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BY JOHN HERBERT SANGSTER, M.A., MATHKAtATIOAL MASl^BB AND LBOTTTREB IN CHESIISTRY AND NATURAL PHILOSOPHY IN THE NORMAL SCHOOL FOR UPPER CANADA. fep>' t>RINTED AND PUBLISHED BY JOHN LOVELL, AND FOB SALE AT THE BOOKSTORES. 1869. . Entered, according to the Act of the Provincial Parliament, in the year one thousand eight hundred and sixty-one, by John LovBLL, in the Office of the Registrar of the Province of Canada. 34-4-S-t ,-i,'-r,»i. iment, in by John vince of PREFACE. The design of the Author in preparing the following pages was to furnish the student with a brief yet compre-- hensive rcsum6 of all that is important in the departments of natural science of which it treats. He believes that the student of chemistry will find included all that is necessary or desirable as an introduction to that important branch of knowledge, without being so diffuse as to extend to several hundred pages. He has aimed throughout at being as clear and as concise as possible in his definitions, explanations, and statements ; and, on that account, he feels confident that teachers who employ his little work as a text-book in their schools will find their pupils master the matter with much more ease than by the use of, it may be, an equally comprehensive but less condensed treatise. Whenever the nature of the subject has permitted the use of problems to illustrate and enforce particular principles, they have been introduced, so that the plaik adopted in Part I* has been steadily kept in view in the preparation of the present book. It is believed that the entire work, including Parts I., and II. is calculated to impart a much more complete and practical knowledge of Natural Phil- oso hy tlian the text-books on that science commonly met m ■ IV FBEFAOE. with in our schools ; however that may be, the Author submits it to the kind consideration of his fellow-teachers, with the hope that it may be found to supply, in a mea- sure at least, the want of a reliable text-book on the subject in our Canadian series of school-books. The Author avails himself of this opportunity of return- ing his thanks to Professor Kingston, of the Toronto Observatory, for valuable information furnished him by that gentl(5man with regard to the declination of the magnetic needle in Canada J Toronto, October, 1861. -jT-r- ithor hers, mea- bject CONTENTS. turn- 'onto a by • the Page Heat, Definitions of ^ 5 '' Theories as regards ....f f.ff 6 " Sources of .....,,. 7 " Transferrence of. , .8 *'■ Conduction of 9 '' Convection of 10 '' Eadiation of 11 " Reflection of 12 " Absorption of , 14 '' Transmission of 14 Theory of Exchanges of Heat 15 Expansion of Gases. 17 Air Thermometer 17 Differential Thermometer 18 Expansion of Liquids 18 Mercurial Thermometer 19 Reduction of Thermometric Scales 21 Intensity and Quantity of Heat 22 Expansion of Solids ' 23 Breguet's Thermometer 24 Pyrometers >. 25 Exceptions to Laws of Expansion 26 Specific Heat 28 Calorimeter , ,', 3t Problems on Specific Heat ^ 31 Specific Heat of Elementary Atoms 33 * Melting Points ,,, 35 Tjatent Heat. I 37 Freezing Mixtures* * . . . -.•T. , 39 ■■■-«. vi CONTENTS. rr- Paob Boiling Points « 40 Nature of Vapours • 42 Elastic Force of Vapours and Gases 44 Ebullition, Theory of Boiling 46 Elevation of Boiling Points 46 , Depression of Boiling Points 4*7 High Pressure Steam ■ 50 Elastic Force of Steam 51 Latent Heat of Steam 53 Spontaneous Evaporation 54. The Cryophorus 55 Hygrometers 56 Spheroidal State 58 Sun and Stars as Sources of Heat 6i The Earth as a Source of Heat 62 Light, Theories as regards 64 '^ Definitions of. 65 Photometry 67 Decomposition of Light f 68 Newton's Spectrum 69 Magnitude of Colored Spaces TO Brewster's Spectrum 12 Absorption of Light '74 Natural Coloration of Bodies .••, "76 Complementary Colors 7*7 Theory of Transverse Vibrations 78 Interference of Light 79 Colors of Thin Films 81 " Grooved Surfaces 85 Heat as a Source of Light 85 Chemical Action as a source of light 87 Phosphorescence as a source of light 89 Fluorescence 90 Catoptrics, Definitions of. 91 Reflection from Plane Mirrors 92 " Concave " 93 " Convex " ,, 94 Problems ....,,., , 95, Formati Problen Dioptric Laws ol Indices Total R Looses. Properti Rules fo Probleir Formati Magnify Spheric£ Chroma The Mic Problem The Teh The Mat The Car The Eye Long an Color Bl Compou: Optical ; Polariza Electrici ti II Electrics Insulatio Sources ( Electrosc Electrom Theories Negative Distribut Intensity ._ wuiiuilOJ Paoi 40 42 44 46 46 , 47 50 51 53 54 55 56 58 6i 62 64 65 67 68 69 . 70 72 74 76 77 78 79 81 85 85 87 89 90 91 92 93 94 95. > ^ CONTENTS. - - ' vJi Formation of Imftgea by Mirrors , , ^9 J Problems on Formation of Images. . . , 104 Dioptrics, Definitions of 105 Laws of Refraction ^ 206 Indices of Refraction 1 0g Total Reflection , ^w Lenses ! . . ' 108 Properties of Lenses * 209 Rules for finding Focal Lengths of Lenses. ..,, m Problems on Focal Lengths of Lenses 1 13 Formation of Images by Lenses 215 Magnifying Power of Lenses 216 Spherical Aberration ^ ■,-,q Chromatic Aberration 22o The Microscope * ,22 Problems on the Microscope ^ 225 The Telescope , .'.'.'!.' 12^ The Magic Lantern . 29 The Camera Obscura , ' ' 131 The Eye and Vision .'..'.'.'.-.'."!.'.'."/.![*.! 131 Long and Short Sight *..*.'..*,*!].'.' 136 Color Blindness Compound Eyes ' Optical Phenomena of the Atmosphere ..',,],[ [ [ [ .' 139 Polwization and Double Refraction .*,'.*.' '' 242 Electricity, Definitions of .' ,. " History of '.''.1*//.'/.*.!"/.*.] 147 " Conductors of. , Electrics and non-Electrics Insulation of Bodies \\\\ Sources of Electricity ' ^ ^ Electroscopes * Electrometers '* \,\,\,,\\\ [[ ^^ ^ Theories of Electricitv "" ^^^ xr^„.. ^eciricity Negative and Positive Fluids , ., Distribution of Free Electricity jg5 Intensity, Tension and Quantity , k^ Induction ^^^ V- 166 i. W I CONTENTS. If Paoi Lftws of Variation in Intensity 151 Flectrical Machine — 158 " « Theory of 169 Electrophorus 1^1 « Theory of 162 Leyden Jar 162 " Theory of 163 Varieties of Discharge 163 EflFects of Discharge 164 Illustrative Experiments 165 Atmospheric Electricity 166 Lightning Rods 168 Dynamical Electricity 169 « « Historyof. iTa Voltaic Piles 1*73 Voltaic Batteries lU Ohm's Formulae of Resistance 181 Effects of Voltaic Current 18t Chemistry of Voltaic Current 190 Electrolysis of Bodies 191 Faraday's Laws of Definite Action 192 Electro-Chemical Theory 193 Electrotype Process 193 Theories of Voltaism 194 Magnetism and Magnets 195 Properties of a Magnet • 19t Dia-Magnetism 200 Formation of Magnets 201 Terrestrial Magnetism 204 Declination of Needle 205 Inclination of Needle 206 Theories of Terrestrial Magnetism. . . : 207 Electro-Magnetism 209 Electric Telegraph 212 Magneto-Electricity i - 215 Therm o-Electricity 2lY Animal Electricity * 217 Miscelianeous Probleiss .,,»...••»?••••»..»•»»». t,i».... ^l-S Paqb .... 157 • . • • 158 .... 159 .... 161 .... 162 .... 162 . ... 163 . . . . 163 .... 164 .... 165 • . • . 166 .... 168 .... 169 .... It2 .... 1Y3 .... 174 .... 181 . • • . lol .... 190 .... 191 .... 192 • • • • X%7iy 193 194 , 195 197 ^ 200 201 204 205 206 207 209 212 215 217 217 ,.... 21S CHEMICAL PHYSICS, HEAT. LECTURE I, DEFINITIONS, THEORIES, SOURCES, CONDUCTION, CONVECTION. DEFINITIONS. 1. Every body contains more or less of that mysterious agent to which the name heat or caloric has been applied. 2. When any substance possesses heat of greater intensity than the human body, it is, in common language, said to be hot or warm ; v/hcn less, it is said to be cold. 3. Cold is merely the absence or abstraction of heat. When wo touch a body not containing heat of as great intensity as the hand, caloric is withdrawn from the latter and a sensation of cold is pro duced. Cold is therefore only a negative property ; and it is necessary to remember, that, since all bodies contain a greater or less amount of heat, heat and cold are simply relative terms, being analogous to the terms positive and negative in electricity. 4. It is customary with some writers to apply the term heat to the sensation experienced by touching a hot body, and tha term caloric to the agent or cause w^hich produces that sensa- tion. In practice this distinction is not very rigidly observed, and in the following pages we shall use the terms indiscrimf* Rately. 9 i 6 NATURE OP HEAT. 5. We commonly speak of a portion of heat as a something that is capable of being added, subtracted, multiplied, divided, conducted, conveyed, radiated, reflected, absorbed, transmitted, &c. ; but it must be distinctly borne in mind, that, as the nature of caloric is a matter of pure hypothesis, these terms must be received with extreme caution, as merely convenient modes of describing facts, and not as explanations of those facts. THEORIES A.S TO THE NATURE OF HEAT. 6. Two theories have been advanced by philosophers for tne explanation of thermal phenomena, and are known as ^ The Corpuscular Theory, s^hich regards heat as being a fluid; and, 2nd. The Wave Theory, which regards heat as being merely a motion. 7. According to the corpuscular theory, heat or caloric may be defined to be a highly elastic imponderable fluid of great tenuity, and of which the particles are possessed of indefinite self-repulsive powers. This fluid is supposed to pervade all space not actually occupied by material atoms, and to enter into the composition of different bodies in different proportions, thereby determining the degree of their fluidity, solidity, &c. 8. The wave theory assumes that every particle of every body in the universe is in a state of perpetual vibration, and that these vibrations, varying in extent and velocity, constitute or produce heat. It further assumes that this vibratory or oscil- latory motion among the atoms of matter, has a constant ten- dency to equalize itself by communication from atom to atom, and from body to body, by means of waves or undulations pro- pagated through the ether which is supposed to fill all space not actually filled with material atoms. Note.— Many of the phenomena of heat aro equally well explained by either of these hypotheses ; others are rendered more intelligible by one than by the other, and some few seem to require a union of both supposi- tions for their satisfactory comprehension. The wave theory is adopted by many philosophers at the present day on account of the striking anal- ogy of heat to light; the rays of heat, like those of light, being capable of reflection, refraction, absorption, polarization, &c. 9. The pi 10. Chen bustion, is, source of he Thus, every evolved duri: mixed with si tare rises aln thrown upon wood. 11. Fricti Thus, Ruml meter, the bo 10,000 lbs., su hours. As other oxj the fact that times take fii friction; the portions of th 12. The < on its dimii whenever a According i ill bodies are londensed in ire diminishe leat, a portio; wet sponge ^s expelled. This fact if Jcrtholet pla< )f heat was g )uc ; the tcmj SOURCES. 7 ^OURCES. 9. The principal sources of heat are : 1st. The Sun and the Earth, 2ncl. Chemical Action. 3rd. Friction. 4th. Compression or Percussion. 5th. Electricity. 10. Chemical action, including as it does all cases of com- bustion, is, after the gun and the earth, the most important source of heat. Thus, every one is familiar with the fact that a large amount of heat is evolved during the burning of wood, coal, spirits, &c. When water is mixed with sulphuric acid in a glass vessel, the temperature of the mix- ture rises almost to the boiling point of water. So also when water is thrown upon quicklime, so much heat is evolved that it sometimes ignites wood. 11. Friction is a very important source of heat. Thus, Rumford found that in boring a brass cannon 8^ inches in dia- meter, the borer making 32 revolutious per minute under a pressure of 10,000 lbs., sufficient heat was generated to boil 18i lbs. of water in 2i hours. As other examples of the development of heat by friction, we may mention the fact that the ungreased axles of waggons, railway cars, &c,, some- times take fire spontaneously ; the custom of savages igniting wood by friction ; the fact that in grinding steel swords, knives, axes, &c., small portions of the metal become incandescent, i. o. red hot. 12. The evolution of heat by compression seems to depend on its diminishing the bulk of the body ; for, as a general rule, henever a body is decreased in size, heat is evolved. According to the corpuscular theory, this is easily accounted for. Thus ,11 bodies are more or less porous, and the insensible heat they contain is londensed in their interstices. Now when a body is compressed, its pores re diminished in capacity, and, no longer capable of containing so much leat, a portion is pressed out and becomes sensible; just as when we take wet sponge in the hand and compress it, a part of the contained water is expelled. This fact is well explained by the action of the coining-press. Thus ertholet placed a piece of copper in a press, and found that the evolution f heat was greatest at the first stroke, and diminished at each succeeding )uc ; the temperature being elevftted at each stroke as follo\YS « W 8 ml TRANSFERENCE. ''■i.M Ist stroke 17 .8** Fahr. 2nd8troke 7.5® " 8rd stroke 1.9° " We have additional examples of the production of heat by compression in the fact that a blacksmith can render a pioco of soft iron rod hot by rapidly hammering it; and a piece of German tinder can be ignited by strongly and suddenly compressing some air contained in a cylinder. 13. The heat produced by electricity, as for example in t'le galvanic battery, is among the most intense that can be ob- tained by artificial means, and is capable of melting many of the most refractory substances known. TRANSFERENCE. 14. When a red-hot ball or other ignited mass is placed in the open air, it rapidly loses its heat, and its temperature sinks until it reaches that of the surrounding bodies. The heat thus lost is transferred by several modes : 1st. A part is carried away by the metallic support or other body on which the ignited mass rests. This process is called conduction. 2nd. A part is conveyed away by certain motions set up in the air. This is known as convection. 3rd. A part is emitted from the surface of the ignited mass in the form of rays, which pass in straight lines and with the velocity of light through a viiciiuiu, and through air and certain other transparent media. This process is termed radiation. Hence heat is transferred in three way.' s : 1st. By Conduction ; 2nd. By Convection ; and 3rd. By Radiation. K'OTE. — The tcrni conducliosi is objcctuinablo, as it Implies that tho particles of a body aro in contact, which wo know to bo impossible. Hence Conduction is properly called Interstitial JicuHation, or radiation from particle to particle apross the intcr-moJocular spaces. 15. Dififf very diflfer convey it, those alon conductors. I1> must bi conductor ar duct heat, b no body tha The foil conducting as 1000 : Gold.. .. Silver, . . Copper.. Iron Zinc. .. 16. Liqr When they rather thai This may water so tha part, when made to boil of the tube t 17. The lens, silk, that these t)heric air, Jductor. The finer t [imprisoned i Clothing. T CONDtCTION. 9 compression Q rod hot by e ignited by ylinder. iple in t'lo jan be ob- ig many of placed in ature sinks J heat thug 't or other process is t up in the ed mass in ! and with hrough air process is CONDUCTION. 15. Different bodies possess the power of conducting heat in very different degrees. Those which, like the metals, readily convey it, are, in common language, called conductors^ while those along which it passes but imperfectly are termed non- conductoTH. It must be remembered, however, that the terms conductor and non- conductor are merely relative terms. In point of fact, all substances con- duct heat, but some much more perfectly than others, and hence there is no body that is absolutely a non-conductor. The following table given by Despr.etz, shows the relative conducting powers of different bodies, expressing that of gold as 1000 : TABLE OF CONDUCTING POWERS. Tin 304 Lead 180 Marble 24 Porcelain 14 Clay 11 lies that the ibio. Hence tliation from I 16. Liquids and gases are very imperfect conductors, and when they become warmed it is generally by the convection rather than by the conduction of heat. This may be clearly illustrated by holding a test-tube nearly filled with water so that the flame of a spirit-lamp may be directed against the upper part, when it will bo Ibund that the upper portion of the water may be made to boil without elevating the temperature of that in the lower part of the tube to any appreciable extent. 17. The advantage of using light porous fabrics such as wool- lens, silk, cotton, fur, &c., for clothing, is referable to the fact that these hold entangled in their meshes a portion of atmos- ]pheric air, which, like all other gases, is a very imperfect con- ductor. The finer the fabric of the cloth, the more perfectly does it hold the air imprisoned among its fibres, and hence ^aw. warmer it is as an article of flothlng. The down of the eider-duck is almost Unrivalled in thia respect. fs- \ i0 (DONVECT161I; In accordarco with this fact, it is found that if the fibres are pressed into close contact, the non-conducting power of the cloth is very much impaired. From the fact that air is an excellent non-conductor, arises the use of double windows for preserving the heat in apartments ; the stratum of air between the windows offering an almost impassable barrier to the escape of the heat contained in the room. Hcnoe also ice-houses are constructed with double walls, and the surface of the ice covered with saw-dust, woollen cloths, straw, &c., in order to preserve it. It is also partly on account of I the air contained within its pores, and partly from the fact that it is itself a very imperfect conductor, that snow acts as a protective covering to the earth, preventing its temperature from sinking as low as it would do otherwise. CONVECTION. 18. When a liquid or a gas is warmed, the process is carried on principally by convection, i.e. by the particles which come in contact with the source of heat flying off and carrying with them a certain amount of caloric, which they distribute among the cooler overlying portions. It follows that when a liquid or a gas is heated from below, currents are produced ; as may be beautifully shown by placing a lighted lamp under a flask con- taining water, with which is mixed some fine insolu- ble powder, as pulverized amoer. The small fragments Of amber will be seen tp rise in the centre, gradually flow off towards the sides, and then fall again towards the bottom. Note.— An important inference from tliis fact is that in a room which has to be warmed by a fire-place or a stove, the grate or the stove must be placed at or near the floor. Fig. 1. are pressed I very much )8 the use of ratum of air o the escape! constructed lust, woollen a account of at it is itself covering to ft would do 3 is carried irhich come Trying with lute among i'ig. 1. EADIATION. "" il LECTURE II. RADIATION, ABSORPTION, TRANSMISSION, THEORY OF EXCHANGES OF HEAT. RADIATION. 19. Heat is emitted from the surface of a^hot body equally in all directions, and always in straight lines. 20. The intensity of radiant heat varies inversely as the square of the distance from its source. Thus, if a certain amount of heat fall upon a given surface at the dis- tance of one foot from tlie ignited mass, at the distance of two feet only, one-fourth as much, and at the distance of three feet only, one-ninth as mUch, will impinge upon it. 21 The rapidity of the radiation of heat from hot bodies is influenced in a remarkable manner by the nature and condition of their surfaces. Thus, it has been found that— Ist. Bright and polished surfaces radiate heat very slowly. 2nd. Metals equally polished radiate heat with equal rapidity. 3rd. The radiating power of a metal is increased by roughening its surface or by coating it with lampblack, or tightly covering it with linen, &o. 4th. The radiating power of a body does not depend altogether on the degree of polish, since glass equally polished with metallic surfaces radiates heat much more rapidly. 5th. The radiating power of a surface is not affected by its color; hence no particular color is better adapted than another for winter clothing. Note.— The absorbing power of bodies for heat depends closely on their color. 22. In order to observe the radi;u ig power of unlike sur- faces, Rumford obtained two similar brass cylinders, both highly polished^ and, having surrounded one with a tight cov- ering of linen, filled them both with boiling water. He then found that the water ia the uncovered cylinder cooled 10 o F. *■. .-u. m '"»!K 12 EEFLECTION. in 55 minutes, while that in the other cooled 10 o P. in 36j min- utes ; or, in other words, Ihe water in the naked vessel re- quired half as long again to cool through a given number of degrees as that in the covered one. 23. Sir John Leslie investigated the radiating power of dif- ferent surfaces by placing hot water in square tin canisters coated with various substances. Presenting these surfaces in succession to a parabolic reflector, he concentrated the radiated heat upon a diflferential thermometer, and then compared the results. The following table expresses the relative radiating powers of the various substances with which the canister was coated,— that of lampblack being represented by 100 : TABLE OF RADIATING POWERS. Lampblack 100 Water (by estimate) 100 Writing paper 98 Sealing-wax 95 Crown-glass 90 riumbago 76 Tarnished lead > 45 relished lead 19 Polished iron 15 Other metals polished 12 It hence appears that lampblack radiates Jive times as much heat as polished lead, nearly seven times as much as polished iron, and about eight and a hay' times &3 much as polished gold, silver, tin, brass, &c. This explains why it is more advantageous to use bright metallic tea-pots than those made of porcelain,— the former keeping the beverage hot much longer than the latter; also the use of bright metallic covers for dishes at table, &c. / ^ 24. When'radiant heat falls on a surface, any one of thre^ things may occur : 1st. It may be reflected ; '2nd. It may be absorbed ; or, 3rd. It may be transmitted. Note.— It may be partly reflected and partly absorbed, partly absorbed and partly transmitted, or partly reflected and partly transmittedk heflection. 25. The rays of heat, like those of light, may be collected irl a focus. Thus if a heated body, as a hot ball of iron (0), bo I \ placed ii divergin t'. direction second c and are 1)etween withdraw Note.— a pair of thermome of snow n live prope meter, whi t'now, anc the formei 26. Tl powers or absorb &c., — wl] which fa but feebl; reflectors A polisl! a large am of the hoa 27. Th by Buff* to lerpendic m REFLECTION. 13 J6J min- ;ssel re- mber of r of dif- sanisters faces in radiated ampared le various ick being 76 45 19 15 12 h heat as kud about > &c. c tea-pots hot much • dishes at of thre^ absorbed d. lected ill (0), toe placed in the focus of a concave parabolic reflector (A), the diverging rays that fall upon the reflector become parallelin F.g. 2. direction ; and if these parallel rays be made to impinge upon a second concave parabolic reflector (B), they become convergent and are reflected to a focus (D.). If a screen be interposed 'oetween the mirrors and some phosphorous be placed at I), upon withdrawing the screen the phosphorous instantly ignites. Note.— If a snowball and a thermometer bo made to occupy tI)o foci of a pair of reflectors, the snowball begins to melt and the mercury in the thermometer falls. This was formerly explained by saying that the ball of snow radiates its cold to the thermometer ; but as cold is merely a nega- tive property, this is evidently impossible. In point of fact, the thermo- meter, which in this case is the hotter body, radiates heat to the ball of tnow, and hence the melting of the latter and the fall of the mercury in the former. 26. There is an intimate connection bet^yeen the radiating powers of different surfaces and their cai)abilities for reflecting or absorbing heat. Thus those surfaces — as lampblack, glass, kc, — which radiate freely, also ;i>)>orb a larjz:o part of the heat which falls upon them ; while tliose that radiate and absorb but feebly, — as, for instance, the bright metals, — are excellent reflectors. A polished metallic reflector remains perfectly cold although it collects a large amount of heat in its focus, while a glass reflector absorbs so much of the heat as to become itself hot. 27. The reflecting powers of difierout .surfaces have been determined by BulT to be as follows;— Of 100 rays incident at an angle of 60 from iho perpendicular, there were reflected — I- 14 ABSORPTION—TRANSMISSION. TABLE OF REFLECTINa POWERS. I By polished gold 76 " " silver or brass 62 " brass, not polished , 52 " polished brass, varnished 41 " looking-glass • • 20 " glass-plate blackened on back 12 " metal-plate blackened 6 ABSORPTION. 28. Color influences to a very great degree the absorbent power of a surface for rays of heat when accompanied by rays of light. Thus with the rays of heat emitted by the sun or by any incandescent mass, the darker the color the more rapid the absorption ; and hence the reason that dark-colored clothes are preferable for winter and light-colored for summer use. TRANSMISSION. 29. Those substances that possess the power of transmitting heat through them as glass transmits light, are termed dia- thermanous or transcalescent. 30. The heat of the sun passes through any transparent body without loss ; but only a portion of the heat from terrestrial sources is permitted to pass, and the amount transmitted in- creases as the temperature of the radiant body rises. Thus when the temperature of the radiant body was 180° F.,-/o-th of all the rays emitted passed through a screen of glass ; when the tem- perature of the radiant body was 360° F., -^^ was transmitted ; and when the temperature was 960° F., :ith was transmitted. 31. Rays of heat which have passed through one plate, are less liable to absorption in passing through a second. Thus Melloni found that out of 1000 colorific rays from an oil flame, 451 were intercepted in passing through four glass plates of equal thickness j and of these 451 rays — 32. A and ind even bla Thus I before tl incident \ Roc! Glas Emc Fhu Roc Suli 33. R< known, heat emii only sub fiities ; a dent ra^ from an Note.— tipon the 1 nature of 34. T\ near on imtil fin io exph ixchange EXCHANGES OP HEAT. 15 . 76 .. 62 ,. 62 . 41 20 . 12 6 381 were intercepted by the first plate. 43 " " second plate. 18 " " third plate. 9 " " fourth plate. 32. All transparent bodies are not equally transcalesccnt, — and indeed some good diathermanous bodies are opaque, or even black. Thus Melloni i)laced plates one tenth of an inch in thickness before the flame of an Argand burner, and found that of 100 incident rays, there were transmitted by — \ TABLE OF TRANSCALESCENCY. 3Sorbent by rays in or by apid the ►thes are smitting led dia- 3nt body irrestrial itted in- 3. Thus th of all the tem- smitted ; litted. ilate, are from an ur glass Rock-salt 92 rays. Glass, rock crystal, and Iceland spar 5'7 " Emerald 29 " Fluor spar and citric acid 15 " Rochelle salt and alum 12 " Sulphate of copper " 33. Rock-salt is the most perfectly diathermanous body known. Not only docs it transmit tte largest amount of the Iieat emanating from a body of given temperature, but it is the only substance that is equally Iranscalescent to heat of all inten- sities; a piece of rock-salt transmitting 92 per cent, of the inci- dent rays of heat, wliether they be radiated from the hand or from an incandescent body. XoTE.— Lenses anrl prisms of rock-salt arc as invaluable in experiments upon the transmission of heat, as those of glass are in researches into the nature of light. THEORY OP EXCHANGES OP HEAT. 34. When several bodies of different temperatures are placed viiiR less and receiving- nion;; and this procoss of ex- changing heat Roes on even alter an ecniilibrium oftemperatui-o has bocu attained, the only dilFercnce being that each body then givca as much heat as it receives. LEC TURK III. EXPANSION OF GASES AND UQUiDS, THERMO- METERS TIIERMOMETRIO SCALES. EXPANSION , 35. All bodies expand under the influence of an increasing temperature, and contract again as their temiieraturc falli^. Thu!? the magnitude of «11 nodies is depondont on ihoh* tompcraturo. A measure that is exactly a yard long in winter, is more than a yard long in summer; ft vessel that ^vill exactly hold a gallon in summer, will hold less than agallon in winter; and the dimensions of all objects are subject to daily and hourly change. This alTiwds an explanation of the irregularity in the movements of our time-pieces. The longer the pendulum of the clock, or the greater the diameter of the balance- wlscel of the watch becomes, the more slowly does it perform !ts oscillations; while if tliO pendulum be shortened or the balance-wheel lessened in diameter, the more rapidly docs it move. Buttho pendulum and the balancc-wlieol are constantly varying in tlieir dimen- Hoiis. o^yingto increase and decrease of the temperature, and theretore the clock or the watch whoso motions they govern does not keep exact time. KencD arises the use of compensation pejididims forelocks, and compen- sation balance--u'1icpls for watches. 36 Of the three forms of matter, gnoses expand most and solids least under tlic same increment of temperature. Thus, heated from the freezing point to the boiling point of water — looc 100( lOOC Note.— Sii tind least pc forces, it foil greater degr 37. Firs air, expand the amount 32« Fahr, j Note.— Tl 38. SkC( its tempera Thus 10 d( applied to a 39. The ter> consis termi^atin €nd passes of some CO and upper -of the tub fdled with the bottle, air expant liquid ; so air contrai owing to t face of th< column of the attach EXPANSION OF OASES. 17 ory of Haling LC tem- ing as :>ar ono 'coivhiff I of ex- las bocu ich licat ,M0- turo. A 1 lonjif in hold less ubjpct to mcnts of etitcr tho >\vly docs ■d or tho ). Buttlio r dimen- thcrefore io\) exact point of 1000 cubic inches of Iron become 1004 1000 " ofWulei- " 1045 1000 " of Atmosi)lieric Air " 13C5 Note.— Since tho attraction of cohesion nets mo8t powerfully in solids ^nd least powerfully in gases, and cohesion and caloric arc autagouistio forces, it follows necessarily that the same increment of heat Avill produce a greater degree of expansion in a gas or liK8arv to cut awiiy part of the cork, no as to altou' ti five oommunicution iK-tuetm the inHi(U' aua tho outsidr ol t lu^ Uail.; : ul h.'. wM- the air in tho uppt'i- part of tlu- l>ott;c uouhl fM^iuJ. uL.i by il« ulubticity couutcrbulanee tlit; ilowuward prtHsurc ol'lUo air iu lUc bottlo. Tho indicatiom of tlio nir-thormomoter arc not to bo relied uu. a« tbo movemcntH of the column of colored li.iui" P'^'^J the air.thermonioter in the receiver of an an-puu.p : dneclly %n e begin to exhaust tho air. the colnmn of colored liuuid beuiuH to descend, own.g to the clastic Ibrco of the uu- iu tlie bulb. Fig. 4 40. Tlic Differential Tlicvmcnieter (invented by Sir John Leslie) consists of a tube lent twice at right iuigles and torniinating in a bulb at each end. The bulbs both contain air iit»d the tube is filled ^vith sulphuric aciu colored ^vith indigo. It is called the differential thermometer because it indicates tlic difference of temperature of the air in the two bulbs. The principle on which it acts will be seen from the following facts : 1 If both bulbs be subjected to the same degree of heat ■ the air in each expands or tends to expand equally and consecpiently the column of licpiid will not move at all. 2 If both bulbs be subjected to the same degree of cold, the air in each contracts or tends to contract equally, and consequently the colored fluid moves neither one way nor the other. 3. If one bulb be subjected to a greater degree of hri-t iha.i the other, the air in that bulb expands and the fluid moves towards the other. 4 If one bnlb be subjected to a greater degree of cold than ti.e i-ther, the air in that bulb contracts and the colored Uquld moves toward it. EXPANSION OF LIQUIDS. 41. First Law.-^// Uqui.h /' than a Thus, m< expansions of ISC'* F., 43. The intensity < 44. Th< are mercu mcasarin^ Note.— IV Alcohol bo alcohol, li and down 1 and down 1 u6. Me other liqu 1st. 2nd. 3rd. 4th. 5th. Note.— it to depri EXPANSION OF LIQUIDS. 19 Thus, wbeti heated fVom the freozing-poiut to the boiUug-poiut of water, Alcohol expands i, or, in other words, 9 nieanures become 10 , Fixed oils" tV, " 1'^ " J^, Water " ...W, " ^25 23 Mercury " ^V " l^^^ ^'* NoTK.-From thi8 it appears that by the aamo increment of heat, alcohol or snirita of wine is about «ix timen as expansible as mercury. In the m.d- die of summer, alcohol will measure 5 per cent, and oil nearly 4 per cent, more than in the d( pHi of winter. 42. S(-ooNi. Law.— Liquids are progressively more expansible at hisfh than at low temperatures. Thus, mercury, which of all liquids is the Itast irregular in its expansions, increases, when heated through successive increments of ISC'* F., as follows :— Heated from 0*^ to ISO^ 1 volume in 55 J « 180° to 360*^ 1 " in 54i « 3G0<> to 540» 1 " in 53 43. The Thermometer is an instrument used for measuring tne intensity of heat. 44 The liquids usually selected for thermometric purposes arc mercury and alcohol ; the former being better adapted for measuring high, and the latter for low degrees of temperature. Note -Mercury boils at 602° Fahr., and freezes at 40° below zero; Alcohol boils at 170° Fahr.. but no degree of cold has yet frozen absolute alcohol. It follows that a Mercury Thermometer will act up to about 600 and down to 35° F., and an Alcohol Thermometer will act up to 150^ lahr., and down to - 150° or - 200°, with tolerable regularity. u6. Mercury is better adapted, for a thermometer, than any other liquid, from the following considerations :— 1st. It can alv/ays be obtained in a state of purity. 2nd. It expands more regularly than other liquids. 3rd. It measures a greater range of temperature. 4th. It does not soil or adhere to the tube. 5th. It is very sensitive, being readily affected by a small increment of heat. NOTE.-The mercury is purified by subliming it, and afterwards boiling it to deprive it of air. % \w iiO EXl»ANSiON Of LIQUIDS. .^•fi 46. The Mercurial Thermometer consists of a capillary ^asg tube, 10 or 12 inches in length, and of equal bore ihrougnout. The lower end of the tube terminates in a thin bulb of moderate size. The bulb and part t" the tube are filled with mercury. The air is expelled from the rest of the tube by expanding the mer- cury in tlie bulb until it rises to the top of the tube, and at that moment directing the flame of a blow-pipe againsSt the open end of the tube and thus hermetically sealing it. As the quicksilver in the thermometer coofs, it recedes from the top of the tube and leaves a vacuum above it. In order to graduate the thermomet.'r, it is attached to a flat piece of wood or ivory and placed in melting ice, when the height of the mercury in the tube is carefully marked. The instrument is next plunged into boiling water and the height of the quick- silver again carefully marked. At the former of these points the number 32 is placed and at the latter 212, to indicate respec- tively the melting point of ice and the boiling point of water. The space between these two points is carefully divided into 180 equal parts or degrees, and similar equal divisions are continued above the boiling point and below the freezing point ]SiOTH.—:Mi3rcury expands 20 times as much as glass and therefore it is that it rises and falls in the thermometer tube \vithe\'ery change of tempe- rature. Thick glass bulbs do not make as sensitive thermometeis as thin bulbs, but the latter are apt to collapse after long use or by sudden exposure to very high or very low temperature^. The greater the bulb and the smaller the diame°ter of the tube, the longer the degrees or divisions of the scale. 47. Three different thermometric scales have been adopted by the chemists of different countries. In all the fixed points are the same, viz., the melting point of ice and tlie boiling point of water but the divisions on the scales are different, being as fol- lows : — In Fahrenheit's scale the melting point of ice = 32«' ; boiling point of water = 212''. ^ In the Centigrade scale the melting point of ice = 0° ; boiling point of water = 100°. ^ In Reaumur's scale the melting pomt' of ice = 0^ ; ^boiling ©oint of water == SO**. That is> melting-p( ice and tl parts. In correspon( it and the 100 and ii I. Tore Rule, j Exam PL Reason. 1 correspond II. To ] Rule. < product 6i EXAMPI Eeason. III. To Rule. J EXAMPI Reason. 32° because IV. Tc Rule. result by EXAMPl Reason. V. To Rule. Examp: Reason. VI. T< Rule. Examp ReaaoK. EXPANSION 01* LIQUIDS. 21 'Ansi boiling boiling That iSi in Fahrenheit's scale 0° or zero is placed 32'' below tlie . melting-point of ice, and the space between the melting puiutof ice and the boiling-point of water is divided into 180 equal parts. In the Centigrade scale and the scale of Reaumur, zero* corresponds to the melting-point of ice, and the space between it and the boiling-point of water is divided in the former into 100 and in the latter into 80 equal parts. EEDUCTION FROM ONE SCALE TO ANO^IER. I. To reduce Centigrade degrees to degrees of Fahr. Rule. Multiply by 9, divide the result by 5 and add 32. Example.— 100^ C. = 212® F. Thus 100 X 9 = 900 -^ 5 = 180 + 32 = 212, Reason. 100° C.=180' F. or 5^ C =0'' F. . • . P C.^f F., and since 0^ C corresponds to 32=^ F., wc add 32^ II. To reduce degrees of Fahrenheit to Centigrade degrees. Rule. Subtract 32, multiply the remainder by 5 and divide the product by 9. Ex AMPLE.— 212° F. =100° C. Thus 212 — 32 = 180 X 5 = 900-^ 9 = 100. Reason. Similar to that in 1. III. To convert degrees of Reaumur into degrees of Fahr. Rule. Multiply by 9, divide the product by 4 and add 32. Example.— 80° R. =212° F. Thus 80x 9 = 720 —A = 180 + 32 rrr 212. Reason. 80° R. = 180° F. . • . 4° II. = 9° F. or 1° K. = f F., and wc add 32° because 0° R. corresponds to 32" F. IV. To convert degrees of Fahr. into degrees of Reaumur. Rule. Subtract 32, multivlv the remainder by 4 and dnvide the result by 9. Ex AMPLE -212° F. =80° R. Thus 212 — 32 = 180 X 4 r^ 720 -H 9 = 80- Reason. Analogous to that in III. V. To reduce degrees of Centigrade to degrees of Reaumur. Rule. Multiply by 4, and divide the product by 5. Ex AMPLE.— 100° C = 80° R. Thus 100 X 4 = 400 -f- 5 = 80. Reason. 100° C. = 80° R., or 5° C. = 4° R. . : . 1^" C. = 4° B. VI. To reduce degrees of Reaumur to degrees of Centigrade. I Rule. Multiply by 5, and divide the product by 4. Example.— 80° R. —100° C. Thus 80 x 5 = 400 -f- 4 = 100. 22 1. 219" F. 2. 117° P. 8. *26°F. 4. 9°F. 6. -4°F. 6.-23=' F. 7. 73=^ C. 8. 49.6° C. 9. 93. 2^ C. 10. 217° C. 11. -t43° C. , , ■ . .. 12. -40.9° C. ' ;.;t 13. 19.7° R. ■,^1 14. -23.4° R. -^a 15. 367.3° R. ili^ 1 16. — 16.9°R. 17. 27.4° R. M 18. 232° R. EXPANSION OP LIQUIDS. » EXERCISE. = 103.8° C. = •- ■ 47.2° C. ■ = = -3.3° C. =: = — 10.2°R. = =z —16° R. = = —24.4° R. = 163.4= F. 121.a ¥. 199.7^ 'F. 173.6 R. -34.4^ R. —32.7 ^R. 76.3° F. —20.6° F. 693.3° F. -21.1° C. 34.2° C. 111.1° C. B3.1° R. 37.7° R. -2.6° R. —12.7° c. —20° c. -30.5° c. 58.4° R 39.6° R 74.5° R 422. 6° F .4° P. -41 .6° F. 48. A thermometer does not measure the quantity of heat present in a body, but merely its intensity. Thus, if from a barrel of water of any tcmporaturo we fill a glass and then place a thermometer in each, the mercury will stand at the same height in each thermometer, although there is obviously much more heat in the barrel of water than in the glass full. 49. Although we can say that one body is hotter or colder than anotlier, we cannot say that one body is twice as hot or twice as cold as another. This arises from the fact that we do not know the true zero points of bodies, or in other words we do not know what is the least bulk into which a given body is capable of being condensed by cold. Now to say that one body is twice as hot as another, is, in reality, to say that " this body exceeds its minimum bulk by twice as much as that body ex- ceeds its minimum bulk." • When the given number of degrees P. is below 32°,flud the number of degrees below 32 and multiply that number by ^ to reduce to Centigrade, and by ^ to reduce to Reaumur. t When tuft given number of degrees C. or R. is—, i. o. below zero,mUi' tiply by g or I »nd subtract 32 to reduce to F. NOTB.