will get through this intermediate cne. 
 But, after it has got through, its plane of vi- 
 bration is changed: it is no longer perpendicu- 
 lar to the axis cf the crystal in front. Hence 
 it will get thiough that crystal. Thus, by 
 reasoning, we infer that the interposition of a 
 third plate of tourmaline will in part abolish 
 the darkness produced by the perpendicular 
 crossing of the other two plates. I have not 
 a third plate of tourmaline ; but the talc or 
 mica which you employ in your stoves is a 
 more convenient substance, which acts in the 
 same way. Between the crossed tourmalines 
 I introduce a film of this crystal. You see 
 the edge of the film slowly descending, and 
 as it descends between the tourmalines, light 
 takes the place cf darkness. The darkness, 
 in factj setmed scraped away as if it were 
 something material. This effect has been 
 called and improperly called depolarization. 
 
 LECTURE IV. 
 
 Chromatic Phenomena produced by Crystals on Polar- 
 ized Light: The Nicol Prism : Polarizer and Ana- 
 lyzer: Action of thick and thin Plates of Selenite: 
 Colors dependent, on Thickness: Resolution of Po- 
 larized Beam into two others by the Selenite ; One 
 of them more retarded than the other: Recom- 
 pounding of the two Systems of Waves by the Ana-r 
 lyzer: Interference thus rendered possible : Conse- 
 quent Production of Colors: Action of Bodies 
 Mechanically strained or pressed : Action of Sono- 
 rous Vibrations: Action of Glass strained or pressed 
 by Heat: Circular Polarization: Chromatic Phe- 
 nomena produced by Quartz : The Magnetization 
 of Light: Rings surrounding the Axes of Crystals: 
 Blaxal and Uniaxal Crystals : Grasp of the Undu- 
 latory Theory. 
 
 We now stand upon the threshold of a new 
 and splendid optical domain. We have to 
 examine, this evening, the chromatic phe- 
 nomena produced by the action ot crystals, 
 and double-refracting bodies gene: ally, upon 
 polarized light. For a long time investigators 
 were compelled to employ plates of tourmaline 
 for this purpose, and the progress they made 
 with so defective a means of inquiry is aston- 
 ishing. But these men had their hearts in 
 their work, and were on this account enabled 
 to extract great results from small instrumen- 
 tal appliances. But we have better apparatus 
 now. You have seen the two beams emer- 
 gent from Iceland spar, and have proved 
 them to be polarized. If we could abolish 
 
SIX LECTURES ON LIGHT. 
 
 one of these beams, we might employ the 
 other for experiments on polarized light. 
 
 These beams, as you know, are refracted 
 differently, and from this we are able to infer 
 that under some circumstances the one may 
 be totally reflected, and the other not. An 
 optician, named Nicol, cut a crystal of Ice- 
 land spar in two in a certain direction. He 
 polished the severed urfaces, and reunited 
 them by Canada balsam, the surface of union 
 being- so inclined to the beam traversing the 
 spar that the ordinary ray, which is the most 
 highly refracted, was totally reflected by the 
 balsam, while the extraordinary ray was per- 
 mitted to pass on. The invention of the 
 Nicol prism was a great step in practical op- 
 tics, and quite recently such prisms have 
 been constructed of a size which enables 
 audiences like the present to witness the 
 chromatic phenomena of polarized light to a 
 degree altogether unattainable a short time 
 ago. The two prisms here before you belong 
 to my excellent friend, Mr. William Spottis- 
 woode, and they were manufactured by Mr. 
 Ladd. I have with me another pair of very 
 noble prisms, still larger than these, manu- 
 factured for me by Mr. Browning, who has 
 gained so high and well-merited a reputation 
 in the construction of spectroscopes. 
 
 These two Nicol prisms play the same 
 part as the crystals of tourmaline. Placed 
 with their directions of vibration parallel, 
 the light passes through both. When these 
 directions are crossed, the light is quenched. 
 Introducing a film of mica between the 
 prisms, the light is in part restored. But 
 notice, when the film of mica is thin, you 
 have sometimes not only light, but colored 
 light. Our work for some time to come will 
 be the examination of these colors. With 
 this view, I will take a representative crystal, 
 one easily dealt with; the crystal gypsum, or 
 selenite, which is crystallized sulphate of 
 lime. Between the crossed Nicols I place a 
 thick plate of this crystal; like the mica, it 
 restores the light, but it produce-^ no color. 
 With my penknife I take a thin splinter from 
 this crystal and place it between the prism;- { 
 its image on the screen glows with the richest 
 colors. Turning the prism in front, these 
 colors gradually fade, disappear, but by con- 
 tinuing the rotation until the vibrating sec- 
 tions of the prisms are parallel, vivid colors 
 again appear, but these colors are comple- 
 mentary to the former ones. 
 
 Some patches of the splinter appear of one 
 color, some of another. These differences 
 ar.: due to the different thicknesses of the. 
 film. If the thickness be uniform, the color 
 is uniform. Here, for instance, is a stellar 
 shape, every lozenge of the star being a film 
 of gypsum of uniform thickness. Each 
 lozenge, you observe, shows a brilliant uni- 
 fcrm color. It is easy, by shaping our films 
 so as to represent flowers or other objects, 
 to exhibit such objects in colors unattainable 
 by art. Here, for example, is a specimen of 
 
 heart's-ease, the colors of which you might 
 safely defy the artist to reproduce. By turn- 
 ing the front Nicol ninety degrees round, we 
 pass through a colorless phase to a series of 
 colors complementary to the foroier ones. 
 Here, for example, is a rose tree with red 
 flowers and green leaves; turning the prism 
 ninety degrees round, we obtain a green 
 flower and red leaves. All these wonderful 
 chromatic effects have definite mechanical 
 causes in the motions of the ether. The 
 principle of interference, duly applied and 
 interpreted, explains them all. 
 
 By this time you have learned that the 
 word " light" may be used in two different 
 senses ; it may mean the impression made 
 upon consciousness, or it may mean the phys- 
 ical agent which makes the impression. It 
 is with the agent that we have to occupy our- 
 selves at present. That agent is the motion 
 of a substance which fills ail space, and sur- 
 rounds the atoms and molecules of bodies. 
 To this interstellar and interatomic medium 
 definite mechanical properties are ascribed, 
 and we deal with it as a body possessed of 
 these p operties. In mechanics we have the 
 composition and resolution of forces, and of 
 motions, extending to the composition and 
 resolu.ion of vibrations. We treat the lumi- 
 niferous ether on mechanical principles, and 
 from the composition, resolution, and inter- 
 ference of its vibrations, we deduc; all the 
 phenomena displayed by crystals in polarized 
 light. 
 
 Let us take, as an example, the crystal of 
 tourmaline, with which we are now so famil- 
 iar. Let a vibration cross this erysta* oblique 
 to its axis ; \vj have seen by experiment tnat 
 a portion of the light will pan through. 
 How much, we determine in this way : Draw 
 a straight line representing the intensity of 
 the vibration before it reaches the tourmaline, 
 and from the two ends of this line draw two 
 perpendiculars to the axis of the crystal ; the 
 distance between the feet of these two per- 
 pendiculars will represent the intensity 01 Lae 
 transmitted vibration. 
 
 Follow me now while I endeavor to make 
 clear to you what occurs when a film of 
 gypsum is placed between the JNicol pri>ms. 
 But, at the outset, let us cstabhsn still 
 further the analogy between the action of the 
 prisms and that ot two plates of tourmaline, 
 The plates are now crossed, and you see that 
 by turning the film round, it may be placed 
 in a position where i' has no povver to -abolish 
 the darkness. Why is this? The answer is 
 that \<\ the gypsum there are two directions, 
 at light angles to each other, which, the waves 
 of light are constrained to follow, ;:nd that 
 now one of these directions is parallel to one 
 of the axes of the tourmaline, and the oiher 
 parallel to the other axis. When this is the 
 case, the film exercises no sensible action 
 upon the light. But now I turn the film so 
 as to render its direction of vibration oblique 
 to the axes ; then you see it has the power, 
 
SIX LECTURES ON LIGHT. 
 
 demonstrated in tneiast lecture, of restoring 
 tne light. 
 
 Let us now mount our Nicol prisms and 
 cross them as we crossed the tourmalines. 
 Introducing our film of gypsum between them 
 you notice tnat in one particular position the 
 film has no power whatever over the field of 
 view. But, when the film is turned a little 
 way round, the light passes. We have now 
 to understand the mechanism by which this 
 is effected. 
 
 Firstly, then, we have this first prism which 
 receives the light emergent from the electric 
 (amp, and which is called the polarizer. Then 
 we have the p ate of gypsum, placed at S 
 (Fig. 17), and then the prism in front, which 
 is called the analyzer. On its emergence 
 from the first prism, the light is polarized ; 
 and in the particular case now before us, its 
 vibrations are executed in an horizontal plane. 
 The two directions of vibration of ; he gypsum, 
 placed at S, are now oblique to the horizon. 
 Draw a rectangular cross upon paper to rep- 
 resent the two directions of vibration within 
 the gypsum. Draw an oblique line to repre- 
 sent the intensity of the vibration when it 
 reaches the gvpsum. Let fall from the two 
 ends of this line two perpendiculars on each 
 of the arms of the cross ; then the distances 
 between the feet of these perpendiculars rep- 
 resent the intensities of two rectangular vi- 
 brations which are the equivalents of the first 
 single vibration. Thus the polarized ray, 
 when it enters the gypsum, is resolved into 
 two others, vibrating at right angles to each 
 other. 
 
 Now, in one of those directions of vibnition 
 the ether is more sluggish than in the other ; 
 and, as a consequence, the waves that follow 
 this direction are more retarded than the 
 others. The waves of both systems, in fact, 
 are shortened when they enter the gypsum, 
 but the one system is more shortened than the 
 
 other. You can readily imagine that in this 
 way the one system of waves may get half a 
 wave-length, or indeed any number of halt 
 wave-lengths, in advance of the other. The 
 possibility of interference here flashes upon 
 the mind. A little consideration, however, 
 renders it evident that, as long as the vibra- 
 tions are executed at right angles to each 
 other, they cannot quench each other, no 
 matter what the retardation may be. This 
 brings us at once to the part played by the 
 analyzer. Its sole function is to recompound 
 i he two vibrations emergent from the gypsum. 
 It reduces them to a single plane, where, if 
 one of them be retarded by the proper 
 amount, extinction can occur. But here, as 
 in the case of thin films, the different lengths 
 of the waves of light come into play. Red 
 will require a greater thickness to produce the 
 retardation necessary for extinction than blue; 
 consequently, when the longer waves have 
 been withdrawn by interference, the shorter 
 ones remain and confer their colors on the 
 film of gypsum. Conversely, when the 
 shorter waves have been withdrawn, the 
 thickness is such that the longer waves re- 
 main. An elementary consideration suffices 
 to show that, when the directions of vibration 
 of prisms and gypsum enclose an angle of 
 forty-five degrees, the colors are at their 
 maximum brilliancy. When the film is 
 turned from this direction, the colors gradu- 
 ally fade, until, at the point where the direc- 
 tions are parallel, they disappear altogether. 
 
 A knowledge of these phenomena ':*> best 
 obtained by means of a model of wood or 
 aasteboard representing the plate of gypsum, 
 ts planes of vibration, and also those of the 
 Dolarizer and analyzer. On these planes the 
 waves may be drawn, showing the resolution 
 of the first polarized ray into two others, and 
 hen the reduction of tne two vibrations to a 
 common plane. Following out ligidly the 
 nte/action of the two systems of waves, we 
 are taught by such a model that a'l the phe- 
 nomena of color, obtained, when the planes of 
 vibration of the two Nicols are parallel, are 
 displaced by the complementary phenomena 
 when the Nicols are perpendicular to each 
 other. 
 
 In considering the next point, for the sake 
 of simplicity, we will operate with monochro- 
 matic light with red light, for example. 
 Supposing that a certain thickness of the gyp- 
 sum produces a retardation of half a wave- 
 length, twice this thickness will produce a 
 retaidation of two half wave-lengths; three 
 times this thickness a retardation of three 
 half wave-lengths, and so on. Now, when 
 the Nicols are parallel, the retardation of 
 half a wave-length, cr of any odd number D 
 half wave-lengths, produces extinction; at ail 
 thicknesses, on the other hand, which corre- 
 spond to a retardation of an even number of 
 half wave-lengths, the two beams support each 
 other, when they are brought to a common 
 plane by the analyzer. S-pposing, then, 
 
30 
 
 SIX LECTURES ON LIGHT. 
 
 that we take a plate of a wedge-form, which 
 grows gradually thicker from edge to back, 
 we ought to expect in red light a series of 
 recurrent bands of light and darkness ; the 
 dark bands occurring at thicknesses which 
 produce retardations of one, three, five, etc., 
 half wave-lengths, uhile the light bands occur 
 between the dark ones. Expeiiment proves 
 the wedge-shaped crystal to show these bands 
 but they are far better shown by this circular 
 film, which is so worked as to be thinnest at 
 the centre, gradually increasing in thickness 
 from the centre outwards. These splendid 
 rings of light and darkness are thus produced. 
 When, instead of employing red light, we 
 employ blue, the rings are also seen ; but as 
 they occur at thinner portions of the film, 
 they are smaller than the rings obtained with 
 the red light. The consequence of employ- 
 ing %uhite light may now be inferred : inas- 
 much as the red and the blue fall in different 
 places, we have iris-colored rings produced by 
 the white light. 
 