— W of another b twice as hot 120° and 6(f because the EXPANSl 24.6° C. -29.2° C. 50. Fir 867.3° C. i7icrement -6° P. 93.6° P. Thus w 232P F. point of ^ is shewn Zinc ex Lead Tin Silver Copper Brass NOTK.— iucremeul is^^-jor^ 32° F. to 2 51. Si sible at h NoTK.- EXPANSION OF SOLIDS. 23 NoTK.— When wo say that the tomperatuic of one body is 120'^ and that of another body 60^ it may, at first sight, appear that the former must be twice as hot as the latter; but it must be borne in mind that the numbers 120° and 60° are merely reckoned from an arbitrary zero-point, adopted because the real zero-point is unknown. LECTURE IV. EXPANSION OF SOLIDS, PYROMETERS, EXCEPTIONS TO GENERAL LAW OF EXPANSION. EXPANSION OK SOLIDS. 50. First Law. — Ml solids do not expand equally under the same i7icrement of teniperature. Thus when heated from the melting-point of ice to the boiling- point of water, the linear expansion of rods of different substances is shewn in the following — j^oxK.— The incromt'nt in bulk is about three times as great as the linear iucremeut. Thus the linear incremoiit of Load is ^^i -^^ the solid increment is 3 .. or -i - • that is, lead increases in ImW 1 part in 117 when heated from HT) 1 117) 32" F. to 212° F. 51. Second Law. — TJie same solid h progreasively more expan- sible at high than at low temperatures. S- TABLE OP EXPANSION. '^f Zinc expands 1 part in 323 Pure Gold expan ds 1 part in ! 682 fc Lead " 1 (; u 351 Iron wire " 1 a ci 812 B Tin " 1 a li 51G Palladium " I U li 1000 Silver " 1 u u 524 Glass " 1 U li 1142 Copper " 1 iC u 581 Platinum " 1 11 11 net Brass " 1 a u 584 Black Marble" 1 u u 2833 1 J NoTR.—riatinum is the most uniform in its cxpansfou. 24 EXPANSION Oi* SOLIDS. a. 52. A compound bar made by soldering two thin plates of bras^ and iron, or of copper and platinum, illustrates very clearly the unequal expansion of these metals. Fig. 5. When heat is applied to the bar, the metals both expand, but the copper much more than the plati- num ; and the result is that the bar becomes curved, so that the platinum is on the inside of the curve. When the bar is subjected to great cold, the reverse takes place : for then the copper contracts more than the platinum, and the latter occupies the outside of the curve. NoTE.-By careful attention to the different degrees of expansibility of metals, a compound bar may be so constructed that its ends shall be the same distance apart, no matter how much its temperature may vary. This is the principle upon which the gridiron pendulum and the balance-wheels of chronometers are constructed. 53. The same principle has been be^u- tifully applied to the construction of a thermometer from solid materials by Bre- guet. This consists of a thin ribbon of silver soldered to a similar slip of plati- num, and the compound slip of metal coiled into a helix or spiral. The upper part of the spiral is fixed to a support, and the lower end terminates in an index which plays over a graduated circle, as exhibited in the figure. Silver is twice as expansible as platinum. When, therefore, the instrument is subjected to an increasing temperature, the unequal expansion of the two metals causes the helix to coil more closely. Similarly, when it is subjected to a decreasing temperature, the unequal contraction of the slips causes the helix to uncoil ; and in either case the movement is measured by the number of degrees through which the index passes. Note.— When the metallic ribbons are very thin, Breguet's thermometer is one of the most delicate and sensitive instruments we possess ; the slightest variation of temperature being measured with precision and rapidity. 54. Wl glass, or surface b the other bar of ni tlic plate on the oil cold is Ji opposite surfiices. 55. F; (letornrm Uaniell. 56. D taining i as the r fitting pi the disj*^ sion whi 57. T tinniu s niont is 0()iitaln( hoiit, thi iiir is r( platinur meter is 53. i Wilson, ill a fu it has { a given of the li the nur of the \ PYROMETERS. 25 54. When hot water is suddenly poured upon a thick plate of glass, or when the flarae of a lamp is directed against it, one t^urface becomes hot and expands before the heat penetrates to the other surface of tlie plate. As in the case of the compound bar of metal (Art. 52), we have here unequal expansion, and tlie plate tends to curve with the heated and ex])anded surface on the outside, but, owing to its iullexibility, it is broken. When cold is applied to a heated plate of glass, it breaks from the opi>osite cause, i. e owing to the unequiil contraction of the two surfaces. PYUOMETERS.. 55. Pyrometers, or fire-mmsurcns, are Instruments used for determining very high degrees of iieat. The best is that of Uaniell. 56. Daniell's Pyrometer consists of a tube of plumbago con- taining a rod of platinum. The tube is closed at one end, and, as the rod of platinum expands, it pushes forward a tightly litting plnf? or wedge at the other or open end. The extent of the disi^lacement of. the wedge measures the amount of expan- sion Avliich the platinum has undergone. 57. The air pyrometer of Pouillet consists of a hollow pla- tiniun sphere fitted wiMi an escape-tube. When this instru- ment is subjected to an increase of temperature, a part of the contained air is expelled ; and the greater the intensity of the heat, the greater the amount of air driven out. The expelled iiir is received over water, and the temperature to which the platinum vessel was subjected is thus measured. This pyro- meter is very accurate and reliable. 53. Another mode of measuring higli temperatures is that of Wilson. This consists in placing a given weight of platinum ill a furnace the heat of wliich is to be measured, and, when it has attained the temperature of the furnace, plunging it in a given weight of water of known temperature. The intensity of the heat to which the jdatinum was subjected, is estimated by tlie number of degrees through which it raises the temperature of the water. 26 EXCEPTIONS TO LAW OP EXPANSION. Thus, suppose that the platinum weighs 1 lb., and that it is plunged in 1 lb. of water at the temperature of 60°, and suppose the temperature of the water to rise to 110°, then the increase of temperature of the water i^ equal to 50"; and to convert this into degrees of Fahrenheit we multiply by 31, because the heat that raises a given weight of water through 1 de- gree would raise an equal weight of platinum through 31 degrees. Then» 60° X 31 = 1550° = temperature of the furnace. Again if the 1 lb. of platinum raise the temperature of 3 lbs. of water 40", then 40° X 3 = 120° = degrees through which the 1 lb. of platinum would have raised 1 lb. of water. And 120° X 31 = 3720° = temperature of tlie furnace. EXCEPTIONS TO GENEUAL LAW OP EXPANSIOX. 50. Certain bodies form remarkable exceptions to the general law that all bodies expand when subjected to an increasing, and contract when subjected to a decreasing temperature. These exceptions are — 1st. Type-metal. 2nd. Rose's Fusible Metal. 3rd. Water. Note.— When clay is exposed to very high temperatures, it contracts, and thus appears to be an exception to the general law. In reality, how- ever, the contraction of the clay is due to the dissipation of the water it contains; and although the clay itself expands, the amount of this expan- sion is more than counterbalanced by the diminution of bulk caused by the loss of the water it originally contained. 60. Type-metal, which is an alloy of lead and antimony, expands as it passes from the fluid to the solid state. This pro- perty causes it to fill the sharp indentations of the mould, and thus enables us to cast many hundreds of type in the same mould, whereas otherwise we should have to shape and cut each sepa- rately. Iron and some other metals possess the same property. 61. Rose's fusible metal is an alloy of— 2 parts by weight of bismuth. 1 « " lead. 1 " " tin. This compound expands regularly like other bodies up to 111^. It then rapidly contracts up to 156^, when it attains its EXCEPTIONS TO LAW OF EXPANSION. 27 plunged in )erature of le water i^ multiply rough 1 do- es. Thou' F water 40", nuin would turo of the 10 general ising, and e. These t contracts, oality, how- tho water it this expan- ausod by the antimony, This pro- Qould, and line mould, each sepa- 3 property. point of maximum density and is less in bulk tiian at 32°. From 156^ it again regularly expands until it melts at 201^. In cool- ing it goes through a similar series of changes. Note.— The sudden expansion which type-metal, iron, and other metals undergo when passing into the solid form, is accounted for by these bodies becoming crystallized and the crystals arranging themselves in angles across one another. This increase is analogous to the sudden expansion of water in freezing. Rose's fusible metal is a chemical compound of such a nature that we sliould almost expect it to be irregular in its expansion. Water is therefore the only well-marked exception to the general law. 62. When boiling water is allowed to cool, it regularly con- tracts until its temperature is about 39^ or 40^, at which point it has attained its maximum density. Wlien cooled below this point, ihstead of contracting, it expands. This fact was very beau- In cooling above 40". In cooling below 40'^. 'k iiii^Q iiiiii'- odics up to attains its tifully illustrated by the experiment of Dr. Hope, who carried into a very cold room a jar containing water of the temperature of 50" F., and having immersed in the water two delicate ther- mometers, one at the bottom and one near tlie surface. As the water cooled, the upper thermometer indicated a temperature higher than the lower till the temperature descended to 40". In other words, -as the surface-water cooled, it became specifi- cally heavier and sunk. AYhcn the lower thermometer had attained the temperature of 40°, it remained stationary until the upper reached the same point. As the cooling still continued, the lower thermometer remained steadily at 40", while the upper thermometer still continued to fall ; or, in other words \4 28 srECIFIO HEAT. the water became specincaliy lighter as it became 00Qle4 below 40" F., and therefore continued at the surface. j,OTK.-The point of nmximum dou.ity of water has been ascertained to bo accurate iy 31)2- F. 63 The fact that water has a point of maximum density in- fluences to a reinarkal)lc extent the duration of the seasons. If water foHowed the same law as other bodies, the upper lay- ers in our lakes, rivers, ponds, &c, as they cooled, would sink until the whole reached the free/.in-point, when it would become solid from the bottom upward. The result would be that our rivers, lakes and poud^ would be converted into solid masses of ice, and the heat of summer would not, in what •ire now the temperate zones, be sunicient to melt them. In point of fact, however, the upper surtace alone freezes ; and the ice thus formed, bein^^ a very imperfect eonductor of heat, protects the underlying mass of water from the cold of the atmosphere. ^ LECTURE y. SPECIFIO HEAT. 64.' Different bodies of equal heights require different amounts of heat to raise their temperatures through the same number of degrees. Thus if two bottles of t!ic ^aTne size, shape, &c., be so placed before a flro that thoy shall receive eciual amounts of heat from the tiro, and one of the J bottles be filled with water and the other with quicksilver, it will bo found that the temperature of the latter will be elevated in a given timo twice as much as that of the former. If equal weights instead of equal volumes be used, the same amount of heat raises the temperature of the mercury 30 times as much as that of the water. Hence all bodies are said to have different capacities for heat; thus water is said to have twice the capa- city for heat that mercury has, bulk for bulk, or 30 times the capacity, weight for weight. 65. If the heat required to raise a given weight of water through a given number of degrees of temperature be represented by 1000 then the heat required to raise an equal weight of any other body through the same number of degrees of temperature is tei'med its specific heat. M SPECII'IO HEAT. 29 00Qle4 rtaincd to nsity In- seasons, iper lay- uld sink ,t would vould be •ted into ill what In point il the ice protects osphere. different the same before a firo i one of the ill be found mo twice as volumes be mercury 30 aid to have !e the capa- tie capacity, ■ of water represented s;ht of any peralure is 66. The following table by Regnault gives the specific heat of various bodies, that of water being 1000 : TABLE OF SPECIFIC HEATS. Water 1000 Sulphur 203 Glass 198 Iron 114 Nickel 109 Zinc 95 Copper 95 Silver 57 Tin 66 Iodine 54 Antimony 51 Mercury 33 Gold 32 Platinum 32 Lead 31 Bismuth 31 Note.— From this table it appears that the capacity of water for heat is 5 time3 as great as that of sulplmr or glass, 9 times as great as that of iron or nickel, 10 times as great as that of copper or zinc, 18 times as great as that of silver, tin, or iodine, 30 times as great as that of mercury, and 31 or 32 times as great as that of gold, platinum, lead, or bismuth. 67. The capacities of dififerent bodies for heat may be deter- mined in four ways : 1st. By the method of warming. 2nd. By the method of cooling. 3rd. By the method of melting. 4thk By the method of mixture. 68.' The method by warming consists in exposing equal weights of different bodies to the same source of heat, and observing to what height their several temperatures rise in a given time. Thus if iron, silver, platinum, and water be exposed to the same source of heat, it will be found that the temperature cf the iron rises 9 times, that of the silver 18 times, and that of the platinum 31 times as rapidly as that of the water. Hence, as they all absorb ar equal amount of heat, the capacity of water for heat is 9 times that of iron, 18 times that of silver, and 31 times that of platinum. So also the capacity of iron is twice that of silver and 3i times that of platinum. 69. The method by cooling, known also as the metho of Dulong and Petit, gives very exact results, but in practice requires several important precautions, such as cooling the bodies in vacuo, &c. It consists essentially in placing several bodies heated to the same temperature in similar circumstances IV, 30 SPECIFIC HEAT. 1" h and observing the rapidity with which they cool. Those bodies which, like mercury, have a low capacity for heat and there- fore contain but little of it, require far less time to cool through a given number of degrees than those which, like water, have a great capacity for heat. 70. The method by melting involves the use of the Calori- meter, and is frequently spoken of as Caloriraetry. The Calorimeter of Lavoisier, Fig. 8, consists of three tin vessels, one within the other. The space between the ves- sels is filled with crushed ice. The body whose specific heat is to be determined is introduced into the inner vessel, c, and the amount of heat it contains is determined by the amount of the ice in the vessel 6 that it melts. The water obtained from the melting ice in 6 passes through the tube e and is collected and carefully measured. The object of having' ice in the outer vessel, a, is to prevent, the external air from melting any of the ice in the middle vessel. Another form of the Calorimeter consists simply of two blocks of ice fit- ting accurately one over the other, and the lower one containing a cavity into which the heated mass is placed. 71. To calculate the specific heat of bodies by the Calori- meter, we proceed as follows : Let 10 = the weight in lbs. of the body introduced into the Calorimeter, t = its temperature in degrees of Fahrenheit, «j' =r the weight in lbs. of the ice melted, s — the specific heat of the body under experiment, ijiien ., ''"'„ z= the lbs. of ice dissolved by the heat that would raise the Fig. 8. f-32- w' iwx (< — 82) temperature of the body in the Calorimeter P. = the lbs. of ice dissolved by the heat that would raise the temperature of 1 lb. of the given body 1°. Then since the latent heat or caloric of fiuidity of water is 142°, we have to' X 142 X 1000. 5=- «j X (< — 32) Interpretation.— MwZ«i>Zi/ the weight, in lbs., of ice dissolved, by 142, and this by 1000, and divide the product by the weight of the given body, in lbs., multiplied by the degrees of temperature it loses t .;• . .' BT^ECIPIO HEAT. 31 Example 1.— If 5 lbs, of charcoal of tho temperature of 752° F. dissolve 6.76 IbH. of ice, what is the apocific heat of tho charcoal? « = w'X 142X1000 6.75X142X1000 6.75x142x1000 «; X (< - 32) - 5 X (762 -32) - 5 X 720 -266.25 Am. Examplk2.— If 3 lbs. of platinum at the temperature of 932'^ F. dissolve •6 of a lb. of ice, what is the specirtc heat of tho platinum? w X 142 X 1000 -6 X 142 X 1000 w X t< - 32) •6X142X1000 3X900 =^^'^^"'- 3 X (932 — 32) - EXBUCI8K, 3. If 4 lbs. of water at tho temperature of 245° F. dissolve 6 lbs. of ice, what is the specific heat of tho water? Am. 1000. 4, If 11 lbs. of mercury at the temperature of 572° dissolve 1.4 lbs. of ice, what is the specific lieat of tlie mercury ? Ans. 33-4. 6. If 6 lbs. of sulphur at tho temperature of 332^ F. melt 2.57 lbs. of ice, what is the specilic heat of the sulphur? Ans. 2027. 6. If.6of alb. of arsenic at the temperature of 882^ F. melts ,0998 of a lb. of ice, what is the specific heat of tho arsenic? Ans. 809. 72. The method by mixture consists in placing in a given quantity of water of known temperature, a known weight of any body of an ascertained higher temperature, and when the two bodies have attained an equilibrium, comparing the loss of temperature of the given body with the gain of temperature of the other. Thus if 1 lb. of water at 100^ F. be mixed with 1 lb. at 50° F., the result- 200° 4- 60° ing temperature will be the mean between 100° and 50°, i.e ^- __ 750 ; out if a pound oi mercury at lod' be mixed with a pound of water at 50°, tho resulting temperature wdl not bo 75°, but only about 51.6°, that is, tho 48.4° lost by the mercury only raises tho temperature of an equal weight of water through 1.6°, or, in other words, mercury has only -i-th the capacity for heat that water has. 73. To ascertain the specific heat of any body by the method * of mixture, — that of water being represented by 1000, — we pro- ceed as follows : Let vo =: the weight in lbs. of the body whose specific heat is to be de- termined. t = its temperature in degrees F. w' = the weight in lbs. of the water. /' = its temperature. 5r=the common temperature after an equilibrium has been attained. Then T— <' = gain of temperature of the ?«' lbs. of water. t — r'=: the loss of the temperature of the V3 lbs. of the othei* l)ody. « r= specific heat of the body. \i- m 32 SPECIFIC HEAT. Ihon the heat gained by the water will bo its specific heat x «» X (T- t'),md tho heat lost by the other body will be its specific heat X w X (< — T) ; or, since tho specific heat of water is represented by 1000 and that of the other body by s, and since tho heat gained by tho water ii Justthatlostby tho other body, we have— axroX(t-T)-imxto'X{T^ V), and thcroforo w' X { T— V)J< 1000 . '~ wx(r-r) Interpretation.— i^wZhpZy together the weight of the water in ' lbs., its gain in temperature, and 1000, and divide the product by the weight of the given body in lbs. multiplied by its loss of temperature. NoTE.-If equal weighta of water and of tho other body are used, the rule becomes- ' _( y_/>) x 1000 . Example l.-Tf 1 lb. of copper at 800^ F. bo plunged into 1 lb. of water at 50° and the resulting temperature be 72°, what is the epeciflc heat of tho copper? Here, since tho weights are equal, we havo .=■\^a'7o -p h" »>liin'"'^Mnto 19 lbs. of 6 If 15 lbs. ofiron at the temperaiure ui ixbi r . u^ pi«.i„— i- water at the temperature of 65°, the resulting temperature is 156^; what ia the specific heat of the iron? ^««- ■^^*- til'EClFIO HEAT. 33 74. The specific beat of gases is determined by transmitting a known weigbt of tbo gas under experiment, heated to 212° F., through a spiral tube contained in a vessel of water, the tem- perature of which is carefully noted at the beginning and end of " the process. 75. In determining the specific heats of bodies, if wo take equal weights we obtain a series of numbers all different, and exliibiting no simple relations among tliemselves ; but if, instead of equal weights, we take (luantities in i)roportion to the chemical equiva- lents or combining numbers of the various bodies, we obtain a series of numbers having a remarkably close relation to one another. Thus, if wo use weights in proportion to their chemical equivalents, the table on page 29 will become— TABLE OP SPECIFIC HEAT OF ELEMENTART ATOMS. Iron 3.093 Load 3.25S Nickel 3.21S Zinc 3.087 Copper 3.017 Sulphur 3.2GG Tin 3.312 Platinum 3.206J Mercury 3.719 Silver 6.174 Gold 6.462 Antimony 6.561 Note.— From this table, it appears thattho elementary atoms of the first nine bodies given have equal capacities for heat, and those of the last three double as much capacity. We may safely conclude that there exists some intimate, though as yet imperfectly understood relation between the thermal and the chemical nature of bodies. Tho same connection has been found to exist in certain chemical compounds, and, both for elemental and compound bodies, may bo stated as follows ;— In bodies of similar chemical constitution, the specific heats are in an inverse ratio to their chemical equivalents or to some simple multiple or submultiple of the latter. 76.* The selection of mercury for thermometric purposes was chiefly determined by its low capacity for heat. It is on account of its low capacity that mercury is sensitive ; i. e., it both warms and cools rapidly, and hence it promptly follows every change of temperature. Note.— Tho thermometer measures only the intensity of heat, while the calorimeter measures the quantitij ,—ox rather the quantity above 32'^ ¥.,i\x9 melting-point of ico» C I 34 CHANGE OF F0R3I. 77. The capacity for heat of different bodies increases as they expand and decreases as they contract ; and hence when a body is suddenly made to expand without the application of heat, its tem- perature Mis because a part of its sensible heat becomes in- sensible. So also when a body is suddenly condensed its tempe- rature rises,— a part of its insensible heat becoming sensible. This is shown very clearly in the case of gases by placing a delicate thermometer in the air contained in the receiver of an air-pump. Upon rapidly exhausting the air, the part Remaining in the receiver expands, and as it expands, its capacity for heat increases ; the consequence is that a part of the sensible heat passes into insensible, and the thermometer sinks. The same principle explains the action of the neplielescopc and the sudden formation of clouds. N0TB.-When the volume of a gas is doubled, its capacity for boat is nearly doubled. One volume of air expanded into two volumes loses from 40^ to5(PF. ; when one volume is compressed into a i volume, its temperature is raised 40° ot5(P F. ; and when suddenly condensed into | of a volume, its temperature is raised sufficiently to ignite tinder. 'A CHANaU OP FORM. 3(J LECTURE VI. CHANGE OF FORM, LATENT HEAT,*" CALORIC OF FLUIDITY, MELTING POINTS, THEORY OF FREEZ- ING MIXTURES. FIRST, CHANGE OP FORM. 78. All solid inorganic bodies that, when subjected to an in-r creasing temperature, do not suflfer decomposition by the heat, finally reach a point at which they melt and assume the liquid form. In this phenomenon two points are to be carefully noticed, viz. : ' 1st. Under the same amount of pressure the melting-point is invariably the same for the same body. 2d. When the solid once begins to melt, its temperature ceases to rise until the whole of the body has assumed the liquid form ; or, in other words, when a body passes from the solid to the liquid state, it does so by the absorption of a certain amount oi' heat. 79. The melting-points of a number of common substances are given in the following — TABLE OF MELTING-POINTS. Iron melts at... . . 2800° F. Tallow melts at.. .. 92° F Gold ... 2016° Oil of Anise .. 50° Silver ... 1873° Olive Oil " .. 36° Zinc ... 773° Ice " . . .. 32° Lead ... 594° Milk " .. 30° Bismuth " ... 476° Wines " .. 20° Tin ... 442° Oil of Turpentine " .. 14° Sulphur " ... 332° Mercury " -39° Wax .. 142° Liquid Ammonia " -46° Phosphorus " ... 108° Ether —47° I 4 Note.— If the bodies are in the fluid form they freeze upon reaching the temperature set opposite them. 86 LATENT HEAT. 1^- ■: \ 80 Water and certain other liquids may, with proper precan- tion, be cooled dowa considerably below their f"«.mg.po.nta wiLut congealing. Thns if » email quantity of water be placed r;a;svelel hiving a perfectly smooth int-or^nrface and Protected from the slightest agitation, it may be cooled down to ?' or ten 5" F., that is 25 or 27 degrees below its proper free.- L» n "before'it becomes solid. When thus cooled be ow 32» F.:the le'ast agitation or the introduction of a small angular frag- „ nt of any substance at once induces the congelation of a part Tf ae water, and the temperature of the ice and remammgwa er instantly rises to 32o. It is,. however, impossible *» »««Jh; temperature of a solid the least degree above its melting-pomt without producing liquefaction. Hence 32<. F-'^PJ^'^"''; °«' The freezing-point of water, which is variable, but the melUnr point of ice, which is constant. kt™.. In freezing, the particles arrange thomselves at certain angles, ^^.^n^thefudden expansion which water undergoes in becoming and hence anses the suaaen expa ^^ ^^^^ ^ •°"*'T^Smrs o7?oe.Ut tie p^tiiloS of water boeome difTerentJy come 1» f ?*'^ '"°'"' f ,;^^„. i, illustrated by the fact that water in the arranged m the act of fr«izmg,«u , ^^ ^^^^^^ cTtntoi^o"! wS. .r;.teTL?coi^^^^^^ lit or e,eTthe most acrid poison, or mix with It the strongest ac.d or any s^Mtuori^Sd, and freeze the compound, gently moving or .g.t.tmg .t r ng Se Toe SB, and the ice formed will be absolutely pure frozen water !S J^tasteless and harmless. The foreign Ingredient, poison, or acid, :^' L , or cToSigmatter, or spirit, has been forced out of the water, and wrtc Lnd eonientratcd in the centre of the mass of ice. 81 The freezing-point of water may be lowered by dissolving any salt in it. Common salt is the most effective agent to use for this purpose, and appears to lower the freezing-point in pro- portion to the amount of it dissolved. Thus, sea-wa^r, which contains ,h of its weight of salt, freezes at 28« F., while water containing i its weight, congeals at 4° F. 82 If 1 lb. of water at 32° be miscd with 1 lb. of water at H4°,' the result will be 2 lbs. of water at a temperature the mean between 32» and 174» i.e., at 103». Bnt if 1 lb. of .ce LATENT HEiiT. 37 ftt 32=^ be mixed with 1 lb. of water at lU^ the result wUl be 2 lbs. of water at 32° ; in other words, the 1 lb. of ice, in passing into the liquid form, absorbs and conceals 142^ of heat. 83 The heat that is thus absorbed by a body in passing inta the liquid state, is discoverable neither by the senses nor by the most delicate thermometer, and is hence called Latent Heat, from the Latin lateo, " to lie hid." 84 The absorption of heat by a body passing from the solid into the liquid, or from the liquid into the gaseous state, may bp illustrated as follows : Let us suppose that a portion of ice at 0° contained in a closed vessel, is placed in a furnace the heat of which is kept so regulated that the ice shall uniformly absorb 1° per minute. For 32 minutes the temperature of the ice will regularly rise, and at the end of that time will be at 32°. The ice then begins to melt, and, although it still continues to absorb 1° of heat per minute, its temperature remains stationary at 32^ until, at the end of 142 minutes, all the ice is converted into water, the temperature of which is 32° F. From this point the temperature again regularly rises at the rate of lo per minute, and this uniform increase goes on for 180 minutes, when the thermometer indicates a temperature of 212^ and the water begins to boil, passing into the form of vapour. Now again the temperature ceases to rise, and for 9t2 minutes remains fixed at 212^ After the lapse of dl2 minutes all the water is converted into* steam at 212°, and the temperature of this steam rises uniformly 1° per minute. 85 The heat that disapears when 9. solid assumes the liquid form is called Caloric of Fluidity ; that which disappears when a liquid assumes the gaseous state is termed Caloric of Elas- ticity. 86 The following table shows the amount of heat absorbed by different bodies in pj^ssing from the soUa to the liquid state ; I k ;^ I ; ■ '*; 38 FEEEzma mixtures; TABLB OF OALORIO OF FLUIDITY. Water Sulphur...* Lead Bees Wax. 142° 145° 162° 175° Zinc 493° Tin ^ Bismuth ^50^ FREEZING MIXTURES. 87 All freezing mixtures depend essentially upon the fact u U nhstracted from the surrounding bodies. 'TLwhenasaltisdissolvcdinw^^^^^^^^^^^^ ^ater. , Nitre, ^oiyns^^iot^^^redn^^^^ ts sal ampaoniac which ic is dissolved lo ^r i» » w"^"' * . dissolved in 19 parts of water, Sf/reZS ?KL'po?&r^'m 'W' r*to lA., or eonsSloraWy Wow "VrThe foUowing table contain, a list of the ingredients used inTommonfteelLI mixtures, and also indicates the degree of cold produced. TABLE OF FKEEZraa MIXTURES WITHOUT ICE^ No. Mixture. Nitrate of Ammonia. Water , Tarts. Thermometer sinks. Degree of cold produced, Muriate of Ammonia Nitrate of Potash Water H from +50° to +4° 5 5 19 I Sulphate of t>o^A «•. Diluted Nitric Atttl. Sulphate of Soda Muriate of Ammonia. Nitrate of Potash — Diluted Nitric Acid. . 3 2 from +50° to +10° 46P from +50° to —3° 40° 53° Sulphate of Soda . . . • Nitrate of Ammonia Diluted Nitric Acid. 6 Sulphate of Soda... Hydrochloric Acid. ti 8 5 from +50° to -10° 60° Phosphate of Soda . . Nitrate of Amin''''nia Diluted Nitric 'i.cid. 5) 4 from +50° to-14° from +50° to 0° 64° 50° from 0° to— 34° 84° No. 1 2 8 4 5 6 7 8 9 10 3 now Comi Snow Comi Salii Snow Comi Sali Nitra Snov Com Nitrs Snov Dilui Sno\ Crys Sno^ Pota SnoA Dilu Sno^ Crys Sno^ Dilu 89. TV up its lai sets free Thisfaci bottle a h ing a the: in the ace cool to th< as long at stopper, s rises, as is be graspe crystalliz . 493P . 550° the fact the ab- applied, ire of the water in mmoniac of water, bly below snta used legree of legree oi cold roQUced. 46P 40° 53° 60° 64° 50° 84° FREEZING MIXTURES. 39 TABLE OF FREKZINa MIXTURES WITH lOB. No. Mixture. Snow or rounded Ice. Common Salt Snow or Pounded Ice. 2 iCommon Salt iSal Ammoniac 8 Snow or Pounded Ice. Common Salt Sal Ammoniac Nitrate of Potash Parts. Thermometer sinks. f} Snow or Pounded Ice. Common Salt Kitrate of Ammonia . . Snow Diluted Nitric Acid. Snow Crys. Muriate of Lime u o a u H a to— 5° Degree of cold. to -12° to -18^ 12] a o 40-26': Snow . . Potash. 10 Snow Diluted Nitric Acid. Snow Crys. Muriate of Lime. Snow Diluted Sulphuric Acid. 1[ 1} 3 from +32° to -30° from +32° to —50^ from +32° to —51° 3i 2; l\ 8 10 } from 0° to — 46P 62° 82° 83P from OP to —66° from —66° to —91° 46° 66° 25° 89. When a body passes from a liquid to a solid state, it gives up its latent heat. Thus, when water assumes the solid form it sets free the 142° of heat it had absorbed in liquefying. This fact can be very clearly shown by placing in a stoppled bottle a hot saturated solution of sulphate of soda, and pass- ing a thermometer air-tight through a cork, as represented in the accompanying figure. Upon allowing the solution to" cool to the ordinary temperature, no crystallization tab«8 place as long as the bottle is closely stoppled ; but upon removing the stopper, solidification at once takes place, and the twnpcrature rises, as is indicated by the thermometer,— indeed, if the bottle be grasped by the hand, it is sensibly warmer than before l*ie crystallization commeuced, k ;/■ 40 BOILINa POINTS. So also if a portion of water containing a themomoter bo carefully cooled below S° say to W or 15° below the ordinary freezing point, upon tht^nli small fragment of ice or any other substance in the water a Torn fs instantly changed into the solid form, ^f^ -^^^^^^^^^^^^^^ ice formed, and the unfrozen water rises at once to 32° by the absorption of the heat disengaged by the part congealed. 90 The fact that water absorbs so large an amount of heat in assuming the liquid form and gives it up again in free.mg has a remarkable influence on our climate and the duration o our seasons. If, by the disengagement of a single degree of heat water could assume the .olid form, the process of freezing would go on with fearful violence, and there would be no gra'dual change from summer to winter ; and if, on the other hand, ice could melt by absorbing a single degree of heat, the yast accumulations of winter would liquefy so rapidly as to inundate the entire country. Both the melting of ice and the freezing of water require time. The 142° of heat have, in the former case, to be absorbed, and in the latter case disengaged, and this serves as an effectual check upon sudden transitions from summer to winter or from wmter to summer. LECTURE VII. SECOND CHANGE OF FORM, VAPORIZATION-BOILING POINTS CALORIC OF ELASTICITY— NATURE OF VAPOURS-ELASTIC FORCE OF VAPOURS-DEN- SITY OF WATER-VAPOUR— EFFECTS OF PRESSURE AND COLD ON VAPOURS AND GASES. 91. When any liquid is subjected to an increasing tempera- ture,'it finally reaches a point at which it begins to boil and pass off rapidly into the state of vapour. In this phenomenon two points require to be carefully noticed, viz. : 1st. The same liquid, under the same circumstances as re- gards the pressure upon its surface, &c., invariably begins to boil at the same thermometric point. 2nd. ' c f< • 11 a 92. The following Hydroc Ether . Sulphic Ammo] Alcoho Water 93. W( heat In p possessio attribute Note.— the steam 94. Tl of water melting Thus si 212°, the t water fro KOTE.- cast-iron every 200 ducting £ 200 cubic 95. I heated BOILING POINTS. 41 (fully cooled point, upon the water, a raturo of tho D absorption int of heat n freezing, iuration of degree of process of e would be if, on the ree of heat, pidly as to }quire time, bed, and in itual check from winter -BOILING lTure of jRS— DEN- PRESSURE ig tempera- )oil and pass )menon two ances as re- , invariably int, 2nd. When a liquid once begins to boil, its teittperature ceases to rise until after it has wholly assumed the form of vapour ; or, in other words, when a liquid passes into tne sli'.te of vapour, it does so by the absorption of a certain iinioiint of heat. 92. The boiling points of a number of bodies are given in the following : TABLE OF BOILtNG POINTS. HydrocUlorio Ether 52° F. Ether 96° Sulphide of Carbon 118'' Ammonia 140° Alcohol 1T3^ Water 212° NitricAcid liSP k\ OR of Turpentine 814° Phosphorus 554° Sulphuric Acid 620° Whale Oil 680° Mercury 662° 93. We have seen (in Art. 84) that water absorbs 972° of heat in passing into the form of steam or vapour, and it is to the possession of this large amount of latent heat that we are to attribute the efficiency of steam as an agent for warming. Note.— The caloric of elasticity of eteam would be almost sufficient, if the steam were a solid body, to render it visibly red hot in day light. 94. The latent heat contained in the steam generated by 1 lb. of water is sufficient to raise nearly 5^ lbs. of water from the melting point of ice to the boiling point of water. Thus since 18(P of hent are required to raise 1 lb. of water from 32° io 212°, the steam generated by 1 lb. of water will raise 972-fl80--:=:52lbs. vH water from S2^ to 212°. NoTE.-When buildings are heated by steam conveyed through them in cast-iron pipes, it is customary to allow one cubic foot of boiler capacity for every 2000 cubic feet of space to be heated ; and it is found that of the con- ducting steam pipe one square foot of surface must be exposed for every m cubic feetof space to be heated to the temperature of 75° F. 95. From the circumstance that the temperature of bodies heated by steam can never be raised above 212° F., and that PI 42 NATtjRE OF VAfOliR^. I consequently all danger of cmpyrcnma is thus avoided, Steam is very much employed for heating extracts, organic substances, &C., and is much preferable to a tire for that purpose. 96. The amount of heat absorbed by different liquids in as- suming the form of vapour is exhibited in the following : TABLE OF CALORIC CI* ELASTICITY. Water 972° F. Alcohol 885^ Ether 162°F. Oil of Turpentine 133° 97. The most important features with regard to the nature of vapours may be illustrated by the following simple experiment : Fig.lO. A glass tube, a, (Fig.lO,) half an inch in diameter and 20 to 25inclic8 long.open atone end and closed at the other, is filled to within half an a inch or so of the top, with mercury, and the remaining space filled with ether. The thumb is then firmly pressed upon the open end, the tube inverted and placed in the mercury contained in a jar, 6, having the same length as the tube, and a diameter three or four times as great. The ether at once rises, owing to its superior levity and occupies the upper or closed extremity of the tube, where, the mercury being at the same level in the tube and jar, it is subjected to a pressure equal to that of the atmosphere. Now il the tube be raised, &c., the following facts will be observed: 1st. If the tube be raised in the jar as high as possible ' without admitting the atmospheric air, a portion of the ether or other liquid becomes converted into va- pour and depresses the column of mercury in the tube ; and if different liquids be tried in succession, it will be found that at the same temperature they depress the mercurial column to an unequal amount,— water, for example, less than alcohol, and alcohol less than ether. Hence, I. Decreasing the pressure upon the surface of a liquid facili' tates its evaporation ; and in a vacuum vapours form instantly, even at ike loioest temperature, KoTE.— Hence, there are many liquids which wouid, if the pressure of the air were removed, become permanently gaseous. If AtijRE OP VAPOCES. 4^ Steam is bstances, ds in as- 1G2° F. . 133° nature of periment : to 25 inches thin half an space filled ) open end, in a jar, 6, reo or four ts superior >f the tube, and jar, it re. Now if ved: ,S possible portion of i into va- . the tube ; m, it will iy depress it, — water, less than juid faciii- \ instantly, ) pressure of tl. 'he elastic force of the vapour of different liquids may be ^measured by the an mnt to which they depress the mercurw-t column in a barometer tube. Thus at a temperature of 80° F. water depresses the mercurial column 1 incA, alcohol 2 tnches, and ether 20 inches. • 2nd. If the end of the tube be grasped in the hand or slightly warmed by exposure to the flame of a spirit lamp, the column of the mercury is still further depressed ; hence, The elastic force of a vapour increases with its temperature. 3rd. If the tube be warmed to the boiling point of the ether, or whatever other fluid is introduced into the upper part of the tube, the mercury is at once depresse' to the same level as that in the jar ; hence, The elastic force of the vapour of any liquid at its boiling point is equal to the pressure of the atmosphere, or, in other words, would sustain a column of mercury 30 inches in height, or is equal to 15 lbs. to the square inch. 4th. If now the tube be allowed to oool, the vapour con- denses and the mercury rises in the tube ; hpnce, ji vapour is condensed into a liquid by decreasing its tempe- rature. 5th. If, in place of cooling the tube, it be depressed in the jar so as to increase the pressure on the vapour con- tained within, a portion of this vapour at once con-^ denses, and the mercury within the tube constantly maintains the same level as that in the jar ; hence, I. J vapour is cmdensed into a liquid by subjecting it to pres^ mre} and II. J vapour is at its point of maximum density when its tempe- rature is the same as the boiling point of the liquid from which it is formed. 