 Some of the cVomatic effects of irregular 
 crystallization are beautiful in the extreme. 
 Could I introduce between our Nicols a pane 
 of glass covered by those frost-ferns which 
 the cold weather renders now so frequent, 
 rich colors would be the result. The beauti- 
 ful effects of irregular crystallization on glass 
 plates, row presented to you, illustrate what 
 you might e.\pect from the irosted window- 
 pane. And not only do crystalline bodies 
 act thus upon light, but almost all bodies that 
 possess a definite structure do the same. As 
 a general rule, organic bodies act in this way; 
 for their architecture implies an arrangement 
 of the ether which involves double refraction. 
 A film of horn, or the section of a shell, for 
 example, yields very beautiful colors in polar- 
 ized light. In a tree, the ether certainly pos- 
 sesses different degrees of elasticity along and 
 across the fibre; and, were wood transparent, 
 this peculiarity of molecular structure would 
 infallibly reveal itself by chromatic phe- 
 nomena like those that you have seen. But 
 not only do todies built permanently by 
 Nature behave in this way, but it is possible, 
 as shown by Brewster, to confer, by strain or 
 by pressure, a temporary double-refracting 
 structure upon non-crystalline bodies, such as 
 common glass. 
 
 When I place this bar of wood across my 
 knee and seek to break it, what is the 
 mechanical condition of the bar ? It bends, 
 and its convex surface is strained longitudi- 
 nally; its concave surface, that next my knee, 
 is longitudinally pressed. Both in the 
 strained portion and in the pressed portion 
 the ether is thrown into a condition which 
 would rend r the wood, were it transparent, 
 double refracting. Let us repeat the experi- 
 ment with a bar of glass. Between the 
 crossed Nicols I introduce such a bar. By 
 the dim residue of light lingering upon the 
 screen, you see the image of the glass, but it 
 has no effect upon the light. I simply bend 
 
 the gla^s bar with my finger and thumb, 
 keeping its length oblique to the directions of 
 vibration in the Nicols. Instantly light 
 flashes out upon the screen. The two sides 
 of the bar are illuminated, the ed.jes most, 
 for here the strain and pressure are greatest. 
 In passing from strain to pressure, we cross a 
 portion of the glass where neither is exerted. 
 This is the so-called neutral axis of the bar 
 of glass, and along it you see a dark band, 
 indicating that the j;lass along this axis exer- 
 cises no . ction upon the light. By employ- 
 ing the force of a press, instead of the lorce 
 of my finger and thumb, the brilliancy of the 
 light is greatly augmented. 
 
 Again, I have here a square of glass which 
 can be inserted into a press of another kind. 
 Introducing the square between the prisms, 
 its neutrality is declared ; but it can hardly 
 be held sufficiently loosely to prevent its 
 action from manifesting itself. Already, 
 though the pressure is infinitesimal, you see 
 spots of light at the points where the press is 
 in contact with the glass. I now turn this 
 screw. Instantly the image of the square of 
 glass flashes out upon the screen. You see 
 luminous spaces separated from each other 
 by dark bands. Every pair of adjacent 
 luminous spaces is in opposite mechanical 
 conditions. On one side of the dark band 
 we have strain, on the other side pressure ; 
 while the dark band marks the neutral axis 
 between both. I now tighten the vise, and 
 you see color ; tighten still more, and the 
 colors appear as rich as those presented by 
 crystals. Releasing the vise, the colors 
 suddenly vanish ; tightening suddenly, they 
 reappear. From the colors of a soap-bubble 
 Newton was able to infer the thickness of the 
 bubble, thus uniting by the bond of thought 
 apparently incongruous things. From the 
 colors here presented to you, the magnitude 
 of the pressure employed might be inferred. 
 Indeed, the late M. Wertheim, of Paris, in- 
 vented an instrument for the determination 
 of strains and pressures by the colors of 
 polarized light, which exceeded in accuracy 
 all other instruments of the kind. 
 
 You know that bodies are expanded by 
 heat and contracted by cold. If the heat be 
 applied with perfect uniformity, no local 
 strains cr pressures come into play ; but, if 
 one portion of a solid be heated and others 
 not, the expansion of the heated portion intro- 
 duces strains and pressures which reveal 
 themselves under the scrutiny of polarized 
 ight. When a square of common window- 
 jlass is placed between the Nicols, you see 
 ts dim outline, but it exerts no action on the 
 polarized light. Held for a moment over the 
 :lame of a spirit-lamp, on reintroducing it 
 between the Nicols, light flashes out upon 
 the screen. Here, as in the case of mechan- 
 cal action, you have spaces of strain divided 
 3y neutral axes from spaces of pressure. 
 
 Let us apply the heat more symmetrically. 
 This small square of glass is perforated at 
 
SIX LECTURES ON LIGHT. 
 
 the centre, and into the orifice a bit of copper 
 wire is intioduced. Placing the square be- 
 tween the prisms, and heating the copper, 
 thft heat passes by conduction along the wire 
 to the glass, through which it spreads from 
 the centre outwards. You sse a dim cross 
 bounding four luminous quadrants growing 
 up and becoming gradually black by compari- 
 son with the adjacent brightness. And as, in 
 the case of pressure, we produced colors, so 
 here also, by the proper application of heat, 
 gorgeous chromatic effects may be produced, 
 and they may be rendered permanent by first 
 heating the glass sufficiently, and then cool- 
 ing it, so that the chilled mass shall remain 
 in a state of strain and pressure. Two or 
 three examples will illustrate this point. The 
 colors, you observe, are quite as rich as those 
 obtained in the case of crystals. 
 
 And now we have to push these considera- 
 tions to a final illustration. Polarized light 
 may be turned to account in various ways as 
 an analyzer of molecular condition. A strip 
 of glass six feet long, two inches wide, and 
 a quarter of an inch thick, is held at the 
 centre between my finger and thumb. I 
 sweep over one of its halves a wet woolen 
 rag ; you hear an acute sound, due to the 
 vibrations of the glass. What is the condi- 
 tion of the glass while the sound is heard ? 
 This . its two halves lengthen and shorten in 
 quick succession. Its t\vo ends, therefore, 
 aie in a state of quick vibration ; but at the 
 centre the pulses from the two ends alter- 
 nately meet and retreat. Between their 
 opposing actions, the glass at the centre is 
 kept motionless ; but, on the other hand, it 
 is alternately strained and compressed. The 
 state of the glass may be illustrated by a row 
 of spots of light, as the propagation of a 
 sonorous pulse was illustrated in a former 
 
 FIG. 1 8. 
 
 lecture. By a simple mechanical contrivance 
 the spots are made to vibrate to and fro. 
 The terminal dots have the largest amplitude 
 
 of vibration, while those at the centre are 
 alternately crowded together and torn 
 asunder, the centre one not moving at all. 
 The condition of the sounding strip of glass 
 is here correctly represented. In Fig. 18, A 
 B represents the glass rectangle with its 
 centre condensed ; while A' B' represents the 
 same rectangle with its centra rarefied. 
 
 If we introduce the glass s s' (Fig. 19) be- 
 tween the crossed Nicols, taking care to keep 
 the strip oblique to the direction of vibration 
 of the Nicols, and sweep our wet rubber over 
 the glass, this may bi expected to occur : At 
 every moment of compression the light will 
 flash through ; at every moment of strain the 
 light will also ilash through ; and these states 
 of strain and pressure will follow each other 
 so rapidiy that we may expect a permanent 
 luminous impression to be made upon the 
 eye. By pure reasoning, therefore, we reach 
 the conclusion that the light will be revived 
 whenever the glass is sounded. That it is so, 
 experiment testifies : at every sweep of the 
 rubber, a fine luminous disk (o) flashes out 
 upon the screen. The experiment may be 
 varied in this way : Placing in fron of the 
 polarizer a plate of unannealed glass, you 
 have those beautiful colored rings, intersected 
 by a black cross. Every sweep of the rubber 
 not only abolishes the rings, but introduces 
 complementary ones, the black cross bein^ 
 for the moment supplanted by a white one. 
 This is a modification of an experiment 
 which we owe to Biot. His apparatus, how- 
 ever, confined the observation of it to a single 
 person at a time. 
 
 But we have to follow the ether still 
 further. Suspended before you is a pendu- 
 lum, which, when drawn aside and then 
 liberated, oscillates to and fro. If when the 
 pendulum is passing the middle point of its 
 excursion, I impart a shock to it tending to 
 drive it at right angles to its present course, 
 what occurs? The two impulses compound 
 themselves to a vibration oblique in direction 
 to the former one, but the pendulum oscil- 
 lates in a plane. But, if the rectangular 
 -hock be imparted to the pendulum whpn it 
 is at the limit of its swing, then the com- 
 pounding of the two impulses causes the sus- 
 pended ball to describe not a straight line, 
 but an ellipse ; and, if the shock be compe- 
 tent of itself to produce a vibration of the 
 same amplitude as the first one, the ellipse 
 becomes a circle. But why do I dwell upon 
 these things ? Simply to make known to you 
 the resemblance of these gross mechanical 
 vibrations to the vibrations of light. 1 hold 
 in my hand a plate of quartz cut from the 
 crystal perpendicular to its axis. This crys- 
 tal thus cut possesses the extraordinary power 
 of twisting the plane of vibration of a. polar- 
 ized ray to an extent dependent on the thick- 
 ness of the crystal. And the more refrangi- 
 ble the light the greater is the amount of 
 twisting, so that, when white light is em- 
 ployed, its constituent colors are thus drawo 
 
SIX LECTURES ON LIGHT. 
 
 asunder. Placing 1 the quartz between the 
 polarizer and the analyzer, you see this 
 splendid color, and, turning the analyzer in 
 front, from right to left, the other colors 
 apoear in succession. Specimens of quartz 
 have been found which require the analyzer 
 to be turned from left to right, to obtain the 
 same succession of colors. Crystals of the 
 first class are therefore called right-handed, 
 ar.d, of the second class, left handed crystals. 
 With profound sagacity, Fresne , to whose 
 genius \ve mainly owe the expansion and 
 final triumph of the undulatory theory of 
 light, reproduced mentally the mechanism of 
 these crystals, and showed their action to be 
 due to the circumstance that, in them, the 
 v/aves of ether so act upon each other as to 
 produce the condition represented by our 
 rotating pendulum. Instead of being plane 
 polarized, the light in rock crystal is circu- 
 larly polarized. Two such rays transmitted 
 along the axis of the crystal, and rotating in 
 
 although the mixture of blue and yellow pig- 
 ments produces green, the mixture of biuc 
 and yellow lights produces white. By en- 
 larging our aperture, the two images pro- 
 duced by the spar are caused to approach 
 each other, and finally to overlap. The one 
 is now a vivid yellow, the other a vivid blue* 
 and you notice that where the colors are 
 superposed we have a pure white. (See Fig. 
 20, where N is the nozzle of the lamp, Q the 
 quartz plate, L a lens, and B the birefracting 
 spar. The two images overlap at O, and 
 produce white by their mixture.) 
 
 This brings us to a point of our inquiries 
 which, though not capable of brilliant illus- 
 tration, is nevertheless so likely to affect pro- 
 foundly the future course of scientific thought 
 that I am unwilling to pass it over without 
 reference. I refer to the experiment which 
 Faraday, its discoverer, called the magnetiza- 
 tion of light. The arrangement for thL 
 celebrated experiment is now before you. 
 
 FIG. 19. 
 
 opposite directions, when brouih': to inter- 
 ference by the analyzer, are demonstrably 
 competent to produce the observed phe- 
 nomena. 
 
 1 now abandon the analyzer, and put in its 
 place the piece of Iceland spar with which 
 we have already illustrated double refraction. 
 The two images of the carbon-points are 
 now before you. Introducing a plate of 
 quartz between the polarizer and the spar, 
 the two images glow with complementary 
 colors. Emplo)ing the image of an aperture 
 instead of that of the carbon-points, we have 
 two complementary colored circles. As the 
 analyzer is caused to rotate, the colors pass 
 through various changes ; but they are al- 
 ways complementary to each other. If the 
 one be red, the other will be green ; if the 
 one be yellow, the other will be blue. Here 
 we have it in our power to demonstrate afresh j 
 a statement made in a former lecture, that, i 
 
 We have first our electric lamp, then n Nicot 
 prism, to polarize the beam emergeni from 
 the lamp ; then an electro-magnet, then a 
 second Nicol prism, and finally our screen. 
 At the present moment the prisms are 
 crossed, and the screen is dark. I place 
 from pole to pole of the electro magnet a 
 cylinder of a peculiar kind of glass, first 
 made by Faraday, and called Faraday's heavy 
 glass. Through this glass the b^am from 
 the polarizer now passes, being intercepted 
 by the Nicol in front. I now excite the 
 magnet, and instantly light appears upon the 
 screen. On examination, we find that, by 
 the action of the magnet upon the ether con- 
 tained within the heavy glass, the plane 01 
 vibration is caused to rotate, thus enabling 
 the light to get through the anaFyzer. 
 
 The two classes into which quartz-crystals 
 are divided have been already mentioned. 
 In my hand I hold a compound piate, ons- 
 
SIX LECTURES ON LIGHT. 
 
 half of it taken from a right-handed and the 
 other from a left-handed crystal. Placing 
 the plate in front of the polarizer, we turn 
 one of the Nicols until the two halves of the 
 plate show a common puce color. This 
 yields an exceedingly sensitive means of ren- 
 dering the action of a magnet upon light 
 By turning either the polarizer or 
 
 the analyzer through the smallest angle, tne 
 uniformity of the color disappears, and the 
 two halves of the quartz show different colors. 
 The magnet also produces this effect. The 
 puce-coicred circle is now before you on the 
 screen. (See Fig. 21 for the arrangement of 
 the experiment. N is the nozzle of the lamp, 
 H the first Nico), Q the biquartz plate, L a 
 lens, M the electro-magnet, and P the second 
 Nicol.) Exciting the magnet, one half of 
 the image becomes suddenly red, the other 
 half green. Interrupting the current, the 
 two colors fade away, and the primitive puce 
 is restored. The action, moreover, depends 
 upon the polarity of the magnet, or, in other 
 words, on the direction of the current which 
 surrounds the magnet. Reversing the cur- 
 rent, the red and green reappear, but they 
 have changed places. The red was for- 
 merly to the right, and the green to the left ; 
 the green is now to the right, and the red to 
 the left. With the most exquisite ingenuity, 
 Faraday analyzed all those actions ar.d stated 
 their laws. This experiment, however, long 
 remained rather as a scientific curiosity than as 
 a fruitful germ. That it would bear fruit of 
 the highest ; mportance, Faraday felt pro- 
 foundly convinced, and recent researches are 
 on the way to verify his conviction. 
 