98. The elastic force of vapours increases very rapidly with their temperature,— each vapour appearing to follow a rate of ♦I P 4i NATURE OP .VAl?OirRS. progression peculiar to itself. That of water-vapour is exhibited ia the following : TABLE OF ELASTIC FOUCB OF WATER-VA^OtTll IS VACUO. Tompcraturo. Elastic force. Water at -'22P F. dopr©68C3 tho mercury in abarora. t"ljo00144inch. 140' 185'' " 212' " *\ 0-0818 01811 0-3608 1-6847 0-2421 6-8583 7-4808 29-9220 <( (( (( (( <( 11 It It It 99. A vapour is said to be at its point of maximum den- sity when we can neither increase the pressure upon it nor decrease its temperature without condensing a portion of it' into liquid. We have seen that a vapour may be produced from a liquid under reduced pressure even at a very low temperature, but in that case the vapour is, comparatively speaking, very rare. The vapour is invariably most dense when at the boiling point of the liquid producing it. The comparative density and weight of water-vapour, at different temperatures, is shown in the following : TABLE OF DENSITY OF WATER- VAPOUR. Temperature. Density. 212PF. 150^ 100° 32" 1-000 0-272 0-074 0-022 0-016 0-009 Weight of lOa cub. in. 14-962 grains. 4-076 " 1113 0-338 0-247 0-136 II II 100. The distinction commonly made between a vapour and a gas is, that the former is more readily made to assume the liquid form= Gases are divided into those which are perma- nently elastic and those which are not permanently elastic ; and the only respect in which the latter differ from vapours, is that I SIQNIFICATION OF VAPOURS. 45 shibited 0. CO. incb. « It tt n (( t€ It tt iim den- ii it nor on of it* ced from jerature, ery rare. Qg point d -weight I in the 3ub. in. ns. t pour and isume the :e penna- ,stic ; and :s, is that they require a greater decrease of tcmperttturo or increase of pressure to condense them. 101. Among the gases which have not as yet been made to assume the liquid form by the conjoined otfects of cold and pressure, may be mentioned oxygen, hydrogen, nitrogen, nitric oxide, carbonic oxide, coal gas, and atmospheric air. Tliese re- fused to liquefy at the temperature of — IGG' F. while subjected to pressures of from 27 to 58 atmospheres. The subjoined table gives the results obtained by Faraday on the cold and pressure required to liquefy certain gases : 1 Gas. Sulphurous Acid. Sulphuretted Hydrogen Carbonic Acid . Tension of vapour in atmos- pheres. Chlorine Nitrous Oxide. . Cyanogen. Ammonia. Hydrochloric Acid. OlefiantGas Hydriodic Acid. . Fluo-silicic ACid. 0-726 1-530 3-000 1-02 i 6 1 14-6 1-14 6-97 22-84 29-09 t 38-50 50 25 37 90 48 44 9 Arsenuretted Hydrogen] 1-8 15-04 ; 26-20 : 4-6 26-? 2-9 0-04 , 6-21 13-19 QPF. 32° 68^ -100° (P 62° -111° —56° 0° 15° 32° 60° 45° 0° 32° 63° 0° 32° 60° -100° 0° 32<^ -105° 0° 0° —160° Becomos a colorless transpa- reut crystalline solid body at —105° F. Becomes a white crystalline translucent bodv, resem- bling camphor, at — 122°. Becomes a white non-crystal- lino solid, resembling snow, or, more nearly, anhydrous phosphoric acid, at a tempe- rature of about — 148°. Does not become solid at— 220° Becomes a transparent crystal- line colorless solid ati— 150° Becomes a transparent crys- talline solid at —30°. Becomes a white translucent crystalline solid at —103°. Does not become solid at Iho lowest attainable tempera^ tare. Becomes a clear solid, like ice, at —60°. Becomes solid at the lowest attainable temperature. -75^ 0° 60° Does not —166°. become solid at •I' 46 EBULLITION, '^! I ''.& LECTURE VIII, EBULLITION— THEORY OF BOILING— MEANS BY WHICH THE BOILING POINTS OF LIQUIDS MAY BE ELE- VATED OR DEPRESSED. 102. When some water is placed in a vessel over a fire, small bubbles of vapour form at the bottom, rise a little way, collapse and disappear. As the process of heating goes on, these bubbles rise higher and higher, and at length reach the surface, where they escape with bubbling agitation, producing the phenomenon of ebullition. • 103. Ebullition takes place in a liquid, or, in other words, the liquid boils, just as soon as the elasticity of the vapour-bubbles is equal to the pressure upon the surface of the liquid. Until the liquid reaches this point, the bubbles of vapour that form near the bottom of the vessel have their elasticity diminished by loss of heat as they rise through the cooler liquid above them, and are thus unable to maintain themselves, and arc consequently crush- ed in and condensed. Hericc the boUing point of any liquid is that point or degree of temperature at wliich the elastic force of its vapmtr is equal to the pressure of the atmosphere. 104. It follows directly from Art. 103 that wo can artificially elevate the boiling point of water or any other liquid by in- creasing the pressure upon its surface. This is well illustrated by Papin's Digester, which is a vessel so contrived that the steam never escapes, but, accumulating in the upper part, exerts a constant and powerful pressure upon the surface of the water. The vessel is fitted with a safety valve, and in it water may be heated with facility to 350° or 400° F. In fact it is said that water may be made red hot in a Papin's Digester and still retain its fluidity. The chief use of the instrument is to intensely heat certain bodies which require a high temperature for their solu- tion. Thus : bones which resist the action of water at 212° arc reduced to a jelly in the Digester. THEORY OF BOILINQ. 47 105. Generally speaking, the j,'reater the apceific gravit/ of a liquid the higher its boiling point, and hence the boiling point of water may bo elevated by dissolving any salt in it. Some suits appear to raise the boiling point more than others, thus: Wutor suturatctl with coinmou salt (100 water to 30 wait) boils ut224^ potawh (too water to 74 potuah) " 238". " " chloiklo of calcium " 264". Note.— This property isofsoiru'prnctical importance, when it is required to subject a body to a nteady tempenituro t>oinowhat above 21ii''. 106. The only modes, then, by which the boiling point of water may bo raised are : ' 1st. l?y increasing the pressure upon its surface ; and 2nd. By dissolving a salt in it so as to increase its specific gravity. It follows that once a liquid boils it can be made no hotter except by one of these two methods. A thermometer plunged in boiling water indicates no change of temperature, no matter how rapidly the process of ebullition may be made to proceed. j,'OTi5.— This fact U of considerable value in domestic economy. Meats, vegetables, soups, &.c., cook just as rapidly when kept gently boiling as when placed on a great fue and made to boil with violence ; the unnecessary oxponditureof fuel in the latter case being altogctlioi- employed in con- verting a portion of the water into steam. 107. It is also evident from Art. 103 that by decreasing the pressure on a surface of a liquid wo lower its boiling point. This may be shown by placing a flask of water considerably below the boiling point or but little above blood heat, inside the receiver of an air-pump and rapidly exhausting the air. After a short time, as the exhaustion becomes tolerably complete, the water enters into a state of violent ebullition. In Leslie's' process for freezing water (Art. 121), where a first-class air-pump is employed, the water may be seen boiling and freezing at one and the same time, or, in other words, it is made to boil at32"F. The same fact, vi/., that decrease of pressure lowers the boiling point, is very beautifully shown by boiling some water in a flask, and while it is in a state of ebullition firmly corking the flask, i^ow if the water be allowed to cool partially and the flask V 'I 48 THEORY OF BOILING. be then plunged in a vessel of cold water, the liquid in the flask again feegins to boil with violence, and the colder the water in the outer vessel the more rapid the ebullition. To understand the reason of this, we have merely to remember that as the flask was corked while the water was boiling, the upper part, or the space between the water and the cork, is filled with vaiwur, and that upon plunging the flask into cold water, this vapour is condensed and thus a partial vacuum produced. The water then boils from the reduced pressure on its surftxce, and as fast as new vapour is generated it is condensed by the external cold water.. Note.— Mr. Howard's patent process for concentrating the syrup of sugar without scorching and browning it, depends upon the facility with which liquids arc evaporated under reduced pressure. Tho boilers con- taining the syrup arc fitted with air-tight lids, and tho air, and tho steam also as fast as it is generated, is pumped ofT by a powerful air-pump worked by a steam engine. By this process sugar syrup may be boiled at 150° F. Tho same process is of great value in inspissating vegetable infusions, i. c. in reducing them to the state of extracts for medical purposes ; as by this means they are obtained without exposure to a very high temperature and consequent loss of a largo amount of tho active principle. .08. Since the pressure of the air is greatest at the level of the sea, and regularly decreases as we ascend into the higher regions of the atmosphere, it is plain that water must boil at a lower temperature on elevations than at the sea-level. Thus travellers assert that at tho summits of lofty mountains, water boils :it so low a temperature that meat and vegetables cannot be cooked. It has been found by experiment that an elevation of 550 feet lowers the boiling point of water 1°, and Saussure ascertained the fact that at the top of Mont Blanc water boil^ at 184'^ F. It is also in close agreement with this fact that as we descend into deep mines, the boiling point of water rises above 212°. 109. The pressure of the air at the level of the sea is differ- ent at different times, causing the mercurial column in the barometer to vary from 27- Tl to 30-G inches in length, and this unequal pressure modifies to a considerable degree the boiling point of wutcr, as seen in the following table : Barometer ii Thus the ence in th fact must I a thermom at 29-92 th no. Bes of pressure a modifyir boils at 2 1: previously heated to coated on 211°. Th( iinperfectl; consideral to be in a tion, or th suspended in the flask le water in understand IS the flask )art, or the 'ajwur, and 5 vapour is The water and as fast :ternal cold the syrup of ! facility with I boilers con- nd tho steam jump worked ed at 150° F. infusions, i. c, cs ; as by this apcraturo auU the level of the higher st boil at a cvcl. Thus taias, water iblcs cannot fl,n elevation ud Saussuro water boils fact that as water rises THEORY OF BOILING. 49 Barometer in inches of mercury. Water boils. 27-74 208° F. 28-29 • 209° 28-84 210° 29-41 ' 211° 29-92 212° 30-6 213° Thus the unequal pressure of the atmosphere causes a differ- ence in the boiling point of water equal to about 5°, and this fact must be attended to in fixing the boiling point of water ou a thermoraetric scale. It is only when the barometer stands at 29-92 that the boiling point of water is 212°. 110. Besides the variation in the boiling point under increase of pressure or density, the nature of the containing vessel exerts a modifying influence. Thus in a rough metal vessel water boils at 212'', in a clean glass vessel at 2 14°. If the glass has been previously well cleaned with hot sulphuric acid, water may be heated to 221o before it boils. On the other hand, in a vessel coated on the inside with sulphur or shell-lac, water boils at 211°. The cause of this phenomenon appears to be but very imperfectly understood, as in the case when it is cooled considerably below its common freezing point, the water appears to be in a condition of unstable equilibrium, and the least agita- tion, or the introduction of an angular body, at once induces the suspended process. sea is diffor- iumn in the ;th, and this e the boiling 50 HIGH PRESSITEE STEAM. LECTURE IX. HIGH PRESSURE STEAM-ELASTIC FORCE OF CON- FINED STEAM— VOLUME OF VAPOURS— RELATION BETWEEN THE SENSIBLE AND THE INSENSIBLE HEAT OF VAPOURS. li ■!• I 'A 111. If the hand or any other part of the body be exposed for a moment or two to the steam generated by water boiling under or- dinary circumstances, it is very severely scalded ; but, when steam of high pressure, and which is consequently much hotter than ordinary steam, escapes through the safety valve of a boiler and issues into the air, the hand may be, with perfect safety, im- mersed in it. This singular property of high pressure steam is explained as fallows : Elastic bodies escaping from a stat« of compression expand beyond their original dimensions. They then contract, after- wards expand, again contract, and thus oscillate, as it were, within narrower and narrower limits until they finally regain their normal condition. Now when steam of high pressure escapes into the air it becomes very greatly expanded, and at the same time so mixed with air that it is prevented fvom subse- quently collapsing. When however steam is mingled with two or three times its volume of air it becomes low pressure steam, is not easily condensed, and has its temperature reduced from 250° or 300° to 120° or 130°, and at this t( mperature is not sufficiently hot to scald the hand. 112. If a portion of steam not accompanied by water be placed in a vessel nnd heated, it does not exert a greater elastic forre than would an equal volume of atmospheric air inclosed and subjected to the same temi)erature. But when water is present, morii steam continues constantly to rise and accumulaxe in the upper part of the containing vessel, and, adding its elastic force to that of the steam previously existing, the pressure becomes enormous. ELASTICITY OP STEAM. 51 113. The elastic force of steam at temperatures above 212^ is de- termined by means of an arrangement illustrated by Fig. 11 :aiss. stout globular copper vessel placed on a stand over the flame of a spirit lamp ; it contains mercury to the depth of about 2 inches, and over that some water, fc is a long tube, open at both ends, and having attached to it a scale carefully graduated in inches. The lower end of this tube reaches nearly to the bottom of the vessel a, and dips beneath the surface of the mercury, c Fig. 11. is a thermometer, the bulb of which is placed just inside the vessel a ; and/ is a stop-cock. The water is boiled for some time with the stop- cock / open so as to expel all the air ; and during this time the thermometer steadily in- dicate^s a temperature of 212°, and the mercury in the tube is at the level of that in the vessel a, thus showing that steam at 212° has an elas- tic force equal to the pressure of the atmosphere. The stop-cock / is now closed, and the steam accumulating in the upper part of the globe acquires increased elastic force, and, pressing on the surface of the water and thus on the mercury, forces the latter to ascend in the gauge-tube 6. For every 30 inches tne mer- cury rises in the tube, the confined steam is said to have a pressure or elastic force of another atmosphere. Thus when the mercury in the tube is at the level of that in the vessel, the steam has an elastic force of one atmosphere. When the mercury in the tube is 30, 60, 90, 120, &c., inches above the level of that in the globe, the steam is said to have an elastic force of 2, 3, 4, 5, &c., atmospheres. 114. The elasticity of steam, at different temperatures, is ex- pressed in atmospheres in the following : 5^ LATENO^ fiEAt 6^ VAlPOUtlS. TABLE OP ELASTIC FORCE OP STEAM. Elasticity in Atmospheres. Temperature. Elasticity in Atmospheres. Temperature. 1 212°F. 8 341 -BOF. li 233-9° 9 350-8° ■ 2 250-5'' 10 358-3° 2J 263-8° 16 392 5° 8 275-2° 20 418-5° 8i 285 -F 26 439-3° 4 293-7° 30 457-2° 4i 300-3" 86 472-7° 5 307-5° 40 486-6° 6 320-4° 46 499-P 7 : 331-2° 60 — s — 610 -ep 115. Equal volumes of different liquids yield very different volumes of vapour. Thus under ordinary circumstances : 1 cubic inch of water yields 169G cubic inches of vapour. 1 « alcohol " 519 " " 1 « oil of turpentine " 192 " " 116. Prom Art. 96 (Chem. Art. 4G) it appears that the denser the vapour the less its latent heat. Thus the caloric of elasticity of water-vapour is 972°, and that of alcoUol-vapour 385°, i. e., water- vapour has 2 J times more latent heat than an equal weight of alcohol-vapour; but since the specific gravity of alcohol- vapour is 2^ times that of water- vapour, it is manifest that equal volumes of the two vapours contain equal amounts of latent heat. 117. Since the latent heat of different vapours is proportional to their volume, it follows that the same expenditure of beat will Latent Heat of Vapours. 53 rature. 8°F. 8° 3" 5° 60 2° ,70 .(JO ■1" different vapour. (( u lO denser lasticity 85°, i. e., m equal avity of manifest ouuts of )ortional heat will generate the same bulk of vapour from all liquids, and hence no advantage would be gained by substituting any other liquid for water in the steam engine. 118. Vapours generated at a low temperature contain more la- tent heat than those generated at a high one. Thus water may be made to boil in a good vacuum at a temperature of 100" or 150^ but the steam produced is much more diffused and rare than that obtained at 212°, and hence (Art. 11) contains more insen- sible or latent heat. It has been determined by experiment that equal weights of steam of all temperatures, when condensed by water, raise the temperature of the water through the same number of degrees, or, in other words, the sensible and the insen- sible heat of steam, added together, amount to a constant quantity. From this we may obtain a simple rule for determining the latent heat of water-vapour at any temperature. Neglecting the heat which it has at O"" F., the sensible heat of steam at 212" is 212^, and (Art. 96) the latent heat of steam at 212° is 972°. Hence the sum of the sensible and latent heat of steam at 212° is 212°+ 9'72° = 1184°. Then to find the latent heat of water-vapour at any other tem- perature, deduct this sensible heat from this constant number 1184°, and the remainder will be the latent heat : Temperature. Latent heat of equal weights of steam. 0° 1184^— 0^ = 82° 1184''- 32° = 100° 1184^—100° =: 150°. 1184°— 150° = 1184° 1152° 1084° 1034° 212° 1184°— 212° = 972° 25(p 1184°— 250° = 934° MP 1184°-300°== 884° 400O 1184°— 400° = 784° Note.— From this it li evident that no fuel is saved by distilling in vacuo; for to convert a cubic inch of water into steam requires the same amount of beat, no matter what the temperature at which the evaporation is effected. B if 54 BVAPOEATION. ■ LECTURE X. SPONTANEOUS EVAPORATION. 119. It has been remarked (Art. 97) that when some water is admitted into a vacuum the latter becomes instantly filled with vapour. The tension of the vapour thus formed depends upon the temperature, and is measured by the amount of depression it causes in the barometric column when admitted into the Torri- cellian vacuum. Thus at —22° F. it lowers the mercurial column 0-0144 of an inch, at 32o F. 0-1811 of an inch, at 86° F. 1-2421 inch, at 140° F. 5-8583 inches, and at 212° F. 29-922 inches. 120. Evaporation always produces cold, since it requires a certain amount of heat to convert a liquid into a vapour. This circumstance may be illustrated by a number of facts. Ist. If some ether be dropped upon the hand and allowed to evaporate it produces a very decided sensation of cold. 2nd The pulse glass (Fig. 12) , which consists of a tube bent twice at right angles and terminated by a bulb at each end, is designed to show the same fact. The instrument is filled par- tially with alcohol and partially vdth alcohol-vapour. When one bulb is grasped by the hand, the warmth im- parted is sufficient to boil the small portion of the liquid that wets the inside, and as this evaporates and distils over into the other bulb, a sensation of cold is produced. 3rd. If a small vessel containing water be covered with a cloth kept moistened with ether, the evaporation of the latter produces sufficient cold to congeal the water. 121. Leslie's process for freezing water by its own evaporation depends on this principle. A little water in a cup is supported over a shallow vessel containing concentrated sulphuric acid,and the whole placed on the plate of an air pump and covered with as small a receiver as possible. All that is required is to pro- duce a good vacuum at first. If this be attained and the sulphuric acid is concentrated, the water-vapour is absorbed by tb- acid as rapidly as it is formed, and the temperature of the watCi *j the cup soon sinks to the freezing point. ■ ,- EV.1- ORATION. 55 water is led with ids upon ■ession it le Torri- 1 column ^ 1-2421 shes. iquires a r. This evaporate ice at right w^ the same and distils cloth kept Qcient cold aporation supported 5 acid,and ered with is to pro- sulphuric tb'* acid watCi *J Note.— Various other bodies, as, for example, chloride of lime, dry parched oatmeal, dry sole lfiathor,&c.,Avould answer for absorbents, though not as well as sulphuric acid. When the acid becomes too much diluted it may be concentrated again by boiling. 122. The C ryophorusjor /ros/-6earer of Wollaston,i3 also employ- ed to show the congelation of water by its own eva- Fig. 13. poration. It consists of a tube terminated in bulbs ^ — >,,^ as in Fig. 13, and containing nothing but water and \\\ water-vapour. When used, all the water is poured 1 1 into the bulb cr, and the instrument placed upright r ^ with the bulb 6 in a freezing mixture. The vapour n^ in the lower bulb is condensed and thus a partial a. vacuum formed, which is immediately filled by a new portion of vapour from the water in the upper bulb. By this means a continuous and somewhat rapid process of evaporation takes place in the up- per bulb, and finally the temperature of the water contained therein sinks to the freezing point. 123. Evaporation into a space filled with air or any other gas follows the same law as evaporation into vacuo ; the only difference being that in the one case the space becomes filled with vapour in- stantaneously, in the other it requires time. The quantity of vapour that rises into a portion of space occupied by air or a gas, is precisely the same as would L ve formed in a vacuum at the same temperature. O If some water be allowed to evaporate into a vacuum a! 8(P F. it will lower the mercurial column 1 inch, or, in other words, the tension of water vapour at 80°F. is ^i^, of the usual tension of the air. So, if some dry air at 80°F. be placed over water, the vapour which rises will increase the tension of that air .J,, if the air bo confined, or Avill increase its bulk ^^j-if the air be allowed ito expand. 124. Evaporation into air goes on at all temperatures ; even in the depth of winter, a large portion of vapour is . formed direct from the snow and ice that cover the face of the country. I The rapidity and degree to which this spontaneous evaporation is carried on depends chiefly on three circumstances : Il1l I 5g HYGHOAiETERS. 1st. The previous dryness of the air. 2nd. Its temperature. 3rd. The rapidity of its movements. Thus, only as much vapour can rise i-^o aportion of air as would rf^e Jo the saL space if it were a vacuum, and hence ,t ,s evident that the amount of vapour that forms and passes into a g'-« ^'vo^ds ulni depend upontheamoun^^^^^^^^^^^^ rort^ofX-thft^ri::r;:ndin;uyevapo.ati.^ which removes the air as fast as it becomes saturated. 125. Humid hot air contains much more vapour than humid cold air; hence when a portion of air saturated with -oisture ha. its temperature lowered, a part of the vapour assumes the liquid form and is deposited in drops, forming dew. ftost brick and stone walls are covered with a prolusion of mo.stuie. 126 Hygrometers and Hygroscopes are instruments designed for measuring the amount of vapour in the air at any particular time. 127. Saussure's Hair Hygrometer is represented in Fig 14. It consists essentially of a human hair freed from grease by immersion in sulphuric ether. This prepared hair is fixed by one end to a hook in the lower part of the frame, is then passed over a pulley carrying the index c, and is attached by the other extremity to a delicate spring, b. As the hair becomes moist it lengthens; and the spring b contracting draws it oyer the. pulley and thus moves the index to the right ; again, Z the hair dries, it contracts and thus moves the index in the opposite direction. Pig. 14. HYOROMETERS. 128 Another mode of determining the amount of rapour in the air is by means of the Psycrometer, or wet-bulb thermometer. This consists of two deli- cate thermometers attached to a frame (Fig. 15), one having its bulb covered with muslin kept con- stantly moistened by water which passes along the string in connection with the reservoir. Evapora- tion produces cold, hence the wet-bulb thermo- meter indicates a temperature lower than the dry- bulb thermometer, in proportion as the evapora- tion is more or less rapid. If the air be very dry this process is very rapid, and the wet-bulb ther- mometer indicates a temperature much lower than the other ; if, on the other hand, the air be satura- ted with moisture, no evaporation takes place, and the thermometers both indicate the same tem^pera- ture. 129. The dew-point is much more easily determined by of Daniell's Hygrometer. This instru- ment consists of a tube terminated in bulbs, as represented in Fig. 10, and containing nothing but ether and ether- vapour. The bulb a is covered with muslin, and the bulb 6 contains a delicate thermometer. The instrument acts upon the principle of the Cryo- phorns. When an observation is to be made, the muslin about a is kept moistened with ether, which by its evaporation produces cold. The va- pour contained in a is thus condensed, and evaporation of the ether in 6 pro- moted. By this means the temperature of h is gradually, lowered, and finally reaches the point at which the sur- rounding air parts with a portion of its moisture and leposits it as dew upon the cooled glass. 57 means Fig. 16. gg SPnEROIPAIi STATE. LECTURE XI. ^ SPHEEOIDAI- STATE OF MATTER. 130 If into a red-hot crucible we pour a small quantity of water, Jcf tinue to keep the crucible ;"f--'>- f f^ '^^,^:r: assumes the form of a sphere, and, gl.dmg w.th a pecula rota tory motion over the bottom of the capsule, evaporates «ry Iwlyand without ebullition ; in other words, it does not read. sIoHing point, and in fact does not become sulhc.ently hot to iiauomufei" , Ti..> witor when in this eondi- scald the hand if thrown upon it. The ^^.^tcl w le tion is said to be in the spheroidal Me. Now, if the crucible be non IS 5.11U reached a temperature not allowed to cool, as soon as it has reaciiui i more than 75» or 80' above the boiling point of -'^ ■ ''" ;P^°_ roidal condition is lost and the liquid begins to boil with explo Bive violence, being rapidly dissipated in steam. K„.._Th. is explained ^V^'^^^^^ ^:'i:^ TT^Z that species of attraction winch wa e ''""f"" '''''""' ^„„„ ^^ter « solid, gives P'-^'r 1'Sl;:r';Xc iocs -o cenlo in contact dropped upon an nitensoly '™\<'* ™3^ ,e„rsteamor vapourof high with it, but becomes surrounded by an ™ v-^»l'» » ' ^^ ,„ .^^.a .eusien. which, being a ve^^erf-^^^^^^^^^^ rc,Crte:rof uirvapouris ^'^^:^^Zr^. Z^rrSdr=n;ft;:^trCrna1rrdaenly bursts into ebullition. .„™.._Aruden.th.tof..^^^^ rruS::^!y hta.rth1 top gUdos over it Without wetUng it, but if not hot enough it adheres and rapidly boils off. ■ 131 Other liquids besides water may be thrown into the sphe- roidal state, and the temperatures at which f^ey pass in o ^ — ri:!C"":ir;ir«fe=r-of"tr r:f tr:c':di:or:LryboLg pom., thethird the tern stance, me ^^^^^^ ^^^^^^ j^^^j^j ^^ ,„ perature at or above h ^ ^^^^^^^^^ ^^^^^^ ^^^ ^^^ ;t; turthl— ture, as indicated by a thermometer, Of the liquid while in the spheroidal state. SPHEROIDAL STATE. 59 Water Alcohol Etber, Hydrochloric Ether that- • . . uj Tke temperature of a li'Vtr^a:g-*^^'->^« r„„idB in the spheroidal sUte >s W o»e Oeg^ ^^^ boiling point, Faraday has succeed., .n even merenry, in a red-hot capsule. BPHEEOIDAL, STATE. 60 lUiuU .ulpl.ur„.» acid bo 'h™» • '^,P^ "^,y „v»porat». Now «hon ami r.aohh,K only li.c "'■"';""2 "^.Jw ,„ Jtho spheroidal acd. tho ,„„.doMocon.«d„.o.ur,,a™.Uc«,««.on«oa^^^^^^^ tbttt iicrfcct immunity from the ™' ^y^^ boUIcs may be secured by P—y--^^^^ ,„,,. the applicatjon - -^« "^^^^^th ether or aleohol, it may be Tlius if tlic liaml l)e moistciieu ^^ moistened with .afely plunged -«.;-""^,. r:;!^, fe miecUnr molten le^ cither of these or w.th wale, t ma be d , ^^^ ^^ or iron without be.ng ^^^J^rl wUh liquid sulphurous S::nr:/:r:rdle:;rnoed While it is immersed in the glowing metal. , • « 1 >.v i\^o fact that when tho moistcnotl hand is N^ ""^^^^ ^uffleiently hot, i.e., always ^"'''^f "\'"^" "^^^^^ temperature of 400» or 500°, or if if thoy --;-f ^f^:;: 1 , e„shio„ of vapour was formed they -o^ f -^*' ^72 iron, and effeetually protected the between the /o"' "" ^,^;.^„„ ^,,„ „„,y heated to 200" Tl^ r Jole in dLt contact with it and was severely bur'ncV T." same explanation holds with rcg.rd »« Pa-ng ^ bar'f red hot iron across the tonguo,-a feat often e.bibit«d b, blacksmiths. 137. Ti the earth incU a str The atr of the 8UI The to ^reat a9 Bel( rises \ BOUaCES OF HEAT. 61 )ndition, ow when acid, tlM) aiu, aud, < plungod iclal stato and other moments, ntKl hand is ► the stato ol' Qhand. Tho caloric than lugh-sliares, f guilt, was tly hot, i. c., )r 500°, or if was formed rotccted the xtcd to 200° was severely to passing a cxiiibitod by APPENDIX TO HEAT, SOURCES OF HEAT. 197 Tub SuN.-The amount of beat annually recelred by ,he earth from ^^^^ sun, has bein estimated to be sumcient to .ncM a stratum of ice 101 feet thick. The atmosphere is supposed to absorb 40 per cent, of the heat of the sun's rays. The total heat emitted l,y U>e B.m is 2361 million times as ^rrcat ..3 that received by the earth. The heat at the surfaee of the sun is seven times as great m Soil blast furnace, the temiK^rature of which . certamly not less than 3500» F. 139 The StAKS.-It is estimated that the fixed stars (all of .h'h alsul) furnish to the earth a„ ^'^-^l^^^^l^^'Z four-fifth. of that supplied by the sun, and ♦^'^^J ''^^"' J™ auxiliary source of heat neither animal nor vegetable hfe could exist upon the earth. m. The EAETH.-The temperature of the earth', surface is not uniform but decreases from the equator to the poles. The temperature falls as we ascend, and also as we descend .„! L Zpoint which is variable, but which >» nowhere -r than 100 feet below the surface. At a distance of ro,n 40 o 100 feet below the surface, the temperature remams unchanged, i_e^ aCs the same as the ».«» temperature "^^^"^^'^^^^^Z »t the moderate depth of * or 5 feet («»«f /''\'''~;„* ; ceases to mark the d«ily range of t^P'^'r'';.,™! and e^ert of from 40 to 100 feet below the surface, the internal and exter nal heat may be said to be balanced. Pelow this stratum of constant temperature the th~eto m,l' F. forevery60_fcet descent. Tliis is Ptovea oy vauv.= 6^ SOUECES OP HEAT. H^Wfl !'V ^ I ,„„„:.„ce. a. aescent into a.p .Ines, AHesUu .eUs, t.e ocCTtrence of thermal springs, &c. ., , nt a vm moacrate depth water would boil, It follow, tha at a ery m ^^^ ,„,aest substances .He distance in which -^::X :^^^^^^^ .t from 21 to 100 m.les. ^^c maj safely ^^ ^^ ^^^ of the earth is not n,o.. ^^^T^^ ^ ,,, ,,„„ thickness .s aswe descend in some places than in others. * ,v,„ earth's centre mnst be inconceivably ^'r trXres:- lid at%50000' .. (A temperature of 'rooo- r. melts the hardest known substances.) .Hence we must regard the e.arth -;^- "/—t^H^ fire which rages beneath us. .He internal heat ^^ -:^Xi:^:r:Z . feffect is scarcely perceptible, not raising than iV of a degree Fahr. .,e internal beat ^^:^:^ -1^^^:::^^:' I year would melt a crust of ice i ot an m . ^ ^ the Lumont,) While ^x::^::^^^!:;::^.. /f ice earth's surface annually fiom tue sun, 101 feet in thickness. (Puillet.) PiUn pnrth's crust was much more rapid formerly The coolmg of the earth '^l of temperature in pro- than now, and consequently, the increase ot i .•^« +n flpqcent was much greater. It has »ecu «» portion to descent w ^^^^^.^^^^ ^^ ^^^^^^ ^^ ^^,. that more than ^^'>^«"/!^^^= J^-/ ^^ .^\.educe the increase for half the .present rate of increase, i. c., to leauce every GO fe.-t descent from 1' to J'' F. 140. Ex records a Captain E of —70^ F shade, an range in latitu'le 1 the raaxii 141. ( combinir Equivi produce evolving When no diser When disenga 142. nized b< they Ua action, ring wi In \ mainta the col always The surron when 50^ or 143 mutui will 1 force heat. Foi (See: SOURCES OF HEAT. 63 ells^ the uld boil, ibstances i mass. m fire with to turikia cious Of the ace, that its )meter more course of a cness, (M. de seived by the I crust of ice •apid formerly rature in pro- een estimated lessen by one- be increase for 140 ExTnEMES OF TEnBESTmALTEMPERATU«E.-Captam Parry X <• KQo v at Melville Island in l»iy , records a temperature of-59o F^ at Melv ^^^ ^^.^^e Ca.nain Black at Fort Reliance, Lat. 60^ 45 ^^- «" i, f WF . Dr Smith records a temperature of lU F in the the maximum of summer and the mm.mum of ^vmtei. .s^ CHEM.OAL AOTION.-Equivalents of the different acids K r^viTh the same base produce the same quantity of heat. X^^ZTomZl bases combining with the sa,j.e ae.d JrdTfferent quantities of beat, the most powerful base "°;rn\*:eri'::^t is converted into an acid salt there is ""^rarrarXconverted >- a ba.c saU there is a disengagement of beat (Graham, Hess, and Andrews) v„^»T»i.LE Heat —The temperature of orga- "ir^bicb i: merely another name for chemical act.on occur- ring within the body of t,,e plant or an,ma ^^ 1 ^ „ ftiTW flporees warmer than tne surruiuuimg always a tew degrees wni hicrher than the :;::;rei::£::« --, - -'«-- -- >- 50^ 0" 00^ above that of the air. U3 Heat and mechanical force, like force and velocity, are Hi. jiB.i ^^^ amount ol beat mutually exchangeable term . u."s B ^j. «iU perform a certain amount of wor 1, and a ce^ ^^ force as friction, or percussion will supply a dehn.te a 'tinst.nee.the— jcale^e^^^^^^^^^^ bushel of coals weighmg 84 lbs., w sumi/i^-uw v (See Part 1, Arts, lt}3, 168.) If >■••' ' », > LIGHT. LECTURJ! XII. THEORIES, DEFINITIONS, PHOTOMETERS ANT PHOTOMETRY. X44. TWO theories have been advanced by phUosophera, with respect to the nature of Light : 1st The Corpuscular Theory of Sir Isaac Newton. 2nd. The Wave Theory of Huygens. 145 The Corpuscular Tlieory, theory of emission, or Newto- nirTh!::y^s:u.es that an — J. b.^^^^^^^^^ emiuin, '"X:::^::':^^'^!::::^:^.^^ velocity. ':::::.:i:^L^^r^o^i"^^-^ ..an, famng u^n the viz. — 1st. 2ad. That ali space is filled by an e.treme^ ^tteethert or medium called the lumi,U/erou> ett«r.-Th« ether s ^p^sed not only to fiU the spac^ ^-j'^^^X ''Trd'stl b:t':^rt:pe^eL^te t^^^tmlsphere, r„: ^n Henstst liids a^d solids occupying their intermolecular spaces. That the particles of a luminous body are in a state of per- lell and very rapid vibration, and that these vibraUng Taniis impinge u'pon the l-n^inifero- ^^^ and ^o^^^^ In it a series of undulations or waves, which, moving w^_ greater or less rapidity, strike upon tiie reuuu .u« .^^~ by give rise to the sensation of light. 147. It V gards light to that in while the optic nerv< nerve. 148. It these theo principal more gent clearer ai proved by and othe: not cleai corpuscu and supp ing an e: to be the 15^ stop < era, with r Newto- onstantly ;heir sub- , velocity, ; upon the iiefly two, astic fluid is ether is )eyoiid the id planets, .tmosphere, [)ying their itate of per- se vibrating and produce oQOving with U auu 111-.-- DEFINITIONS. 65 T. -11 ihns be Observed that the corpuscular theory re- 147. It will thus be oDserv analogous gards light as acting upon f ^P*^^ f J^n the olfactory nerve ; I that in which odorous ^f -^/^^//Xn ,f light upon the ,bile the wave theory expUmB^^^^^^ ^^^.^^^^ optic nerve as being similar to that of souna p nerve. ,,S. It . not Absolutely „ece.a^ ^^ ^ ^Tthl these theories in order to ^^I'^l^^^'^^^Ti^.^ory ■. the principal '-'-"/"ff'^^/^'f.; aay, because it affords a -- ^^»'f C^:tple e^i^t n of L^y phenomena, as is clearer and more »"°f; !!,» yo„ng,Fraunh6fer,Herschel, proved by the researches of F"™^'' J » «' ^^^^.^able facts are Ld others-, y^^^^f^T^lllZ^y^i^^^^; and the ::: « :r-r.tou -.^ed^..-^^^:-^ ::r^:ir= :::inr f^::^-^ - notappear to be the result of undulations. DEFINITIONS, &C. U9. Light is that which enables us to see bodies 160. All visible bodies are divided into ; I Self-luminous bodies, ll! Non-luminous bodies or lilaminated bodies. m All self-luminous bodies discharge, and all illuminated boto rtaect, light of the same color as themselves. point is visible. 163. Light moves in straight lines, and consists of separate and distinct parts called rays of light. 164. A ray ^i "g slop or allow to pass, smallest portion that we can either 66 FORMATION OV SHADOWS. 155. A pcucil of light consists of a greater or smaller numoer of rays. 156 Transparent bodies are those whieh allow so ^^S^^l^^'^- tity of S" to pass, that objects are distinctly vs.ble through their substance. 157. Opaque bodies are those which do not permit light to pass through their substance. 158 Translucent bodies are those which are semi-transparent 158. iiansiui^cu .. r„i.t tr» niss but not sufficient to or which allow a measure of light to pass, Dut n enable one to discern objects through them. ^ ur^Av ict nlaced before a luminous one, , ''"■ ""iZuX^So^ tJTettter being unable to pass the rays ofl.ght proceeaing completely inter- :S'r.:""-TXtion ora sJ.do.on that :me of the opa,ue body remote from the source ofl.ght. 160 If the luminous body be one of any «»»-«''-''';■ "^f^"; -"r rr^iyT^oruS/c^tr^x::^^^^^^ lai If the luminous body be a mere point there is no shading off from the dark umbra to the illuminated porfon , m other words there is no penumbra. .„..« « shidow having the same geo- 163. A body always casts a shaaow uav g metrical outline as itself. ::;;g;'wm "depend upon the distance between the opaque and luminous bodies ; and 3rd. If the sha wil ops 164. T circumstj ist. The 2nd. The 3rd. The 4Lh. The 2nd. B 3rd. T 167. a whii the pr opaqn Theli the r( same lights rod. are e the times PHOTOMETRY. 67 iimoer , quan- irough Lght to 5 magnii- sts of a . and an 3 shading in other ame geo- before a -then tious body, infinity. s, the sha- h which it stween thy 3.d. If the opaque body be larger than *;'-'». ^.j'^tg; no! shadow will diverge, and the rapidity of its aivergti wm dljnd upon th^ distance between the luminous and opaque bodies. PHOTOMETERS, &0. 164. The Illuminating power of a light depends upon several circumstances. I3t. The absolute intensity of the light. 2nd The color of the light. ,, .r^gle - wMch -^rays Jligbt «^^^^^ Gth. The degree to which the rays are through the air, &c., &c. 165 Photometers are instrument^ designed to measure the rela. illuminated. ,68 The methods of photometry commonly employed are ; ul Romford's method, by -mparison of shadows. 2nd. Ritchie's method, by comparison of lUuminatea opaque body, the more intense light «»^^; *^ "^ ^^^^ „f r:ir:bTr::r:rt::et;c^^^^^ rmllpth or intensity, when the '"-jj^ ^J th lights are compared by comparing their 'J'^'*°<^'^ " rod Thus the magnitude of the lights being the same_if hey tim«s as great, &c. PHOTOMETRY. Fig. 17. 68 163. Ritchie's Pho- tometer, Fig. It, con- sists of a box a 6 about 8 or 10 inches in length and 1 inch in diameter, and having in the mid- dle a small triangular wedge, with its sides, 7?i s,mg, inclined at an angle of -^<^-' ''''\^^ ^.^ge is^^::^S7^o;^ed with white paper. At th^ summ ^^ ^^^ ^.^^^^ ^^ ^^ an aperture d .o ^j^^^yj^ / ^^^ ^^^..^d in position until examined are ^^-^l^^^^^^^^^^^ equally illuminated, when, r:^::: m— SpowL of the UgMsareasthe squares of their distances from the central point m. i«o The method by extinction of shadows depends upon the 169. The metioa y .^ ^^^^^^^^ ^ following prmc^le . f an oi q ^^ ^ ^3^^^^^ ^^^ .^ ^ ^^^^^^ lummous body and a cree ^^^ ^^^^^^^^^ ^^ ^^^ ^^^'^ '\f : 3Ct obUtZ^^^^^^ tLe of the shadow. The ::r.: ol «ts are then as the squares of their distances from the screen. LECTUBEXIII. \.r.^riMT>,->=iITION OF LIGHT, NEWTON'S SPECTRUM, ''''bbbwItS spectrum, calorific rays, SiSM, FRAUNHOFER'S SPECTRAL LINES. 170 White light, as emitted from the 8un o. any luminous bodvis a componM of differently colored lighta and maybe d:cL;osedranaly..-a, or separated iuto it. elementary parts by two methods, viz : / 131. £>y i\Clla^-'v!^»"• 2nd. By Absorption. ith white )e having tits to be tion until 5d, when, le squares upon the aetween a f a second nee to the low. The distances >ECTRUM, 3 RAYS, LINES. ly luminous md may be ntary parts NEWTON'S SPECTRUM. ' ,; • -T'" NEWTON'S SPECTRUM. ,,. gUss prism opposite a smai r>^ perforated ^^ ^^^^^ ^^^ e o /larlrpnpd Toom, and causing tuc isinXgU ti.: prisn., upon t.e wall or a screen. Fig. 18. 