 A few words more are necessary to com- 
 plete our knowledge ot the wonderful inter- 
 action between ponderable molecules and the 
 ether interfused among them. Symmetry of 
 
 molecular arrangement implies symmetry on 
 the part of the ether ; atomic dissymmetry, 
 on the other hand, involves the dissymmetry 
 of the ether, and, as a consequence, double 
 refraction. In a certain class of crystals the 
 structure is homogeneous, and such crystals 
 produce no double refraction. In certain 
 
 other crystals the moJ r jles are ranged sym- 
 metrically around ? Certain line, and not 
 around others. A'ouglhe former, therefore, 
 the ray is undivided, while along ail the 
 others we have double refraction. Ice is a 
 familiar example ; it is built with perfect 
 symmetry around the perpendiculars to the 
 planes of freezing, and a ray sent through 
 ice in this direction is not doubly refracted ; 
 whereas, in all other directions, it is. Ice- 
 land spar is another example of the same 
 kind : its molecules are built symmetrically 
 round the line uniting the two blunt angles 
 of the rhomb. In this direction a ray suffers 
 no double refraction, in all others it does. 
 This direction of double refraction is called 
 the optic axis of the crystal. 
 
 Hence, if a plate be cut from a crystal of 
 Iceland spar perpendicular to the 3x:s, a." 
 rays sent across this plate in the dirction c 
 the axis will produce but cne image. Bu* 
 the moment we deviate from the parallelism 
 with the axis, double refraction sets in. If, 
 therefore, a beam that has been rendered 
 conical by a converging lens be sent through 
 the spar so that the central ray of the cone 
 passes along the axis, this ray only will es- 
 cape double refraction. Each of the others 
 will be divided into an ordinary and extraor- 
 dinary ray, ihe one moving more slowly 
 through the crystal than the other ; the one, 
 therefore, retarded with reference to the 
 other. Here, then, we have the conditions 
 
SIX LECTURES ON LIGHT. 
 
 for interference, when the waves are reduced 
 by the analyzer to a common plane. A 
 highly beautiful and important source of 
 chromatic phenomena is thus revealed. 
 Placing the plate of spar between the crossed 
 prisms we have upon the screen a beautiful 
 system of iris rings surrounding the end of 
 the optic axis, the circular bands of color 
 being intersected by a black cross. The 
 arms ^f th : s cross are parallel to the two 
 directions of vibration in the polarizer and 
 analyzer. It is easy to see that those rays 
 whose planes of vibration within the spar 
 coincide with the plane of vibration of either 
 prism, cannot get through both. This com- 
 plete interception produces the arms of the 
 cross. With mono-chromatic light the rings 
 would be simply bright and black the 
 bright rings occurring at those thicknesses of 
 the spar which cause the rays to conspire ; 
 the black rings at those thicknesses which 
 cause them to quench each other. Here, 
 however, as elsewhere, the different lengths 
 of the light-waves give rise to iris-colors 
 when white light is employed. 
 
 Besides the regular crystals which produce 
 double refraction in no direction, and the 
 nniaxal crystals which produce it in all direc- 
 tions but one, Brewster discovered that in a 
 large class of crystals there are two directions 
 in which double refraction does not take 
 place. These are called biaxal crystals. 
 When plates ot these crystals, suitably cut, 
 , are placed between the polarizer and analyzer, 
 Ihe axes are seen su -ounded, not by circles, 
 but by curves of ano her order and of a per- 
 fectly definite mathem* : cal character. Each 
 band, as proved experimentally by Herschel, 
 forms a lemniscata ; but the experimental 
 proof was here, as in numberless other cases, 
 preceded by the deduction which showed that, 
 according to the undulatory theory, the bands 
 must possess this special character. 
 
 I have taken this somewhat wide range 
 over polarization itselt and over the phenom- 
 ena exhibited by crystals in polarized light, 
 in order to give you some notion of the firm- 
 ness a. id completeness of the theory which 
 grasps them all. Starting from the single 
 assumption of transverse undulations, we 
 first of all determine the wave-lengths, and 
 find all the phenomena of color dependent 
 on this element. The wave-lengths may be 
 determined in many independent ways, and, 
 when the lengths so determined are compared 
 together, the strictest agreement is found to 
 exist between them. We follow the ether 
 into the most complicated cases of interac- 
 tion between it and ordinary matter, "the 
 theory is equal to them all. It makes not a 
 single new hypothesis ; but out of its original 
 stock of principles it educes the counterparts 
 of all that observation shows. It accounts 
 for, explains, simplifies the most entangled 
 cases ; corrects known laws and facts ; pre- 
 dicts an 1 discloses unknown ones ; becomes 
 the guide of its former teacher Observation ; 
 
 and, enlightened by mechanical conceptions, 
 acquires an insight which pierces through 
 shape and color to force and cause. "* 
 
 But, while I have thus endeavored to illus- 
 trate before you the power of the undulatory 
 theory as a solver of all the difficulties of 
 optics, do I therefore wish you to close your 
 eyes to any evidence that may arise against 
 it ? By no means. You may urge, and 
 justly urge, that a hundred years ago another 
 theory was held by the most eminent men, 
 and that, as the theory then held had to 
 yield, the undulatory theory may have to 
 yield also. This is perfectly logical ; but let 
 us understand the precise value of the argu- 
 ment. In similar language a person in the 
 time of Newton, or even in our time, might 
 reason thus: " Hipparchus and Ptolemy, 
 and numbers of great men after them, be- 
 lieved that the earth was the centre of the 
 solar system. But this deep-set theoretic 
 notion had to give way, and the theory of 
 gravitation may, in its turn, have to give 
 way also." This is just as logical as the first 
 argument. Wherein consists the strength of 
 the theory of gravitation ? Solely in its com- 
 petence to account for all the phenomena of the 
 solar system. Wherein consists the strength 
 of the theory of undulation? Solely in its 
 competence to disentangle and explain phe- 
 tnomena a hundred-fold more complex than 
 those of the solar system. Be as skeptical, 
 if you like, regarding the undulatory theory ; 
 but if your skepticism be philosophical, it 
 will wrap the theory of gravitation in the 
 same or greater doubt, f 
 
 LECTURE V. 
 
 Range of vision incommensurate with Range of Radi- 
 ation: The Ultra- Violet Rays: Fluorescence: 
 Rendering Invisible Rays visible: Vision not the 
 only Sense appealed to by the Solar and Electric 
 Beam : Heat of Beam : Combustion by Total Beam 
 at the Foci of Mirrors and Lenses: Combustion 
 through Ice-Lens: Ignition of Diamond: Search 
 for the Rays here effective : Sir Villiain Herschel's 
 Discovery of Dark Solar Rays: Invisible Rays the 
 Basis of ttie Visible : Detachment by a Ray-Filter 
 of the Invisible Rays from the Visible; Combustion 
 at Dark Foci: Conversion of Heat- Rays into 
 Light-Rays: Calorescence : Part played in Nature 
 by Dark Rays: Identity of Light and Radiant 
 Heat: Invisible Images: Reflection, Refraction, 
 Plane Polarization, Depolariz.ition. Circular Polar- 
 ization, Double Refraction, and Magnetization of 
 Radiant Heat. 
 
 THE first question that we have to con- 
 sider to-night is this : Is the eye, as an organ 
 of vision, commensurate with the wnole 
 range of solar radiation is it capable of re- 
 ceiving visual impressions from all the rays 
 emitted by the sun? The answer is nega- 
 tive. If we allowed ourselves to accept for a 
 
 * Whewell. 
 
 t The only essay known to me on the Undulatory 
 Theory, from the pen of an American writer, is an 
 excellent one by President Barnard, published in ths 
 Smithsonian Report for 1862. 
 
SIX LECTURES ON LIGHT. 
 
 35 
 
 moment that notion of gradual growth, 
 amelioration, and ascension, implied by the 
 term evolution, we might fairly conclude that 
 there a*-e stores of visual impressions awaiting 
 man far greater than those of which he is 
 now in possession. For example, here be- 
 yond the extreme violet of the spectrum there 
 is a vast efflux of rays which are totally use- 
 less as regards our present powers of vision. 
 But these ultra-violet waves, though incom- 
 petent to awaken the optic nerve, can so 
 shake the molecules of certain compound sub- 
 stances as to effect their decomposition. The 
 grandest example of the chemical action of 
 light, with which my friend Dr. Draper has 
 so indissolubly associated his name, is that of 
 the decomposition of carbonic acid in the 
 leaves of plants. All photography is founded 
 on such actions. There are substances on 
 which the ultra-violet waves exert a special 
 decomposing power ; and, by permitting the 
 invisible spectrum to fall upon surfaces pre- 
 pared with such substances, we leveal both 
 the existence and the extent of the ultra- 
 violet spectrum. 
 
 This mode of exhibiting the action of the 
 ultra-violet rays has been long known ; in- 
 deed, Thomas Young photographed the ultra- 
 violet rings of Newton. We have now to 
 demonstrate their presence in another way. 
 As a general rule, bodies transmit light or 
 absorb it, but there is a third case in which the 
 light falling upon the body is neither trans- 
 mitted nor absorbed, but converted into light of 
 another kind. Professor Stokes, the occupant 
 of the Chair of Newton in the University of 
 Cambridge, one of those original workers 
 who, though not widely known beyond 
 scientific circles, realiy constitute the core of 
 science, has demonstrated this change of one 
 kind of light into another, and has pushed 
 his experiments so far as to render the invisi- 
 ble rays visible. 
 
 A long list of substances examined by 
 Stokes when excited by the invisible ultra- 
 violet waves, have been proved to emit light. 
 You know the rate of vibration corresponding 
 to the extreme violet of the spectrum ; you 
 are aware that, to produce the impression of 
 this color, the retina is struck 789 millions of 
 millions of times in a second. At this point, 
 the retina ceases to be useful as an organ of 
 vision, for, though struck by waves of more 
 rapid recurrence, they are incompetent to 
 awaken the sensation of light. But, when 
 such non-visual waves are caused to impinge 
 upon the molecules of certain substances 
 on those of sulphate of quinine, for example 
 they compel those molecules, or their con- 
 stituent atoms, to vibrate ; and the peculiar- 
 ity is, that the vibrations thus set up are of 
 slower period than those of the exciting 
 waves. By this lowering of tr.e rate of vi- 
 bration through the intermediation of the sul- 
 phate of quinine, the invisible rays are ren- 
 dered visible. Here we have our spectrum, 
 and beyond the violet I place this prepared 
 
 paper. The spectrum is immediately elonga- 
 ted by the generation of a new light beyond 
 the extreme violet. President Morton has 
 recently succeeded in discovering a substance 
 of great sensibility which he has named 
 Thalloie, and he has been good enough to 
 favor me with some paper saturated with a 
 solution of this substance. It causes a very 
 striking enlongation of the spectrum, the 
 new light generated being of peculiar bril- 
 liancy. To this change of the rays from a 
 higher to a lower refrangibility, Stokes has 
 given the name of Fluorescence. 
 
 By means of a deeply-colored violet glass, 
 we cut off almost the whole of the light of 
 our electric beam ; but this glass is peculiarly 
 transparent to the violet and ultra-violet 
 rays. The violet beam now crosses a large 
 jar filled with water. Into it I pour a solu- 
 tion of sulphate of quinine : opaque clouds, 
 to all appearance, instantly tumble down- 
 wards. But these are not clouds : there is 
 nothing precipitated here : the observed ac- 
 tion is a > action of molecules, not of particles. 
 The medium before you is not a turbid me- 
 dium, for, when you look through it at a 
 luminious surface, it is perfectly clear. If we 
 paint upon a piece of paper a flower or a 
 bouquet with the sulphate of quinine, and ex- 
 pose it to the full beam, scarcely anything is 
 seen. But on interposing the violet glass, 
 the design instantly flashes forth in strong 
 contrast with the deep surrounding violet. 
 Here is such a design prepared for me by 
 President Morton with his thallene : placed 
 in the violet light it exhibits a peculiarly 
 vivid and beautiful fluorescence. From the 
 experiments of Dr. Bence Jones, it would 
 seem that there is some substance in the hu- 
 man body resembling the sulphate of quinine, 
 which causes ail the tissues of the body to be 
 more or less fluorescent. The crystalline 
 lens of the eye exhibits the effect in a very 
 striking manner. When I plunge my eye 
 into this violet beam, I am conscious of a 
 whitish-blue shimmer filling the space before 
 me. This is caused by fluorescent light gen- 
 erated in the eye itself ; looked at from with- 
 out, the crystalline lens at the same time 
 gleams vividly. 
 
 But the waves from our incandescent car- 
 bon-points appeal to another s^nse than that 
 of vision. They not only produce light as a 
 sensation; they also produce heat. The mag- 
 nified image of the carbon-points is now upon* 
 the screen, and with a suitable instrument the 
 heating power of that instrument might be 
 demonstrated. Here, however, the heat is 
 
 pread over too large an area to be intense. 
 By pushing out the lens and causing a mova- 
 ble screen to approach our lamp, the imige 
 becomes smaller and smaller : the rays be- 
 come more concentrated, until finally they 
 are able to pierce black paper with a burning 
 
 ing. Rendering the beam parallel, and re- 
 ceiving it upon a concave mirror, the rays are 
 
36 
 
 SiX LECTURES ON LIGHT. 
 
 brought to a focus ; and paper placed at the 
 focus is caused to smoke and burn. This 
 may be done by our common camera with its 
 lens, an.d by a concave mirror of very mod- 
 erate power. 
 