172. When a ray of light is thus de- composed, its ele- mentary constitu- ents arrange them- selves on the screen so as to form an elongated colored • figure, called the Pris- matic Spectrum. X..,«Ki».r..o.w,.eU,Hp..i««^^^^^^ slmtter B F, and «>™ 'h""*'' '^ '"*^?Lht line toP.i5l>ei>toutof to beam oflight, totoad "f P^*"* »" "^nfbibiting the seven prismatic course and dispersed into a speotram, K L, exn colors. „3. The order of the colors in the prismatic s^ctrnm .s, com- J'cing with the least refrangible ray, as follows . Red. Orange Yellow Green. Blue. Indigo. Violet. „.. The cause of the dispersion o";." ^^I^^^^P^^^^^^^^^^^ .„™ IS found in ^^^^^^-^jtXl^^'^ o^-^^^^^ -^ r^:\i::::^s"rrrer least r^^^^^^^ --. the most remote, is called the u^ost r^fiaug... m rjQ NEWTON'S SPECTRUM. a ;« ihP anectrum are not all ^equally ''"■ ""1 "ted t ra y « ItU.r U. d.tect the boun- Iong,-aad indeed it is a "^'yj ^f^, „any trials, tZX = - r :St: deter..ea tUe W. of the colored spaces to l)e as follows :- MAGNITUDE OF COLORED SPACES. ~~'' ~ , „„^ annnosed to bo dividod into 360 Fraunhofer's are for a prism of flmt glass. . • c n y? of precisely the same kind, be ,,e. If a second pr.m a 5 ^^ ^2^ ^^ ,^[ ,3, ,,e seven pris- applied to the first, BAC, as inmc ^ ^^ ; and the niatic colors are recomi,ounde'i into ord a J .^^^j^.^i, beam of light thus produced V^^^l ^^^^ Jen interposed, it would have fallen had no pnsm wuatevei ^^ So if a circular disc of card-board be divided into colored spaces, as m Fig. i^, and be made to rapidly rotate upon its axis, the colors, if pure, blend into one another and produce white light. Simi- larly , impalpable powders of the different colors may be mixed together so as to appear white or grayish white. ^^ 177 The illuminating power of the spectrum is greatest in the X space, and decreases towards either end. T«-e-- Ing the maximum illuminating etteci, as in ^uc jellow, by - , we have the following ; R 9^ 178. By portions c each color that the c side the i Knglefield different ( 180 elit or lengths «| is made equally e boun- f trials, lengths kind, be even pris- i ; and the on which iterposed. atest in the 1, represent- \v, by 1000, NEWTON S SPECTRUM. TABLE OP ILLUMINATING POWERS. 71 94 640 1000 480 ItO ..« Bv placinc delicate thermometers in the differently colored !f of the spectrum and allowing them to remain until 'Tlofhid exert d its maximum effect, Herschel discovered each color ^ad ex^^^^^^^^ , intensity are accumulated out- r t Cctum a mtle beyond the red space. Sir Henry t^JlZ^^^^^^ ^^termined the heating powers of the different colors to be as follows : Blue, Green, Yellow, Red, Beyond Red 56=* 58° 12° 79^ . 4-nii n is a strong douWe lino in the orange ; ^ is in the green ; i^is a very strong hne in the blue ; G is in the indigo, and /fin the violet. NOTE.-These lines are of great use in enabling us to measure accurately the refractive and dispersive powers of different bodies. BREWSTER'S SPECTRUM. 182. When a beam of white light is transmitted through a piece of blue glass, the transmitted Ight is of a blue color. This blue, however, is not a simple homogeneous color like the blue of the spectrum, but is composed of all the colors of white light which the glass has not absorbed. When we Interpose a piece of glass of this description between the prism and the spectrum, the latter is found to be deficient in a certain portion of its colored rays Thus the glass is found to have absorbed a great part of the red, the whole of the orange, most of the green, a considerable part of the blue, less ot the indigo, and very little of the violet. In the spe( trum, the vellow, which is scarcely at all absorbed, is found to extend over the space formerly occupied by the orange on the one side and the green on the other. Hence, by absorption, orange light has been decomposed into red and yellow, and green light into yp.i= low and blu^, 183. Frc different c< that the sc equ leng The mi die of th the prime and the dary bet of each mities oi beewster's spectrum. 73 1 found f main- These wefully Subse- )f 2000, lectrum. s of the Fig. 20. Violet. II G ' ' \ ■=: : A Red. logeneous of all the id. When tween the cient in a .3 found to he orange, less of the ^trum, the extend over le side and ;e light has U4. ?»» + rk irpl- 183. From-repeaTe'd obs.rv;«on, on the abson-Uon of UgW by aifferen. colored .nedia, »-;- ^ ^-^ : ^Id^^ectra .f that the solar spectrum consists ot three supcu v equ lengths, viz. : Ist. A Red Spectrum. 2nd. A Yellow Spectrum. 3rd. A Blue Spectrum. The maximum of the primary r.i spectrum ■-^"^IZ'^'^ aie of the red space of »•'^ -^^^^^^ ^ yXw soace : the pri,»«ry yellow, spectrum .s m '^^'^^11 ./;„ ^^e boun- ,„d the maximum of the pr«arWu spectrum. ^^^^^ aary betweeu the blue -^l^^^^^^^^^^ ,, ,^ t^ ,.tre- of each of the three primary spectra coinciuc ' mities of the solar spectrum. ■ 184. The coloration of the different parts ^ the solar sj^c trum is accounted for by Brewster's theory as follows . I. Bed, yellow, and blue U,Ut exist in every part of the spec rum. primary colors combine „, , ,e red -^^ ^^X^J,^:^! light to produce white light ^^ uncombined tion of the spectrum. ,v. M the spa. next f:;^-^:!^:::^^:. rfr:r U^; -/C surpms red and yellow unite 10 produce orange. the remaiuing yellow colors the space. ^ **^,.=o twn colors combine to form green. ^ BUrpiu:i Wi iiivff^- - -•- f r If ff^ ABSORPTION OF LIGHT. VII lo the blue portion, all the red, nearly all the yellow an.t a small part of the blue, or more properly of the indigo, combine to form white, and the remaining mdigu unites with the remaining yellow to produce blue light. Vin The indigo band is produced by all the red and all the yellow, requiring a small portion only of the indigo for the production of white light. IX In the violet space there is rather more red than yellow, and the surplus red combining with the indigo, or the 80 called blue, produces violet light. X By absorbing at any point of the spectrum the excess of any color above what is necessary to produce white light, we may cause white light to appear, and this white light cannot be decomposed by refraction. XI Representing primary red light by the letter R, primary yellow by Y, and primary blue by B, the proportions of these primary colors that enter into the composition of different parts of the spectrum are as follows :— j^QLOR PROPOETION OP PRIMARY RAYS. While....' 20R+ 30 Y+50B Red ^^ Orange *^'^+ '^ ^ Yellow ''Y Green 10 Y + 10 B ■ Blue CY+12B T A-\ . 13B Indigo Violet 5R +15B LECTURE XIV. ABSORPTION OF LIGHT, NATURAL COLORATION OF BODIES, COMPLEMENTARY COLORS. ABSORPTION OF LIGHT. 185. When a ray of light falls on a surface, it may be either Absorbed, Transmitted, or - , ; Reflected. ^ ABSORMION'O^ LIOHT. n low, and indigo, f indigu lue light. [ all the ndigo for 1 yellow, fo, or the excess of lice white , and this on. I, primary jortions of position of IB 5 B 5B )B A.TION OF y be either „c partly absorbed and partly reflected. 18e. Bodies differ very greatly In their capacity fo^ '^''-^■ng colon light ■, but oven the most 'J-PJ-j^:; ^t^ absorb some, and the amount -"'".^bed mc eas s w ti „09S of the stratum of the transmitting medium throng the light passes. T.u.ontUosu.n.it«oflony.ount^^^^^^^^^^^ 187. In the following list the bodies decrease In absorptive power from the first to the last : Roclf Crystal Selenite Glass Mica Water and transparent fluids Air and colorless gases Charcoal Coal Hot Nitrous Add Gas Metals Black Hornblende Black Pleonaste Obsidian °,raSe:rrxrtirrx^«o,atr.n,autt.a..^^^ Bilver blue light. x f,^ 188 Several hypotheses have been advanced to account fo 188. &evera ,i ^ ^ ^^^ as yet, been the phenomenon °^^'°'^'^2' vrinciv<^^ are the following :- ' T4TC^^ - -^' "^ "«" "^ actually stopped by L particles of the body, and remain within .t, m the form of imponderable matter (Brewster). „. That tbrTght is Lt Within the body by ^^^^^ of the different parts of the ray, which after tekmg two ■ rontes of different lengths, meet again in a condition to interfere (Herschel). i m I 76 NATURAL COLORATION OP BODIES. III. The absorption of light arises from the multitude of re- flections in the interior of the body (Newton). NATURAL COLORATION OF BODIES. 189. Ink, black pleonaste, obsidian, and some other bodies absorb ail the colors of the spectrum equally ; but as a genera thing, the different colored rays are not absorbed m equal proportions. Hence arises the natural color of bodies. 190 The principal points to remember in regard to the causes of the reflected colors of bodies are the following :— I A body which absorbs all the white light that falls upon it, appears black. II A body that reflects all the light that falls upon it, or * that reflects all the prismatic colors in proper propor- tions to compose white light, appears white. HI A body which appears red, when placed in ordinary white light, exhibits that color because it absorbs all the blue and yellow rays, and reflects*)nly the red. IV A body that appears blue in ordinary white light, exhibits that tint because it absorbs all the yellow and red rays, and reflects only the blue V A body that appears orangp. in ordinary white light, ex- hibits that color because it absorbs all the blue rays, and reflects the red and yellow rays in proper propor- tion to form orange, &c. VI All bodies, of whatever color, exhibit that color only when placed in white light, or in compound light of which that color is a constituent, or in homogeneous light of that color. VII Thus a body which reflects homogeneous red when placed in white light, and which does not reflect white light from its outer surface, will appear black if placed in homogeneous blue, or indigo, or green, or yellow light, because it absorbs all these colors. Jf placed in homo- geneous red light, it reflects all the light thrown upon it and appears red ; 'f in homogeneous orange, or '..,... .1^ _i ~ur. 11,0 Troiinw o.r hhie. and reflects violet iignx, ii au3wiw= tuv j^ -- - -,. the red, and hence appears red. VIII. Simi 11 r t I J 191. T absorptio vays stop by their to the tri 192. ( produce of the \) length I color, tl R Y C i I 1 p of re- : bodies general in equal tie causes tils upon )on it, or r propor- ordinary )3orbs all red. ;, exhibits red rays, light, ex- blue rays, jr propor- olor only i light of nogeneous hen placed i^hite light placed in rilow light, d in bomo- rown upon orange, or ind reflects COMPLEMENTARY COLORS, ^« VTTT Similarly a body which is green in white light, is blue ''''' 't itLgeneoL blue light, since t-e^^^^^^^^^^^^ yellow in homogeneous yellow hgnt reflect: is since there is no biue 'to"reflectris yellow in orange light because it absorbs the red and reflects the yellow ; and is black in homogeneous red light. COMPLEMENTARY COLORS. leX. The transmitted tints of bodies arise from the nnequa, .,h oration or refleetion of the colored rays of light. All the lied eUher by reflection or absorption, orboth.w.U torm :^S:l: 1 comUd color, which will be complementary to the transmitted color. > ... Complem^n.^^^^^ Igth of the spectrum; then setting one leg on the giveu color, the other will fall on the complementary color. TABLE OP COMPLEMENTARY COLORS. Bluish Green. Red Blue. Orange In'iigo. Yellow ^. , ^.. 1 , ... Reddish Violet. Green Orange Red. . Blue ^_ „ . Orange Yellow. Indigo » . . Yellow Green. Violet White. Black Black. White :^:r V h 78 THEORY OF TRANSVERSE VIBRATIONS. LECTURE XV. THEORY OF TRANSVERSE VIBRATIONS, CAUSE OP COLOR AND BRILLIANCY OF LIGHT, INTERFE- RENCE OF LIGHT. THEORY OP TRANSVERSA VIBRATIONS. 193 According to the Wave theory, light is caused by undu- ,^ TZ eL, and, since it is P-^^^^^ -~^^^ observation that light travels at the rate of 192,000 miles per LconZthe undulBtionB that produce light must advance with that velocity. 194 In p™ducing these undulations, the particles of ether dc noimove fonvard at all, but simply vibrate or osc.Uate up ■ aud down, and thus, without advaneing themselves, propagate au onward motion. (See Part I. Art. 349, Note.) « . . Thi, ^-a be satisfactorily iUuBtrated by tying one end of a tolcr- """ -^^1 °: .11 rand I'oldin? the other end in the hand so as not abiy .0 >g rope "> »;'f '' ^JX. tt» iteo extremity up and down. A series :;.'':.°.fi u"d"u tnslsTXa a"cd aiong the rope, but the parHc.es of wMch tte rope is eomposed do not themselves advance m the least. 195 It is customary to malce a distinetion between the tenns •undulation and vibration, the latter being regarded as the cause and the former as the effect. Thus, in the experiment with the rope, the movement of the ha,.d up and down .s the vibration or cans. , and the wave-like movement that runs along the rope is the undulation or effect. 198 The vibrations that produce luroinou. undubitioM In the ether are always transver.» .0 lb direction of the "ly, Thus, if the ra, is running north an! «.M(h, (he vihratior. pro- ducing it was »mde east and west, ..r In ..ny other direc i.m «t Ml LgUn to a line running north and so.llh. The undulation, ot the air, on the contrary, wi.i.l, «!«. rise to tlm phenomena of sound, are VtlAmA I// vltnatlons wblcll are nonnal to the r„ 1 e fflttfle (a tiM Mniii 4ircction i» wlncti (he M|r is moving. PHENOMENON OF INTERFERENCE. 79 E OF ERFE- r undu- lomical lies per ,ce with )f ether Ihite up opagate he terms i as the :periment rn is the uns along a (ions In the my. itirm pfo- •ectiott at id Illations iJH'fjorncnH iial to tItH he r*|r id by the tmmveree vibrations of **°«^«';'"" °^„ ,„ggoBted that other d'ueed by nonnal v«>™«-» '° *« '"^^Tr sles of «ght, or hearing. rrnr-e:"l:rntrS:;o'Icedbyno.n^,anaintheatn.o. phe^yrodaecd by tr»mver«, vibrationB. ,e,. The amplitude of ^^^^^Xt^^^^^ ■aeterniihed by the ^!---;;j;:^,:: ^te of -t, or, the below its original position m the ttum in „j;j^ amplitude of tbe wave may be said^^ be - - ,^ magnitude of the excursions of the vibrating p 198 The length of a wave is measured by the distances be- t„e!n'the crests of two adjacent waves or undulations. ,,e. The — cy of allUgh^^^^^^^^^^^^ „f the waves producing it,-theg eat rth ^P ^^^ ^^^ ^^^ brilliant 'he ligbt. ^be color ot S ^^^^ ^^^ length of the w^e -the on.-^^^^^ ^^, l^^^^^^^ i,„„th rotrXsrl;lolr'of their refrangibility. PHESOMENOS OP ISTBEFEREXCB. ,nn If two ethereal undulations in opposite phases meet, 200. It two einerea oroducing quiescence among ,hcy destroy each other's effect, an^d,producgq the particles of the ether, give rise to blacknm or of light. In this case the waves are said to vnt^rfere. ,^„ , _wave,.rcsaidtobeinoppo,i..p..a,e,when.heeonvexit,ot th^n cor Jp^hd. to the concavity of the other. .,„.li„.ohenomononoocurBln the iWionce of sound, ■ r:w^:orrjr:&- ;- and destroy one another, produdng sileuce. (iieoPart I., Art^l) J ^+ u.tprftre if thev mee after they have oiii Two waves do not interrf re m mc^r ^ t"rien:o,-gh parts of eqna,U„^HorJbro..gh paths that . . it I ., 1 7^4 5. «c., entire waveii, differ in length by 1, I, ^^> *» '^' * ' .o {ntPrfrre when they have traveH*4 throu|^ 202. Two waves ^^'^^Trn length not being M, 3, 4, 5, „„cMual paths, the "-^f f^^ „,,.^,„^^„^^ ., c„mpleU wl.on the ^c, entire waves ) ^--^ •"- PHENOMENON OF INTERFERENCE. 80 inequality in the lengths of the paths of the two waves is Ml, 21, 3^,4^, &c., waves. 'A .f thP mode in which the wave lengths of light ment Through a pin-hole, s, Fig. 21, Fig. 21. in the shutter of a darkened room, allow a sunbeam to enter, and at a short dis- tance from the aperture place a thin wire, a &, (seen endwise in the figure ) horizontally across the centre of the admitted beam of light ; finally, a little bevond the wire, set a white paper . ^ Tt \vill now be found that there is a white stripe, followed on each side by a darK c^ one these in turn succeeded by white bars, and so o,. The explanation is simple. The waves ot .ght bend rn,md the wire, just as water waves, and also sound- res beadCnd angular or rounded bodies. Whereve two waves fall upon the screen haymg travelled through paths that arc eciual - ^ ^^/^f^ ,^/,^^^^^^^^^ p,„duce irnTiictif :;:/-- .;- -; -1 -r: other's effects, and produce » ^^•"'*^/;; .^f ;e lore or less '•- ^^r.-^:;^^"^tLL:;rrst:peintoa„o- perfectly, and hcucu arx^c^ xnt- - .. .,- ther. y hn, flight itinuous I white, Fig. 22. 3 produce ng from e nolher^y jsult is a eet after at / the 3 differing . band ; at ugh paths 2xalt each Between )re or less e into ano- COLORS OF THIN FILMS* 81 Now if we can ascertain the difference in the length of the two HnesTf and 6/, we get the length of the wave by simply doubling I and, since we may make the experiment with any colored llht we may determine the wave-lengths of the various colors. '204 v"y carefully conducted investigations by Newton Br^wtierVrschel, and others, with respect to the wave-lengths, &c oTligl^t, have given results represented in the following . ' TABLE OP WAVE-LENGTHS OF LI GHT. I Color of Rays. Lengths of waves in parts ofanmch. Red Orange Yellow Green Blue Indigo Violet Extreme Violet.. White. . •> 0-0000256 0-0000240 0-0000227 0000211 0-0000196 0-0000185 0-0000174 0-0000167 0-0000225 No. of waves in one incli. 39180 41610 44000 47460 51110 54070 57490 59750 44444 No. of waves that impinge during one second. 477 Trillions. 506 " 535 577 622 658 699 727 541 a (( (( a a (( Srr4rana Jhliror^now ...M „„e.ana.a..a,ni»e, a, .0.. a, the violet wave. . ^ ^ ^j. dividing 192000 NOTE 2-The «"'* «"« ; *::7by ,t n mbcr, i„ ,|,e flrs. line, or by S«^i»ctrn iS^Ue „«»be« oHUo ,oeo„a H„o. LECTURE XVI. COLORS OF THIN FILMS, - ^.ORS OP GROOVED COLOK.. "' SURFACES. 205 The b-U!iaot colors displayed by soap babbles thm 205. ine J. „„,,_ transparent bodies, are due to the plates of glass m,.« other t- p ^^^ ^^^^^^^ ^^.^ interference o '"e ray^ reflected „„a„i,«o„ altoge- interference, if '^"^^^^^^^^Zil^m, the amount of retar- ther and produces blackness , on y p , ^^^ dation determines the peculiar tint leflectea. g2 COLORS OP THIN FILMS. color ira,.mltto4 by the film or thi- plate is complementary to that reflected. (^^ 906 In order to observe the colors of thin films of air csirTKewton placed a double convex lens, whose radms of \ Z, In feet upon a ground plate of glass. Upon lenses: Transmitted Rings. Reflected Kinqs ..White. B^^^^ '.!. Yellowish Red. ^^^^ !'.;.... Black. White TT- i^f .......Violet. Yellow „, Blue. ^^^ ' ' ....White. Vi«l^^ ;.... Yellow. ^^"^ ; Red. ^^^"^ Violet. Y^^^«^ Blue. ^'^•' ;; Green. I^^^P^^ Yellow. ^^^^ :; .Red. Green Yellow .W; Greenish Blue. ^'^ ; Red. ^^^^^^ Bluish Green. Red.-. P , Greenish Blue parts oi au men, -^^ pi»-cs .-- -a--; produce the different colored rings : C0L0E8 OF THIN FILMS. 83 Series or . Orders \ of Colors.! Colors seen by Reflection First.... Very black ' Black Blackish Pale sky-blue..... •••• White (like polished silver) Straw color ; , Orange-red(driedorge-peel) I Red(geranium sanguineum) f Violet (vapour of iodine) . Ilndigo i 'I Blue ••• ••••' ^^ , , J Green (that of the sea). iSecond . . i Lemon-yellow ; Orange (fresh orange nnd). Bright red 1 J«;^-^ l^Dusky red > ^^ *^' i Purple (flower of flax) Indigo Prussian blue Third . . "{ Grass-green • • • •• ^^ . Pale yellow Rose-red . 1^ Bluish-red Fourth.. f Bluish-green J Emerald-green 1 Yellowish-green \^ Pale rose-red Fifth < Sea-green • • ' (Pale rose-red c Greenish-blue Sixth "* ^ Pale rose-red , C Greenish-blue jj Seventh. ^ p^^^ reddish-white < ' '•S 84 COLORS OF THIN FILMS. Bv aid of this tabic, the thickness of thin lilms of air. water, or glass, nvay be rLdflydLminek by observing the c^^^^ thickness of plates of two substances, reflecting the same color, are m the inverse ratio of their indices of refraction. NOTE l.-It thus appears that a film of air less than one half of the mUlionth part of an inch in thickness, and films of water and glass less than ^e^Wrd of the millionth part of an inch in thickness, cea«e to reflect light, and appear, consequently, black. Note 2 -These rings may be observed by placing two pieces of clear window glass together and pressing them in the centre by means of a pointed body. The plates need not be very thm. NOTE 3 -When observed in homogeneous light, the rings simply exhibit the samccolor as the light itself, and alternating with dark or non-lumi- nous rings. 208 Other examples of the production of colors by thin films are met with when a small quantity of any volatile oil is spread over the surface of a liquid or of a solid, the films of certain chemical compounds deposited by galvanic electricity upon the surface of metals, the film of oxide formed on plates of lead, the films gradually deposited on the window-panes of stables, &(• &c. The beautiful iridescent and opalescent paper of De la Rue owes its peculiar loveliness to the action of a thin film. A very minute quantity of spirit varnish is thrown on the sur- face of water, and spreads itself out on all sides until it forms a film of exquisite thinness. A sheet of paper or any other sub- stance is then introduced beneath the film, and carefully and gently raised so as to bring with it the film of varnish, which, upon the evaporation of the water, remains permanently attached to the surface of the paper and exhibits the prismatic colors with marvellous clearness. 209. The colored rings observed by looking at the flame of U candle or at the sun, or at any other luminous body, through a glass plate having minute particles of dust, lycopodium seed, &c , or of water as bj breathing on it, or small fibres, as those ot silk or cotton, scattered over it, are due to the interference of the luminous waves inflected round the atoms or fibres. The !.__„ 4U„ ^„^t;nioa nf rlnat or moisture, &c., the more distinct are the colors produced. 210. Th( bf'pearl an hainutely i ference, th minute in« rays of lig white wax of mother same play Note.— T the inch. 1 of iridescei SOURCE CHE PHC FLU 211. T I 212. ■ by the a our oth< not by 1 envelop of an ai our SUB 213. All soi SOURCES OF LIGHT. 86 COLORS OF GBOOVBD SCBFACE». 210 The beautiful tints presented by the surface of mo(/«r- ,^ll and other natural or artificial bodies W -'-- tainutely grooved or striated, are likewise produced by mter fe enre the depressions being of such a depth as to cause a mnue inequality in the lengths of the paths of the me.dent "vs If "ght. That this is the case is proved by '.he fact that >f Xe w X or sealing-wax is softened and pre.e nf jrreen-light compound. - Observe same cautiu" m unAxng .«« .- c- f the air. I are most d in some re particu- er rays of :e, but tend Ktremity of phorus are le phospho- ! spectrum sphorus. red to fail nine or of Dwn baclc. cceedingly which the le name of ■, " on the thorough- Qd his ex- ny persons dispersion. blue light change in are capa- gible — the nfluenced. is always i light in le wave is shed. ed to the CATOPTRICS. 91 phenomenon just described, because it does not, like the terms epipolic action, internal dispersion, true diffusion, &.c., involve any theory. 230. The number of fluorescent bodies is somewhat limited. Fluor-spar, a solution of sulphate of quinine, and an aqueous solu- tion of horse-chestnut, diffuse a blue light ; many compounds of sesqui-oxide of uranium give a greenish-blue light, especially the nitrate of the glass called canary-glass (glass colored yellow by oxide of uranium) ; a decoction of madder and alum gives a yel- low or orange-yellow fluorescence ; tincture of turmeric, a green- ish light ; and an alcoholic solution of chlorophyll diffuses a red light. • Note.— Fluorescence is entirely dependent upon the incidence of certain rays, and is therefore quite distinct from phosphorescence ; and, although the famer may give a blue light very much resembling the latter, they are by no means to be confounded. As a general rule, phosphorescent bodieg are not fluorescent. LECTURE XVIII. CATOPTRICS. REFLECTION PROM PLANE MIRRORS, REFLECTION FROM CONCAVE MIRRORS, REFLECTION FROM CONVEX MIRRORS, RULES FOR FINDING THE FOCAL DISTANCE OF MIRRORS. 231. Catoptrics is that branch of optical science which inves- tigates the laws that govern the reflection of light by mirrors, &c. 232. Mirrors are highly polished solid bodies capable of reflect- ing a large proportion of the rays of light incident upon them. The term mirror is commonly restricted to reflectors made of glass coated on one side with an amalgam of tin. Specula are highly polished metallic reflectors. They are made of steel, of silver, or of the so called speculum metal, the best variety of which consists of 32 parts copper and 15 parts tin. Note.— Specula are better reflectors than glass mirrors, as in the latter a portion of the incident rays are reflected from the first surface and render the image less perfect than that obtained by the use of a speculum. .Mi' 92 REFLECTION PROM PLANE MIRRORS. Hi L ■:''' I ft* i 233. Mirrors and specula are plane, concave, or convex. A plane mirror is a flat surface like a common looking-glass ; a concave mirror is a reflecting surface curved like the inside of a watch-glass ; a convex mirror is a reflecting surface curved like the outside of a viratch-glass. 234. Parallel rays of light are such as lie in the same plane and being produced ever so far both ways do not meet. Converging rays are such as continually approach each other in one direction, so that if sufficiently produced they will meet in a point. Diverg- ing rays are such as continually recede from each other. 235. When a ray of light falls upon a surface and is reflected, the angle contained by the line of incidence and the perpendicular to the point at which the ray strikes the surface is called the angle of incidence ; the angle contained between the perpendicular and the line of reflection is called the angle of reflection. The two fol- lowing facts are to be carefully noted : 1st. The incident riy^ the perpendicular to the point of incidence, and the reflected ray, are all in the same plane. 2nd. Vie angle of reflection and the angle of incidence are equal. Note.— In order to trace the course of a ray of light incident on a plane mirror we draw a line at right angles to the mirror at the point of incidence and make the angle of reilection equal to the angle of incidence. For a concave mirror, we join the point of incidence with the geometrical centre of curvature, and, considering this as the perpendicular, make the angle of reflection equal to the angle of coincidence. For a convex mirror, we join the point of incidence with the geometrical centre of curvature, and continuing this line through the mirror, we regard it as the perpendicular. BEFLECTION FROM PLANE MIRRORS. 236. Rays of light incident upon a plane mirror retain their rela- tive directions after reflec- Fig. 23. tion, i. e. parallel incident^ ^ . £ eee, rays continue to be parallel after reflection ; diverging incident rays continue to to converge after reflectioa, ^ onvex. A g-glass; a 5 inside of ice curved i plane and 'onverging B direction, t. Diverg- er. fleeted, the ndicular to the angle of lar and the e two fol- f incidence^ :idence are it on a piano of incidence noe. For a >trioal centre I the angle of r,>ve join the 1 continuing I their rela- s continiue REFLECTION FROM CONCAVE MIRRORS. REFLBOTION FROM CONCAVE MIRRORS. 93 S87. Parallel rays incident upon a concave mirror are reflected to a point whose distance from the face of the mirror depends upon Fig- 24. the curvature of the mirror. /^A —~i 238. The point to which a con- ^ cave mirror reflects the rays inci- dent upon it, is called the focus or " fire place " of the mirror ; and the focus for parallel rays, as Fin Fig. 24, is called its principal focus. 239. Diverging rays incident upon a concave mirror are reflect- ■Sf Fig. 25. ed to a focus, /, Fig. 25, which is always more remote from the mir- ror than its principal focus, F. If the radiant point, P (Fig. 25), be made gradually to approach to- wards the mirror, the following facts are observed :— I. As P approaches the mirror /recedes from it. IT. When P coincides with C, the geometrical centre of curva- ture, / is also coincident with c. III. Whfen P approaches the mirror so as to take the position/, the focus /has receded so as to assume the position P. IV. When P reaches the point F, the incident rays are reflected so as to be parallel to one another, i. e. the point/, or the focus, has become infinitely remote. V. When P passes beyond F, the rays falling from it upon the mirror are reflected so as to diverge. 240. From the foregoing illustration it appears that when the radiant point is at P the rays are reflected to a focus in/, but when the radiant point is at/ the rays are reflected to a focus in P. On account of this relation between P and /, the radiant point and the focus, they are called conjugate foci. The distance/D is called the conjugate focal distance of the mirror to distinguish it from F i), which is the principal focal distance. 94 REFLECTION PEOM CONVEX MIRRORS. NoTB.— When the radiant point is between the mirror and its principal focus, the reflected rays diverge from the face of the mirror. Now if these diverging rays be considered as passing back through the mirror, they will appear to converge towards a point behind it; this point is called their vtr- tual/ocua. 241. Converging rays incident upon a concave mirror are reflect- ed to a focus which is always nearer to the mirror than the princi- pal focus. The conjugate focus is virtual, i. e. is behind the mirror. Here we note Vie following facts : — I. As the convergence of the rays is decreased, the focus ap- proaches the principal focus, and the virtual conjugate focus recedes indefinitely. II. As the convergence is increased, both foci approach the mirror. REFLECTION PROM CONVEX MIRRORS. 242. Parallel rays incident upon a convex mirror are reflected so as to diverge from one another. Their focus is virtual, and.for rays falling on the mirror near its middle point, the distance is about half the radius of curvature. 243. Diverging rays incident on a convex mirror are reflected so fis to diverge more rapidly. Their focus is virtual, and their focal distance is always less than half the radius of curvature, but con- tinually approaches that magnitude as the radiant point recedes from the mirror. 244. Converging rays incident on a convex mirror are reflected parallel if the incident rays converge towards the principal vir- tual focus ; convergent, if the incident rays converge towards a point nearer to the mirror than the principal virtual focus ; and divergent, if the incident rays converge towards a point beyond the principal virtual focus of the mirror. RULES FOR FINDING THE FOCAL DISTANCE OP MIRRORS. 245. Let /= focus,/' = virtual focus, F = principal focus, r = radius of ciirvntiirfi of* fhn pfiiprnr taT>f»n /^♦• ..o/IJa*.* »x„s„j. ji _!_x....i .-j. of convergence. ts principal S^ow if these 3r, they will ed their vir- ire reflect- the princi- >ehind the focus ap- conjugate >roach the eflected so id,for rays 36 is about eflected so their focal 3, but con- nt recedes e reflected icipal vir- towards a 3CU3 ; and at beyond lORS. = radiu8 of iriuui pijiui CONCAVE MIRRORS. 95 OOHOAVB MIRBOBS. t»ABALLEl. Eats. F. = i r. i Ot : Principal focus U eqttal to half the radius of curvature. ExAK^LB l.-^What iB the principal focal diatanco of a concave mirror having a radioa of curvature of 40 feet? SOLUTIOir. F = Jr = iof40 = 20ft. ^M*. Example 2— What ie the principal focal distance of a concave mirror having a radius of curvature of 17 ft. 11 inches? SOLUTION. » F = i r = J of 17 ft. 11 inches = 8 ft. 11 j inches. Ans. dr DiYBRQiNa Rays. /= 2d-r. Or: The conjugate focus is found by multiplying the distance of the radiant point by the radius of curvature of the mirror, and dividing the product by the difference between twice the distance of the radiant point and the radius of curvature. Example 3.— What is the conjugate focal distance, for divergent rays of a concave mirror whose radius of curvature is 25 ft.-the radiant point bei'nir 40 feet from the mirror? SOLUTION. Here d = 40 feet and r = 26 ft. dr 40x26 1000 „« „ 2x40-26 = "IT = ^® ^- Vf ^"C^©*- ^«*. Iho./=^ = Example 4.— What is the coi^ugate focal distance for divergent rays on a concave mirror whose radius of curvature is 64 ft., the radiant point beina 64 ft. from the mirror? SOLUTION. Here d = 64 feet and r = 64 feet 64x64 64X64 Then/=^=;,, •^ 2<*-r *-2x64~64~ 64 =64 ft;. Ana. Examples.— What is the conjugate focal distance for diverging rays in- cident on a concave mirror whose radius of curvature is 19 feet, the radiant point being 9 ft. 6 in. in front of the mirror? SOLUTION. Here dr=9J ft. and »• =19 ft. !rhen/=-^=3.-^se that fall r is limited ve or con- ion in con- i by rejlec- in form. jed a curve . the caustic I is viHual much the le object principal por. set as the ture is to 'vature, CONSTRUCTION OF IMAGES. 103 IV. When the object is in the principal axis of the mirror, the image is also in the principal axis ; but when the object is on one side of the principal axis, the imag-e is on the same side. 258. To construct an image of a body by means of a convex or concave mirror use the following — RULE. I. Take any point in the object and froin it draw a secondary axis. ^ II. Take any ray whatever incident from the assumed point and join the point of incidence with the centre of curvature of the mirror. This line will be perpendicular to the mirror at that point, and will shew the angle of incidence. III. Draw from the point of incidence, on the other side of the perpendicular^ a straight line, making with it an angle equal to the angle of incidence. IV. The line thus determined is the path of the reflected ray, and, being produced until it meets the secondary axis, determines the spot in which the image of the assumed point is formed. V. Determine the position of several other points of the object in the same manner. 259. The position, size, and distance of the image may be determined by Art. 245, VI and VIII of Art. 253, and III and IV of Art. 25T. £xAMFLUi 1.— An object 17 inches long is placed 11 feet before a concave mirror whose radius of curvature is 18 feet; if the head of the figure coin- cides with the principal axis of the mirror, determine the position and magnitude of the image. SOLUTION. I. Here d = 11 Ibet, and r == 18 feet. dr \ _ 11 xl8 198 2 X 11-18 Then/: = — .— = 49 ft. fi inches = distance of 2dr-r image from mirror. II. Dis. of object: dis. of image: : size of object : size of imago. Or; 11 i\. ; 49 ft. 6 in. : : 17 inches : —=^~- = 76^ in. = length of image. III. The image is inverted, having its head still .^incident with the prin- cipal axis of the mirror, 104 PROBLEMS. I 1 Example 2,— ^ n object 22 inches in length is placed 6 feet from a convex mirror whose radius of curvature is 14 feet; the foot of the object being below the principal axis, determine the distance, size, and position of the image. Here d = 6 and r = 14. BOLUTION. ''''\-r4 = -i=3«««m 2d+r 2 X 6 + 14 ~ 26 virtual image from the mirror. II. 72 inches: 38 |Q inches: : 22 inches: ^^ ^ ^ inches == distance of = 11 [] inches := 72 ~ ** r 3 size of image. III. Image is erect, with its foot on the same side of the principal axis as the foot of the object. EXEUCI8E. 3. An object 2 feet in length is placed 5 feet before a concave mirror whose radius of curvature is 8 feet; the centre of the object being in the principal axis of the mirror, determine the distance, position, and size of the image. Ans. Distance from mirror = 20 feet; length of image = 8 feet; image inverted, and still has its centre in the principal axis. 4. An object 14 inches long is placed 43 inches before a plane mirror; determine the position and size of the image. Ans. Distance from mirror = 43 inches ; length of image = 14 inches : image erect. 5. An object 16 inches in length is placed 27 inches before a convex mir- ror whose radius of curvature is 40 inches ; determine the distance, position, and size of the image, the object being completely above the principal axis of the mirror. Ans. Distance from mirror = lllf inches; length of image =s6|f in. ; image erect, and completely above the principal axis. 6. An object 12 inches in length is placed 4 feet 7 inches before a concave mirror whose radius of curvature is 30 inches ; the head of the object being above the principal axis, determine the distance, position, ^nd size of the image. Ans. Distance from mirror = 11 \\ inches ; length of image = 2 t in. image inverted, and has its head below principal axis. 7. An object 20 inches in length is placed 100 feet before a convex mir- ror whose radius of curvature is 15 feet; the head of the object being in the principal axis of the mirror, determine the position, distance, and size of the image. ^ns.^Distance from mirror = 6 feet uU inches; length of image = Hi inches; image inverted, with its head still in principal axis. 8. An object 4 inches in diameter is placed 22 inches before a convex mirror whose radius of curvature is 1 foot 10 inches ; the lower edge of the object being below the principal axis of the mirror, determine the position distance, and size of the image. ' Ans. Distance from mirror = 7 J inches ; diameter of image =11 inches ; ; ^ image erect, with lower edge still below the principal axis. am a convex >bject being 9ition of the listance of inches = ipal axis as cave mirror teing in the and size of feet; image ane mirror; = 14 inches: jonvex mir. ice, position, incipal axis B =5 elfin.; re a concave object being I size of the ;e=2f in.; ionvex mir- )Ct being in tee, and size of image = al axis, e a convex •edge of the the position, =1 J inches; is. ' ^ ■ BIOPTRICS. 105 LECTURE XX. DIOPTRICS. DEFINITIONS, LAWS OF REFRACTION, INDICES OP RE- FRACTION, PHENOMENON OP TOTAL REFLECTION, DIFFERENT KINDS OF LENSES, PROPERTIES OF DIFFERENT KINDS OF LENSES, RULES FOR FIND- ING THE FOCAL LENGTHS OF LENSES. DEFINITIONS. 260. Dioptrics is that branch of optical science which inves- tigates the progress of those rays of light which enter transpa- rent bodies and are transmitted through them. 261. When a ray of light is passing through the same medium it invariably preserves its rectilineal course ; but when it passes from one medium into another of different density, it becomes bent or refracted out of its original path. Thus, if a straight rod is placed obliquely, partly immersed in water, it appears bent just at the surface of the water. If a shilling be placed in the bottom of a basin on a table, and the observer move back until he has completely lost sight of the coin, it again becomes visible to him upon a second person carefully pouring water into the bowl. 262. Let B C, Fig. 30, be the surface of some water in a vessel, and S Jl a ray of light incident on it at A, N AN the perpendicular to the sur- face, A R the direction of the reflected ray, and A T the direction of the re- fracted ray ; then : The angle S A Nis the angle of inci- dence. The angle N A R is the angle of reflection. The line NA N ia called the nori: -' The angle T A N is called the angle of refraction. Let A a he taken equal to A b, and from a and b let fj?U the perpendiculars a m and 6 n to the normal NAN] then : The line a w is called the sine of the angle of incidence - ' ' 106 LAWS OP REFRACTION, i^,."' i.'» I i 'B-m The line 6 n is called the sine of the angle of refraction. The ^ne a m divided by the line 6 n is called the index refraction, and is commonly represented by the letter n. of 263. The general laws of the refraction of light may be thus Stated : I. The incident ra^^ the refracted ray, and the normal, are all in the same plane, and the sine of the angle of incidence bears a constant ratio, in the same medium, to the sine of the a m angle of refraction, o^-rzr =n = a constant quantity. n. When a ray of light passes from a rarer into a denser medium, as from air into water, it is bent towards the normal or perpendicular. til. When a ray of light passes from a denser into a rarer medium, it is refracted from the normal or perpendicular. IV. When a ray of light is incident perpendicularly on a refracting surface, it suffers no refraction or change in its direction. V. The index of refraction is always the same for the same me- dium, whether the angle of incidence be great or small. 264. The index of refraction diflfers from diflferent bodies, being, as a general rule, greatest in combustible bodies, and increasing also with the density of the body. The indices of refraction of a few of the most remarkable bodies are exhibited in the follow- ing— TABLB OF INDICES OF REFRACTION. SUBSTANCE. INDEX OF REFRAC. Vacuum 1*000000 Hydrogen 1-000138 Oxygen 1 •000272 Nitrogen 1 • 000300 Air 1-000294 Water 1-366 SUBSTANCE. INDEX OF REFRAC. Crown Glass ........ 