 We \vi!l now adopt stronger measures with 
 the radiation from the electric lamp. In this 
 camera of blackened tin is placed a lamp, in 
 all particulars similar to those already em- 
 ployed. But, instead of gathering up the 
 rays from a carbon-point by a condensing 
 tens placed in front of them, we gather them 
 up by a concave mirror, silvered in front, 
 and placed behind the carbons. By this 
 mirror we can cause the rays to issue through 
 the orifice in front, either parallel or conver- 
 gent. They are now parallel, and therefore 
 to a certain extent diffused. We place a 
 convex lens in the path of the beam ; the 
 light is converged to a focus, and at that 
 focus you s;e that paper is not only pierced 
 and a burning ring formed, but that it is in- 
 stantly set ablaze. Many metals may be 
 burned up in the same way. In our first lec- 
 ture the combustibility of zinc was mentioned. 
 Placing a strip of sheet-zinc at this focus, it 
 is instantly ignited and burns with its charac- 
 teristic purple flame. (In the annexed figure 
 m m f represents the concave mirror, L the 
 
 4 " 
 
 lens, at the focus C of which combustion is 
 effected). Dr. Scoresby succeeded in ex- 
 ploding gunpowder by the sun's rays con- 
 verged by large lenses of ice ; the same effect 
 may be produced with a small lens, and with 
 a terrestrial source of heat. In an iron mould 
 we have fashioned this beautiful lens of 
 transparent ice. At the focus of the lens I 
 place a bit of black paper, with a little gun- 
 cotton folded up within it. The paper ignites 
 and the cotton explodes. Strange, is it not, 
 that the beam should possess such heating 
 power after having passed through so cold a 
 substance ? 
 
 In this experiment, you observe that, before 
 the beam reaches the ice-lens, it has passed 
 
 ' through a glass cell containing water. The 
 beam is thus sifted of constituents, which, it 
 permitted to fall upon the lens, would injure 
 its surface, and blur the focus. And this 
 leads me to say an anticipatory word regard- 
 ing transparency. In our first lecture we 
 entered fully into the production of colors 
 by absorption, and we spoke repeatedly 
 of the quenching of the rays of light. Did 
 this mean that the light was altogether 
 annihilated? By no means. It was sim- 
 ply so lowered in refrangibility as to 
 escape the visual range. // was converted 
 into heat. Our red ribbon in the green of the 
 spectrum quenched the green, but if suitably 
 examined its temperature would have been 
 found raised. Our green ribbon in the red 
 of the spectrum quenched the red, but its 
 temperature at the same time was augmented 
 to a degree exactly equivalent to the light ex- 
 tinguished. Our black ribbon, when passed 
 through the spectrum, was found competent 
 to quench all its colors ; but at every stage of 
 its progress an amount of heat was generated 
 in the ribbon exactly equivalent to the light 
 lost. It is only when absorption takes place 
 that Jieat is thus pi'oduced ; and heat is always 
 a result of absorption. 
 
 Examine this water, then, in front of the 
 lamp, after the beam has passed a little time 
 through it : it is sensibly warm, and, if per- 
 mitted to remain there long enough, it may 
 be made to boil. This is due to the absorp- 
 tion by the water of a portion of the electric 
 beam. But a certain portion passes through 
 unabsorbed, and does not at all contribute to 
 the heating of the water. Now, ice is also 
 transparent to the latter portion, and there- 
 fore is not melted by it ; hence, by employ- 
 ing this particular portion of the beam, we 
 are able to keep our lens intact, and to pro- 
 duce by means of it a sharply-defined focus. 
 Placed at that focus, black paper instantly 
 burns, because the black paper absorbs the 
 light which had passed through the ice-lens 
 without absorption. In a subsequent lecture, 
 we shall endeavor to penetrate further into 
 the physical meaning of these and other simi- 
 lar actions. I may add to these illustrations 
 of heating power, the ignition cf a diamond 
 in oxygen, by the concentrated beam of the 
 electric lamp. The diamond, surrounded by 
 a hood of platinum to lessen the chilling due 
 to convection, is exposed at the focus. It is 
 rapidly raised to a white heat, and when re- 
 moved from the focus continues to glow like 
 a star. 
 
 Placed in the path of the beam issuing from 
 our lamp is a cell with glass sides containing 
 a solution of alum. All the light of the beam 
 passes through this solution. The beam is 
 received on a powerfully converging mirror 
 silvered in front, and is brought to a focus by 
 the mirror. You can see the conical beam 
 of reflected light tracking itself through the 
 dust of the room. I place at the focus a 
 scrap of white paper : it glows there with 
 
SIX LECTURES ON LIGHT. 
 
 37 
 
 dazzling btightness, but it is not even charred. 
 On removing the alum-cell, however, the 
 paper instantly inflames. There must, there- 
 fore, be something in this beam besides its 
 light. The light is not absorbed by the 
 white paper, and therefore does not burn the 
 paper ; but there is something over and above 
 the light which is absorbed and which pro- 
 vokes combustion. What is this something? 
 In the year iSoo Sir William Herschel 
 passed a thermometer through the various 
 colors of the solar spectrum, an I marked the 
 rise of temperature corresponding to each 
 color. He found the heating effect to aug- 
 ment from the violet to the red ; he did not, 
 however, stop at the red. but pushed his 
 thermometer into the dark space beyond it. 
 Here he found the temperature actually higher 
 than in any part of the visible spectrum. By 
 this important observation, he proved that 
 the sun emitted dark heat-rays which are en- 
 tirely unfit for the purposes cf vision. The 
 subject was subsequently taken up by See- 
 beck, Melloni, Miiller, and others, and within 
 the last few years it Ins been found capable 
 of unexpected expansions and applications. 
 A method has been devised whereby the solar 
 or electric beam can be so filtered as to detach 
 from it and preserve intact this invisible 
 ultr.-red emission, while the visible and ultra- 
 violet emissions are wholly iatercepted. We 
 are thus enabled to operate at will upon the 
 purely ultra-red waves. 
 
 In the heating of so.id bodies to incandes- 
 cence this non-visual emission is the neces- 
 sary basis of the visual. A platinum wire is 
 stretched in front of the table, and through 
 it an electric current flows. It is warmed by 
 the current, and may be felt to be warm by 
 the hand ; it also emits waves of heat, but no 
 1 ght. Augmenting the strength of the cur- 
 rent, the wire becomes hotter ; it finally 
 glows with a sober red light- At this point 
 Dr. Draper many years ago began an inter- 
 esting investigation. He employed a vol- 
 taic current tD heat his platinum, and he 
 studied by means of a prism the successive 
 introduction of the colors of the spectrum. 
 His first color, as here, was red ; then came 
 orange, then yellow, then green, and lastly 
 all the shades of blue. Thus as the tempera- 
 ture of the platinum was gradually aug- 
 mented, the at^rns were caused to vibrate 
 more rapidly, shorter waves wer3 thus pro- 
 duced, until finally he obtained the waves 
 corresponding to the entire spectrum. As 
 each successive color was introduced, the 
 colors preceding it became more vivid. Now, 
 the vividness, or intensity of light, like that 
 of sound, depends, not upon the length of the 
 wave, but on the amplitude of the vibration. 
 Hence, as the red grew more intense as the 
 more refrangible colors were introduced, we 
 are forced to conclude that, side by side with 
 the introduction of the shorter waves, we had 
 an augmentation of the amplitude of the 
 longer ones. 
 
 These remarks apply, not only to the vis- 
 ible emission examined by Dr. Draper, but 
 to the invisible emission which preceded 
 the appearance of any light. In the emis- 
 sion from the white-hot platinum wire now 
 before you the very waves exist with Avhich 
 we started, only their intensity has been in- 
 creased a thousand-fold by the augmentation 
 of temperature necessary to the production 
 of this white light, i oth effects are bound 
 together: in an incandescent solid, or in a 
 molten solid, you cannot have the shorter 
 waves without this intensification of the 
 longer ones. A sun is possible only on 
 these conditions; hence Sir William Her- 
 schel's discovery of the invisible ultra-red 
 solar emission. 
 
 The invisible heat, emitted both by dark 
 bodies and by luminous ones, flies through 
 space with the velocity of light, and is called 
 radiant heat. Now, radiant heat may bj 
 made a subtle and powerful explorer of 
 molecular condition, and of late years it 
 has given a new significance to the art of 
 chemical combination. Take, for example, 
 the air we breathe. It is a mixture of oxygen 
 and nitrogen; and with regard to radiant 
 heat it behaves like a vacuum, being incom- 
 petent to absorb it in any sensible degree. 
 But permit the same two gases to unite 
 chemically; without any augmentation of the 
 quantity of matter, without altering the gase- 
 ous condition, without interfering in any 
 way with the transparency of the gis, the 
 act of cherr.ical uni n is accompanied by an 
 enormous diminution of its diathermancy, or 
 ^erviousness to radiant heat. The reseaiches 
 which established this result also proved the 
 elementary gases generally to be highly 
 :ransparent to radiant heat. This, again, 
 ed to the proof of the diathermancy of ele- 
 mentary liquids, like bromine, an t of solu- 
 tions of the elements sulphur, phosphorus, 
 and iodine. A spectrum is now before you, 
 and you notice that this transparent bisul- 
 Dhideof carbon has no effect upon the colors. 
 Dropping into the liquid a few flakes of 
 odme, you see the middle of the spectrum 
 cut away. By augmenting the quantity of 
 'odine, we invade the entire spectrum, and 
 inally cut it off altogether. Now, the iodine 
 ,vhich proves itself thus hostile to the light is 
 perfectly transparent to the ultra-red emis- 
 sion with which we have now to deal. It, 
 herefore, is to be our ray-filter. 
 
 Placing the alum-cell again in front of the 
 electric lamp, we assure ourselves, as before, 
 of the utter inability of the concentrated 
 J ight to fire white paper. By introducing a 
 cell containing the solution of iodine, the 
 ight is entirely cut off. On remov- 
 ng the alum-cell, the paper at the dark focus 
 s im tantly set en fire. Black paper is more 
 bsorbent than white for these ultra-red rays; 
 and the consequence is, that with it the sud- 
 denness and vigor of the combustion are aug- 
 mented. Zinc is burnt up at the same place, 
 
38 
 
 SIX LECTURES ON LIGHT 
 
 while magnesium ribbon bursts into vivid 
 combustion A sheet of platinized platinum 
 placed at the focus is heated to whiteness. 
 Looked at through a prism, the white-hot 
 platinum yields all the colors of the spectrum. 
 Before impinging upon the platinum, the 
 waves were of too slow recurrence to awaken 
 vision; by the atoms of the platinum, these 
 long and sluggish waves are in part broken 
 up into shorter ones, being thus brought 
 within the visual range. At the other end of 
 the spectrum, Stokes, by the interposition of 
 suitable substances, lowered the refrangibil- 
 ity so as to render the non- visual rays visual, 
 and to this change he gave the name of 
 Fluorescence.. Here, by the intervention of 
 the platinum, the refrangibility is raised, so 
 as to render the non-visual visual, and to 
 this change we give the name of Calorcs- 
 cence. 
 
 At the perfectly invisible focus where these 
 effects are produced, the air may be as cold 
 as ice. Air, as already stated, does not ab- 
 sorb the radiant heat, and is therefore not 
 warmed by it. Place at the focus the most 
 sensitive air-thermometer : it is not affected 
 by the heat. Nothing could more forcibly 
 illustrate the isolation, if I may use the term, 
 of the luminiferous ether from the air. The 
 \vave-motion of the one is heaped up, without 
 sensible effect, upon the other. I may add 
 that, with suitable precautions, the eye may 
 be placed in a focus competent to heat plati- 
 num to vivid redness, without experiencing 
 any damage, or the slightest sensation either 
 of light or heat. 
 
 These ultra-red rays play a most important 
 part in Nature. I remove the iodine filter, 
 and concentrate the total beam. A test-tube 
 containing water is placed at the focus : it 
 immediately begins to sputter, and in a min- 
 ute or two it boils. What boils it? Placing 
 the alum solution in front of the lamp, the 
 boiling instantly ceases. Now, the alum is 
 pervious to all the luminous rays; hence it 
 cannot be these rays that caused the boiling. 
 I now introduce the iodine, and remove the 
 alum ; vigorous ebullition immediately re- 
 commences. So that we here fix upon the 
 invisible ultra-red rays the heating of the wa- 
 ter. We are enabled now to understand the 
 momentous part played by these rays in Na- 
 ture. It is to them that we owe the warming 
 and the consequent evaporation of the tropi- 
 cal ocean ; it is to them, therefore, that we 
 owe our rains and snows. They are 
 absorbed close to the surface of the 
 ooean, and warm the superficial water 
 while the luminous rays plunge to 
 great depths without producing any 
 sensible effect. Further, here is a large flask 
 containing a freezing mixture. The aqueous 
 vapor of the air has been condensed and 
 frozen on the flask, which is now covered 
 with a white fur. Introducing the alum-cell, 
 we place the coating of hoar-frost at the in- 
 tensely luminous focus ; not a spicula of the 
 
 frost is melted. Introducing the iodine-cell, 
 j and removing the alum, a broad space of the 
 frozen coating is instantly removed. Hence 
 we infer that the ice which feeds the Rhone, 
 the Rhine, and cthei rivers which have 
 glaciers for their sources, is released from its 
 imprisonment upon the mountains by the 
 invisible ultra- red rays of the sun. 
 