1 • 500 Flint Glass 1*639 Sulphur 2-040 Phosphorus 2-224 Diamond 2*487 Chromate of Lead. . . 2 *936 265. The preceding table gives the index of refraction of a ray of light passing from a vacuum into various media. In order to li icHon. the index of er n. may be thus lal, are all in cidence bears e sine of the luantity. mser medium, he normal or 'arerm£dium, ir. '. a refracting s direction. he same me- r small. >odies, being, id increasing refraction of a the follow- OF BEFBAC. 1-500 1'639 2-040 2-224 2-487 2-936 TOTAL REFLECTION. 107 Jtion of a ray In order tg determine the index of refraction for light passing from one medium into another, we must divide the index of refraction of the second medium by that of the first. Thus, the index of refraction of a ray of light passing A-om water into crown glass is -g^g = 1-098; of a ray of light passing from water into 1 '000294 air the index of refraction is —:,-^. — z=:0-132, &c. 266. When light falls upon a polished metallic reflector, it is partly reflected by the surface, and partly absorbed or otherwise lost ; when it falls upon a glass reflector or other transparent medium, a second portion is refl'^ cted from the second surface. In all cases the amount of light reflected by the first surface is greatest when the incident rays are perpendicular to the surface. The number of rays reflected out of 100 rays incident at diffe- rent angles by different reflectors is shown by the following — TABLE SHEWING BAYS EBFLECTED OUT OF 100 INCIDENT BAYS. Angle of Crown Plate Flint Speculum Polished Incidence. Glass. Glass. Glass. Metal. Steel. 10<» 3-608 3-546 3-819 70-85 60-52 20O 3-837 3-790 4-117 69-43 30° 4-189 4-164 4-574 68-11 58-69 40 o 4-767 4-778 5-320 66-91 50O 5-810 5-882 6-656 65-87 54-96 60° 7-964 8-155 9-369 65-03 70° 13-448 13-891 16-015 64-41 80° 32-396 33-155 36-422 64-04 90° 75-776 74-261 72-074 63-91 53-60 TOTAL BBPLECTXON. 267. Under ordinary circumstances, when light falls upon a transparent body, it is partially reflected by the first and second surfaces, and partially transmitted ; when, however, the rays fall very obliquely upon the second surface of the transparent medium, they are wholly reflected, i. e. they do not pass through the sur- face into the rar«r medium beyond. This phenomenon is known 108 lENSES. • )■> ,„ fts the Total Reflection of light, and can, of course, (Art. 563,) only take place when the exterior medium is less dense than that in which the rays are passing. 268. When light passes from a denser to a rarer medium, the angle of refraction is greater than the angle of incidence. Jn the case of water and air, the angle of refraction is 90'* when the angle of incidence is 48° 35'; so that if a ray of light passes through water making an angle greater than 48<* 35' with the perpendi- cular, the refracted ray makes an angle greater than 90° with the perpendicular, and consequently does not pass from the water at all. Light passing through common glass at an angle greater than 41° 49', suflFers total reflection. Note 1.— To an eye placed beneath the surface of water all the objects above the horizon would be seen within an angle of 97° 10', or double the angle of total reflection for water. Note 2.— The brilliancy of the light which has suffered total reflection far exceeds that reflected from the best metallic reflectors. Thia may bo shown by nearly filling a wine-glass with water and holding it up so that the surface of the water may be seen from beneath. When thus placed it presents an appearance equally brilliant with that of burnished silver, on account of the perfect reflection of the incident light. No object above the surface of the water in the glass will be visible. LENSES. 269. A -jENS is a transparent body, as glass or crystal, having one or both of its sides segments of spheres. 270. The principal lenses and other optical glasses are shown in sections in Fig. 31 Fig. 31. I. An optical prism A, is a triangular prism having two plane surfaces, A R, A S, called refracting surfaces, and a face, R S, called the base of the prism. The angle R A 8 is called the refracting angle of the prism. 11. A Plane Glassj B, is a plate of glass baring two parallel plane surfaces, abj c d. (Art. 563,) se than that ledium, the nee. Jn tlie n the angle les through le perpendi- )0« with the , the water ogle gveater 11 the objects ►r double the [ reflection far This may be it up so that hus placed it led silver, on ject above the stal, having 9 are shown J!l f two plane and a face, [eRAS is WO parallel LINSES, III. A Spherical Lens, C, is a geometrical sphere, 109 IV. A Double Convex Lens, D, has both its surfiaces convex, either equally or unequally. V. A Piano-Convex Lens, £, has one surface plane and the other convejf. VI. A Double- Concate Lens, F, has both its surfaces concave, either equally or unequally. VII. A Piano-Concave Lens, G, has one surface plane and the other concave, VIII. A Meniscus, H, has one surface concave and the other con-- vex, and their relative curvatures are such that they meet if produced. Since the centre of the meniscus is thicker than its edge, it may be regarded as a convex lens. IX. A Concavo- Convex Lens, I, has one surface concave and the other convex, but their curvatures are such that they would never meet if continued. The concavo-convex lens has its centre thinner than its edge, and may hence be regard' ed as a concave lens, NoTK.— In Fig. 31 these lenses, &c., are seen only in sections, so that if they were revolved around their central axis, M N, they would severally, except the prism, describe the solid lenses they are designed to represent. 271. When a ray of light passes through a prism near the refract- ing angle, it is turned towards the back of the prism, and hence the image is removed towards the refracting angle. Thus, let a ray of light, a b, be incident upon the surface, A C, of the prism A C B; it is first refracted in the direction b /, and upon emerging it is still further refracted to d. An eye placed at d would therefore see an object at a as though it occupied the posi- tion a'. If the refracting angle be turned down, all objects appear to be elevated when seen through the prism. NoTB.— The angle aea* is called the angle qf deviation. Fig. 32. «,£* 110 ACTION OP LENSES. '! m 272. The following particulars in conuection with a prism aro to be carefully noted : I. The points / and c are called the geometrical centres, or centres of curvature. II. The point d is the optical centre, a b is the aperture^ c/ is the principal axis, and any other Viae, m n, passing through rf is a secondary axis. III. All the rays, such as m n,fc, Sfc, that pass through the opti- cal centre, d, are called principal rays. 273. A double convex lens may be regarded, in its action upon light, as being formed of two prisms, of small refracting angles, placed back to back. Fig. 34. Thus, lot a 6 c and rf 6 c be two prisms of small refracting angles placed back to back. We have seen that light, iu its passage through a prism, is bent towards the back of the prism ; hence parallel rays,/, h, k, m, g, falling upon the surface a 6 d are refracted to a focus in F-, or if diverging rays from F fall upon the surface acd, g they are refracted as parallel rays/, h, k, m, g. 274. A double concave lens may be regarded, in its action upon light, as being formed of two prisms of small refracting angles, united by those refracting angles. Thus, let ros and a t v hQ two such prisms united by their refracting angles at s. Then, as before, since a ray of light in passing through a prism is bent towards the back, the parallel rays e, Ti,f, falling on the surface o st, become di- vergent in passing through the lens; and converging rays, g, n, k, falling on the surface rsv, emerge on the other side as parallel rays, e, h,f. 275. Convex lenses are proved by the laws of refraction to possess the following properties : I. Every principal ray that falls upon a convex lens of limited thickness is transmitted unchanged in direction. II. Incident rays parallel to the axis of the lens are refracted to a focus ; and the focus for these parallel rays is called the principal focus of the lens. I prism aro 33. ;h the opti- ction upon ng angles, 34. FOCAL LENGTHS OV LENSES. Ill its action refracting n Taction to ! of limited n. e refracted s is called III. Rays diverging from the principal focus of a convex lens are refracted parallel. IV. Diverging rays, emanating from a point in the axis more distant than the principal focus, converge after refraction, and the point of convergence approaches the principal con- jugate focus as the point, from which the rays radiate, recedes. y. Rays radiating from a point in the axis nearer than the principal focus diverge after refraction, but the diver- gence of the refracted rays is less than that of the inci- dent rays. 276. The principal properties of concave lenses are the follow- ing : 1. Rays parallel to the axis are rendered divergent by a con- cave lens. II. Diverging rays are made still more divof gent by a concave lens. III. Incident rays converging towards the principal focus are made parallel by a concave lens. IV. Incident rays that converge towards a point more remote than the principal focus are rendered divergent by passage through the lens. V. Incident rays that converge to a point between the lens and its principal focus, are refracted to a point beyond this principal focus. ^ 277. The focal lengths of glasses of all kinds may be found by the following formulas : Let r = radius of curvature of one surface. /= " " « other " d = distance of source of light. d'= " " virtual point of convergence of the rays incident upon the lens. t = thickness of lens. X 112 FOCAL LENGTHS OF LENSES. CONYIX LBNBIB. '^1 i " '^raii'i Parallel Rays, I. Double equi-convex lens, F = r. II. Double unequi-convex lens, F = 2 rr' r + f III. Plano-convex lens with plane surface exposed to rays, F =: 2 r. IV. Plano-convex lens with convex surface expose a to rays, F = 2 r — | /. 2 rr' V. Meniscus, " F = Diverging Rays. VI. Double equi-convex lens, F = VII. Double unequi-convex lens, F = VIII. Plano-convex lens, F = r— ^ dr IX. Meniscus, F = d— r. 2drr' d(r + r')— .2r/. 2dr d—2r. 2drr' d\r—r')+2rr'. Converging Rays. d' r X. Double equi-convex lens, F =17— — > a -\- r. XI. Double unequi-convex lens, F = — XII. Plano-convex lens, F = XIII. Meniscus, F = 2rr'd' d'(r+r')+2rr'. 2d'r d'+2r. 2d'rr' d'(r—f)+2rr'. CONCAVE TENSES. Parallel Rays. The virtual focus is found by formulas, I II, III, and IV, and for the concavo-convex lens by formula V. Diverging Rayh^ The virtual focus is found by formulas X, XI, XII, XIII. VI, VII, VIII and IX. M PROBLEMS. 113 BXAMPLBS. fixAMrLX 1.— What in the focal length, for parallel rays, of a convex lent whoie radii of curvature are each 7 inches? SOLUTION. Formula 1. F = r = 7 inchea. An$. KxAMPLK 2.— What is the focal length, for parallel rays, of a glass sphere who«e diainet«r is 2^ inches? SOLUTION. Formula 1. F = r=i of 21 = li inches. Ana. ExAMPLK8.->-Ti'hat is the focal length, for parallel rays, of a menlsoui whoso ra^'U of curnture are respectively 6 and 5 inches? Foi-mula V. F, = 2rr' SOLUTION. 2x6x6 60 = --=60 inches. Ana. r — r* 6 — 5 Example 4.— What is the focal length of a double convex lens whose rarfii are respectively 4 and 5 inches, for rays emanaHng from a point 20 iieet dl»tant? Formula VII. F = SOLUTION 2drr' 2 x240x 4x6 9600 '2120' =4*628 inches. Ana. d(r+r')—2rr' 240(4 + 6) — 2 x 4 x 6 KxAMPLB 6.— What is the focal length, for parallel rays, of a piano-con- vex lens whose radius of curvature is 10 inches, and thickness J an inch, the rays felling on the convex side? SOLUTION. Formula IV. F=:2r-f « =2 X 10-f of i = 20-i=19f inches. ExAMPLK 6.— What is the virtual focal length of a double concave lens whose radii of curvature are 11 inches and 9 inches, the incident rays con- verging towards a point 20 inches from the lens? Formula VII. F = 3960 202 SOLUTION. 2drr' d(r-\-r') — 2rr '' =: 19'6 Inches. Ana. 2 X 20 X 11 X 9 20(114-9)-2xllx9 — Example 7.— What is the virtual focal length of a concavo-convex lens whose radii ofcurvature are 11 and 7 inches, the rays emanating from a point 5 feet before the lens ? SOLUTION. fix 60x11 X7 9240 _ - V -/.".-. - -ggj- « 28 -45 Inches. Ana, 114 PROBLEMS. BXBHOISIi. 8. What is the focal length, for parallel rays, of a glass lens in tlio fdna of a sphere having a diameter of one-sixteenth of an inch? Ans. ^ of an inch. 9. What is the focal length of a double convex lens whose radii of curva- ture are 10 and 3 inches, for incident rays appearing to converge to a point 40 inches from the mirror? Ana. 41379 inches. 10. What is the focal length, for parallel incident rays, of a meniscus whose radii are 16 and 17 inches ? Ans. 265 inches. 11. What is the virtual focal length of a double concave lens whose radii of curvature are 16 and 20 inches, for incident rays diverging from a point 20 feet from the lens ? Am. 16-56 inches. 12. What is the focal length, for parallel rays, of a double convex lens whose radii are each one-third of an inch? Ans. ^ of an inch. 13. What is the focal length of a plano-convex lens whose radius of cur- vature is 8 inches, for rays converging towards a point 4 inches from the lens? ./Ins. ^ of an inch. 14. What is the focal length of a meniscus whose radii of curvature are 8i and 83 inches, for rays diverging from a point 40 inches before the lens? Ans, B'j'p^ inches. 16. What is the focal length of a double convex lens whose radii of curva- ture are 14 inches and 13 inches, for divergent rays proceeding from a point 100 inches distant from the lens ? Ans. 15^^ inches. 16. What is the focal length of a meniscus whose radii of curvature are 21 and 29 inches for incident rays convergent towards a point 10 feet from the lens? -4ms. 67107 inches. 17. What is the focal length, for parallel rays, of a plano-convex lens whose radius of curvature is 20 inches, and thickness IJ inches, the rays being incident upoi its convex surface? Ans. 39 inches. m 18. What is the focal length of a concavo-convex lens whose radii of cur- vature are 16 and 16 inches, for incident rays converging towards a point 100 inches from the lens? Ana. 82|| inches. 19. What is the focal length of a plano-convex lens, for parallel rays, the radius ofcurvatureofthe lens being 30 inches and the plane face being exposed to the rays? - -4ns. 60 inches. FORMATION OF IMAGES BY LENSES. 115 tiio f6xm »f an inch. iof curva- ) to a point 379 inches. , meniscus 255 inches. lens whose Dg from a ■55 inches. nvex lens of an inch. liusof cur- 9 from the of an inch. vaturo arc e the lens? Q^ inches. [ii of curva- •omapoint &5^ inches. Tature are feet from •107 inches. onvex lens )s, the rays I. 89 inches. •adiiofcur- rds a point i2|| inches. lei rajrs, the face being s. 60 inches. Fig. 36. LECTURE XXI. FORMATION OF IMAGES BY LENSES, MAGNIFYING POWER OF LENSES, SPHERICAL ABERRATION, CHROMATIC ABERRATION. 278. Images are formed by lenses in precisely the same man- ner as by mirrors, and are, like those produced by the latter, either real or virtual. 279. The formation of an image by a convex lens may be understood by a reference to the accompanying figure. ^ JB is an object on one side of the lens, L M, and ftirther re- moved from it than its principal focus, F. All the rays that emanate from A,ViS,Aea,Aca, Ada, are made to converge to a focus, a, where they paint an image of the point A. Similarly all the rays that proceed from B are united in the point h, and thus an inverted image is formed on the remote side of the lens. 280. If the object be placed between the lens and its princi- pal focus, as in Fig. 37, the rays diverge on leaving the lens, and form a virtual, magnified, and erect image on the same side as the object and more remote from the lens. Let A B be an object placed before the len8,i M, and within its focus. Then all the rays that emanate from jff9» A e F,Ac a\ Ada', are refracted by tlieir passage through the lens and appear to pro- ceed from a. Similarly, the rays from B, m B d F, ^ jy. . Ji c b', B e h', appear to proceed from o. The result is that an eye placed at F receives the rays of light issuing from the object A B as though they proceeded from a b. 116 MAGNIFYING fOWER OP LENSES. Note.— As the image a b and the oltject A £ both subtend the sanvj angle, a cb, to the eye and the former is more remote, it is of course mag- nified. The enlarged image obtained by a single lens used as a microscope is of the kind here described. 281. A concave lens gives a reduced virtual image on the same side of the lens as the object. 282. The following points are to be remembered in connec- tion with the formation of images by lenses : I. All real images are inverted, and all virtual images are erect. II. The size of the image is to the size of the object as the distance of the image from the lens is to the distance of the object from the lens. III. if an object be placed before a double equi-convex lens at the distance of twice its focal length, the image is on the otBer side of the lens, at an equal distance from it, and of equal size. IV. As the object approaches nearer to the convex lens, the image recedes, and vice versa. V. Wherf the object is in the focus of the convex lens, the rays are refracted parallel, and the image is infinitely distant. VI. When the object is more remote from the convex lens than its focus, the image is real and inverted. It is mag- nified if the object is distant less than twice the focal length of the lens, but is diminished if the object is distant more than twice the focal length of the lens. VII. When the object is between the convex lens and its focus the image is virtual, erect, and magnified. VIII. The larger the lens, the greater the number of rays of light it receives from the object, and consequerftly the brighter the image. MAGNIFYING POWER OF LENSES. 283. The apparent size of an object depends upon the angle at which It is seen, because the eye judges of the magnitude of an object by the direction or divergence of its limiting rays. nd tlio sanv! ' course mag- a microscope age on the v.. in connec- images are ject as the le distance ex lens at nage is on ce from it, c lens, the c lens, the 3 infinitely : lens than [t is mag- i the focal i object is he lens. I its focus, of rays of [ueiflly the MAaNiPtma power op lensiss* 117 the angle gnitude of iting rays. Thus, if the lines drawn from the extremities of an object placed at a certain distance before the eye, meet on the retina making an angle of say 30°, the object will appear twice as large as when removed to such a distance as to subtend an angle of only 15°, &c. The nearer, then, an object can be brought to the retina, the greater will be the angle under which it is viewed, and consequently the greater its apparent mag-* nitude. 284. If a man be placed at the distance of say 200 feet from the eye, the image formed on the retina subtends so small a visual angle, and is hence so indistinct, that we are unable to discern bis features with any degree of clearness. Now suppose we place midway between the man and the eye a convex lens of 50 feet focal length, we shall obtain (Art. 282, IIL) an inverted image of the man 100 feet behind the lens, and this image will be of the size of life. The eye now, being only 6 inches from the image, can examine minutely the details of his personal appearance. The effect of the lens has therefore been to bring the man from the distance of 200 feet to the dis- tance of 6 inches, or, in other words, to bring him 400 times nearer to the eye, and it has hence apparently magnified him 400 times. 285. By using a lens of less focal length, we might have actually as well as apparently magnified the image of the man. Thus, suppose the man to be, as before. 200 feet from the eye, and that between the eye and the man, 25 feet before the latter, we place a lens whose conjugate focal lengths are 25 and 175 feet. Then the man being 25 feet before the lens, his image will be 175 feet behind it, and will be magnified in the proportion of 175 to 25, i. e. 7 times. At the same time, the lens has had the effect of bringing the image 400 times nearer the eye, and hence its apparent magnitude has been increased 7 X 400 = 2800 times. 286. If, in the last case, We change the relative positions of the object and the eye, the image would be actually diminished 7 times in magnitude ; but as it is still brought 400 times nearer '»il 118 SPHERICAL ABERRATION. 1'^ M the eye, its apparent magnitude will be increased ^90 = 57^ times. 287. The distance of distinct vision is, for most persons, about 10 inches, i. e. unaided by glasses they perceive a small object when placed at that distance from the eye more clearly than when at a greater or less distance. This arises from the fact that in order to produce a clear well-defined image on the retina, the rays must enter the eye very nearly parallel to one another. When we bring an object very near to the eye, we give it great apparent magnitude, but it becomes very indistinct. But if we bring an object nearer to the eye than the limit of distinct vision, and then by any contrivance cause the rays that proceed from it to enter the eye in a state of parallelism, we magnify the image without militating against its clearness. Now we have seen that when the rays emanate from the focus of any lens, they emerge parallel, so that when an object, or the image of one. is placed in the focus of a lens held close to the eye, and having a short focal length the rays will enter the eye under the conditions requisite to give clearness of vision, and the image will be magnified in proportion to the proximity of the object to the eye. Thus, suppose the focal length of the lens is i of an inch, then its magnifying power will be 10 inches, the limits of distinct vision, divided by i of an inch, the focal length of the lens, i. e. 40 times ; and since the apparent superficial magnitude is always as the square of the apparent linear magnitude, the magnifying eflfect of such a lens is describ- ed t)j saying it is equal to 40 linear or 1600 superficial powers. SPHERICAL ABERRATION. 288. The rays refracted from a convex or concave surface do not all meet in the same point, but those which enter the lens at its principal axis are refracted to a focus more remote or nearer the lens that those which enter at its edge. This imperfection is called the spherical aberration of lenses. 289. Let L M, Fig. 38, be a piano-convex lens with its plane surface exposed to the parallel rays B L, C K, D P, E M, Let SPHERICAL ABERRATION. 119 A9Q = 57f rsons, about mall object ilearly than •om the fact Q the retina, ne another. Lve it great But if we tinct vision, ceed from it r the image have seen they emerge >laced in the focal length isite to give I proportion )se the focal ^ power will I of an inch, he apparent le apparent i is describ- d powers. surface do • the lens at a or nearer erfection is h its plane E M. Let C k and D p be inci- dent upon the lens g very near its princi- pal axis, ^ m n Fj ^ and let F be their A Fig. 38. focus tion after refrac- , also let B L and E i be incident near the edge of the lens ; it will be found upon tracing their course by the rules before laid down, that they meet in a focus, /, much nearer the lens than F. Let the rays Mf&nd Lfhe con- tinued till they meet G H, a plane perpendicular to the line A F, then The distance Ff is called the longitudinal spherical aber- ration of the lens. The distance G H ]& called its lateral spherical aberration. 290. The following will give some idea of the amount of longitudinal spherical aberration of different lenses : L In a plano-convex lens, placed as in Fig 38, the aberration is to equal 4-5 times m n, the thickness of the lensl IL In a plano-convex lens with its convex side exposed to the parallel rays, the aberration is only 117 times its thick- ness. III. In a double equi-convex lens the aberration is 1-67 of its thickness. IV. In a double convex lens having its radii of curvature as 2 to 5, the aberration is about 4-5 times the thickness of the lens if the side whose radius is 5 is turned towards the parallel rays, but is only about M 7 times the thickness of the lens if the other side is exposed to the parallel rays. V. The lens with least spherical aberration is a double convex lens whose radii are to each other as 1 to 6. When the more convex side of such a lens is exposed to the parallel rays, the aberration is only 1-07 of the thickness ; but when the flatter side is thus exposed, the aberration is as much as 3-45 of the thickness of the lens. -I 4 3 120 CliROMATlO ABEftRATlOI^* i'l ir '1 291. The effect of spherical aberration is, obviously, to produce indistinctness in the image. This arises from the fact that all the rays that emanate from any point of the object are not refract- ed to the same focus, and the result is the production of several images-— the rays from which cross one another and interfere, so as to produce caustics and render the principal image obscure. 292. Lenses whose sections are ellipses or hyperbolas are perfectly free from spherical aberration ; but owing to the great difficulty of accurately grinding them to these forms, the lenses employed in optical instruments are always simply convex or concave, and other means are made use of to obviate the difficulty arising from aberration. 293. These means are chiefly two in number. The first and simplest consists in placing, between the lens and the object, a perforated metallic or other disc, which is technically called a diaphragm. The perforation is of such a size as to aUow only those rays to enter the lens that would fall upon its dwdle part"; in other words all the rays that would pass through the lens near its margin are stopped, and the image is thus rendered much more distinct. The second method consists in uniting a menis- cus with a double convex lens. The radii of curvature of these lenses have to bear certain proportions to one another, and these proportions have been computed by Sir J. Herschel. When used as a magnifying glass, as in the microscope, the meniscus is directed to the object, but when used for form- ing an image, or as a burning glass, the convex lens is directed to the object, CHROMATIC ABERRATION. 294. When a ray of ordinary white light Is refracted by a lens of any form, consisting of a single refracting medium as glass or a gem, it is decomposed as by a prism, and dispersed into a more or less perfect spectrum. It follows that when a single lens is placed before an object, the rays of white light pro- ceeding from the latter are decomposed. The violet raya. being most refrangible, are refracted to a focus nearer to the lens than the focus of the yellow rays, and these latter nearer than that of ly, to produce fact that all re not refract- ion of several i interfere, so age obscure. rperbolas are g to the great Qs, the lenses ly convex or i the difficulty riie first and the object, a ally called a 3 aUow only nmidle part ; I the lens near tide red much Ing a menis- ture of these another, and J. Herschel. Toscope, the ed for form- 3 is directed CHROMATIC ABERRATION, 12X racted by a medium as id dispersed hat when a ite light pro- , ravfl- beinff he lens than than that of the red rays. The image formed by such a lens is coloured vio- let if it is formed in the focus of the violet rays, and is bordered or fringed with red and yellow ; it is yellow if formed in the focus of the yellow rays, and is fringed with blue or red, &c. This imperfection in lenses is known as chromatic aberration. 295. It was believed by Newton that it was impossible to refract light without decomposing it, and he was led to this belief by supposing that the dispersing power of a body was always in proportion to its refracting power. It is now known, however, that the dispersive power of a body is not always proportional to its index of refraction. Crown-glass and flint-glass have very nearly the same index of refraction, yet the dispersive power of the latter is nearly twice that of the former. This circumstance.enables us to correct, chromatic aberration. 296. A convex lens causes the violet rays of light to conve e more powerfully than the red rays, while a concave lens causes the violet rays to diverge more powerfully than the red, rays. It is plain that, by combining together a convex and a concave lens, we may overcome the difficulty of chromatic aber- ration ; but if we make both lenses of the same kind of glass, the concavity of the one will be exactly equal to the convexity of the other, and the magnifying power of the convex lens is destroyed. Now flint-glass disperses twice as powerftilly as crown-glass, so that a concave lens of flint-glass, which is just sufficient in power to correct the chromatic aberration of a convex lens of crown-glass, is not capable of completely neutralizing its mag- nifying power. A compound lens of the kind here described is called an achromatic lens. Note.— The combination employed to produce achromatism overcomes also to a certain extent the spherical aberration of the lens. M ■iii 122 OPTICAL INSTRUMENTS. LECTURE XXII. OPTICAL INSTRUMENTS. THE SIMPLE MICROSCOPE, THE COMPOUND MICRO- SCOPE, THE TELESCOPE, THE MAGIC LANTERN, THE CAMERA OBSCURA. 207. A Microscope (micros^ " small," and skopeo, " I see") is an optical instrument used for magnifying very small objects in order to enable us to examine them more minutely. 298. Microscopes are simple or compound, achromatic or non- achromatic. 299. A Simple Microscope consists essentially of a single lens, which magnifies the object by enabling us to bring it in close proximity to the eye without rendering it indistinct. (See Arts. 280, 287.) Note.— rwo or three or more lenses may be combined so as to act as a single lens, and constitute a simple microscope. Two lenses thus act- ing constitute what is called a dmblet, three lenses a triplet, &c. 300. The fbllowing are the principal simple microscopes occasionally employed : I. A minute hole perforated in a piece of black card-board by a fine needle. II. A drop or globule of Canada balsam suspended in a hole made in card-board or a sheet of metal. III. A glass sphere or globule made by holding a glass thread in the flame of a spirit lamp until it melts and runs into a sphere. IV. A drop of water, oil, varnish, or Canada balsam, suspended from the lower surface of a clear glass plate. V. A glass magnifying lens properly ground and polished. VI. A lens of garnet, diamond, or other precious stone simi- larly ground and polished. VTT TliP WoUaston lens- which ia formed bv two nlano-convex or double-convex lenses placed in a brass cup or tube md separated by a diaphragm of blackened wood. rD MICRO- LANTERN, "I see") is 11 objects in latic or non- a single lens, it in close tinct. (See [so as to act mses thus act- microscopes ird-board by d in a hole glass thread Its and runs n, suspended te. polished. I stone simi- Dlano-convex i. cup or tube }d wood. THE MICROSCOPE. 123 VIII. The Coddlngton lens, the most perfect of all simple microscopes, consists of a spherical lens with its equa- torial portions ground away so as to limit the central aperture. 301. A Compound Miorosooph consists of two or more lenses so arranged that one forms an enlarged image of the object, and the others magnify this image. The mode in which this is aooomplished itiay be understood by a refer- ence to Fig. 39. The minute object, Jlf iif, to be exatoined, being placed a short distance beyond the principal focus of the ob- ject-glass A B, & magni- fied inverted image is formed at mn. A second lens, the eye-glass, being so placed that this image shall &il in its principal focus, acts as a simple microscope in enlarging the image. Conunonly, however, a third lens E F, called the field-glass, is placed between the object-glass A B, and the eye- glass OD, It has the effect of intercepting the extreme pencils of hght m n, which would otherwise not have fallen on the eye-glass. Note.— The lens ABia called the olQect-glaas or ol^ective. the lens E F the field-glass, and the lens CD the eye-glass or ocular; and the first and last may, like the simple microscope, be doublets, triplets, &c. 302. Compound Achromatic Microscopes are of various forms, and are supplied with a variety of delicate mechanical contri- vances to enable the operator properly to adjust the instru- ment. In all, however, the essential parts are the three lenses above described. Commonly the object-glass consists of a triple achromatic objective, and the field-glass and eye-glass are combined into one eye-piece, and are so arranged as to correct both the spherical and the remaining chromatic aberration. 303. The angular aperture of a microscope is the angular breadth of the cone of light the object-glass receives from "the object and transmits through the instrument. Of course it depends upon the diameter of the lens and the distance of the object. 124 THE MICROSCOPE. Note.— The principal English manufacturere of microscopes are Boss, Powell, Smith, and Beck. A iirstrrate achromatic compound microscope made by one of these is worth from iJlOO to $500, and in some cases a single lens of high power costs from $20 to $50. The prize microscope of Mr. Boss had the following lenses ; linch focal length 27° angular aperture. 1 .1 .< 60° " > i « " 118° J « «' 107° " ^,« , « 185° 304. The following are the principal rules with regard to the power of a microscope : I. The illuminating power varies nearly as the square of the angular aperture. II. The penetrating power varies directly as the angular aperture. III. The visual power varies as the square root of the angular aperture. IV. The disturbance arising from spherical aberration varies as the square of the angular aperture. V. The defining power, or sharpness of minute detail, varies as the degree of perfection with which the spherical and chromatic aberration is corrected. VI. The magnifying power of the compound microscope is found by multiplying together the magnifying power of the objective and of the eye piece. 305. The Solar Microscope is simply a variety of magic lan- tern (Art. 314) in which the light of the sun is thrown by an inclined reflector through the back of the instrument upon the object to be magnified so as to strongly illuminate it, and thus allow higher magnifying powers to be used. The image is cast on a screen, and may thus be exhibited to many persons at the same time. 306. The Oxy-hydrogen Microscopb differs from a solar mi- croscope merely in employing the light obtained by casting a burning jet of a mixture of hydrogen and oxygen gases upon a piece of chalk or lime. As in the case of the solar microscope, P SB are Boss, mioroscopo iases a single cope of Mr. ture. THE MICROSCOPE. 125 gard to the lare of the le angular lie angular n varies as )tail, varies e spherical Toscope is ying power magic lan- •own by an t upon the t, and thus lage is cast :sons at the a solar mi- y casting a ises upon a microscope. the image is cast npon a screen, and so as to be viewed by many persons at once. 307. The magnifying power of a microscope maybe computed as follows : Let d = distance of distinct vision = 10 inches, p = magnifying power of lens, and /= focal length of lens. Then for a simple microscope or a magnifying lens p = . Example 1.— What is the magnifying power of a lens whose focal length is t of an inch? SOLUTION. d 10 P = y =T''=^ linear, w 60« =2500 superficial dimensions. Example 2.- What is the magnifying power of a simple microscope whose focal length is liu of an inch? SOLUTION. d 10 i» = -F = T = 300 linear, or 8002 =90000 superficial dimensions. JFbr a compound microscope, find the magnifying power of the objective by dividing the distance of the image formed by it by the distance of the object^ and multiply the result by the magnify- ing power of the eyc'viece as obtained above. Example 3.— In a compound microscope the object is placed i of an inch from the objective, and the eye-glass has a focal length of J of an inch, the distance between the objective and the focus of the eye-glass being 8 inches, what are the linear and superficial magnifying powers of the microscope? SOLUTION. 8 inches -f- 1 = 64 = linear magnifying power of objective, rf __ 10 40 P = -?■ — ^ '-r- = 13J = linear magnifying power of eye-glass. Then 64 x 13^ =853} = linear, and (853}) « = 7281771: = superficial mag- nifying power of the combination. DXSBCISie. ^ 4. What is the magnifying power of a lens whose focal length is | of an ^^^^^ -4ns. 26* linear dimensions. u. TT uat IS tne magnilying power of a simple microscope whose focal length is i inch to a person whose limit of distinct vision is 7 inches? /iw5, 28 linear dimensions. 126 THE TELESCO^lS. 6. Wlmt is the magnifying power to a good eyo of a compound mloro* pcopo, having an oye-pioco i iucli focai length, the object being placed i of an inch from the objective, and the distance between the objective and the focus of the eye-piece 6 Inches? What would be the magnifying power to a nerson whoso limit of vision is only 6 inches? Ans. (1) 480 linear dimensions. (2) 288 linear dimensions. 7 The eye-piece of a microscope has a focal length of | of an inch, the object is placed j of an inch from the objective, and the distance between this latter and the focus of the eye-pioco is 11 inches. What is the magni- fying power of the instrument? Ans. 102G} linear dimensions. 8. A near-sighted person whose limit of distinct vision is only 4^ inohen, uses a compound microscope with an oyo-pioco i inch focal length, the object being J inch from the objective, aid the focus of the eye-glass 8 inches from the objective. What, to him, is the magnifying power of the instru- jj^Qjj^^f Ana 192 linear dimensions. THB TBLKSCOPB. 308. Telescopes (from tele, "far off," and sfcop«o, "J see") are instruments constructed for viewing distant objects. Tiiey are eitlier refracting or reflectitg, the Jatter differing from tlie former merely in having one or more reflecting mirrors or specula NoTB.— The telescope was invented in the thirteenth century, and was introduced into England by Roger Bacon. James Gregory was the first to describe, and Sir Isaac Newton the first to construct, a reflecting telescope. 309. The Astronomical Telescope consists of two convex lenses, viz. an object-glass and an eye-glass. The object-glass is placed at one end of a tube longer than its focal length, and the eye-glass in a smaller tube which slides in and out of the larger, so as to allow of the focus being properly adjusted. An inverted image of any distant object is formed by the object-glass in the focus of the eye-gla':s, and this latter magnifies it, and transmits the rays in a state of parallelism to the eye. The astronomical telescope always gives an inverted image, and its power is found by dividing the focal length of the object-glass by the focal length of the eye-glass. (See Fig. 40.) 310. The Telescope op Galileo (used in 1609, and the oldest in form) consists of a double convex lens of long focus, used as an object-glass, and a concave lens of short focire as eye-piece. The lenses are placed at a distance apart equal to the difference THE TELESCOPE. U7 of their principal foci. The light from a distant object col- lected by the large surface of the convex lens is converged towards a focus beyond the concave eye-glass, and is by it refracted to the eye in a state of parallelism. The magnifying power is found, as in the astronomical telescope, by dividing the focal length of the objective by the focal length of the eye- piece. Note 1.— The telaicopo of GaUloo gives an erect and very clear image of the object, but owing to the divergence of the rays through the eye-glass, the field of view is small. NoT» 2.— The opera-gloss consists of two small Galilean telescopes placed side by side, so as to bo used by both eyes at once. NotB 8.— The Mght-glass used by seamen is formed like a large opera- glass. It has low magnifying power, but concentrates a largo number of the rays emitted by a distant object, and transmits them to the eye in tlie condition required for distinct vision. 