 The growth of science is organic. The 
 end of to-day becomes to-morrow the means 
 to a remoter end. Every new discovery is 
 immediately made the basis of other discov- 
 eries, or of new methods of investigation. 
 About fifty years ago, CErsted, of Copen- 
 hagen, discovered the deflection of a mag- 
 netic needle by an electric current ; and 
 Thomas Seebeck, of Berlin, discovered that 
 electric currents might be derived from heat. 
 Soon afterwards these discoveries were turned 
 to account by Nobili and Melloni in the con- 
 struction of an apparatus which has vastly 
 augmented our knowledge of radiant heat. 
 The instrument is here. It is called a thermo- 
 electric pile; and it consists of thin bars of 
 bismuth and antimony soldered together in 
 pairs at their ends, but separated from each 
 other elswhere. From the ends of this ' 'pile" 
 wires pass to a coil of covered wire, within 
 and above which are suspended two magnetic 
 needles joined to a rigid system, and carefully 
 defended from currents of air. The heat, 
 then, acting on the pile, produces an electric 
 current ; t'he current, passing through the 
 coil, deflects the needles, and the magnitude 
 of the deflection may be made a measure of 
 the heat. The upper needle moves over a 
 graduated dial far too small to be seen. It 
 is now, however, strongly illuminated. Above 
 it is a lens which, if permitted, would form 
 an image of the needle and dial upon the 
 ceiling, where, however, it could not be con- 
 veniently seen. The beam is therefore re- 
 ceived upon a looking-glass, placed at the 
 proper angle, which throws the image upon 
 the screen. In this way the motions of this 
 small needle may be made visible to you all. 
 
 The delicacy of this instrument is such 
 that in a room like this it is exceedingly dif- 
 ficult to work with it. My assistant stands 
 several ieet off. I turn the pile towards him: 
 the heat from his face, even at this distance, 
 produces a deflection of 90. I turn the in- 
 strument towards a distant wall, which I 
 udge to be a little below the average temper- 
 ature of the room. The needle descends and 
 passes to the other side of zero, declaring by 
 Lhis negative deflection that the pile feels the 
 chill of the wall . Possessed of this instrument, 
 of our ray-filter, and of our large Nicol prisms, 
 we are in a condition to investigate a subject 
 of great philosophical interest, and which 
 'ong engaged the attention of some of our 
 foremost scientific workers, Forbes being the 
 first successful one the substantial identity 
 of light and radiant heat. 
 
 That they are identical in all respects can- 
 not of course be the case, for if they vr re 
 
SIX LECTURES ON LIGHT. 
 
 they would act in the same manner upon all 
 instruments, the eye included. '1 he identity 
 meant is such as subsists between one color 
 and another, causing them to behave alike ^s 
 regards reflection, refraction, double refrac- 
 tion, and polarization. As regards reflection, 
 \ve may employ the looking-glass used in our 
 first lecture. ^larking any point in the track 
 of the reflected beam, and cutting off the | 
 light by the iodine, on placing the pile at the 
 marked point, the needle immediately starts 
 aside. This is tru? for every position of the 
 mirror. So that both for light and heat the 
 same lav/ of reflection holds good; for both 
 of them also the angular velocity of the re- 
 flected beam is twice that of the reflecting 
 mirror. Receiving the beam on a concave 
 mirror, it is gathered up into a cone of re- 
 flected :ight; marking the apex of the cone, 
 and cutting off the light, a moment's expo- 
 sure of the pile at the marked point produces 
 a violent deflection of the needle. (See Fig. 
 23, where m 111 is the mirror, P the pile, and 
 T the opaque solution.) 
 
 This beam of light now enters a right- 
 angled prism and is reflected at the hvpothe- 
 nuse, i:i a direction perpendicular to its for- 
 mer one. The reflection here is total. Cut- 
 ling off the light, we prove the reflection of 
 the heat to be total also. The formation of 
 
 invisible images by lenses and mirrors may 
 also be demonstrated. Concentrating the 
 beam, and cutting off the light, at the dark 
 focus the carbon-points burn their images 
 through a sheet of black paper. Placing a 
 sheet of platinized platinum at the focus, 
 when the concentration is strong an incan- 
 descent image of the points is immediately 
 stamped upon the platinum. 
 
 And now for polarization and its attendant 
 phenomena. Crossing our two Nicol prisms, 
 
 B, C, Fig. 24, and placing ou pile D be- 
 hind the analyzer, neither heat nor light 
 reaches it; the needle remains undeflect- 
 cd. Introducing the iodine, the slight- 
 est turning of either prism causes the 
 heat to pass, and to announce it- 
 self by the deflection of the needle. Like 
 light, therefore, heat is polarized. Crossing 
 the Nicols ag^in, the heat is intercepted and 
 
 the needle returns to zero. Plunging into 
 the dark space between the prisms our plate 
 of mica, the needle instantly starts off, show- 
 ing that the mica acts upon the heat as it did; 
 upon the light : we have in both cases the 
 same resolution and recompounding of vi- 
 brations. Removing the mica, the needle 
 falls to zero ; but, on introducing a plate of 
 quartz between the prisms, the consequent; 
 deflection declares the circular polarization of 
 the heat. For double refraction it is neces- 
 sary that our images should not be too large- 
 and diluted : here are the two disks pro- 
 duced by the splitting of the beam in Ice- 
 land spar. Marking the positions of the disks, 
 and cutting off the light, the pile finds in its 
 places two heat-images. The needle now- 
 stands near 90, and, on turning the spar,, 
 the deflection remains constant. Transfer- 
 ring the pile to the other image, the cU-fiec-. 
 lion of 90 is maintained ; but on turning 
 the spar the needle now falls to zero. The 
 reason is manifest. Permitting the light to- 
 pass, we find the luminous disk at some dis- 
 tance from the pile. We are dealing, in 
 fact, with the extraordinary *bcam which 
 rotates round the ordinary So that fr.~ > t-at 
 as well as for light we have double refraction, 
 and also an ordinary and extraordinary ray. 
 (In the adjacent figure, which shows the ex- 
 perimental arrangement, N is the nozzle cf 
 
40 
 
 SIX LECTURES ON LIGHT. 
 
 the electric lamp, L a converging lens, B the 
 bircfracting spar, and P the thermo-electric 
 pile.) 
 
 If time permitted we might finish the series 
 of demonstrations by magnetizing a ray of 
 heat as we magnetized a ray of light. 
 
 We have finally to determine the position 
 and magnitude of the invisible radiation 
 which produces these results. For this pur- 
 pose we employ a particular form of the 
 thermo-electric pile. Its face is a rectangle, 
 which by movable side-pieces can be ren- 
 dered as narrow as desirable. Throwing a 
 concentrated spectrum upon a screen, 
 by means of an endless screw, we move this, 
 rectangular pile through the entire spectrum. 
 Its surface is blackened so that it absorbs all 
 the light incident upon it, converting it into 
 
 a curve which exhibits the distribution of 
 heat in our spectrum. It is represented in 
 the adjacent figure. Beginning at the blue, 
 the curve rises, at first very gradually; then, 
 as it approaches the red more rapidly, the 
 line CD representing the strength of the ex- 
 treme red radiation. Beyond the red it shoots 
 upwards in a steep and massive peak to B, 
 whence it falls, rapidly for a time, and after- 
 wards gradual'y fading from the perception 
 of the pile. This figure is the result of 
 more than twelve careful series of measure- 
 ments, for each of which the curve was con- 
 structed. On superposing all these curves, 
 a satisfactory agreement was found to exist 
 between them. So that it may safely be 
 concluded that the areas of the dark and 
 white spaces respectively represent the rela- 
 
 FIG. 25. 
 
 heat, and thus enabling it to declare its power 
 by the deflection of the magnetic needle. 
 
 When this instrument is brought to the 
 violet end of the spectrum, the heat is found 
 to be almost insensible. As the pile grad- 
 ually moves from the violet towards the red, 
 it encounters a gradually augmenting heat. 
 The red itself possesses the highest heating 
 power of all the colors of the spectrum. 
 Pushing the pile into the dark space beyond the 
 red, the heat rises suddenly in intensity, and.at 
 some distance beyond the red, attains a max- 
 imum. From this point the heat falls some- 
 what more rapidly than it rose, and afterwards 
 gradually fades away. Drawing- an hori- 
 zontal line to represent the length of the 
 spectrum, and erecting along it, at various 
 points, perpendiculars proportional in length 
 to the heat existing at those points, we obtain 
 
 I tive energies of the visible and invisible 
 I radiation The one is 7.7 times the other. 
 
 But in verification, as already stated, con- 
 sists the strength of science. Determining 
 in the first place the total emission from the 
 electric lamp ; then by means of the iodine 
 filter determining the ultra-red emission ; the 
 difference between both gives the luminous 
 emission. In this way, it was found that the 
 energy of the invisible emission is eight times 
 that of the visible. No two methods could 
 be more opposed to each other, and hardly 
 j any two results could better harmonize. I 
 I think, therefore, you may rely upon the ac- 
 j curacy of the distribution of heat here as- 
 signed to the prismatic spectrum of the elec- 
 i trie light. There is nothing vague in the 
 j mode of investigation, nor doubtful in its 
 conclusions. 
 
SIX LECTURES ON LIGHT. 
 
 LECTURE VI. 
 
 of Spectrum Analysis: Solar Chemistry: 
 Summary and Conclusions. 
 
 We have employed, as our source of light 
 in these lectures, the ends of two rods of coke 
 rendered incandescent by electricity. Coke 
 is particularly suitable for this purpose, be- 
 cause it can bear intense heat without fus on 
 or vaporization. It is also black, which 
 helps the light; for, other circumstances be- 
 ing equal, as Ihown experimentally by Bal- 
 
 four Stewart, the blacker the body the 
 brighter will be its light when incandescent. 
 Still, refractory as carbon is, if we closely ex-' 
 amine our voltaic arc, or stream of light be- 
 tween the carbon-points, we should find there 
 incandescent carbon-vapor. We might also 
 | detach the light of this vapor from the more 
 ! dazzling light of the solid points, and obtain 
 ! its spectrum. This would be not only les.i 
 ' brilliant, but of a totally different character 
 j from the spectra that we have already seen. 
 i Instead of being an unbroken succession t>f 
 ' colors from red to violet, the carbon-vapor 
 
42 
 
 SIX LECTURES ON LIGHT, 
 
 would yield a few bands of color with spaces 
 of darkness between them. 
 
 What is true of the carbon is true in a still 
 mo r e striking degree of the metals, the most 
 refractory of which can be fused, boiled, ind 
 reduced to vapor by the electric current. 
 From the incandescent vapor the light, as a 
 general rule, flashes in groups of rays of 
 definite degrees of refrangibility, spaces ex- 
 isting between group and group, which are 
 unfilled by rays of any kind. But the con- 
 templation of the facts will render this sub- 
 ject more intelligible than words can make it. 
 Within the camera is now placed a cylinder 
 of carbon hollowed out at the t<"p to receive 
 a bit of metal; in the hollow is placed a frag- 
 ment of the m;tal thallium, and now you see 
 the arc of incandescent thallium-vapor upon 
 the screen. It is of a beautiful green color. 
 What is the meaning of that green? We 
 answer the question by subjecting the light 
 to prismatic analysis. Here you have its 
 spectrum, consisting of a single refracted 
 band. Light of one degree of icfrangibility, 
 and that corresponding to green, is emitted 
 by the thallium-vapor. 
 
 We will now remove the thallium and put 
 a bit of silver in its place. The arc of silver 
 is not to be distinguished from that of thal- 
 lium; it is not only green, like the thallium- 
 vapor, but the same shade cf green. Are 
 they, then, alike? Prismatic analysis en- 
 ables us to answer the question. It is per- 
 fectly impossible to confound the spectrum 
 < f incandescent silver vapor with that of 
 thallium. Here are two green bands instead 
 of one. Adding to the silver in our camera 
 a bit of thallium, we obtain the light of both 
 metals, and you see that the green of the thal- 
 lium lies midway between the two greens of 
 the silver. Hence this similarity of color. 
 
 But you observe another interesting fact. 
 The thallium band is now far brighter than 
 the silver bands ; indeed, the latter have won- 
 derfully degenerated since the bit of thallium 
 was put in. They are not at all so bright as 
 they were at first, and for a reason worth 
 knowing. It is the resistance offered to the 
 passage of the electric current from carbon 
 to carbon that calls forth the power of the 
 current to produce heat. If the resistance 
 were materially lessened, the heat would be 
 materially lessened ; and, if all resistance 
 were abolished, there would be no heat at 
 all. Now, thallium is a much more fusible 
 and vaporizable metal than silver; and its 
 vapor facilitates the passage of the current to 
 such a degree as to render it almost incom- 
 petent to vaporize the silver. But the thal- 
 lium is gradually consumed ; its vapor di- 
 ininishes, the resistance rises, until finally 
 you see the two silver bands as brilliant as 
 they were at first. The three bands cf the 
 two metals are now of the same sensible 
 brightness. 
 
 We have in these bands a perfectly unal- 
 terable characteristic of these two metals. 
 
 You never get other bands than these two 
 green ones from the silver, never other than 
 the single green band from the thallium, 
 never olher than the three green bands from 
 the mixture of both metals. Every known 
 metal has its bands, and in no known case 
 are the bands of two different metals al;ke. 
 Hence these spectra may be made a test for 
 the presence or absence of any particular 
 metal. If we pass from the metals to their 
 alloys, we find no confusion. Copper gives 
 us green bands, zinc gives us blue and red 
 bands ; brass, an alloy of copper and zinc, 
 gives us the bands of both metals, perfectly 
 unaltered in position or character. 
 