811. The Tbrrestrial Tblbscopb differs from the AstronomicaJ merely in having two additional lenses for the purpose of refracting the image to the eye in an erect position. The terrestrial telescope is shown in Fig. 40. If the lenses E F and O Hho removed, and the eye placed at L, we have the astronomical tele- scope and see the Image inverted. The lense E F serves to erect the Fig. 40. c ^ Image, but it, at the same time, renders tbe rays convergent ; the second Ions, O H, throws these rays into a state of parallelism, and they reach the eye in the condition requisite for distinct vision. 312. Reflbctino Telescopes are of various forms, and are named after their inventors. They are used almost exclusively for astronomical purposes, and many of them are of very great power. The instrument described by Gregory, and hence called the Gbboorian Tblbscopb consists of a concave mirror having the centre cut away. The rays of light emanating from a dis- tant object are collected by this, and reflected so as to form an n f. L 11-- 128 THE TELESC01»E, inverted image. They are then received on a small concave mirror placed fronting the great one, and are thus reflected through the orifice of the latter, giving an erect image which is properly magnified by an eye-glass. - The Newtonian Telescopb consists of a concave speculum placed in the bottom of a tube with the axis parallel to that of the tube. The rays reflected from it are received by an inclined plane mirror, by which they are reflected so as to form an image on the side of the tube. This image is, as before, properly mag- nified by an eye-piece. Herschbl's Telescope consists of a metallic speculum set in a tube with its axis inclined towards the side of the latter, so as to cast the image (of course inverted) outside of the tube. The image is then exammed by the aid of a magnifier. - In the Gregorian telescope the observer faces the object, in 'the Newtonian he faces the tube, and in Herschel's he has his back towards the object. NOTB 1 -Newton was the first to construct a reflecting telescope, and the one made with his own hands is yet in the possession of the Royal So- cietv Sir William Herschel constructed 200 seven-feet Newtoman reflect- i^g teles opes, 160 ten-feet, and 80 twenty-feet focal length. He finished his Seat telescop;, 40 feet in length, on the 27th August, 1789, and on the same fay discovered with it the sixth satellite of Saturn. Its speculum was 49J inches in diameter and weighed 2118 lbs. ^ ^ x- NOTB 2 -The celebrated telescope of Lord Rosse is the largest reflecting telescopeever constructed. It was several years in being made, and was ^oZet^dln 1845. The tube is of wood hooped with iron and is 7 feet in Seter and 64 feet in length. The speculum is 6 feet in diameter, and ""'iC^TsSum of this telescope is 72 inches in diameter, if we as- sume hfpupU of th. human eye to be ,1, of an inch in diameter sume '"«!'*' ^ J diameter as the human eye, and l^^^'dmH. gr-^ iorrftoo'sow if ono-h^f of the light be .o»t by "Sorftom fhe mirror, we .hall have concerned in forming the unage S^ame, ae much light a» ordinarily enters the eye. TU» wUl. m a measure, account for the remarkable power of the mstrumont. 313 The lensea employed in good astronomical telescopes require to be achromatic, and it is difficult to obtain tliem of large size. This arises irom luc xa^t, vx...., .... s, r- .■ the achromatic lens consists of a convex lens of crown-glass combined with a concave lens of flint-glass, and in practice, it THE TELESOOPE. 129 loncave sflected hich is >eculuin that of inclined I image ly mag- set in a r, so as tie tube. bject, in has his »e, and the Royal So- ian reflect- inisbed his n the same m was 49J t reflecting e, and was is 7 feet in meter, and r, ifwe as- diameter^ n eye, and be lost by the image a will, in a telescopes I them of RTTilained. — i- — / own-glass sractice, it is found almost impossible to obtain flint-glass in large pieces of uniform density, free from flaws, veins, and other imperfec- tions. Note 1.— Some idea of the difficulty of obtaining large achromatic lenses may be gleaned from the fact, that the object-glass of the great achro- matic reflecting telescope of Cambridge, Mass., is but sixteen inches in diameter, and yet it cost, unmounted, the enormous sum of $15000. And it is recorded as a perfect marvel that a Mr. Bontemps, in the employ- ment of Chance Brothers & Co. of Birmingham, has succeeded in pro- ducing a disc of flint-glass 29 inches in diameter, 2^ inches thick, weighing 200 lbs., and so free from imperfections as to be very nearly faultless. When combined with a crown-glass lens into an achromatic objective, it will be worth many thousands of pounds sterling, Note2.— The Magnifying Power of a telescope is measured by the apparent enlargement of the image. The Illuminating Power of a telescope is the amount of light which it collects from the object and transmits to the eye, as compared with the amount of light the unaided eye would collect from the same object. The Penetrating Power of a telescope is the ratio of the distance from which the eye and the telescope would collect, for the purposes of vision, an equal amount of light. The penetrating power is equal to the square root of the illuminating power The Visual Power of a telescope is found by multiplying the pene- trating power by the magnifying power, and extracting the square root of the product. If X)=r diameter of the objective, d = diameter of the pupil of the eye, »=number of lens through which the light has to pass before reaching the eye, x = the amount of light transmitted by each lens, commonly about ■^0, r=: visual power, P = penetrating power, /= illuminating power, ^nd Jtf= magnifying power. Ti, w — focal leng th of object^glaaa JD» a?» ^"®" focal length of eye-glass ' * '' ^=v'^ = fv-ir=V^-l^|* 814. The Magio Lantern is an instrument used for projecting on a screen a magnified image of an object painted in transpa- rent colors on glass. 130 .'*» THE MAOIO ". LANTERN. Fig. 41. It coneists essentially of a dark chamber, or tin box, A A', which contains the source of illumination, the lenses, &c. The parabolic reflector p g, receives the diverging rays of light from the lamp, X, and reflects them parallel upon the convex illuminating lens, m. The lens m concentrates the light upon the object which is painted on a slide that fits into e d. The rays proceeding from the strongly illuminated object pass through a second .convex lens, n, by which they are converged upon a screen, so as to give a magnified image. (See Art. 279.) 315. The magnifying power of the Magic Lantern is equal to the distance of the screen from the lens n, divided by the dis- tance of the object from the same lens. It follows that we may increase the size of the image at pleasure by either increasing the distance between the lantern and the screen, or by decreasing the distance between the object and the lens, proper adjustments being attached to the instrument to enable us to do the latter. Since, however, the amount of light transmitted through the lens n remains unchanged, the brilliancy of the picture decreases as its size is enlarged. Note,— In order to enable us to cast a large picture, we may make use of a more powerflil light than that obtained by a common lamp, as for ex- ample the Bude light or the oxy-n. the permanent he eye inward, It is frequently in cutting the ye proper are hi, emanating n through the so as to regu- or of the eye. lens and the n the retina so ed imaj^e, ADJUSTMENTS OP THE EYE. 135 IV. The pigmentum nigrum absorbs the rays of light as soon as they have passed through the retina, in order to prevent them from being reflected from one part of the interior of the eye to iiuother, as this would cause much indistinctness of vision. Note.— The pupil in man is circular and contracts circularly— varying iu diameter from one-tenth to one-fourth of an inch. It cannot contract or enlarge instantly, but requires a certain length of time. Hence if a person comes from a darkened room into one brilliantly illuminated he is at flrat dazzled by too much light being admitted into the eye. On the other hand when a person goes from a light apartment into the open air at night it is forae time before the pupil enlarges so as to allow sufficient light to enter to enable him to see surrounding objects. . In the owl the pupil 'is so large that during the daytime he cannot contract it sufficiently to protect liis eye from the sun's light, and hence lie is nearly blind by day. In the cat, and ether beasts of prey that leap up and down in pursuit of food, the opening of the iris is iu the form of an ellipse with its long diameter vertical; in herbivorous animals, on the other hand, which require a long horizontal range of vision, the pupil is elliptical with its long diamMer horizontal. 331. Spherical aberration is overcome in the eye in part by the difference in curvature of the cornea and crystalline lens - the latter being more convex in front than behind, and in part by the iris, which acts as a stop or diaphragm so as to allow the admitted rays of light to fall only upon the centre of the crystalline lens. 332. Chromatic aberration is corrected by the different den- sity of the several humors, the increased density of the crys- talline lens from its circumference to its centre, and the action of the iris as Si diaphragm. 833. We have seen that the distance of the image from the lens hich forms it, varies with the distance of the object, and that in he telescope, the microscope, and other optical instruments, some lechanical contrivance is employed to adjust the instrument a proper focus. The healthy eye possesses this power of ad- ustment, to see near or distant objects in the utmost perfection, )ut of the mode in which this is accomplished very little is cnown. It is however, supposed, that either the position or the bym, or perhaps both the position and the form of the crystalline i 136 CONDITIONS OP DISTINCT VISION. lens are changed so as, under all circumstances, to throw the image on the retina. 334. Near Sightbpnbss ariaes commonly from the too great convergent power of the eye. The cornea or the crystalline lens or both are too convex, and hence they bring parallel or slightly divergent rays to a focus before they reach the retina ; \>y bring ing the object nearer, however, the rays that proceed to the eye are more divergent, and their focus is therefore removed further back so as to fall upon the retina. The remedy for this defect is the employment of concave spectacles, which neutralize the too great convexity of the eye. NoTB.— Care should bo taken by persons who wear spectacles always to employ lenses whose power is not too great, as such have a tendency to increase rather than remedy the imperfection of the eye. 335. Long Sightedness is for the most part peculiar to old persons, and arises from the partial flattening of the eye, and consequent loss of refractive power. The result is that the divergent rays which proceed from a near object are refracted to a focus behind the retina, and the image on the latter is indis- tinct. Long sightedness is remedied by the use of convex glasses which assist the eye in bringing the rays to a focus on the retina. 336. The principal conditions of distinct vision are the following I. The object must be situated at such a distance as to form on the retina an image of some appreciable magnitude. II. The object must be sufficiently illuminated to produce a distinct impression on the retina. III. Distinct vision is obtained only by rays that are sensibly parallel or very slightly divergent. Note 1.— The minimum limit of distinct vision varies in different eyes- being commonly about lO inches but in some as low as 3 inches. The maximum distance to which an object may be seen varies with its sue, color, and degree of illumination. A white object illuminated by the light of the sun can be seen by a good eye to the distance of 17260 times its own diameter, a red object about half as far, and a blue one somewhere about one-third as far. The diBtanee to which an eye can penetrate depends, however, very ruuuh upon habit and training. As a general rule, dark-colored ©yes can sco farther than light-colored ones. KOTE 2.- wlmtcver m although th( inversely as Bions of the distance; lu which it is s Note. .3.- gay of 8 or 1 that the coi have an anj dent that tl slightly div 337. Tl of an erec among si that up a retina, ar and dowi eye, he is ground a 338. I acquired to appre( bouring ; angle, & 339. ] fact that a single mind, in habit, si retina > results. upon the same ob object. NOTE.- finger, ai IN. J, to throw the BINaLU AND ERECT VISION. \ 137 KoTE 2. -The apparent brightness of tho object remftins constant whatever may be its dlatance f oin the oyo. This arises from the fact that although the amount of light leceivod by tlie eye from the object varies the too great iuverscly as the square of the distance of the object, the superficial dimen- sions of the image on the retina also varies inversely as the square of the distance; hence as tho amount of light received decreases, the space over which it is spread decreases ia the same proportion. crystalline lens allel or slightly tina; t)y bring seed to the eye emoved further for this defect neutralize the ctacles always to ve a tendency to peculiar to old )f the eye, and lit is that the are refracted to latter is indis- convex glasses, a focus on the v^ision are the ice as to form litude. I to produce a at are sensibly n different cycs- as 3 inches. The Tics with its size, inated by the light r250 times its owu somewhere about 'wCvuf , vCfy liiUch ored eyea can sco Note. .3.— When the eye is adjusted for viewing an object at the distance say of 8 or 10 inches from it, the pupil is contracted to about i^(T of an inch, so that the cone of light entering the eye from any point of the object will have an angular divergence of only about half a degree. Hence it is evi- dent that the rays that produce distinct vision are either parallel or very slightly divergent. 337. The mode in which an inverted image gives us the idea of an erect object has long been a matter of much discus,' m among scientific men. The simple explanation appears to e that up and down, with reference to the image formed on the retina, are merely relative terms, up meaning towards the sky, and down towards the earth. When a man stands before the eye, he is seen erect, merely because his^feet appear towards the ground and his head towards the sky 338. Ideas of the distance and magnitude of an object are acquired only by experience, by means of which the eye is enabled to appreciate its size and the distance by comparison with neigh- bouring familiar objects, its dimness or distinctness, the visual angle, kc. 339. Many theories have been advanced to account for the fact that, though an image is cast upon the retina of each eye, only a single object is seen. This blending of the two images, by the mind, into a single perception, is apparently chiefly the effect of habit, since, when the two images do not fall upon parts of the retina which are accustomed to act together, double vision results. But it must be remembered that the mind does not look in upon the retina, and although there are two images depicted of the same object, the mind instinctively acquires the idea of but one object. NoTB.-If one eye be forced a little to one side by pressure \7ith thd finger, an object examined by both eyes will appear double* / i38 COLOIt BtiNbNfisS. 310. An impression on the retina 13 not instantaneously mado, nor does it instantaneously die out. The former is shown by the fact that we cannot see a rifle ball or a cannon ball when we stand at right angles to the course of its flight, but, if the pro- jectile is approaching us or is going from us, it preserves the same direction long enough to produce an impression, and we see it. With regard to the time the impression remains on the retina, it may be remarked that it is well known that winking does not interfere with distinct vision, because the image re- mains on the retina so as to give the sense of continuous vision. Moreover if a lighted stick be whirled rapidly in a circle its track appears to be a continuous ring of fire. By carefully conducted experiments it has been ascertained that the time an impression remains on the retina varies in different eyes from -j^j to i of a second. 341. Color blindness is a peculiar affection of the retina, by which the eye is rendered unable to distinguish certain colors of the spectrum. Some can only discern yellow and blue in the spectrum ; some mistake orange for green or green for orange ; some can only distinguish with certainty yellow, white, and green ; some cannot distinguish by color the ripe cherries on a tree from its leaves, &c. 342. The Simple Eye, of which that of man is the most perfect type, belongs peculiarly to the vertebrate kingdom, but is occa- sionally found also in invertebrate animals. The snail and kindred creatures have this simple eye,mounted on the tip of a long stalk or pedicle. In spiders the eyes are simple, usually eight in number, and are situated on the top of the head. The larvae of many insects possess simple eyes only, but the eye of the per- fect or fully developed insect is compound. These compound eyes have the same general form as simple eyes, and are placed either on the side of the head as in insects, or are supported on pedicles as in crabs. When examined by aid of a magnifying lens, the compound eye is found to consist of many hexagonal facettes or eyes, each being the large end of a cone, which is about six times as long as it is broad, and which receives a single filament of the optic nerve. In one species of beetle the eye OPTICAL PHENOMENA. 139 icoiisly madp, in shown by in ball when >ut, if thepro- jreservea the ision, and we nains on the that winking tie image re- nuous vision, ircle its track ly conducted n impression iV -o i of a lie retina, by tain colors of blue in the for orange ; , white, and iherries on a most perfect but is occa- 3 snail and e tip of a long isually eight The larvae e of the per- e compound 1 are placed upported on magnifying y hexagonal tie which is i receives a >eetle the eye contains 25,000 of these facettes ; in the eye of the butterfly 17,000, in that of the dragon-fly 12,500, and in that of the house- fly 4,000 have been counted. The number of facettes is evidently intended to compensate for the immovability of the eye— each facette being a perfect eye in itself, although having, on account of its fixed axis, but little range of vision. LECTUBE XXIV. OPTICAL PHENOMENA OP THE ATMOSPHERE. 343. The diffused light of the atmosphere is due to the reflection of the rays by its individual particles and by the earth's surface. Were it not for this scattering of the rays of light, the atmosphere would not be illuniinated at allj and we should, even at mid-day, see the stars shining forth from an intensely black ground. 344. The dark vault of heaven appears, during a fine day, of a fine blue tint. This is due to the unequal reflection of light by the particles of air — the blue rays being for the most part reflected, and the yellow and red absorbed. The darkest blue is always in the zenith, the atmosphere near the horizon appearing much lighter in Color. And as we ascend into the higher regions, the blue deepens until at length it becomes black. S^OTB.— The evening and morning red depends in all probability upon the vapour contained in the air. 345. Twilight is the partial illumination of the atmosphere that intervenes between sunset or sunrise and total darkness. It is due to the rays of the sun striking the higher regions of the atmosphere and being refracted to the earth. In Canada, the twilight continues till the sun is 18" below the horizon, but in equatorial regions the twilight is of much shorter duration. 346. Looming is a term applied to the apparent elevation of objects, at sea, above their true level. Thus, islands and vessels seem raised above the water^ or very distant vessels !■»;• 140 THE RAINBOW. M " I) I appear above the horizon. In all these cases the appearance i^ due to extraordinary atmospheric refraction. Note,— Occasionally a voseel appears suspcndod in the clouds with nii inverted imago beneath it, producing an appearance known as the Fata Morgana. 847. The Miragb, often seen in hot sandy deserts, is an ap- pearance in which distant objects seem to be reflected in the waters of a placid and beautiful lake. It is caused by the par- tial rarefaction of the lower stratum of air, which rests upon tlie heated surface of sand, causing the rays that emanate from remote objects to pass in a curvilinear path to the eye. 348. Atmospheric refraction causes all the heavenly bodies which are not in the zenith to appear nearer to that point than they really are. In the horizon such bodies are lifted out of their true position about half a degree, so that we actually sec the lower limb of the sun or moon before the upper has come to the horizon, and in the evening continue to see the lower limb until the upper has, in fact, sunk beneath the horizon. Similarly the stars all appear to rise before they in reality come above the horizon, and are visible for some time after they have set. Atmospheric refraction also, by acting more upon the lower limb of the sun and moon, when on the horizon, relatively lifts that portion up so as to give these luminaries an apparently oval form. THE RAINfiOW. 349. The Rainbow consists of one or more circular arcs of pris- matic colors seen when the observer is standing with his back to the sun and rain is falling between him and a cloud, which serves as a screen on which the bow is depicted. When the bow is double, i. e. When two boWs are seen, the inner or brightest is called the primary, and the outer one, which is not so bright, is termed the secondary. In the primary bow the order of colors is, beginning with the innermost or lowest, violet, indigo, blue, green, yellow, orange and red ; in the secondary bow this order of colors is reversed. The inner bow is not seen when the sun is more than 42' above the horizon, and the outer one does not appear when the elevation of the sun is more than 64«>. When the sun is in the horizon both bows extend to semicircles but become f}T\ftller arcs of a circle as he is higher above the hoi'ljctv pearance i^ )ud8 with an M the Fata 3, is an ap- Jted in the by tlio par- ts upon the Einato from e. mly bodies point than fted out of dually sec IS come to lower limb Similarly ome above ' have set. lower limb y lifts that '■ oval form. ircsof pris- 1 his back oud, which len the bow brightest is ) bright, is r of colors digo, blue, jT bow this seen when the outer more than extend to I is higher THE RAINBOW. 141 860. The rainbow is caused by the refractions and reflections of light by the falling drops of rain. In the primary bow thera is one reflection and two refractions, and in the secondary thero are two reflections and two refractions. Let /» bo a lino drawn from the eye of the obsorvor to the centre of the rainbows, and lot A, IJ, C, and D, E, F, bo ephorical drops of rain in th« act of falling. Then of tho cones of light that fall upon each of the drops ABC, the rays that pass through or near tho axis are refracted to a fyona Fig. 43. behind the drop, but those that fall upon the upper side of the drop wdl be refracted, the rod least and the violet most, and will fall upon t^ back of the drop so obliquely that some of them will he reueciea as ecuwu )n Fig 48. Upon again passing out of the drop they are refracted tt 3io ^^^^ %*mi>m.»*' ..JttM 142 POLARIZATION OF LIGHT. i M\ '*%'. eye at 0, and, since the red i8 refracted least and the violet most, the red wMl form the outer color of the spectrum perceived by the eye, and the violet the lower or innermost band. Similarly, by tracing the course of the rays that en er the lower part of the drops D, E, F, it will be found that the rays are reli^cted twice and reflected twice so as to finally reach the eye, depicting on the retina a spectrum having the violet for the outer band and the red for the inner. The bows are circular arcs because of the many rain-drops that compose the shower. Those only can reflect red to the eye that make an angle with P O equal to the angle ^ O P or the angle FOP; those only can reflect violet to- the eye that make angles equal to C O P or D O i^, &c., and these drops must necessarily be for the moment in the arc of a circle having P for its centre. 351. Haloes are prismatic rings occasionally seen around the sun and varying from 2o to 45 «» in diameter. They are caused by reflection from minute crystals of ice floating in the higher regions of the air. 352. Coronas are rings circling the moon and are said to gen- erally indicate the approach of a storm. They ftre caused by reflection of light from the external surface of watery vapour floating across the face ofHhe moon. 353. Parhelia (false suns, sun dogs) are bands of light which are sometimes seen surrounding the sun and*sometimes passing through it. They are attributed to reflection from minute crystals of ice in the air. LECTURE XXV. POLARIZATION AND DOUBLE REFRACTION OP LIGHT. polarization. 354. When a ray of light, ab Fig. 44, is incident upon a plane glass plate A D /Ki./.vr the outer tat compose :e an angle le only can DOF, &c,, I of a circle in around They are ng in the lid to gen- caused by ry vapour ight which es passing ,te crystals ./ P LIGHT. le glass plate he most part w if the ray on a second plane glass plate C D (also blackened at the back) parallel to A B, the ray be will bo incident on C D at an angle of B4i°. In this position of the plates their planes Fig. 44. of reflection are coin- cident, and the ray be will be reflected like any ray of ordinary light; but if we turn the plate CD in such a manner that the ray be forms the axis of rotation, the angle of incidence will remain the same, but the par- allelism of the mirrors will cease, and conse- quently their planes of reflection will no longer be coincident. Under these circumstances we shall find that, while revolving the plate C D, the brilliancy of the twice reflected light alternately dies out and is renewed. When the plate C D is turned 90° or a quarter round, the ray be is no longer reflected from the plate C D as it would be were it a ray of ordinary light ; when we have revolved the plate C D, 180° or half round, the planes of reflection of the plates are again coincident, and the ray be is totally reflected ; when tho plate G D has been revolved 270^ or three quarters round, their planes of reflection are once more at right angles to one another, and the ray be is not reflected at all ; and so on. Of course at intermediate points of revolu- tion the ray is partially reflected. 355. It appears then that when a ray of common light has suffered reflection from a glass surface at an angle of 54 J**, it has acquired certain remarkable and peculiar properties. It is resectable on one side but not on the other, so that its opposite sides have opposite properties. Under these circumstances the ray of light is said to be polarized. Note.— The Plane of polarization is the plane in which tho ray can be eompletely reflected by the second mirror, and is, of course, coincident wiih the plane of reflection from the first mirror. 856. A PoLARiscoPB or polarizing apparatus is an arrangement of mirrors or reflectors, by means of which the efiects of polar- ization can be examined, the angle of polarization measured, &c. Polariscopes are of various forms. 357. We have seen that the vibrations in the ether which produce light are made transverse "to the course of the ray • ^ ' s%t Ik.-*" A' 144 CIBCtJLAR POLARIZATION. in all conceivable planes. Now it is supposed that the diflfer- ence between polarized light and common light consists m the fact that the vibrations producing the former are all made in one single plane. 358. Certain crystals seem to possess the power of polarizing all the light that passes through them in particular directions. This appears to be due to their partly absorbing the light, and causing the remainder to vibrate in a single plane. Thus if a transparent tourmaline be cut paraUel to its principal axis into plates ia of an inch in tWckness, and two of these bo taken and polished, they exhibit with great beauty the property of polarizing light. The light is readily transmitted through either plate separately or through both when they are held lengthwise parallel to one another, but if the second plate is made to cross the first, it totally obstructs the Ught. A tourmaline plate therefore affords a means of olarizing light and also of determining whether a ray has already been polarized by other means. When used for the latter purpose, the plate of tourmaline, or other smtable substance is called an analyzer. 359. When a ray of light is polarized by reflection from the first or second surface of a transparent body a part of the trans- mitted light equal to it is also polarized by refraction. The whole amount of light transmitted, however, greatly exceeds the part polarized, so that, In common language, we say that light is only partially polarized by a single refraction. "When however a ray of light is transmitted obliquely through anumoer of parallel plates of glass or other transparent medium, a new portion is polarized by each plate, until at length the whole of the trans- mitted beam is polarized. 360. The kind of polarization we have been hitherto desc -ibing is called Plane Polarization, to distinguish it from Elliptical and Circular Polarization. In order to effect the plane polarization of light by reflection from metallic surfaces, it must be reflected many times at the proper angle of polarization. 361. If a ray of light has been twice reflected from the second „„«f«/.na of boHi^a nt thftir anerle of ereatest polarization, or if Dill Itt\,v-:7 Vi *,i;.*.^-^ — t -S.5-.- ^, ^^ it has been reflected but once at that angle from a metallic sur- face, it appears to consist of light vibrating in two planes only DOUBLE REPEACTION. 146 le diflfer- s in the made in olarizing irections. ight, and Lcipal axis taken and zing light, or through , but if the ht and also her means, aer suitable from the the trans- ion. The cceeds the at light is however a of parallel portion is the trans- dese nbing iphcal and 3larization e reflected the second ktion, or if 'tallic sur- danes only at right angles to one another, and the phase of vibration in one is retarded i of a vibration. Under these circumstances the light is said to be circularly polarized. 362. Elliptical Polarization takes place when a ray of light is reflected once from a metallic surface at its angle of maximum polarization. It appears as light vibrating in two planes which are not at right angles to one another, and the light vibrating in the one plane is retarded less than i of a vibration behind that vibrating in the other plane. 363. The colored phenomena dependent on polarization, plane, circular and elliptical, are exceedingly beautiful and varied. All of these colors are produced by the interference of rays. The polarization of light has now become a most reliable means of investigation in the hands of the analytical chemist, as it often enables him to detect the slightest adulteration in a solution. DOUBLE REFRACTION. 364. By Double Refraction, we mean a property possessed by certain crystals, as Iceland spar, of splitting or dividing a single incident ray into two emergent ones. Thus, when a crystal of Iceland spar is laid upon a d"rk line on paper, it conveys to the eye the impression of two parallel Imes removed from one another by a small intervening space. 365. A crystal of Iceland spar is rhombohedral in form, and a line drawn from an obtuse angle of the crystal through the centre to the opposite obtuse angle is called its principal axis. Now the two emergent rays are distinguished as ordinary and extraordinary ; the former in the case of Iceland spar being that which appears most removed from the principal axis, and the latter that nearest the principal axis. 366. Crystals, like Iceland spar, which refract the rays as indicated in Art. 365, arc called Positive Crystals ; while those in which the ordinary ray lies nearer to the principal axis than the extraordinary ray are called Negative Crystals. Note.— Some crystals have two axes of double refraction, as for example mica, topaz, gypsum, nitrate of potash, &c. I' !?>>: f 146 ELECTRICITY. 367. When the two rays that emerge from a crystal of Ice- land spar or other crystal possessing the power of double re- fraction are examined, they are both found to consist of light totally polarized-the one being polarized at right angles to the other. Hence the various ways by which light may be polarized are reflection, refraction, absorption, and double refraction. Note 1 -A crystal of Iceland spar, one of tourmaline, is among the most valuable pieces of polarizing apparatus we possess. The former cut into the form of a prism (a Nicol's prim) is UBQd for throwing the ordinary image out of the field of view, as it transmits only the extraordinary ray. Note 2.— Thin plates of double refracting crystals exhibit colored rings of exquisite beauty marked by a black cross, when viewed in certain directions by polarized light. ELECTRICITY. LECTURE XXVL DEFINITIONS, SKETCH OF THE HISTORY OP THE SCIENCE, IDIO-ELECTRICS AND AN-ELECTRICS, CONDUCTORS, NON-CONDUCTORS, INSULATION. 368. Electricity (Greek, electron "amber") is the name given to a highly elastic, attenuated and imponderable agent which per- vades the material world, and which is visible only in its eflFects. It is susceptible of a very great degree of intensity, and has a tendency to equilibrium unlike that of any other known agent. The word 'fluid," as applied to electricity, must be taken in a conventional sense only. 369. When a rod of glass or of sealing wax is smartly rubbed for a few moments by a piece of warm flannel or silk, it acquires the power of acting upon light bodies, so as to attract and repel them. While this transiCnx puvvCf lasts, the ?od is sai;^ to be elec- trified or charged ; the piece of paper or other light body is said to be attracted when it approaches the rod, and is said to be HISTORY OF ELECTRICITY. 147 I of Ice- ouble re- ; of light les to the polarized among the former cut 16 ordinary iinary ray. iored rings in certain OF THE ECTRICS, iTION. lajne given which per- . its eflfects. and has a )wn agent. taken in a is smartly il or silk, it I to attract to be eke- body is said said to be repelled when it recedes from it. In the dark a faint light is seen to follow the track of the rubber, and this is accompanied by a crackling noise. Thc^s are the fundamental phenomena of electricity. 370. The science of electricity was to some extent known and studied by the ancients. The discoveries were, however, very few, and the science was not systematized until the time of Franklin and Du Fay, between the years 1733 and 1Y60. The following is a brief sketch of the history of electrical science : B.C. 600 371. A.D. 1600. 1670. t709. Thalbs of Miletus discovered that amber, when txcited by friction, attracts light substances. Thbophrastus, a pupil of Aristotle, noticed the electric properties of the mineral called tourmaline. Pliny and Aristotlb were acquainted with the pecu- liar effect resulting from the touch of the torpedo, but had no idea that it was referable to the same cause as the properties already observed in amber and in tourmaline. De. W. Gilbert, physician to James I. of England, in an appendix to a valuable work on the magnet, pub- lished a variety of electrical experiments on gems, glass, gums, &c. Mr. Robt. Boyle added to the number of electrics, and discovered the electric light emitted by the diamond when rubbed in the dark. Otto GuEaiOKB, in Germany, contemporary "with Mr. Boyle, mounted a globe of sulphur upon an axis, and thus procured electricity in greater quan- tities. He also discovered electrical repulsion. Sir Isaac Newton discovered that glass does not pre- vent electrical attraction and repulsion. Mr. Hawksbbb mounted a glass instead of a sulphur globe. / 148 A.D. 1129. 1735. 1U2. It46. 1752. 1771. HISTORY OF ELECTEICITY. Mr. Stephen Gray, of the London Charter-House, first observed the fact that some substances are conductors, and others non-conductors, and hence he discovered a method of insulating bodies. Du Fay inferred the existence of two fluids to which he gave the names of Vitreous and Rednous Electri- cities. Professor Bozb, of Wurtemberg, added the Pnme Conductor of the globe machine. It was at first sup- ported by a man standing on a cake of resin, after- wards it was suspended by a cord of silk from the ceiling. Mr. WiNCKLER, of Leipsic, about the same time, sub- stituted a cushion instead of the hand, which had hitherto been used as a rubber to excite the globe. Professor Muschenbr(ECk, of Leyden, in conjunction with his associate Cunbus, by accident discovered that electricity could be collected in a glass vessel containing water. VoN Klbist, dean of a cathedral in Germany, made the same discovery simultaneously with Prof. Muschen- broeck. Sir W. Watson, Smeaton, Bbvis, Wilson, and Canton, all distinguished members of the Royal Society of London, improved and extended the discovery of Muschenbroeck, and gave us the Leyden Jar in its present form. Dr. B. Franklin discovered the identity of electricity and lightning ; introduced points for protection ; combined several Leyden jars into a battery ; and gave his hypothesis of a single fluid. Cavendish and (Epinus investigated the hypotheses of Du Fay and Franklin. Watson and Canton fused metals hj electricity. Beccaria decomposed water by means of electricity, ELECTRICS AND NON-ET ^CTRICS. 149 [ouse, first onductors, discovered to which us Electri- the Prime t first sup- esin, after- : from the time, sub- which had [Q globe. ;onj unction discovered ;lass vessel /f made the '. Muschen- id Canton, Society of iscovery of Jar in its ' electricity protection ; ittery ; and hypotheses city, jctricity, A.D. 1186. VoLTA invented the Elecirophorus, Coulomb, by means of his torsion electrometer^ reduced electricity, the most subtile of all physical agents, beneath the rigorous sway of mathematics, and thus placed it at once among the Physical Sciences. Profbssob Faraday and Sm W. Snow Harris are per- haps the most distinguished cultivators of electrical science at the present day. 871. Those substances which, under ordinary circumstances, readily evince electrical properties by friction, are termed elec- trics or idio-electrics. They are exhibited in the following : TABLB OF ELECTRICS, OR NON-CONDUCTORS. 1 Shellac Brimstone Amber Jet- Resin Gums Gun-Cotton Glass Diamond Gems Bituminous Substances Silk Fur Hair Wool Feathers Paper Turpentine Oils All dry Gases Atmospheric Air Steam of high elasticity Ice at 0® Fahr., &c. 372 Those substances which do not readily evince electricity under" ordinary circumstances by friction, are called non-elec- trics or an-electrics. They are shown in the following : TABLE OF NON-ELECTRICS, OR CONDU0TOB8. AH Metals Well burned Charcoal Plumbago Acids Saline Fluids Water Steam Flame Smoke Animal and vegetable sub- stances containing mois- 1* ture, &c. 1 150 SOURCES OP EXCITATION. 873. Electrics are also called non-conductors^ from the fact that they transmit electricity very imperfectly, but non-electrics are generally speaking very good conductors. Some substances be- come conductors or non-conductors by a change of temperature — thus glass, when heated to redness, becomes a conductor ; water, when in the state either of steam of high elasticity, or of ice at or below O** Fahr., becomes a non-conductor. 374. When a metal rod is subjected to friction, electricity ia developed upon its surface, but the metal being a good conduc- tor it is conveyed away by the hand as fast as it is generated. If, however, the metal rod be attached to a glass handle, the fluid accumulates upon the rod and becomes visible in its eflfects. Hence electricity may be developed on any one of the so called non-electrics if it be insulated. 375. A body is said to be insulated when it is supported by a non-conducting substance, such as a rod of glass or shellac. LECTURE XXVII. SOURCES OF ELECTRICAL EXCITATIO:?^ AND KINDS OF ELECTRICITY, ELECTROSCOPES AND ELECTRO- METERS, THEORIES AS REGARDS THE NATURE OF ELECTRICITY. SOURCES OF EXCITATION. 876. The principal sources of electrical excitation and the kind of electricity developed by each, are as follows : I. Fkiction producing Frictional, Statical, Tensional, Com- mon or Machine Electricity.