 But we are not confined to the metals ; the 
 salts of these metals yield the bands of the 
 metals. Chemical union is ruptured by a 
 sufficiently high heat, the vapor of the metal 
 is set free and yields its characteristic bands. 
 The chlorides of the metals arc particularly 
 suitable for experiments of this character. 
 Common salt, for example, is a compound of 
 chlor.nc and sodium ; in the electric lamp, it 
 yields the spectrum of the metal sodium. 
 The chlorides of lithium and cf strontium 
 yield in like manner the bands of these 
 metals. When, therefore, Bunsen and Kirch- 
 hoff, the celebrated founders of spectrum 
 analysis, after having established by an ex- 
 haustive examination the spectra of all known 
 substances, discovered a spectrum containing 
 bands different from any known bands, they 
 immediately inferred the existence of a new 
 metal. They were operating at the time 
 upon a residue obtained by evaporating one 
 of the mineral waters of Germany. In that 
 water they knew the new metal was con- 
 cealed, but vast quantities of it had to be 
 evaporated before a residue could be obtained 
 sufficiently large to enable ordinary chemistry 
 to grapple with the metal. But they hunted 
 it down, and it now stands among chemical 
 substances as the metal Rubidium* They 
 subsequently discovered a second metal, 
 whic:i they called Casium. Thus, having 
 first placed spectrum analysis on a safe foun- 
 dation, they demonstrated its capacity as an 
 agent of discovery. Soon afterwards Mr. 
 Crookes, pursuing this same method, ob- 
 tained the salts of the thallium which yielded 
 that bright monoc- romatic green band. The 
 metal itself was first isolated by a French 
 chemist. 
 
 All this relates to chemical discovery upon 
 earth, where the materials are in our own 
 hands. But Kirchhoff showed how spectrum 
 analysis might be applied to the investigation 
 cf the sun and stars, and on 1 is way to this 
 result he solved a problem which had been 
 long an enigma to natural philosophers. A 
 spectrum is pun in which the colors do not 
 overlap each other. We purify the spectrum 
 by making our slit narrow and by augment- 
 ing the number of our prisms. When a pure 
 spectrum of the sun has been obtained in 
 this way it is found to be furrowed by in- 
 
SIX LECTURES ON LIGHT. 
 
 43 
 
 O 
 
 numerable dark lines. Four of them were 
 lirst seen by Dr. \Vollaston, but they were 
 afterwards multiplied and me.*sured by Fraun-. 
 hofer with such masterly skill that they are 
 now universally known as Fraunhofer's lines. 
 To give an explanation of 
 these lines was. as I have said, 
 a problem which long chal- 
 lenged the attention of philos- 
 ophers. (The principal lines 
 are lettered according to Fraun- 
 hofer in the annexed sketch of 
 the solar spectrum. A, it may 
 be stated stands near the ex- 
 treme red, and J near the 
 extreme'violet.) 
 
 Now, Kirchhoff had made 
 thoroughly clear to his mind 
 the priric pies which linked to- 
 gether the emission of light 
 and the absorption of light ; 
 he had proved their insepara- 
 bility for each particular kind 
 of light and heat. He had 
 proved, for every specific ray 
 of the spectrum, the doctrine 
 that the body emitting any ray 
 absorbed with special energy a 
 ray of the same refrangibility. 
 Consider, then, the elfect of 
 knouhdge, such as you now 
 possess, upon a mind prepared 
 like that of Kirchhoff. We 
 have seen the incandescent va- 
 pors of metals emitting defi- 
 nite groups of rays ; accord- 
 ing to Kirchhoff 's principle, 
 those vapors, if crossed by solar 
 light, ought to absorb : ays of 
 the same refrangibility as those 
 which they emit. He proved 
 this to be the case ; he was 
 able, by the interposition of a 
 vapor, to cut out of the solar 
 spectrum the band correspond- 
 ing in color to that vapor. 
 Now. the sun possesses a pho- 
 tosphere, or vaporous enve- 
 lope doubtless mixed with vi- 
 olently agitated clouds and 
 Kirchhoff saw that the power- 
 ful rays coming from the solid, or the 
 molten nucleus of the sun, must be inter- 
 cepted by this vapor. One dark band of j 
 Fraunhof r, for example, occurs in the 
 yellow of the spectrum. Sodium vapor h 
 demonstrably competent to produce that dark 
 band ; hence Kirchhoff inferred the exist- 
 ence of sodium-vapor in the atmosphere of 
 ,the sun. In the case of metals, which emit 
 a large number of bands, the absolute coi - 
 c dence of every bright band of the metal 
 with a dark Fraunhofer line, raises to the 
 highest degree of certainty the inference that 
 the metal is present in the atmosphere of the 
 sun. In this way solar- chemistry was found- 
 ei on spectrum analysis. 
 
 FIG. 27. 
 
 But let me not skim, so lightly over this 
 great subject. I have spoken of emis- 
 sion and absorption, and of the link that 
 binds them. Let me endeavor to make plain 
 to you, through the analogy of sound, their 
 physical meaning. I draw a fiddle-bow across 
 this tuning-fork, and it immediately fills the 
 room with a musical sound; this may be re- 
 garded as the radiation or emission of sound 
 from the fork. A few days ago, on sounding 
 this fork, I noticed that, when its vibrations 
 were quenched, the sound seemed to be con- 
 tinued, though more feebly; The sound ap- 
 peared to come from under a distant table, 
 where stood a number of tuning-forks of dif- 
 ferent sizes and rates of vibration. One of 
 these, and one only, had been started by the 
 fork, and it was one whose rate of vibration 
 was the same as that of the fork which 
 started it. This is an instance of the ab- 
 sorption of sound of one fork by -another. 
 Placing two. forks near each other, sweeping 
 the bow over one of them, and then quench- 
 ing the agitated fork, the other continues to 
 sound. Placing a cent-piece on each prong 
 of one of the forks, we destroy its perfect 
 synchronism \\ith the other, and then no com- 
 munication of sound from the one to the other 
 is possible. 
 
 1 will now do with //^vfc/what has been here 
 done with sound. Placing a tin spoon con- 
 taining sodium in a Bunsen's flame, we ob- 
 tain this intensely yellow light, which corn-- 
 sponds in refrangibility with the yellow band 
 of the spectrum. Like our tuning-fork, it 
 emits waves at a special period. I will send 
 the white light from our lamp through that 
 flame, and prove before you that the yellow 
 flame intercepts the yellow of the spectrum 
 S S, Fig. 28; in other words, absorbs wrves 
 of the same period as its own, thus produc- 
 ing, to all intents and purposes, a da:k 
 Fraunhofer's band in the place of the yellow. 
 (A Bunsen's flame contained within the chim- 
 ney C is placed in front of the lamp L. Tha 
 tin spoon with its pellet of sodium is plunged 
 into the flame. Vivid combustion soon seU 
 in, and, when it does, the yellow of the speo 
 trum, at D, is furrowed by a dark band. 
 Withdrawing and introducing the sodium. 
 flame in rapid succession, the sudden disap- 
 pearance a id reappearance of the strip of 
 darkness arc observed). 
 
 Mentally, as well as physically, every age 
 of the world is the outgrowth and offsprin^ 
 of all preceding ages. Science proves itself 
 to be a genuine product of Nature by grow- 
 ing according to this law. We have no so- 
 lution of continuity here. Every great dis- 
 covery has been duly prepared for in two 
 ways: first, by other discoveries which form 
 its prelude; and, secondly, through the 
 sharpening, by exercise, of the intellectual 
 instrument itself. Thus Ptolemy grew ouij 
 of Hipparchus, Copernicus out of both, Kep- 
 ler out of all three, and Newton out of all 
 the four. Newton did not rise suddenly Jrora 
 
44. 
 
 SIX LECTURES ON LIGHT. 
 
 the sea-level bf the intellect to his amazing 
 elevation. At the time that he appeared, the 
 table-land of knowledge was already high. 
 He juts, it is true, above the table-land, as a 
 massive peak; still he is supported by it, and 
 a great part of his absolute height was the 
 height of humanity in his time. It is thus 
 
 'rn thediscowies of Kirchhoff. Much had 
 beta previously accomplished; this he mas- 
 tered, and the*: by the force of individual 
 genius went beyond it. He replaced uncer- 
 tainty by certaiuty, vagueness by definite- 
 ness, confusion Ly oxler; and I do not think 
 that Newton has a s:i er claim to the discov- 
 ciies that have made h';s name immortal than 
 Kirchhoff has to the credit of gathering up the 
 fragmentary knowledge of his time, of vastly 
 extendir it, and of infusing into it the life 
 of great principles. Splendid results have 
 since been obtained with whk ii the names of 
 Janssen, Muggins, I,ocl:>?r, Respighi, 
 Young, and others, are honorably associated, 
 but, splendid as they are, they are but the 
 sequel and application of the principles es- 
 tablished in his Heidelberg laboratory by the 
 celebrated German investigator. 
 
 SUMMARY AND CONCLUSION 
 
 My desire in these lectures hz?s bt a .n to 
 show you, with as little breacn of continuity 
 as possible, the past growth and p/esent as- 
 pect of a department cf science, ;'n which 
 have labored some of th^ gre; test i itellccts 
 the world has ever seen. My f "lend Profes- 
 sor Henry, in introducing me at Washington, 
 spoke of me as an apostle; but theonh apos- 
 tolate that I intended to fulfil was to place, 
 
 in plain words, my subject before you, and to 
 permit its own intrinsic attrsctions to act up- 
 on your minds. In the \vayof experiment, I 
 have tried to give you the best which, under 
 the circumstances, could be provided; but I 
 have sought to confer on each experiment a 
 distinct intellectual vaLie, for experiments 
 ought to be the representatives and exposi- 
 tors of thought a language addressed to the 
 eye as spoken words are to the ear. In as- 
 sociation with its context, nothing is more 
 impressive or instructive than a fit experi- 
 ment; but, apart from its context, it rather 
 suits the conjuror's purpose of surprise than 
 that purpose of education which ought to be 
 the ruling motive of the scientific man. 
 
 And now a brief summary of our work 
 will not be out of place. Our present mas- 
 tery over the laws and phenomena of light 
 has its origin in the desire of man to know. 
 We have seen the ancients busy whh this 
 problem, but, like a child who uses his aims 
 aimlessly for want of the necessarv muscular 
 exercise, so these early men speculated vagut- 
 ly and confusedly regarding light, not having 
 as yet the discipline needed to give clearness 
 to their insight, and firmness to their grasp 
 of principles. They assured themselves of 
 th~ rectilineal propogation of light, and that 
 the angle of incidence was equal to the angle 
 of reflection. For more than a thousand 
 years I might say, indeed, for more than 
 ritteen hundred years subsequently the 
 scientific intellect appears as if smitten with 
 paralysis, the fact being that, during this 
 time, the mental force, which might have run 
 in the direction of science, was diverted in- 
 ;o other directions. 
 
 The course of investigation as regards 
 light was resumed in 1100 by an Arabian 
 philosopher named Alhazan. Then it was 
 taken up in succession by Roger Bacon, Vi- 
 tellio, and Kepler. These men, though fail- 
 ing to detect the principle which ru.ed the 
 facts, kept the fire of investigation constantly 
 burning. Then came the fundamental dis- 
 covery of Snell, that corner-stone of optics, 
 as I have already called it, and immediately 
 afterward we have the application by Des- 
 cartes of Snell's discovery to the expianaticn 
 of the rainbow. Then came Newton's 
 crowning experiments on the analysis and 
 i synthesis of white light, by which it was 
 proved to be compounded of various kinds 
 of light of different degrees of refrangibiiity. 
 
 In 1676 an impulse was given to optics by 
 astronomy. In that year Olaf Roemer, a 
 learned Dane, was engaged at the Observa- 
 tory of Paris in observing the eclipses of Ju- 
 piter's moons. He converted them into so 
 many signal-lamps, quenched when they 
 plunged into the shadow of the planet, and 
 relighted when they emerged from the shadow. 
 They enabled him to prove that light 
 requires time to pass through space, and to 
 assign to it the astounding velocity of 190,- 
 ooo miles a second. Then came the English 
 
SIX LECTURES ON LIGHT. 
 
 45 
 
 astronomer, ' Bradley, who noticed that the 
 fixed stars did not really appear to be fixed, 
 but describe in the heavens every year a little 
 orbit resembling the earth's orbit. The re- 
 sult perplexed him, but Bradley had a mind 
 open to suggestion, and capable of seeing, in 
 the smallest fact, a picture of the largest. He 
 was one day upon ihe Thames in a boat, and 
 noticed that, as long as his course remained 
 unchanged, the vane upon his mast-head 
 showed the wind to be blowing constantly in 
 the same direction, but that the wind ap- 
 peared to vary with every change in the di- 
 rection of his boat. " Here," as Whewell 
 s.iy~, " was the image of his case. The boat 
 was the earth, moving in its orbit, and the 
 wind was the light of a star." 
 
 We may ask in passing, what, without the 
 faculty which formed the ''image," would 
 Bradley's wind and vane have been to him ? 
 'A wind and vane, and nothing more. You 
 will immediately understand the meaning of 
 Bradley's discovery. Imagine yourself in a 
 motionless railway-train with a shower of rain 
 descending vertically downward. The mo- 
 ment the train begins to move, the rain-drops 
 begin to slant, and the quicker the train the 
 greater is the obliquity. In a precisely sim- 
 ilar manner the rays from a star vertically 
 overhead are caused to slant by the motion o'f 
 the earth through space. Knowing the 
 speed of the train, and the obliquity of the 
 falling rain, the velocity of the drops 
 may be calculated ; and knowing the speed 
 of the earth in her orbit, and the obliquity of 
 the rays due to this cause, we can calculate 
 just as easily the velocity of light. Bradley 
 did this, and the "aberration of light," as his 
 discovery is called, enabled him to assign to 
 it a velocity almost identical with that de- 
 duced by Roemer from a totally different 
 method of observation. Subsequently Fizeau, 
 employing not planetary or stellar distances, 
 but simply the breadth of the city of Paris, 
 determined the veloci y of light : while after 
 him Foucault a man of the rarest mechanical 
 genius solved the problem without quitting 
 his private room. 
 
 Up to his demonstration of the composition 
 of white light, Newton had been everywhere 
 triumphant triumphant in the heavens, tri- 
 umphant on the earth, and his subsequent 
 experimental work is for the most part of 
 immortal value. But infallibility is not the 
 gift of man, and, soon after his discovery of 
 the nature of white light, Newton proved 
 himself human. He supposed that refraction 
 and dispersion went hand in hand, and that 
 you could not abolish the one without at the 
 same time abolishing the other. Here Dol- 
 land corrected him But Newton committed 
 a graver error than this. Science, as I 
 sought to make clear to you in our second 
 lecture, is only in part a thing of the senses. 
 The roots of phenomena are embedded in a j 
 region beyond the reach of the sen es, and \ 
 less than the root of the matter will never ' 
 
 satisfy the scientific mind. We find, accord- 
 ingly, in this career of optics, the greatest 
 minds constantly yearning to pass from the 
 phenomena tD their causes to explore them 
 to their hidden roots. They thus entered 
 the region of theory, and here Newton, 
 though drawn from time to time towards the 
 truth, was drawn still more strongly towards 
 the error, and made it his substantial choice. 
 His experiments are imperishable, but his 
 theory has pa-sed away. For a century it 
 stood like a dam across the course of discov- 
 ery ; but, like all barriers that rest upon 
 authority, and no. upon truth, the pressure 
 from behind increased, and eventually swept 
 the barrier away. This, as you know, was 
 done mainly through the labors of Thomas 
 Young, and his illustrious French fellow- 
 worker Fresnel. 
 
 In 1808, Malus, looking through Iceland 
 spar at the sun reflected from the window of 
 the Luxembourg Palace in Paris, discovered 
 the polarization of light by reliection. In 
 1811 Arago discovered the splendid chro- 
 matic phenomena which we have had illus- 
 trated by plates of gypsum i:: polarized ligtit ; 
 he also discovered the rotation of tne plane 
 of polarization by quartz-crystals. In 1813 
 Seebeck discovered the polarization of light 
 by tourmaline. That same year Brewscer 
 discovered those magnificent bane's of color 
 that surround the axes of biaxal crystals. In 
 1814 Wollaston discovered the rings of Ice- 
 land spar. All these effects, whkh, without 
 a theoretic clue, would leave the human 
 mind in a hopeless jungle of phenomena 
 without harmony or relation, were organically 
 connected by the theory of undulation. The 
 theory was applied and verified in all direc- 
 tions, Airy being especially conspicuous for 
 the severity and conclusiveness of his proofs. 
 The most remarkable verification fell to the 
 lot of the late Sir William Hamilton, of Dub- 
 lin, a profound mathematician, who, taking 
 up the theory where Fresnel had left it, ar- 
 rived at the conclusion that, at foar bpjjial 
 points at the surface of the ether-wave in 
 double-refracting crystals, the ray was divided 
 not into two parts, but into an infinite num- 
 ber of parts ; forming at these points a con- 
 tinuous conical envelope in3tead of two 
 images. No human eye had ever seen this 
 envelope when Sir William Hamilton inferred 
 its existence. Turning to nis fri nJ Dr. 
 Lloyd, he asked him to test experimentally 
 the truth of his theoretic conclusion. Lloyd, 
 taking a crystal of arragomtc, and following 
 with the most scrupulous exactness the indi- 
 cations of theory, cutting the crystal wheie 
 theory said it ought to be cut, observing it 
 ..here theory said it ought to be observed, 
 found the luminous envelope which had pre 
 viousiy been a mere idea in the mind of the; 
 mathematician. 
 
 Nevertheless this great theory cf undula- 
 tion, like many another truth, which i i the 
 long-rua has proved a blessi ig ti humanity. 
 
SIX LECTURES ON LIGHT. 
 
 had to establish, by hot conflict, its right to 
 existence. Great names were arrayed against 
 it. Ic had been enunciated by Hooke, it had 
 been applied by lluyghens. it had been de- 
 fended by Euler. But they made no impres- 
 sion. And, indeed, the theory in their hands 
 was more an analogy than a demonstration. 
 It first took the form of a demonstrated verity 
 in the hand of Thomas Young. He broug'ic 
 the waves of light to bear upon each other, 
 causing them to support each other, and to 
 extinguish each other at will. From their 
 mutual actions he determined their lengths, 
 and applied his determinations in all direc- 
 tions. He showed t at the standing difficulty 
 of polarization might be embraced by the 
 theory. After him came Fresnel, whose 
 transcendent mathematical abilities enabled 
 him to give the theory a generality unattained 
 by Young. He grasped the theory in its 
 entirety , followed the ether into its eddies 
 and estuaries in the hearts of crystals of the 
 most complicated structure, and into bodies 
 subjected to strain and pressure. He showed 
 that the facts discovered by Malus, Arago, 
 Brewster, and Biot, were so many ganglia, 
 so to speak, of his theoretic organism, deriv 
 ing from it sustenance and explanation. 
 \Vith a mind too strong for the body with 
 which it was associated, that body became a 
 wreck long before it had become old, and 
 Fresnel died, leaving, however, behind him a 
 name immortal in the annals of science. 
 
 One word more I should like to say regard- 
 ing Fresnel. There are things, ladies and 
 gentlemen, better even than science. There 
 are matters of the character as well as matters 
 of Uu intellect, and it is always a pleasure to 
 those who wish to think well 01 human na- 
 ture, when high intellect and upright charac- 
 ter are combined. They were, I believe, 
 combined in this young Frenchman. In 
 thosi hot conflicts of the undulatory theory, 
 he stood forth as a man of integrity, claiming 
 no more than his right, and ready to concede 
 their rights to others. He at once recognized 
 and acknowledged the merits of Thomas 
 Young. Indeed, it was he, and his fellow- 
 countryman Arago, who first startled England 
 into the consciousness of the injustice done 
 to Young in the Edinburgh Review. 1 should 
 like to read you a brief extract from a letter 
 written by Fresnel to Young in 1824, as it 
 throws a pleasant light upon the character of 
 the Frenc ) philosopher. '* For a long time," 
 says Fresnel, "that sensibility, or that 
 vanity, which people call love of glory, has 
 been much blunted in me. I labor much 
 Jess to catch the suffrages of the public than 
 to obtain that inward approval which has 
 always been the sweetest reward of my efforts. 
 Without doubt, in moments of disgust and 
 discouragement, I have often needed the spur 
 of vanity to excite me to pursue my researches. 
 But all the compliments I have received from 
 Arago, De la Place, and Biot, never gave me 
 so much pleasure as the discovery of a theo- 
 
 retic truth, or the confirmation of a calcu'a- 
 tion by experiment." 
 
 This, ladies and gentlemen, is the core of 
 the whole matter as regards science. It 
 must be cultivated for its own sake, for the 
 pure love of truth, rather than for the ap- 
 plause or profit that it brings. And now my 
 I occupation in America is wellnigh g'7ie". 
 Still I will bespeak your tolerance for a few 
 concluding remarks in reference to the me;i 
 who have bequeathed to us the vast body of 
 knowledge of which I have sought to give 
 you some faint idea in these lectures. What 
 was the motive that spurred them on? what 
 the prize of their high calling for whici they 
 struggled so assiduously? What urged them 
 to those battles and those victories over reti- 
 cent Nature which have become the heritage 
 of the human race? It is never to be for- 
 gotten that not one of those great investiga- 
 tors, from Aristotle down to Stokes and 
 Kirchhoff, had any practical end in view, 
 according to the ordinary definition of the 
 word "practical." They did not propose to 
 themselves money as an end, and knowledge 
 as a means of obtaining it. For the most 
 part, they nobly reversed this process, made 
 knowledge their end, and such money as 
 they possessed the means of obtaining it. 
 
 We may see to-day the issues of their 
 work in a thousand practical forms, and this 
 may be thought sufficient to justify i'c, if not 
 ennoble their efforts. But they did not work 
 for such issues ; their reward wa:, of a totally 
 different kind. We love clothes, we love 
 luxuries, we love fine equipages, we love 
 money, and any man who can point to these 
 as the result of his efforts in life justifies 
 these efforts before all the world. In Ameri- 
 ca and England more especially he is a 
 " practical " man. But I would appeal con- 
 fidently to this assembly whether such things 
 exhaust the demands of human nature? The 
 very presence here for six inclerr.ent nights 
 of this audience, embodying so much oi the 
 mental force and refinement of this great 
 city, is an answer to my question I need 
 not tell such an assembly that there are joys 
 of the intellect as well as joys of the body, 
 or that these pleasures of the spirit consti- 
 tuted the reward of our great investigators. 
 Led on by the whisperings of natural truth, 
 through pain and self-denial, they often pur- 
 sued their work. With the ruling passion 
 strong in death, some of them, when no 
 longer able to hold a pen, dictated to their 
 friends the result of their labors, and then 
 rested from them forever. 
 
 Could we have seen these men at work 
 without any knowledge of the consequences 
 of their work, what should we have thought 
 of them ? To many of their contemporaries 
 it would have appeared simply ridiculous to 
 see men, whose name 3 are now stars in the 
 firmanent of science, straining their atten- 
 tion to observe an effect of experiment al- 
 most too minute for detection. To the un- 
 
SIX LECTURES ON LI3I-IT. 
 
 initiated, they might well appear as big 
 children playing with not Very amusing toys. 
 It is so to this hour. Could you watch the 
 true investigator your Henry or your Dra- 
 per, ior example in his laboratory, unless 
 animated by his spirit, you could hardly un- 
 derstand what keeps him there. Many of 
 the objects which rivet his attention might 
 appear to you utterly tiivial ; and, if you were 
 cO step forward and ask him what is the use of 
 his work, the chances are that you would 
 confound him. He might not be able to 
 express the 1:33 of it in intelligible terms. 
 He might not be able to assure you that it 
 will put a dollar into the pocket of any 
 human being living or to come. That 
 scientific discovery may put not only dollars 
 into the potkets of individuals, but millions 
 into the exchequers or nations, the history of 
 science amply proves ; but the hope of its 
 doing so never was and never can be the 
 motive power of the investigator. 
 
 I know that 1 run some risk in speaking 
 thus before practical men. 1 kno,v what De 
 Tocqueville says of you. " The man of the 
 North," he says, "has not only experience, 
 but knowledge. He, however s does not care 
 for science as a pleasure, and only embraces 
 i with avidity when it leads to useful appli- 
 cations." But what, I would ask, are the 
 hopes of useful applications which have 
 drawn you so many times to this place in 
 ipite of snow-drifts and biting cold? What, 
 1 may ask, is the origin of that kindness 
 which drew me from my work in London to 
 address you here, and which, if I permitted 
 it, would send me home a millionaire ? Not 
 because I had taught you to make a single 
 cent by science, am I among you to-night, 
 but because I tried to the best of my ability 
 co present science to the world s an intellec- 
 tual good. Surely no two terms were ever 
 so distorted and misapplied with refe.ence to 
 man in his higher relations as these terms 
 ustful and practical. As if there were no 
 nakedness of the mind to be clothed as well 
 as naktdness of the body no hunger and 
 thirst of the intellect to satisfy. Let us ex- 
 pand the definitions of these terms until thty 
 embrace all the needs of man, his highest in- ! 
 teliectual needs inclusive. It is specially on ' 
 this ground of its administering to the higher I 
 needs of intellect, it is mainly because I be- 
 lieve it to be wholesome as a source of know- 
 ledge, and as a means of discipline, that 1 
 virge the claims of science this evening upon 
 your attention. 
 
 But. with reference to material needs and 
 joys, surely pure science has also a word to 
 say. People sometimes s-pc-ak as if steam had 
 not been studied before James Watt, or elec- 
 tricity before Wheatstone and Morse ; where- 
 as, in point of fact, Watt and Wheatstone 
 and Morse, with all their practicality, were 
 the mere outcome of antecedent forces, which 
 acted without reference to practical ends. 
 This also, I think, me its a moment's 
 
 J tion. You arc delighted, and with good 
 (reason, wilh your electric telegrrphs, prcud 
 of your steam-engines and your factories, 
 and charmed with the productions of pho- 
 tography. You see daily, with just elation, 
 the creation of new forms of industry 
 new powers of adding to the wealth and 
 comfort of society. Industrial England is 
 heaving with forces tending to this end, and 
 the pulse of industry beats still stronger in 
 the United States. And yet, when analyzed, 
 what are industrial America and industrial 
 England? If you can tolerate freedom of 
 speech on my part, I will answer this ques- 
 tion by an illustration. Strip a strong arm, 
 and regard the knotted muscles when the 
 hand is clenched and the arm bent. Is this 
 exhibition of energy the work of the muscle 
 alone ? By no means. The muscle is the 
 channel of an influence, without which it 
 would be as powerless as a lump of plastic 
 dough. It is the delicate unseen nerve that 
 unlocks the power of the muscle. And, 
 without those filaments of genius which have 
 been shot like nerves through the body of 
 society by the original discoverer, industrial 
 America and industrial England would, I 
 fear, be very much in the condition of that 
 plastic dough. 
 
 At the, present time there is a cry in Eng- 
 land for technical education, and it is the ex- 
 pression of a true national want ; but there 
 is no cry for original investigation. Still 
 without this, as surely as the stream dwindles 
 when the spring dries, so surely will "tech- 
 nical education" lose all force of growth, all 
 power of reproduction. Our great investi- 
 gators have given us sufficient work for a 
 time , but, if their spirit die out, we shall 
 find ourselves eventually in the condition of 
 those Chinese mentioned by Ue Tocqueville. 
 who, having forgotten the scientific origin of 
 what they did, were at length compelled to 
 copy without variation the inventions of an 
 ancestry who, wiser than themselves, had 
 drawn their inspiration direct from Nature. 
 