* n. Chemical Action producing Dynamical Electricity, Voltaism or Galvanism. III. Difference of Temperature in connected metallic bars— civine rise to Thermo-Electricity. * See Art. 391, Note. the fact that lectrics are )stances be- aperature— jtor ; water, )r of ice at lectricity is od conduc- I generated, handle, the in its effects, be so called ported by a shellac. W KINDS ELECTRO- A.TURB OF on and the ional, Com- ity,Voltaism ;allic bars— . ELECTROMETERS. 15l IV. Magnetic Action developing Magneto-Electricity. V. Living Animal Mattbr— Animal Electricity. NOTB -Change of form, mere contact, simple pressure, change of tem- nerature, fcc, also give rise to the manifestation of electrical force, but these must be regarded as coming property under the head of one or other of the above five sources. BLEOTROBCOPES. 377. ELB0TROSCOPB8 are instruments used to detect the presence of free electricity. The principal electroscopes in use are the following : I The Pith-Ball Elkotbosoopb. This consists of two small pith baUs insulated by silk threads. When brought into the neighbour- hood of an excited body, the balls become similarly electrified, and repel each other. II Thb GOLD-LBAF Eleotroboopb. This consists of two slips of gold- leaf property insulated and inclosed in a gl^ss jar. When brought into the neighborhood of an electrified body, the leaves diverge and thus indicate the presence of the electric fluid. Fig. 45. III. BOHNBNBBBGER'8 GOLD-LBAF ElBOTROSCOPK. This consists of a small Zamboni's pile* a b placed horizontally and having each extremity connected by a wire to perpendicular metallic plates p and m. One of these plates is therefore the positive and the other the negative electrode of the pile. A metallic disc o n is connected by a wire c d to a slip of gold-leaf d g which hangs midway between the two platesp, m, being equal- ly attracted by each. When, however, the slight- est trace of electricity is communicated to the disc o n, the leaf instantly moves towards the plate, which has the opposite polarity. ELEOTROMETERS. 378. Electrometers are instruments employed to measure the m^miiyofelectricalforce, and, like electroscopes, they depend for their action upon electrical attraction and repulsion. Electrometers differ from electroscopes merely in having attached a gra- duated arc or some other means by which to compare the intensities of dififerent accumulations of the fluid. The chief electrometers in common use are the folio wins : ii m * See Art. 437. 152 ELECTROMETERS. I. Thb Quadbant Blectromktbe. This consiats Fig. 46. of a light pith ball a attached by an exceed- ingly thin and light insulating rod / to an upright, metallic bar b c. The rod m f movea freely round the pivot /which is the centre of the circle of which the graduated ivory semi- circle iS is an arc. The number of degrees of the graduated arc through which the rod / «i is driven when the instrument is placed on a charged conductor, is, in a measure, indicative of the intensity of the accumulated fluid. It is obvious that no amount of electricity, no matter what its intensity, can repel the ball beyond 90°. Fig. 47. n. Coulomb's Torsion Electrometer consists of a tube,ac, 8 or 10 inches in length, having a flat graduated plate at the top and terminating downwards in a glass jar c d. Through the tube there passes a fine thread of glasSt^or of shellac, or of unspun silk, which terminates upward in a button and index on the plate d and down" wards in a horizontal bar of gum-lac, 6 having a smal gilt pith bail p at one extremity and a paper vane n, to arrest oscillations, fixed at the other. Through another aperture/ in the top of the glass case, a second rod of shel- lac, called the carrier rod, with a gilt pith ball at its extremity, is introduced. On the glass case there is a graduated circle mtn,hy which the number of degrees through which the ball p is repelled, may be measured. Coulomb has demonstrated that the reactive force of an elastic filament or its tendet-iy to return to its previous state is exactly pro- portional to its torsion. Hence the nun.ber of degrees through which the ball p is repelled by the charged ball at the end of the car- rier rod is the measure of the electrical force accumulated on the latter. Suppose the electricity accumulated on the carrier ball repels the ball p through 20°, and it is required to ascertain the torsion force necessary to maintain the ball at a divergence of 10°. If when the balls are 20° apart we begin to turn the button on the plate a back- wards we shall gradually bring the haXlp nearer to the charged ball at the end of the carrier rod, and upon thus turning the index on a back through 70° we shall have brought the balls within 10" of each other "Novr the filament of glass or lac is twisted 10° to the right, and 70° to the left so that its torsion is represented by 80°, and hence we have the numbers 20 and 10 for the relative values of the repulsive forces at the distances of 20° and 80° ; and since the numbers 20 and 80 are in the proportion of 1 to 4, we infer that electrical repulsion and attraction vary inversely as the square of the distance. Fig. 46. it its intensity, Fig. 47. c, te > of an elastic is exactly pi o- grees through end of the car lulated on the )all repels tbo torsion force , If when the plate a back- 3 charged ball tie index on a in 10" of each right, and 70° hence we have jpulsive forces 10 and 80 are in repulsion and ELECTEICAL THEORIES. ELECTRICAL THEORIES. 153 379. Tho theories that have been advanced in explanation of electrical phenomena are chiefly two, viz : I. The one-fluid theory, or theory of ^^ranklin. II. The two-fluid theory, or theory of Du Fay. NOTB.-Of these theories that of Franklin is the simpler, but Du Fay's is considered to be the more philosophical. 380 The one-fluid theory, or theory of Franklin, assumes the existence of a single elementary imponderable fluid of extreme tenuity and elasticity, existing in a state of equable distribu- tion throughout the material world. This fluid is supposed to be repulsive of its own particles, but attractive of the par- tides of all other matter. Every body has a certain amount of capacity for this fluid, and when it contains its natural share is said to be in a state of electrical quiescence or repose. When however, by friction or other mechanical or chemical means' we increase or diminish its quantity in a body, there ensues a powerful action arising from the tendency of the body to re- gain its natural share, if its original quantity has been diminished, or to throw it off to other bodies, if it has been increased. 381 According to Franklin's theory a body naving more than its natural share of electricity is said to be pontively electrified or -H electrified ; one having less than its natural share is said to be negatively electrified, or — electrified. 382 The two-fluid theory,or theory of Du Fay assumes the ex- istence of an infinitely attenuated fluid, highly elastic and impon- derable and pervading all bodies. It is supposed to be compounded of two elementary fluids possessed of distinct and opposite proper- ties and called vitreous and resinous electricities. These elemen- tary fluids are further assumed to be each repulsive of its own particles but attractive of the particles of the other, so that when combined in proper proportions they coraplet^i- ^'^_^ c? neutralize each other, thus produ-^o; perfect electrical repose When, however, by fric^icjs or other mechanical or chemical _,..»„ ^. ^«ooT"^"'='' ♦>ii« r.omnound, the vitreous and resinous fluids we separated, one adhering to.the surface of the rubber. 154 ELECTRICAL THEORIES. and the other to the surface of the excited substance, and hence in no case of electrical excitation can we obtain one kind of electricity, without the simultaneous development of the other. 383. The two-fluid theory is the one commonly adopted by scien- ti6c men, but in3tead of using the terms vitreous and negative electricities, the terms positive and negative are employed. It is hence necessary to note carefully that : Positive or Vitreous electricity is that kind of electricity that adheres to the surface of glass when it is excited by friction with a silk rubber. Negative or Rennous electricity is that kind of electricity that adheres to the surface of resin when it is excited by friction with a sil/c rubber NoTE.-In the former caao the electricity adhering to the rubber is negative, in the \a,itoritiB positive. 384. No general rule can be given as to which kind of elec- tricity will be developed by friction on a given substance, this depending upon the material that forms the rubber, and even then the question can be determined only by experiment. The following table of substances is given by Faraday, and is so arranged that each body becomes excited positively by friction with those below it in the list, and negatively by those above it. 1. Catskin or Bearskin 2. Flannel 3. Ivory • 4. Quill 5. Rock Crystal 6. Flint-glass t. Cotton 8. Linen Canvas 9. White Silk 10. Black Silk 11. The Hand 12. Shellac 13. Wood 14. Metal 15. Sulphur. Note.— Of all known substances catskin is most euBceptible of positive and, perhaps, sulphur of negative, electricity. and hence ne kind of the other, ed by scien- ,d negative loyed. It i3 itricity that rietion with jtricity that fiction with the rubber is nd of elec- )Stance, this r, and even ment. The ', and is so 1/ by friction Dse above it. ble of positive riBXaiBUTlON OF FREE ELECTRICITY. 155 LECTURE XXVIII. DISTRIBUTION OP ELECTRICITY ON CHARGED BODIES, ACTION OP POINTS, THEORY OF INDUCTION, TEN- SION, INTENSITY AND QUANTITY, LAW OF VARIA- TION IN FORCE OF ELECTRICAL ATTRACTION AND REPULSION. DISTRIBUTION OF FRBB BLBCTRICITY. 886. Electricity, in its natural or compound state, appears to be diflfused equally throughout any given mass of matter, but when separated into its component elements, each appears con- fined to the surfpee of the body in which it has been set free in the form of an exceedingly thin layer, not penetrating sensibly into the substance of the mass. 380. As free electricity resides on the surface only, of bodies, the quantity that can be accumulated in a given body neces- sarily depends upon the extent of surface, and when the same quantity of electricity is thrown on surfaces of different magni- tudes the force exerted by the charged surfaces will vary inversely as their squares. 887. When a spherical body is charged, the electricity dis- tributes itself equally over every part of the surface ; but in a spheroid it becomes accumulated at the extremities, and the more elongated the spheroid, the greater the disproportion between the force exerted at its extremities, and that manifested at its middle part. 888. On a flat disc or plate, with sharp edges, the electric fluid increases in depth or quantity from the centre to the edge, but the increase is not regular, being much more rapid near the edge than towards the middle of the plate. 880. Electricity is always given off rapidly from points. This Arises from the fact that the fluid accumulates in such quantities, ♦ iu^ »»4»/>w.:*:Aa r.f r»«;nfa na in Rroiiire sufficient tension to overcome the small amount of atmospheric pressure that can i 156 INPUCTION. there be exerted, and accordingly flows oflf in a continuous stream to the surrounding bodies. NOTB -So rapidly is the fluid given off from points, that it is impossihle to charge the prime conductor of an electric machine, if a point he attached to it, or if a point he presented to it. INTENSITY, TENSION AND QUANTITY. 390. By the intensity of a charge of electricity, we mean its attractive force upon surrounding bodies as measured by the electrometer. The intensity varies as the square of the quantity accumulated in a given amount of surface. Thus ifthere be three egMai mrfacea so charged that the second shall have accumulated upon it twice as much and the third three times as much as the first, then the intensities of the charges will he as 1, 4 and 9; the squares of the numbers 1, 2 and 3. Note —The same distinction exists between the terms intensity and quantity in electricity as in heat. The intensity of the latter agent ia determined, it will be remembered, by the thermometer, while the quantity in a given body is ascertained by the calorimeter; so the intensity of elec tricity is measured by the electrometer but its quantity by the amount of chemical decomposition it can effect 391. The term tensim, as applied in electricity, is employed to denote the power or ability possessed by an accumulation of the fluid to pass or force its way through any resisting: medium. NoTB -Electricity as set free by friction is of high tension, but is small in quantity; i. e. its mechanical or disruptive power is immense, but it can but feebly perform such offices as chemical decomposition; hence its name temimal electricity. It is called statical to distinguish it from dynamical electricity, the latter moving constantly in currents, while the former appears to be in a state of rest except at the moment of discbarge. The origin of the names comnumjrictwmh and machine, are obvious. INDUCTION. ^ a92. When an electrified body is placed near a conducting body (n its natural state, the whole of the latter becomes oppositely elec- . trifled unless it be insulated, in which case the extremity next the electrified body becomes oppositelv and that farthest from it sim- ilarly electrified. The electricity thus acquired by the second body is called induced electricity, or is said to be produced by i.iduction. continuous is impossible f a point be re mean its ured by the the quantity ) second shall times as much ,4 and 9; the intensity and latter agent is le the quantity tensity of elec the amount of ! employed to ilation of the medium, in, but is small mse, but it can hence its name Tom dynamical lile the former liscbarge. The bvious. iducting body rppositely elec- jmity next the 3t from it sim- dj the second produced by LAW OP ATTRACTION AND REPULSION, 157 393. All non-conductors allow induction to take place through them, and are from this circumstance, called di-eledrics. Sir W. S. Harris gives tlie following list of di-electrics, in which it will be observed that air is the worst and shellac the best. SUBSTANCE. SPECIFIC INDUCTIVE CAPACITY. Air, 1-00 Rosin, I'll Pitch, 1'80 Wax, 1-86 Glass, 1-90 Sulphur, 1-93 Shellac, 1-95 394. According to Faraday's theory, induction is essentially physical action, occurring between contiguous part* les only, and never taking place at a distance without polarizing the molecules of the intervening di-electric, causing them to assume a peculiar constrained postion, which they retain as long as they are under the influence of the indue- Fig. 48, tive body. For example, if P, Fig. 48, represent a body charged posi- tively, and abed, &c„ intermediate molecules of air or any other di-electric, then the free electricity in P acts upon the © a Q body N hy polarizing these intermediate particles. 'Thus, since r\ ^ /-\ positive electricity repels positive and attracts negative, the ^ q?> ^ stratum of atoms lying adjacent to P is acted upon by the © (, q electricity resident in the latter in such a manner that the side ,-. ^'^ of each next P becomes negatively and tiie side remote from ® ^ © P positively electrified. This stratum of atoms acts sin^i- Q ^ (^ larly upon tlie molecules next beyond, and so on until the ^-^ © ^^ action is carried to the body N. ' ©^__^ LAW OF ATTRACTION AND REPULSION. L^|J 395. Bodies similarly electrijied repel each other ^ ^^ and those differently electrified attract each other^ with a force varying inversely as the square of their distance apart. 396. Electricity is transferred silently from a charged body by the double power of conduction and convection. When a body is carefully insulated upon a resinous support, the rapidity 158 ELECTRICAL MACHINES. witn which it parts with its cliarge by exposure to the air, depends principally upon the amount of moisture in the latter. Bodies imperfectly insulated, as by silk or uncoated glass, lose an additional portion by its escape along the imperfectly insu- lating support. NoTE.-rarticles of dust in the air act as carriers in conveying away a charge from an insulated electrified body. LECTURE XXIX. ELECTRICAL MACHINES AND GENERAL THEORY OF THEIR ACTION, THE ELECTROPHORUS, THE LEY- DEN JAR, DISCHARGES OF ACCUMULATIONS OF FLUID. 397. The two kinds of electrical machines in common use are the plate and the cylinder machines, of which the. former is by far the most powerful and convenient. The Plate Electrical Machine consists of a circular glass Fig. 49. plate p, of any diameter from 10 to 40 or 50 inches, a prime con- ductor j) c, insulated on a glass pillar i, and furnishid with points EtECTRICAL MACHINES. 159 ) the air, he latter. 3S, lose an ctly insu- )ying away 20RY OF HE LEY- [OXS OF ion use are •mer is by Hilar glass > I prime con- i with pQuits /n, to collect the fluid from the revolving plate ; a negative con- ductor n c, likewise insulated on a glass pillar 6, and having a metallic connection with the rubber r. A silk bag s is made to enclose the lower half of the plate for the purpose of re- taining the fluid on its surface till it reaches the points »i, in connection with the prime conductor.— The rubber r is com- monly formed of two cushions of buckskin stuffed with horse hair, and the degree of pressure is regulated by a small screw a, near the negative conductor. When the machine is in action either the prime or the negative conductor is connected with the ground by a brass chain. 398. The theory of the action of the electrical machine is, according to the one-fluid hypothesis, as follows : Upon turning the handle of the machine the glass plate becomes positively electrified at the expense of the rubber, and in revolv- ing gives up this surplus fluid to the prime conductor as it passes between the points of the latter. The prime conductor thus becomes charged positively, while the rubber is left negatively electrified. After a few revolutions the process ceases on ac- count of the negative condition of the rubber, but when this latter has a metallic or other proper connection with the earth, it draws the electric fluid from the latter as fast as it is carried to the prime conductor by the plate, and thus the produc- tion of free electricity may be continued for any length of time. Note 1.— It is manifestly impossible, according to this view of the na- ture of electricity, to charge a body positively without at the same time charging some other body (commonly the earth) negatively, because we cannot give one body more than its natural share of the fluid without removing a portion from some other body. Note 2— Adopting this theory we may liken the action of the electric&l machine to the action of a common pump. Thus, we may regard The earth as the ivell, The chain as the loioer pipe of the pump, The rubber as the batrel, The plate or cylinder as the piston, The silk as the spout, and The prime conductor as the pail. 'i ^gQ E1.ECTRICAL MACHINEB. 300 According to the two-fluid hypothesis, the action of the electrical machine is not so simple, nor are electricians agreed as rits precise rationale. The following is the explanation ^'onlurlTthe glass plate or cylinder the electricity natu- rally present in it becomes decomposed^the positive adhering to the surface of the glass and the negative to the rubber, the positively electrified portions of the glass coming, dur- ing each revolution, in close proximity to the prime conduc tor act powerfully by induction upon the electricity natu- rally present therein-decomposing it into its component ele- ments and attracting the negative, which, being accumulated Tn a state of tension at the points of the conductor darts off towards the plate, to meet the positive fluid, and thus re-con- iTut the neutral compound. The prime conductor is thus left powerfully positive, not by ac,uvrins electricit^from the revolv^ns llass, hut by giving up its own negative fluid to the latter, 400 From this explanation it appears that the negative conductor is connected with the earth in order to afford a route for the escape of the negative fluid from the rubber. Tcn-ri. rortain strong objections have been urged against this explana- tion of the aSn of thfelectrical machine. The most important of these tion of the actum '^°^ ^ tj^^t the amount of combmed elec objections IB that it Practically ^«« j^^^ ^1^,,^ is no limit to SeTufn^^^'fTerpTsS^^ ^-- ^^ '^''^ luhdraw n^^^^^^ electricity. To this it may be answered : withdrawing ntg „,pptricitv is liable to lead to misap- ist. That the *^'^«^.*.-'^^f,r\^^^^^^^^^^ light and heat, a mere prehension. ^ f ncit[ s qm^^^^^^^^ ^^^^ ^^^^^^ the motion among the particles o^^tter in ^.^ ^^ ^^^, ^vavesor undulations produced by ^^f^'^fZ^^^^^ e the stop- tralize those produced by positive ^^J ^^^"^^^^^^^^^^ ^et with increased page of the former must necessarily allow the latter lo Td^hat in order to obtain an i'^^-itd. great a^^^^^^^^^^^ tricity, the conducting or containmg ^"••^^^;'/; .^' '^^J'l^^^^^ there inner surface of Leyden jars,) must be 'f''^'lyJll^l;^''X',n up to the will be an infinitely great amount of negative flmd to be given up revolving plate. _ ^ ^^ ^^^^.^^^ ^.^^ ^^^^ ^^.jl 3rd. That the amount oi me e^^?"- "";"' T'-"^^^^^ estimated .11 THE ELECTRO PIIORUS. 161 of the agreed anation y natu- idhering rubber, ig, dur- conduc- y natu- lent ele- imulated darts off J re-con- thus left revolving negative i a route 3 cxplatia- at of these ibined elec- no limit to ; by simply ed: I to misap- leat, a mere regard the ing to neu- , e. the stop- :h increased (Ositivc elec- tor and the hence there n up to the a very small sen estimated Ives as much electricity as is contained in a vivid flash of lightning, or, to use his own words, a certain electro-chemical arrangement produced " as much elec- tricity in a little more than three seconds of time as a Leyden battery charged by thirty turns of a very large and powerful plate electrical machine in i\ill action. This quantity, though sufficient if passed through the head of a rat or a cat, to have killed it as by a flash of iJightning, was evolved by the mutual action of so small a portion of zinc wire and water in contact with it, that the loss of weight sustained by either would be inappreciable by our most delicate instruments. It would appear that 800,000 such charges as I have referred to above, would be necessary to supply electricity sufficient to decompose a single grain of water." 401. During the development of maohine electricity a pecu- liar odour like that of phosphorus is evolved. This odour arises from the formation of a substance called ozone, which is con- sidered to be an allotropic form of oxygen. (See Chem., Arts. 93 and 110.) 402. The development of machine electricity is greatly faci- litated by the use of an amalgam, applied to the rubbers, con- sisting of two parts zinc, one of tin, and six of mercury, heated together in a crucible, and afterwards formed into a paste with lard. It is supposed that the oxidation of the amalgam aids the evolution of electricity. THE BLECTROPHORUS, Fig. 50. 403. The Electrophorus, invented by Volta, consists of a cir- cular metallic dish, a b, Fig. 50, having a rim about a third of an inch high. This dish is filled with a mixture of 1 part Venice turpentine, 1 part shellac, and 1 part resin, melted together at a gentle heat, and, after being poured into the dish, allowed to cool gradually, so as to acquire a smooth surface. A second cir- cular conducting disc c, called the cover, Rnd furnished with till iUBUmtillg ^mss iiaiiUXC, u, m,s UJ-U« ttiv Uj^Jr'-l i;jvt»inw Ui n«c resinous plate, 16: THE LEYDEN JAR, 404. The resinous plate of the Electrophorus becomes nega- tively electrified when rubbed with dry flannel or fur. Upon replacing the cover, this being insulated, is acted upon induc- tively by the charged plate of resin— the positive electricity being attracted to the lower surface, and the negative repelled to the upper. Upon now presenting the knuckle to the cover a spark of positive electricity passes from the hand to the cover, so ae to neutralize the free fluid upon its upper surface. If, under these circumstances, the cover is raised beyond the imme- diate influence of the excited plate of resin, it is found to be charged with free positive fluid ; and upon again presenting the knuckle to it a spark passes from it to the hand. If OTK —Since no electricity is taken from the plate, it is manifest that one excitation of it is sufficient under favorable circumstances, for the development of any amount of electricity. The theory of the action of the electrophorus according to the Frank- linian theory is as follows : When the plate is rubbed with fur it loses electricity and becomes negor tivehj charged, and, acting inductively upon the cover, it attracts a portion of its fluid to the under surface, leaving the upper negatively charged. Upon now presenting the hand, electricity passes from it to the cover, but when the cover is subsequently removed beyond the inductive influ- ence of the plate, the fluid which was held to the under surface becomes free, and upon again presenting the knuckle it passes from the cover to the hand. THE LEYDEN JAU. 405. Tlie L-YDEN Jar consists of a wmc mouthed glass vessel coated with tin-foil, both inside and outside, to within two or three inches of the top. It is closed by means of a dry wooden stop- per, through which passes a metallic rod terminating upwards in a brass knob, and connecting, by means of a wire or chain at the other end, with the inside coating of the jar. When the jar is charged, the two electricities are held on the opposite sides of the glass by their mutual attraction— the metallic coatings merely serving 93 good cond\i .tors and never accumulaiing in themselves any electricity. AVhcu: :i jar is discharged under ordinary circumB uuices, there remains in the jar, after the first airy Fig. 51. ELECTRICAL DISCIIARaES. 163 BS nega- r. Upon n induo- lectricity repelled cover a lie cover, face. If, he imme- nd to be nting the mifost that es, for tbo the Frauk- omcs negor ts a portion ly charged, f the cover, tctive influ- ce becomes he cover to :iass vessel vo or three oden stop- jtallic rod knob, and r chain at coating of d, the two osite sides ttraction — serving '^a cumnlaling Vh('h ;i jar ?umHUiace», e fust tliS" charge, a residual charge off- of the quantity originally accu- mulated. In passing from one coat to the other of the jar, electricity travels at the rate of 288000 miles a second. Note.— lu the above figure of the Leyden jar, the tin-foil on both sides roaches as high as the line a b. The jar is discharged by making a metal- lic or other conducting connection between the outside and inside coat- ings*. Two or more Leyden jars having their terminal knobs connected ^y wires, constitute what is called a battery of Leyden jars. 406. The instrument represented in Fig. 52 is called a Jointed Discharger. It con- sists of a glass handle, 6, with F'g- ^2. two curved metallic wires, c c, having metallic balls at their extremities. The wires aie movable round a joint, «, so as to be set at any required distance apart. When used for discharging a charged jar or battery, it is held by the glass handle, and one knob being placed in connection with the outside coating, the other is brought to one of the terminal balls of the battery. LECTURE XXX. ELECTRICAL DISCHARGES AND THEIR EFFECTS, ELECTRICAL EXPERIMENTS. ELECTRICAL DISCHARGES. 407. Discharges of accumulated electricity are of three kinds, viz : I. The Disruptive Discharge. * II. The Convective Discharge. III. The Conductive Discharge. Under the term disruptive discharge are included all varieties of electric discharge accompanied by the manifestation of light. The Convective discharge consists in the accumulated fluid being conveyed away silently by small particles of ponderable matter 164 EFFECTS OF ELECTRICAL DISCHARGES, floating in the atmoaphere ; the Conduclive discharge, in the conveyance of electricity from particle to particle of matter- without any change of place among the particles themselves. EFFECTS OF ELECTRICAL DISCIUBGBS. 408. The effects of the electrical discharge may be classed I. Physiological. II. Chemical. III. Mechanical. 400 The physiological effects are experienced when the dis- charge" is transmitted through the animal body. It causes the muscles to contract momentarily with convulsive energy, and produces a peculiar wrmchin^ sensation in the limbs through which it passes. If sufficiently powerful, it destroys life. Note -The shock may be transmitted through any number of persons at the same time. The Abbe NoUet sent it through a chain of 600 persons, in a verTlong chain the effects ai'e slightly stronger at the extremities than at the centre. 410 The chemical effects of statical electricity are very feeble' It produces, however, a slow and feeble decompositiou in certain chemical compounds when in the fluid state, such as iodide of potassium, ammonia, sulphuric acid, water, Ac, and it causes a mixture of two volumes of hydrogen nnd one of oxy- gen to combine instantly with explosive violence 411 The mechanical effects of the electrical discharge are seen when the passage of the fluid is impeded by meeting with a bad conductor. Under these circumstances the fluid either rends the obstacles asunder, or, in forcing its way through it, developes sufficient heat to ignite it if combustible. . 412 A variety of amusing and instructive expeyments are commonly exhibited in the lecture room to illustrate the nature of electricity, and the physiological, chemical and mechanical effects of its discharge. A few of the most interesting of these are the following : , -„ ... •.,.,.;.,./.'«« »tr^ii78G SuiGi Galvani, of the University of Bologna, discover- ed the convulsive movements set up in frogs' legs by contact with metals. 179G YOLTA, of the University of Pavia, discovered the fact that electricity is apparently produced by mere coutac;, and was hence led to the invention of his " Pile " and " Couronne des tasses," which are the tvpes of all the arrangements at present used for the production of dynamical electricity. Volta did not communicate his invention of the " Pile " or " Cou- ronne des tasses" to the world till the year 1800. 1800 Nicholson and Cablisle, of London, decomposod water and other compounds, by the agency of the voltaic battery or pile. IBOY ISm H. Davy succeeded in decomposing potassa and proving it to be the oxide of the metal potassium. He subsequently decomposed other oxides, and thus discovered several new metuli 1807 Professor (Ersted, of Copenhagen, called attention lu the analogy between magm'tlsni and voltaic t>kc- tricity, and thus laid tiie fuuiidfttjon of the icience of electro-magnetism. 1807 M. Arago discovered that thi* fleofrical current po sesses the power of imparting magnetism to atcfcl and iroii ; anU M. Ampere anKo-anrcu m;: iitsaig t.r - magnetism is induced by 'Jocular currenti* aromi'l the magnetized bod/. VOLTAIC PILES. 173 ! more impor- greeable and lat when the of dissimilar ontact, a pe- He was not :h electricity, at the metals ng motion in ;s the nerves gna, discover- frogs' legs by vcred the fact ced by mere vention of his which are the it used for the Volta did not ile" or <' Con- year 1800. I, decomposed agency of the jg potassa and etal potassuini. xidcs, and thus Ifd attention to id voltaic ««lec- jf llie icience of mi current p [netjsm to steel . his ilmoij *bat currenttf aroiin'l 1830 1831 1837 M. Ampere suggested the application of defle. .ed needles for telegraphic purposes. Faraday laid the foundations of the science of mag- neto-electricity. M. Alexander, of Edinburgh, exhibited in London an electric telegraph on Ampere's principle, but his instrument was so complicated as to be useless, a separate needle and insulated wite being remiired for each letter of the alphabet. [Since 183 1 between two and three hundred patents have been granted for different methods of telegraphic correspond- ence, nearly all depending upon electricity.] LECTURE XXXIII, ARRANGEMENTS EMPLOYED FOR THE PRODUCTION OF DYNAMIC ELECTRICITY. VOLTAIC PTLES. 433. The arrangements employed for the development of dynamical electricity are of two kinds, viz. : I. Voltaic Piles : and II. Voltaic Batteries. 434. Voltaic or Galvanic Piles consist, iissentially, of an ar- rangement of metallic discs separated by discs of paper or of cloth. The two metals commonly used are either silver and zinc, copper and zinc, or zinc and binoxide of manganegi-, and the pile is composed of an alternation of several hundred of these discs bound together by screws, or packed in a glass tube. VoltaiQ piles are either : I. Moist riles: or n Dry Piles. 4iB. The MomT pile, originally invented by VoitA, h formed by J.rHlng dlacs of nilver, zinc, and cloth or p*per, about one fc/.d ^ half mv.\m in diameter, and alternating them, always obser/ing i|lg 05.fip|.^./ij..j. /'loth, silver, /inc. cloth, silver, — so as to have ilic bottom plate zinc and the top one silver, The outaiae disc* V\ 174 VOLTAIC BATTERIES. are each supplied with a conducting wire ; and the intermediate cloth or paper discs are kept moist by brine or weakly acidu- lated water. Note.— The paper or cloth discs are made eoniewhat emaller than tbo others, and the whole arrangement is placed between throe or lour glass rods and hold together by binding screws. 430.. De Luc's dry pile is formed by soaking sheets of thick writing paper in milk or honey, or other analogous animal fluid, and then gumming on one side of it a sheet of tin-foil or zinc- foil, and a coating of black oxide of manganese on the other. • The sheets are now cut into discs about the size of shilling pieces, and these are packed in a glass tube so that all the zinc or tin-foils face the same way. Two or three hundred, or even thousands, are placed in the same tube and are pressed together by metallic caps and screws. NoTK.— One end of the pile is positive and the other negative, and this disturbance is exhibited for a great length of time ; but, if the paper discs are artificially dried before being packed, no electrical excitement is pro- duced, thus proving that here, as elsewhere, the evolution of the fluid is due to chemical action. 437. Zamboni's dry pile consists of several thousand discs of metallic paper, having one side coated with zinc and the other with gold-leaf, packed in a tube as in the case of De Luc's dry pile. BATTERIES. 438. A Voltaic Battery properly consists of a combination of two or more simple voltaic circles, but the term is also ap- plied to the simple circle itself when capable of producing any considerable effects. 439. The essential elements of a simple voltaic circle are : L An elementary body (zinc) and a compound body (dilute acid), which act chemically upon one another in such a mannner that the elementary substance (zinc) is substi- tuted for a constitutcnt (hydrogen) of the compound, whi'^.h constituent is expelled ; and « IL A conducting substance (platinum, copper, silver, charcoal, &c.), which is nqt chemically acted upon, but merely fur- nishes a roule for the passage of the electrical fluid?, to recombinc with one another continually. SINGLE FLUID BATTERIES. 175 intermediate ?akly acidu- »ller than tlu- I or lour glass ets of thick animal fluid, foil or zinc- n the other. of shilling all the zinc Ired, or even ;sed together ative, and this the paper discs itemeut is pro- of tho fluid is sand discs of nd the other De Luc's dry combination a is also ap- roducing any circle are : body (dilute ler in such a nc) is substi- le compound, ver, charcoal, ut merely fur- •ical fluid?, lo 440. Voltaic batteries may, for the most part, be divided into the following classes j viz. : I. Those formed of two different metals and one fluid. II. Those formed of two different metals and two dissimilar liquids. III. Those formed of two different fluids and one metal. SINGLE-FLUID BATTERIES. 441. The principal single-fluid batteries arc the following : I. Cruickshank's trough. II. Wollaston's battery. III. (Ersted's trough. IV. Hare's spiral or calorimotor. V, The sulphate of copper battery. VI. Smee's battery. 442. Cruickshank's Though, the first improvement on Volta's couronnc des tasses, consists of zinc and copper plates cemented water-tight into grooves in the sides of a porcelain trough so as to be parallel to one another and a short distance apart. ' IS'oTK. — One of the chief objections to this battery is the tediousness of filling the separate cells with the exciting liquid. It was impioved by Davy and Nicholson, who soldered the metallic i)latcs to a rod of metal so as to immerse tlie arrangement with ease. 443. Wollaston's Battery resembles Cruickshank's as \\\\- ])roved by Davy and Nicholson, the essential difference being :hat each zinc plate is placed between two copper [dates. The active «Burface is thus doubled, and the battery rendered m;>re effective in the same proportion. Note.— The battery oin])loy d by Davy in his inimoi-tal discovery of tho metals of the alkalies (Octob'-r, 1807' \v;\? of this kind, and was made in 1808. It consisted of a combiuatK'ii of l\ plates twelve inches square, 100 plates six inches, and bJO plates four ihches, the whole being equivalent to 274 plates four inches s(]uare. The celebrated battery of the London Koyal Institution, nmde iu 1810, was also of this construction, and consisted of 2000 couples arranged in 200 giass troughs, each trougii cnniainiug 10 couples, and each pjato having an elfective surface of 22 square inches r 176 BINGLE-FLUID BATTERIES. 444 (Ersted's Trough consists of copper compartments, which 'contain the exciting fluid, and in winch the zinc is placed so as not to touch the other metal in any part. 445 Hare's Spiral, or C alobimotor, is formed by rolling zinc and copper sheets into a spiral or coil, so that the plates are everywhere about half an inch asunder. Several of these arrangements are placed in cells containing dilute acid. NOTK -The original calorimotor of Dr. Hare consisted of 20 zinc and 80 copper plates each 19 inches square, rolled into a coil and combined ni a box L such a manner as to form but two large elements of 50 square feet each or^ square feet of active surface in both members. Iheenor- mou; bltt^y of Mr. Children was also of this construction, being formed of 16 pairs of plates, eachC feet wide and 23 ieet long. Pepys' spiral at the London Institution was formed of sheets of copper and zfnc each 60 feet long .ad 2 feet wide. These were coiled upon one Itrwm' horse-hair r'opes between them. Each bucket contamed 5o gallons of the exciting fluid. 446 The Sulphate of Copper Battery consists of an outer and an inner cylinder of copper, the intermediate space containing a solution of sulphate of copper in very dilute sulphuric acid. A zinc cylinder is plunged bctweea.the coppers into this excit- ing liquid, rtnd is kept from contact with the copper by wooden rings. 447. Smee's Battery is the most efficient of all single fluid batteries. It consists of two plates of zinc, c, z, clamped to a piece of wood w by means of the clamp b. Be- tween the zinc plates there is a plate of platinum foil, p, or of platinized silver, the exciting fluid is formed by diluting sul- phuric acid with from seven to sixteen times its bulk of water. two-fluid r.ATTEUlES. 44S. The principal two-fluid batteries in- volving the use of two dissimilar metals or iheir ■substitutes are : Fig. 53. TWO-FLUID BATTERIES. 177 mpartments, nc is placed r rolling zinc e plates are ral of these icid. : 20 zinc and 30 combined in a ■ 50 square feet ers. The enor- , being formed leets of copper oiled upon one Bt contained 55 if an outer anil ce containing ilpliuric acid. to this excit- )er by wooden Fig. 53. Fig. 54. I. DanicU's Constant Battery. II. Grove's Nitric Acid Battery. HI. Bunsen's Carbon Battery. 449. Daniell's Constant Battery consists of a copper vessel, C, Fig. 54, in which is placed a porous earth- enware cell, p, containing a cylinder of zinc. The copper vessel is filled with a^ saturated solution of sulphate of copper, and the earthenware cell with dilute sulphuric acid. Finally, as in all other compound bat- teries, the zincs and coppers are alternately connected by copper wires. Note.