 To keep society as regards science in 
 healthy play, three classes of workers are 
 necessary : Firstly, the investigator of natural 
 truth, whose vocation it is to pursue that 
 truth, and extend the field of discovery for 
 the truth's own sake, and without reference 
 to practical ends. Secondly, the teacher of 
 natural truth, whose vocation it is to give 
 public diffusion to the knowledge already won 
 by the discoverer. Thirdly, the applier of 
 natural truth, whose vocation it is to make 
 scientific knowledge available for the needs, 
 comforts, and luxuries of life. These three 
 classes ought to coexist and interact. Now, 
 the popular notion of science, both in this 
 country and in England, often relates, not to 
 science strictly so called, but to ths applica- 
 tions of science. Such applications, espe- 
 cially on this continent, are so astounding 
 they spread themselves so largely 2nd um- 
 bragcously before the public eye as to shut 
 
SIX LECTURES ON LIGHT. 
 
 out from view those workers who are engaged 
 in the quieter ami profounder business of 
 original investigation. " 
 
 Take the electric telegraph as an example, 
 which has been repeatedly forced upon my 
 attention of late. I am not here to attenuate 
 in the slightest degree the services of those 
 who, in England and America, have given 
 the telegraph a form so wonderfully fitted for 
 public use. They earned a great reward, and 
 assuredly they have received it. But I should 
 be untrue to you and to myself if I failed to 
 tell you that, however high in particular re- 
 spects their claims and qualities may be, prac- 
 tical men did not discover the electric tele- 
 gn.ph. The discovery of the electric telegraph 
 implies the discovery of electricity itself, and 
 the development cf its laws and phenomena. 
 Such discoveries are not made by practical 
 men, and they never will be made by them, 
 because their minds are beset with ideas 
 which, though of the highest value from one 
 point of view, are not those which stimulate 
 the original discoverer. 
 
 The ancients discovered the electricity of 
 amber ; and Gilbert, in the year 1600, ex- 
 tended the force to other bodies. Then fol- 
 lowed other inquirers, your own Franklin 
 among the number. But this form of elec- 
 tricity, though tried, did not come into use 
 for telegraphic purposes. Then appeared the 
 great Italian, Volto, who discovered the 
 source of electricity, which bears his name, 
 and applied the most profound iasight and 
 the most delicate experimental skill to its de- 
 velopment. Then arose the man who added 
 to the powers of his intellect all the graces of 
 the human heart, Michael Faraday, the dis- 
 coverer of the great domain of magneto- 
 electricity. CErsted discovered the deflection 
 of the magnetic needle, and Arago and Stur- 
 geo:i the magnetization of iron by the elec- 
 tric current. The voltaic circuit finally found 
 its theoretic Newton in Ohm, while Henry, 
 of Princeton, who had the sagacity to recog- 
 nize the merits of Ohm while they were still 
 decried in his own country, was at this time 
 in the van of experimental inquiry. 
 
 In the works of these men you have all 
 the materials employed at this hour in all the 
 forms of the electric telegraph. Nay, more ; 
 Gauss, the celebrated astronomer, and Weber, 
 the celebrated natural philosopher, both pro- 
 fessors in the University of Gottingen, wish- 
 ing to establish a rapid mode of communication 
 between the observatory and the physical 
 cabinet of the university, did this by means 
 of an electric telegraph. The force, in 
 short, had been discovered, it- laws investi- 
 gated and made sure, the most complete 
 mastery of i's phenomena had been attained, 
 nay, its applicability to teleg.aphic purposes 
 demonstrated, by men whose sole reward for 
 their labors was the noble joy of discovery, 
 and before your practical men appeared at 
 all upon the scene. 
 
 Are we to ignore all this ? We do so at 
 
 our peril. For I say agaia, that, behind all, 
 your practical applications, there is a region 
 of intellectual action to which practical men 
 have rarely contributed, but from which they 
 draw all their supplies. Cut them off from 
 this region, and they become eventually help- 
 less. In no case is the adage truer, '"Other 
 men labored, but ye are entered in o their 
 labors," than in the case of the discoverer 
 and the applier of natural truth. But now a 
 word on the other side. While I say that 
 practical men are not the men to make the 
 necessary antecedent discoveries, the cases 
 are rare in which the discoverer knows how 
 to turn his labors to practical account. Dif- 
 ferent qualities of mind and different habits 
 of thought are needed in ;he two cases; and, 
 while I wish to give emphatic utterance to 
 the claims of those whose claims, owing to 
 the simple fact of their intellectual -elevation, 
 arc often misunderstood, I am not here to 
 exalt the one class of workers at the expense 
 of the other. They are the necessary supple- 
 ments of each other; but remember that one 
 class is sure to be taken care of. All the ma- 
 terial rewards of society are already within 
 their reac.i; but it is at our peril that we neg- 
 lect to provide opportunity for those studies 
 and pursuits which have no such rewards, 
 and from which, therefore, the rising genius 
 of the country is incessantly tempted away. 
 
 Pasteur, one of the most eminent members 
 of the Institute of Fiance, in accounting for 
 the disastrous overthrow of his country and 
 the predominance of Germany in the late 
 war, expresses himself thus: " Few persons 
 comprehend the real origin of the marvels of 
 industry and the wealth of nations. I need 
 no further proof of this than the employment, 
 more and more frequent in official language, 
 and in writing of all sorts, of the erroneous 
 expression applied science. The abandon- 
 ment of scientific c reers by men capable oi" 
 pursuing them with distinction was recently 
 complained of in the presence of a minister 
 of the greatest talent. This statesman en- 
 deavored to show that we ought not to be 
 surprised at this result, because /;/ our day 
 the reign of theoretic science yielded place to 
 tJiat of applied science. Nothing coald bs 
 more erroneous than this opinion, nothing, I 
 venture to say, more dangerous, even to 
 practical life, than the consequences whicn 
 might How from these words. They have 
 rested on my mind as a proof of the imperi- 
 ous necessity of reform in our superior edu- 
 cation. There exists no category of the 
 sciences to which the name of applied science 
 could be given. ll'e have science, and the 
 applications of science which are united to- 
 gether as the tree and its .\ruit." 
 
 And Cuvier, the great comparative anato- 
 mist, writes thus upon the same theme- 
 "These grand practical innovations are the 
 mere applications of truths of a higher order, 
 not sought with a practical intent, but which 
 were pursued for their own sake, and solely 
 
SIX LECTURES ON LIGHT. 
 
 through an ardor for knowledge. Those 
 who applied them could not have discovered 
 them; those who discovered them had no in- 
 clination to pursue them to a practical end. 
 Engaged in the high regions whither their 
 thoughts had carried them, they hardly per- 
 ceived these practical issues, though born of 
 their own deeds. These rising workshops, 
 these peopled colonies, those ships which fur- 
 row the seas this abundance, this luxury, 
 this tumult all th's comes from discoverers 
 in science., and it all remains strange to 
 them. At the point where science merges 
 into practice, they abandon it; it concerns 
 them no more." 
 
 When the Pilgrim Fathers landed at Ply- 
 mouth Rock, and when Penn made his treaty 
 with the Indians, the new-comers had to 
 bui'.d their houses, to chasten the earth into 
 cultivation, and to take care of their souls. 
 In such a community, science, in its more 
 abstract forms, was not to be thought cf. 
 And, at the present hour, when your hardy 
 Western pioneers stand face to face with 
 stubborn Nature, piercing the mountains 
 and subduing the forest and the prairie, the 
 pursuit of science, for its own sake, is not to 
 be expected. The first need of man is food 
 nnd shelter ; but a vast portion of this con- 
 tinent is already raised far beyond this need. 
 The gentlemen of New York, Brooklyn, Bos- 
 ton, Philadelphia, Baltimore, and Washing- 
 ton, have already built their houses, and very 
 beautiful they are ; they have also secured 
 their dinners, to the excellence of which I 
 can also bear testimony. They have, in 
 fact, reached that precise condition of well- 
 being and independence when a culture, as 
 } i^'h as humanity has yet reached, may be 
 justly demanded at their hands. They have 
 reached that maturity, as possessors of wealch 
 and leisure, when the investigator of natural 
 truth, for the truth's own sake, ought to find 
 among them promoters and protectors. 
 
 Among the many grave problems before 
 them they have this to solve, whether a re- 
 public is able to foster the highest forms of 
 genius. You are familiar with the writings 
 of De Tocqueville, and must be aware of the 
 intense sympathy which he felt for your insti- 
 tutions ; and this sympathy is all the more 
 valuable, from the philosophic candor with 
 which he points out, not only your merits, 
 but your defects and dangers. Now, if I 
 come here to speak of science in America in 
 a critical and captious spirit, an invisible 
 radiation from my words and manner will 
 enable you to find me out, and will guide 
 your treatment of me to-night. But, if I, in 
 no unfriendly spirit in a spirit, indeed, th~ 
 reverse of unfriendly venture to repeat be- 
 fore you what this great historian and analyst 
 of democratic institutions said of America, I 
 am persuaded that you will hear me out. He 
 wrote some three-and-twenty years ago, and 
 perhaps would not write the same to-day ; 
 but it will do nobody any harm to have his I 
 
 words repeated, and, if necessary, laid to 
 heart. In a work published in 1 850, he says : 
 " It must be confessed that, among the civil- 
 ized peoples of our ^ge, there are few in 
 which the highest sciences have made so 
 little progress as in the United States."* 
 He declares his conviction that, had you been 
 alone in the universe, you would speedily 
 have discovered that you cannot long make 
 progress in practical science, without culti- 
 vating theoretic science at the same time. 
 But, according to De Tocqueville, you are 
 ' not thus alone. He refuses to separate 
 America from its ancestral home ; and it is 
 here, he contends, that you collect the treas- 
 ures of the intellect, without taking the 
 trouble to create them. 
 
 De Tocqueville evidently doubts the ca- 
 pacity of a democracy to 1 osier genius as it 
 was fostered in the ancient aristocracies. 
 " The future," he says, "will prove whether 
 the passion for profound knowledge, so rare 
 and so fruitful, can be born and developed 
 so readily in democractic societies as in aris- 
 tocracies. As for me," he continues, " I 
 can hardly believe it." He speaks of the un- 
 quiet feverishness of democratic commu- 
 nities, not in times of great excitement, for 
 such times may give an extraordinary impe- 
 tus to ideas, but in times of peace. There 
 is then, he says, "a small and uncomfortable 
 agaitation, a sort of incessant attrition of 
 man against man, which troubles and dis- 
 tracts the mind without imparting to it cither 
 animation or elevation." It rests with you 
 to prove whether these things are necessarily 
 so whether the highest scientific genius can- 
 not find in the midst of you a tranquil homo. 
 I should be loath to gainsay so keen an ob- 
 server and so profound a political writer, but, 
 since my arrival in this country, I have been 
 unable to see anything in the constitution of 
 society to prevent a Sstudent with the rocl; ot 
 the matter in him from bestowing the most 
 steadfast devotion on pure science. If great 
 scientific results are not achieved in Ameri- 
 ca, it is not to the small agitations of society 
 that I should be disposed to ascribe the de- 
 fect, but to the fact that the men among you 
 who possess the endo.vments necessary for 
 scientific inquiry are laden with duties of ad- 
 ministration or tuition so heavy as to be 
 utterly incompatible with the continuous and 
 tranquil meditation which original investi- 
 gation demands. It may well be aske 
 whether Henry would have been transformer 
 into an administrator, or whether Draper 
 would have forsaken science to write history, 
 if the original investigator had been honored 
 as he ought to be in this land ? I hardly 
 think they would. Still I do not think thia 
 
 * II faut reconnaitre, que parmis les peuples civil- 
 ises de nos jours, il en esc p'eu chez qui les hautes 
 sciences aient fait moins de progres qu'aux Etats- 
 Unis, ou qui aient fourni moins de grands artis r es, 
 de poetesillustres, et de celebrcs ecrivains. (Ds U 
 Democratic en Amerique, etc., toius ii., p. 36.) 
 
SIX LECTURES ON LIGHT. 
 
 state of things likely to last. In America 
 there is a willingness ou the part of individu- 
 als to devote their fortunes, in the matter of 
 education, to the service of the common- 
 wealth, which is without a parallel elsewhere; 
 and this willingness requires but wise di- 
 rection to enable you effectually to wipe away 
 the reproach of De Tocqu^ville. 
 
 Your most difficult problem will be no 1 " to 
 build institutions, but to make men ; not to 
 form the body, but to find tne spiritual em- 
 bers which, shall kindle within that body 
 a living soul. You have scientific genius 
 among you ; not sown broadcast, believe 
 me, but still ' cattered here and there. Take 
 a'l r.nnecessary impediments out of its way. 
 Drawn by your kindness I have come here to 
 give these lectures, and, now that my visit to 
 America has become almost a thing of the 
 past, I look back upon it ns a memory with- 
 out a stain. No lecturer was ever rewarded 
 
 as I have been. From this vantage-ground, 
 howtver, let me remind you that the work of 
 the lecturer is not the highest work ; that in 
 science, the lecturer is usually the distributor 
 of intellectual wealth amassed by better men. 
 It is not solely, or even chiefly, as lecturers, 
 but as investigators, that your men of genius 
 ought to be employed. Keep your sympa- 
 thetic eye upon the originator of knowledge. 
 Give him the freedom necessary for his re- 
 searches, not overloading him either with the 
 duties of tu tion or of administration, not de- 
 manding from him so-called practical results 
 above all things, avoiding that question 
 which ignorance so often addresses to genius, 
 " What is the use of your work ? " Let him 
 make truth his object, however unpractical 
 for the time being that truth may appear. 
 If you cast your bread thus upon the waters, 
 then be assured it will return to you, though 
 it may be after many days. 
 
 CONTENTS. 
 
 Xscr. I. INTRODUCTORY 2 
 
 II. Origin of Physical Theories ... 8 
 
 III Relation of Theories to Experience . 18 
 IV. Chromatic Phenomena produced by 
 
 Crystals 27 
 
 LECT. V. Range of Vision and Range of Ra- 
 diation 34 
 
 VI. Spectrum Analysis. ...... 45 
 
Tyndall, 
 
 J. 
 
 
 ' Select 
 Tyndall 
 
 works of Jo] 
 
 in 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ~~s* 
 
 M300991 
 
 QtV 
 
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 THE UNIVERSITY OF CALIFORNIA LIBRARY