— In order to keep the solution in the outer vessel saturated, some crystals of sulphate of copper are placed on a perforated shelf at the top of the jar— extending from the porous cell to the copper vessel. This is necessary, because as the zinc oxi- dizes in the inner cell, the sulphate of copper in the outer cell is decomposed. 450. This battery is called the constant battery on account of the permanence of its action, and this permanence is to be accounted for as follows : In all other forms of voltaic battery the particles of hydrogen, and ultimately those of oxide of zinc, as they are liberated or formed, are deposited on the copper, and thus mar its conducting capacity : the action of the battery becoming weaker and weaker, from this cause, till it finally ceases entirely. In Daniell's battery this is obviated as follows : The hydrogen of the decomposed water is not given off in bubbles at the copper surface, as in other batteries ; but the sulphate of copper in the outer cell being decomposed atom for atom with the decomposed water, the hydrogen takes the oxygen of the oxide of copper, and metallic copper is deposited on the inner surface of the outer cell. Thus the metallic copper surfiice is constantly renewed ; and if the zinc be well amalgamated, and means be taken for renewing the strength of the acid solution, the battery remains in unimpaired action for many hours. 45l. ijSOVES iJATTifiKi cousiots of a cylinder oi Zinc open both ends, and containing a porous earthenware cup, in which 178 TWO-FLUlD BATTERIES. " !■ ■■ ' . r'^!' -j a is immersed a slip of platinum foil. The arrangement is plunged in an outer glass or porcelain vessel containing dilute sulphuric acid. The porous earthenware cup is filled with s4rong nitric acid, and the zinc and platinum elements are properly connected bv conducting wires. 452. Guove's Batteuy is the most powerful and energetic voltaic arrangement in use, the platinum in it being estimated to be equivalent to 18 times as much copper surface in Daniell's battery. Four cells with platinum foil 3 inches long and half- an inch wide decompose water rapidly ; and an arrangement of from 20 to 50 such cells forms a battery of amazing power. 453. In Grove's Battery tliere is a double decomposition, and consequentlv an increased evolution of electricity. The water is decomposed, as in other batteries ; but the hydrogen, in place of being evolved, decomposes the nitric acid in the porous cell, and combines with part of its oxygen. Copious fumes of peroxide (,f nitrogen are given off, and so vitiate the surrounding air as to render i't important to keep the battery while in action in a good draught of air. A part of the peroxide of nitrogen being ab- sorbed by the nitric acid, colors it green. .454, BrsMRNS Carbon Battery is the same in principle as Grove's nitric acid battery, the same fluids and porous cups being used in each. The essential difference is that Bunsen sub- stitutes a cylinder of baked coke in the porous cup in place of a slip of platinum foil. BATTERIES OF ONE METAL AND TWO FLUIDS. 455. The third class of batteries, or that comprehending the arrangement in which two dissimilar fluids and but one metal are used, includes : I. Becquerel's Battery. II. Reinsch's Charcoal Battery. III. Grove's Gas Battery. 450. Becquerel's Battery consists of a U-shaped tube of gl«ge^ one arm. of which contains a solution of caustic potash or uny other powerful base, and the other arm sulphuric acid or any other strong acid,— the two fluids being separated by a plug TWO-FLUID BATTERIES. 179 it is plunged te sulphuric trong nitric [y connected id energetic g estimated in Daniell's ig and half- angement of power. )osition, and The water ^en, in place porous cell, s of peroxide ing air as to ion in a good 'n being ab- principle as porous cups Bunsen 3ub- p in place of s. jhending the it one metal iped tube of austic potash huric acid or ted by a plug of clay or some other porous substance. A strip of platinum having a wire attached is immersed in each arm, and upon bring- ing these wires into contact an electrical current passes. 457. Rbinsch's Charcoal Battery is formed of a glass jar containing a porous earthenware cell— both the outer vessel and the inner cell being filled with coarsely powdered charcoal. The charcoal in the outer vessel is saturated with sulphuric acid, and that in the porous cell with nitric acid. Finally a rod of coke or charcoal, having a wire attached, is placed in each com- partment. 458. Grove's Gas Battery consists of a glass tube, JC, having a series of legs attached alf' right angles to it, and a series of glass jars Z B, each, except Z, having fixed in it two platinum plates, one long and narrow and the other shorter and wider. The wide plate of each cell is placed higher than the narrow one, and is connected to the narrow plate of the next cell by a platinum wire. The glasses are filled to the top of the narrow Fig. 55. • c plates with acidulated water. In the vessel Z is a piece of zinc supported on a little tripod, and surrounded by dilute sulphuric acid. The stopper being removed from the tube, J C, the legs are immersed in the cells so that each narrow plati- num plate may be inclosed in a leg, the wide ones being ex- cluded and exposed to the air . the hydrogen evolved in the 1 ^ _ .11 ._•__ -„ J £11 art ^^r^aAMttrf tVio fttmnsnheric air. VeSSci £, "wiii ri3C aliU lUl ./JV, cApvimtg v.... 1- The glass stopper is then to be inserted into f, and ihe genera- tion of b vdroi^en will continue until the piece of zino bec(^mes I 180 TWO-FLUID BATTERIES. I uncovered with acid : then the narrow slips of platinum will be exposed to an atmosphere of hydrogen in the legs of the tubf, the wide ones being exposed to the oxygen of the air. A current of electricity will thus be generated, the wire connected with the narrow plate conveying negative, and that connected with the wide plate positive electricity. 459. Although the electrometers^ as they are termed, are commonly zinc, copper, and dilute sulphuric acid, or zinc, platinum, and dilute sulphuric acid, many other substances may be employed for the production of a current of electricity. 460. In the following table the 'iiost positive metals are placed first in the list, and the least positive or those less oxidi- zible last. The generating plate of the battery is, therefore, made of one of the plates first in the list, and the conducting plate of one of the last ; and the more remote the two metals stand from each other in the scale, the more decidedly will the electrical current be produced. TABLE OP TENSION. Positive. 1. Zinc. 2. Lead. 3. Cadmium. 4. Tin. 5. Antimony. 6. Bismuth. 7. Iron. 8. Brass. 9. Copper. 10. Silver. 11. Gold. 12. Platinum. 13. Graphite. 14. Charcoal. Negative, Note.— A secoad zinc plate of the same kind as the first can never act as a conducting plate, because it generates a current opposed in direction to the first. But if the geneiating plate be of rough cast zinc, a plate ot rolled zinc, made perfectly smooth,will act as a conducting plate, and cause the evolution of a weak current. n1 \» employed is cither dilute sulphuric acid or hydrochloric acid. Nitric acid la objectionable on account of the production of lum will be af the tubt', A current lected with nected with :ermed, are id, or zinc, stances may icity. metals are e less oxidi- s, therefore, conducting ! two metals ;dly will the CONDUCTORS OF ELECT^RICITY. i8t can never act )d in direction zinc, a plate ol (late, and cause ^chloric acid, roduction of fumes of peroxide. The acid may be used of any degree of strength, but usually, if sulphviric acid, it is diluted with from 8 to 16 times id weight of wu er* ■ -/ LECTURE XXXIV. CONDUCTORS AND NON-CONDUCTORS, OHM'S FOR- MULAE FOR COMPARATIVE STRENGTH OF VOLTAIC CURRENTS, MEANS OF INCREASING THE INTENSITY OR THE QUANTITY OF THE FLUID IN THE CURRENT. CONDUCTORS. 462. We have seen that bodies may be divided into the two classes of conductors and Ron-couductors of statical electricity, and the same division holds with respect to voltaic — the same bodies being good or imperfect conductors of both. Faraday has shown that it is a general law that the so-called solid non- conductors "assume the conducting power during liquefaction and lose it during congelation." . Thus, water ia a tolerably good conductor, ice is a very imperfect conduc- tor; liquid chloride of lead is a good conductor, solid chloride of lead a very imperfect conductor. 463. As a class the metals are excellent conductors, out they differ among themselves very much in this respect — copper being among the best and potassium among the worst metallic conductors. In the following table the conducting power of copper is expressed by 100. TABLE OF CONDUCTING POWERS. Copper. 100 Gold 93 Silver 73 Zinc 28 Platinum 16 Iron 15 Lead 8 Mercury .,». 3 Potassium 1 464. Since all metals are more or less imperfect conductws, it follows that : IMAGE EVALUATION TEST TARGET (MT-3) ./o % ^ /.^ #^^ 1.0 fM IIIIIM = .r 111 oo I.I ■^ III 6' 1.8 1.25 11.4 ill 1.6 V] <^ /a 7: 'c3 % ■s'f o^: W Photographic Sciences Corporation 23 WEST MAIN STREET WEBSTER, N.Y. 14580 (716) 872-4503 182 ohm's formulae op resistance. II : * • :■■! 'it 154' I. The resistance oflFered by a metallic conducting wire in- creases with its length. II. The resistance afforded by a metallic conducting wire decreases as its sectional area increases, and hence III. The resistance of a conductor, of the same substance throughout, varies directly as its length, and inversely as its sectional area, or / roc — s where r = resistance, / = length, and s = sectional area of the conducting wire. Note,— From this it appears if we have a conducting wire of uniform tliickness— doubling its length halves the quantity conducted, &c. On the contrary, if the length remains unchanged, doubling the sectional area doubles the quantity conducted, &c. 465. If Z s c respectively represent the length, the sectional area, and the conducting power of a wire of any given metal, and /' s' c' the same elements of a wire of a different metal : their resisting effects will be equal when : I I' sc s'c' '■ 466. From the expression in the last article we may easily determine the reduced length of any wire, i. e. the length of a wire of any substance which shall produce the same retarding effects in a given length of a given wire. For since = — — - sc sc l'sc=lsY orl' = ycr of cells is >rtion to the all, and the !" colls in the Ell resistance T of couples, J is yery OHM S FORMULiE. 183 gmall unless n is also very great ; for if / is very great com- pared with n, 1_ is v^ry giyjat, and therefore E is very small. n — M Hence it is necessary to employ a very large number of cells in a compound battery when a considerable amount of resistance is to be overcome, as in the electrolysis of imperfect conductors, the production of the voltaic arch, the transmission of a mes- sage through a long telegraphic circuit, &c. 473. In connecting the metallic elements of the cells toge- ther we commonly unite the zinc of one cell with the copper of the next, and so on. By this arrangement, we form a compound battery of many cells. If on the contrary we take a number of cells and connect all the zincs together, and also all the coppers together, the arrangement will constitute but a simple circle, and will be, in effect, equal to that produced by two plates each possessing a surface equal to the aggregate surface of the seve- ral plates of the coresponding metal. In this case if there be m cells the internal resistance will necessarily be only the m part of what it would be in|a com- pound battery of the same numbei of cells. Hence in vi cells having all the zincs connected together and also all the coppers, E Iqc i+L, Hence if I be very great compared with r the intensity will be but little increased by enlarging the size of the plates. NoTB — If I he very small in proportion to m the intensity is much increased by tliis method, and it l = o or be infinitely small / OC ^»^> i. e. r the intensity will be proportional to the extent of surface acting as a single couple. 474. Since the quantity of electricity developed is in direct proportion to the amount of metallic surface chemically acted upon, we have the quantity of electric fluid that passes in a unit of time through any part of the circuit equal to the electromor yive force divided by the sum of all the resistances, \, e.. E qOC — — 186 ohm's formula. and henco in the case referred to in Art. 4Tl, E q oC l+r n and in that alluded to in Art. 473, yoc E When the cxtenml resistance / becomes so small as to be ne- glected these formulic severally become E mE yoc - and q OC r -y It hence ajjpears that : I. Increasing the number of cells in a battery, when the external resistance is not great, does not increase the quantity of electricity in current. n. When the external resistance is not great, increasing the size of the plates increases in the same proportion the quantity of fluid in the current. 475. If, at the same time, we increase both the number of cells, and also the size of the plates, , E i or (7 OC 1 — I r n* in From which we learn that : ]J\ I, the external resistance, is great, as in ike electrolysis of im- perfectly conducting fluids, it is most advantageous to unite many cells into a compound battery ; hut if the external resistance he small, as in electro-magnetic experiments, greater advantage may he obtained by uniting all the zincs together and all the copper^ together so as to form a couple of large extent of surface. s to be nc- , when the he quantity reasing tlio le quantily number of ylysis of im- unite many ice be small, ge may be the copper^ c. EPrECTS OF VOLTAIC CURRENT. 187 LECTURE XXXV. EFFECTS OF THE VOLTAIC CURRENT, HEATI ?G EFFECTS, LUMINOUS EFFECTS, PHYSIOLOGICAL EFFECTS, CHEMICAL EFFECTS, ELECTROTYPE PROCESS, THEORIES AS REGARDS VOLTAIC ELEC- TRICITY. CALORIFIC EFFECTS. 476. Heat ia evolved whenever a current of electricity is sent through an imperfectly conducting body ; and, since the intensity of the heat developed is greatest when large plates are employed, we may infer that the calorific effects of the battery are due rather to the quantity than to the intensity of the current. 477. The heat of the current has been carefully measured by Becquerel and others, by means of a close spiral inclosed in a glass calorimeter. It has thus been determined that when a cur- rent traverses a homogeneous wire the heat evolved in a unit of time is proportional. I. To the resisting power of the metal forming the wire. II. To the square of the quantity which passes in the current. Thus, if we link together platinum, silver, iron, and copper, and pass a current of electricity through them, the platinum, being the worst conduc- tor, becomes most heeted, and the copper least. 478. "When a fine wire of platinum is made to connect the poles of a battery, it becomes, if sufficiently short, incandescent and finally melts. If such a wire be immersed in water con- tained in any small vessel it causes it to boil, if in alcohol or ether it ignites it, or if we carry it through phosphorus or o-uu- powdcr it inflames it. Note.— An arrangement of this kind has been employed for blasting in mines, and in submarine blasting, In England it has been exten- sivcly adopted for blasting the chalk cliffs. A number of holes are bored and filled with powder, each havinga strip of platinum wire placed in it. These platinum wires are connected to one another by means of copper wire, and the whole properly connected with the battery. The in- stant the battery is set in action the pieces of platinum become sufficiently hot to explode, at the same moment, the gunpowder in every hole. v.- -1 ''.' M 188 LUMINOUS EFFECTS 479. The principal experiments illustrative of the calorific 11 agency of the battery are the following : I. When the positive electrode is formed of carbon, and is fash- ioned into a small crucible, gold, silver, platinum and other substances arc rapidly melted, deflagrated and volatilized. Under these circumstances it is found that gold burns with a blue light ; silica with a fine green ; sodium, yellow ; potassium, violet; strontium, red; calcium, violet-red; barium, reddish-yellow, &c. 11. Sapphire, quartz, lime, slate, &c., are readily fused, and the earths reduced to their metallic bases. III. A piece of diamond placed in the charcoal cup. when the other pole is brought over it so as to bring the voltaic arch over the gem, melts, boils up, and presently spreading bpen is converted into coke, thus showing that diamond and coke or charcoal are but different modifications of one and the same body. LUMINOUS EFFECTS. 480. The luminous effects of the battery follow directly from its heating power, for all solid bodies become incandescent when heated to about 1000° F. (Draper.) 481. The luminous efi'ects of the battery are seen on a small scale whenever the circuit, even of a very weak arrangement, is closed or opened. In using the very powerful battery of the Royal Institution (2000 couples), Sir H. Davy discovered, in 1809, that when charcoal points are attached to the poles they may be separated to the distance of two or three inches, and that the intermediate space becomes occupied by an ovoid of light of the most dazzling brilliancy. To the light thus produced he gave the name voltaic arch. 482. Although other substances may be employed as the terminal poles of the battery for developing the voltaic arch- graphite or well-burned charcoal is by far the most effeptiyc. The arch may be produced equally well in atmospheric air, a vacuum, in nitrogen, in carbonic acid, under water, &c , and therefore cannot be connected with the conabustion of the char- coal. Of VOLTAId CURREN*. I8d le calorific tnd is fash- 1 and other volatilized, burns witli n, yellow ; violet-red ; ed, and the . when the the voltaic y spreading it diamond fications of rectly from iscentwhen on a small igement, is tery of the covered, in poles they ,es, and that I of light of iced he gave lyed as the taic arch— 3t effeptiyc. heric air, a r, &c , and of the char- 483. When charcoal or other points are employed, it is always found that the charcoal on the positive electrode becomes cup- ped or hollowed out, while that on the negative electrode be- comes elongated by a small cone which exactly fits into the opposite depression. It is hence inferred that the electrical light is due to the transference of highly incandescent particles from the positive to the negative electrode. The rushing and hissing noise which accompanies the voltaic arch is due to this mechanical removal and passage of the particles of carbon or other body forming the points. 484. In order to develop the arch under water or in air, or in any other gas, it is necessary to first approach the points into contact, and then gradually remove them to the maximum dis- tance (two, three, or even four inches). This appears to be ow- ing to the fact that the energy or intensity of the current is not sufficiently powerful to enable it to penetrate the intervening stratum of air or other non-conductor ; but when the points are brought into contact and then separated, the projection of mate rial atoms commences, and the flow of the current is estab- lished. NoTB.-Herschel has shown that when the points are approached within an inch or two of one another, and a charge from a Leyden jar transmitted through them, it at once determines the formation of the voltaic arch- doubtless by commencing the transportation of atoms. Note 2.- In a vacuum it is not necessary to approach the points into actual contact in order to commence the flow. This is owing to the fact that a vacuum does not offer so much resistance to the passage of the elec- tric fluid. The voltaic arch is extinguished by a strong wind. 485. The electric light is, like solar light, unpolarised. It explodes a mixture of hydrogen and chlorine, and acts upon chloride of silver and other photographic agents like the sun. Like the solar light it imparts phosphorescence to the diamond, fluor-spar, and other bodies. Fraunhofer has shown that the spectrum formed by electric light differs from the solar spectrum in having a very bright line in the green and another rather bright one in the red. Note— By means of an arrangement called "Duboscq's photo-electric lantern " the electric light may be used to replace the sun in all experi- ments requiring simply a strong white light. It is preferred to solar light 190 CHEMICAL EPF4:CTa for taking microscopic photographs. Fizcau and Foucault havo, in co!ri'< paring tho photographic power of tho voltaic arch witli tlmt of tlio nun, found that a double Borios of 92 carbon couples gave an eflbct 3 of that pro- duced by the sun two hours above tho moridiuu on a clear August day. In tho same series of experiments they found that tho Drummond light is only equal in effect to j^^ that of tlie sun. liunsen found the voltaic arch produced by 48 carbon couples equal in intensity to that ol'572 candles. PHYSIOLOGICAL EFFECTS. 486. A shock of voltaic electricity transmitted through the animal frame produces effects which differ from those resulting from machine electricity in being less violent and sudden but more continued, and accompanied by a peculiar sensation of prickly heat on the surface. Note 1.— It requires a battery containing a large number of elements to exhibit marked physiological effects. The shock from a battery of 300 or more couples is capable of producing dangerous or even fatal results. When the shock is sent through a chain of many persons their hands should be moistened with salt and water, and even then the effects are sen- sibly less in the centre than near the poles. Notb2.— A voltaic current sent through the ear produces a roaring sound; thrown upon the tongue it gives rise to a metallic taste; trans- mitted through the eye it is accompanied by a flash of light. Note 3. — In all cases tho peculiar physiological effects are only experienced when the contact is in the act of being broken or renewed. CHEMICAL EFFECTS. 487. In describing the chemical effects of the voltaic battery it has become customary to employ certain technical terms that were first suggested by Faraday. These terms, with their deri- vations and accepted significations, are as follows : I. Electrode {electron, "electricity," and hodos, "a way,") corresponds to the old term pole or terminal wire. The negative electrode is the way or direction in which the current enters the battery, the positive electrode, the way or direction in which it leaves the battery. II. Anode (ana " upwards," " as the sun rises," and hodos) is the side of a substance or decomposing cell by which the current enters. III. Cathod (kata " downwards," " as the sun sets," and hodos, IS the side of the body or decomposing cell by which the current flows out. OF VOLTAIC) CtJRRENT. I9I NoTB.— Let a person suppose himself to be standing with hia fhco towards tlio north, l^onting a battery placed on the ground with its positive end to the east, and tlio wires bent in the form of an arch in tlie direction whicii the sun takes in his daily motion ; hit liim f\irtlier Huppose that tlio terminal wires instead of Joining, pass into a decomposing cell ; then the current will pass out at the eastern side, will flow over to the west end, and again enter the battery. The positive electrode is the eastern or right-hand side of the battery, and of course the western or left-hand side is the negative electrode. The anoile is tho eastern or receiving side of the decomposing cell, and the western side is tho cathode. IV. Electrolysis (electron " electricity," and luo " I loosen,") is the term applied to tho chemical decomposition of a body by means of electrical agency. V. Electrolytes (same derivation) are bodies susceptible of decomposition by electricity. VI. Ions (ion neuter of einii " to go,') are the chemical con- stituents of the electrolyte. VII. Anions (ana and ion) are the ions that go to the anode, and consequently correspond to what are otherwise called electro- negative bodies. VIII. Cations (kata and ion) are the ions that go to the cathod, and hence correspond to electro-positive bodies. 488. No electrical current, or only a very weak one, can pass through a compound fluid without effecting, to a certain extent, its decomposition. One of tho simplest and most striking examples of this decomposing power of the electrical current is seen in the electrolysis of water. The ap- paratus commonly employed to exhibit this analysis. Fig. 56. is exhibited in Fig. 56, which consists of two tubes filled with water and inverted in a glass jar. The terminal wires of the battery (marked + an^ — , in the figure) are attached to two platinum plates, one in the lower end of each of the tubes. The water is slightly acidulated with sulphuric acid in order to render it a better conductor. Upon transmitting the current from a battery of several cells through this arrangement.bubbles of hydrogen gas are evolved at the negative pole, and of oxygen at the positive pole, and the gases are collected in the tubes by displacing the water. After continuing the experiment for a short time it will be seen that twice as much gas is evolved at the — electrode as at the + electrode, hence proving that water is a compound ol tv/o voh umes hydrogen and one volume oxygen. 192 CHEMICAL EFFECTS 480. The electrolysis of salts may bo exhibited by Blling a U-shaped tube with a solution of the salt, colored purple by the infusion of red cabbage, and then immersing in each end of it a platinum plate connected with the poles of a battery. When the connection is complete and the battery set in action the solution becomes red at the positive electrode from the liberation of the acid, and green at the negative from the alkali set free. If a haloid salt (see Chem. Art. 288) be employed, the chlorine or other halogen appears at the positive pole, and bleaches or discharges the color of the cabbage infusion, and the metal appears as a basic oxide at the negative pole producing the same green tint as before. Note 1.— On account of oppositely olectriflod bodleB attracting ono an- other, the constituent wliicli goes to tlio positive pole is called the electro- negative body, and vice. vera&. NOTB 2.— Bodies differ very much in the degree of facility With which they suffer decomposition by the agency of electricity. Thus : Iodide of Potassium in solution \ Fused Chloride of Silver | are very eisily decomposed. Fused Proto-chlorido of Tin * Chloride of Lead fused Iodide of Lead fused Hydrochloric acid Dilute Sulphuric Acid Note 3. —No body is electrolyzable except when fluid, i. e. either dissolved or melted. The same element or constituent is not invariably electro- negative or electro-positive. Thus, hydrogen is highly electro-positive when compared with oxygen or chlorine, but it is uniformly electro-negative in connection with the metals. Oxygen however is electro-negative in every combination, and potassium electro-positive. 490. The principal laws of electrolysis are known under the title of Faraday's Laws of Definite Action. They are as follows : I. The quantity of a given electrolyte, resolvable into its ions by electrolysis, depends solely upon the quantity of the electric fluid which passes into it — being quite independent of the form of apparatus used, the dimen- sions of the electrodes, the strength of the solution, &c. II. In every case of electrolysis the ions are sepaiated in atOEiic proportions, and v/hen the current is made to are electrolyzed with more difficulty. OF VOLTAIC CURRENT. 193 traverse several electrolytes in succession in the same circuit the whole series of ions are set free in atomic proportions to each other. III. The oxidation of an atom of zinc in the battery generates exactly as much electricity as is required to decompose one atom of water, or of any other electrolyzable pro- toxide. 491. The principal sequences or corollaries to this theory are : . 1st. The source of voltaic electricity is chemical action ex- tI clusively. 2nd. The forces termed chemical affinity and electricity, if not absolutely one and the same, are at least very intimately related to one another. 492. The Voltameter is an instrument designed to measure the quantity of electricity evolved by any arrangement. It con- sists of a graduated tube into which the combined gases set free as in Pig. 56 are collected and measured, and depends upon the principle above stated, viz : that the quantity of electrical fluid that passes into an electrolyte determines the amount of chemical decomposition. 493. The Electro-Chemical Theory of Sir H. Davy assumes that every body has a kind of natural appetency for the assump- tion of either the positive or the negative electrical fluid, and that bodies thus naturally possessed of these opposite kinds of elec- tricity attract one another so as to unite and form a compound. It thus reduces chemical affinity to a mere case of electrical attraction, and of course regards all compounds as binary in their nature. electrotype process, 494. One of the most interesting and important applications of the decomposing power of the voltaic circuit is seen in the process of tlectro-metallurgy, or the precipitation of metals from their salts in solution, 194 THEORIES OF VOLTAISM. m 495. The most simple form of electrotype ap- Fig. 57. paratus is shown in Fig. 57. In a glass jar, as for example a tumbler, is placed a satu- rated solution of a metallic salt, as sulphate of copper. Within this is a porous earthenr^v^are cylinder P (or a lamp-glass with a piece of bladder tied firmly over the lower end) filled with dilute sulphuric acid. The meijal to be copied, or the substance to be coated with the deposited metal, is suspended in the metallic solution by a wire attached to a zinc plate which is immersed in the dilute acid ' '^he inner vessel. The whole thus forms a C( , \d cell resembling that of Daniell. When the arrange- ment jmplete, the chemical action excited on the zinc disen- gages electricity, which passes over the wire to the suspended medal, and thence escaping into the metallic solution decom- poses it. The precipitated metal is deposited upon the surface of the metal, and of course (Art. 490) for every equivalent of zinc dissolved, an equivalent of copper, or other metal used, is deposited, or for every 32-5 grains of zinc combined 32 grains of copper are precipitated. Note 1.— In practice, when medals or engraved plates are to be e»pied, revorsed casts of the objects to be copied are made in wax or fusible metal, and these are subjected to the electrotype process ; the back being pro- tected by varnish to prevent the deposition and consequent adhesion of the metal to it. Note 2.— In the electro-plating process by silver or gold, the metallic solution employed is alkaline instead of acid. Even alloys, as brass, bronze, German silver, &c., may bo precipitated from solution by the process of electro-metallurgy. THEORIES OF VOLTAISM. 496. With regard to the theory of the voltaic battery three views have been advanced and are known respectively as : I. Volta's Contact Theory. II. The Chemical Theory. III. The Molecular Theory. 497. Volta's Contact Theory attributes the effects of the MAGNETISM. 195 battery to the simple contact of unlike metals— each decomposing the neutral electricity resident in tjie other, so that one becomes positive and the other negative. It assutoes that the chemical action set up merely furnishes conductors for the passage of the unlike electricities from one metal to the other so as to recombine and again form the neutral compound. Note.— The contact theory of Volta is strongly advocated by most of the continental philosophers of Europe,and especially by those of Germany. It seems, however, to bo disproved by the simple fact that Faraday has obtained the evolution of copious currents of Voltaic electricity without the use of dissimilar metals. 498. The Chemical Theory, supported by Fabbroni, Davy, Wollaston, Faraday, Becquerel, De la Rive, &c., supposes that chemical action is the exclusive source of the electric current, and that indeed voltaic excitement and chemical action are the reciprocals of each other. Note.— It appears to be proved by the researches of Faraday and others that chemical action is requisite to the production of the electric current,' and that the energy of the former is in exact proportion to the power of the latter. 499. The Molecular Theory of Peschell takes a sort of middle ground between the contact theory and the chemical theory. It assumes that when electricity is generated in any Voltaic ar- rangement, it results from a molecular change, brought about in the touching bodies by the adhesive force which subsists between them. MAGNETISM. LECTUKE XXXVI. DEFINITIONS, NATURAL AND ARTIFICIAL MAGNETS, PROPERTIES OP A MAGNET, THEORIES OF MAG- NETISM, BIAMAGNETISM. 500. Magnetism is the name applied to that peculiar power of attracting iron, which is possessed by the lodestone. The phenomenon of magnetic attraction was noticed by some of the earliest writers of antiquity-thus Thalcs, Pythagoras, Plito, Aristotle, J'llny, Cicero, Lucretius, and others, make mention of it in their works. 196 MAGNETS. 501. The name magnet or magnetism is by many supposed to be derived from Magnesia, in Asia Minor, in which locality the lodestone was first discovered. Others derive the term, " magnet" from " Magnes" a shepherd who first noticed its attractive force for his iron crook when tending his flocks on Mount Ida- Iipio and Euripides call it the Herculean Stone, because it commands iron, the strongest of the metals. The Jews in their Talmud, call it " the stone which attracts." The Chinese call it thsvrchy, " the love stone," i. e, the stone loving to- wards iron. They also term it " hy-thy-chy," or " the stone which snatches up iron." In the Sanscrit it is named ayaskanta " the stone loving towards iron." The Hungarians call it magnet A« " the love stone." The French term it Vaimant " the loving stone." All the above terms are derived from its attractive force; others are descriptive of its directive power, thus. The Burmese call it d'anamtcWiim or " the stone which shows the south." The Swedes term it segel-sten " the seeing stone." The Icelanders give it the name leiderstein " the leading stone." The English know it as the todesroces3 s6ve- :tremity hav- tucbed. ae two. Wer I pole of the ling them in md re turning 3peat several to the same ignets are tied their opposite d south poles soft iron, and brass, and the the middle ot le bar shall be oagnets. The ir but not over is drawn over irefuUy moved ly having been f the bar, it 13 e saturated with icribed above for a on one side, the B same manner. ictive influence )n hardened by ns more or less magnetizing a t'oiiMAtioM ot^ Ari^rtftotAt. MAaNfel\s. 20:^ steel bar, is by placing: it within the coils of an electro-inag- netic spiral, as in Fig. 58. If the helix is direct (see Art. 555), ^^S- ^^' the north pole of the magnet is next the zinc plate, but with a left handed helix the north pole is next the copper plate. With a spiral of sufficient magnitude^ any steel bar may at once be magnetized to saturation. A bar of the horse-shoe form may be magnetized by winding cop- per wire about it from end to end, and then connecting the extremities of the wire with the zinc and copper plates of a voltaic battery. Note.— Wlien a stool bar is brought to the tempering heat and then placed in an electro-magnetic helix connected with a voltaic battery, and after remaining within the spiral for a few moments, plunged into cold water, or, brine, or oil, or better still a cold solution of yellow prussiate of potassa, it becomes permanently and very powerfully magnetic. 526. The circumstances affecting the value of magnets are chiefly the following, viz : I. The mode of keeping ; II. The form and proportion of its parts ; and III. The natiwe and hardnass of the steel composing it. 527. To preserve the power of a magnet (horse-shoe) it is essen- tial that it should be suspended, having an armature or bar of soft iron connecting its poles ar;.d attached to a weight«iearly or quite as great as the magnet can sustain. Thus kept, the mag* net gains power, but if left unarmed its magnetism is rapidly dissipated. Bar magnets should be arranged in pairs parallel to one another and with reversed poles, and having their extremities connected by means of armatures or transverse bars of soft iron. 528. The dimension of a bar magnet should be in the propor- tion of about 60, 3, and 1, 1. e., its width shou e three times its i Ml' 4 fn ^Q4 »tfiRRESl?AlAti MAGNETISM* thickness and its length about twenty times its width, tti a &oe "agnot the distance between the poles should not be g Iter than L width of one of the poles, the faces should be smooth and even, and the whole surface highly polished. TC..V 1 If a maenet is loaded with tho l\ill weight it is able to support. graduaU; increasing tho load. KnT» 2 Tho Bowor of a compound magnet Is less than the sum of the pot^^'li^rrtohar. Assumlj^th^^^^^^^^^^ rrrs"t:^o3;:::;rrd"h7^r:.'i-„gtoc^^^^^^ The power of a single bar is equal to ^ « two bars combined ^^ ,. _ ,, n 150 " tour ,, If i<4 eight '^ " ^^^ K0TK8.-The power of a magnet islostor ^^f ^^^^^ .[»[^«J^,"« /^^^^^^ • „ ^«.rth of time without its keeper, by allowing it to lall, or gmng [tTnTsuddentar L^^^^^^^^^ l-«"^ '' -"^^' ^^^?;'" tkT '" ir, by suddenly wrenching the keeper or its load Irom .t, &c. LECTURE XXXVIII. ' • TERRESTRIAL MAGNETISM. DEFINITIONS, VARIATIONS, DIP OF THE NEEDLE, &c. DEti'INITIONS. / 829 A magnetic needle suspended so as to move in a horizon- tal plane is called the horizontal needle. The W«' <-/ tkenagneUe TlrUian is a plane perpendicular to the horizon, and pas^n through the north and south poles of the magnetic needle whU nT directive position. The direction of the magnet^e »er«fe« s Istr^ght lin' passing through the poles of the suspended t!di* Th» dMion or variation of the magnet is the angle ™:drbetwe';n the magnetic meridian and the geographical meri- dian of the place in which the magnetic needle is suspended. VARIATION IN DECLINATION. 205 ath. In a uld not be should be ed. » to support, he power of power of a 1 weight and ) aura of the hat coniposo ;h bar ia ftieu Coulomb : 2 6 2 12 llowing it to fallTor giving inary temper- &c. 2EDLE, &c. in a horizon- ' the magnetic and passing needle while letic meridian le suspended ; is the angle ■aphical raeri- Buspended. NoTE.~Tho horizontal noodlo i» loaded at ono extremity so aa to over. conic, in a measure, ita tendency to dip or inoUno towjirds th« north or eouth pole. 530. The horizontal needle docs not point towards the north pole except at isolated localities on the earth's surface. At all other places it is directed to a point more or less east or west of the true north, and at each place the amount of this declination varies from time to time. VARIATION. 531. At each place the declination undergoes periodic changes depending on tlie hour of the day^ and called diurnal variations, by which it increas'cs from its minimum to its maximum value, and returns to its minimum again in the course of 24 hours, thus oscillating about a certain middle value called the mean declina- tion of the day. At Toronto the hours at which the needle occupies its extreme and mean positions are not exactly the same throughout the year, but taking one month with another, it is at its most easterly limit about 8 A.M., it passes through its mean between 10 A.M. and U A.M., reaches its westerly limit about 1 P.M., returns again to its mean position about 10 P.M., and attains its most easterly limit as before at 8 A.M. 532. Tlie amplitude of the diurnal variations or the angle be- tween the greatest eastern and western positions of the needle in the course of the day is about 10' on the average of the year, but it is not the same in all the months, being more than 12' on the average of the six months from April to September inclusive, and less than T on the average for the rest of the year. 533. The declination proper to any hour, as well as the daily mean declination, undergoes periodic changes depending on the time of year, and called annual variations, and it oscillates in the twelve months about a certain middle value called the mean declination of the year relative to each hour and to the mew of all hours. - 534. At Toronto the months in which the needle occupies its extreme positions are not the same for all hours, but taking one hour with another it is in its most easterly position in January, and in its most westerly position in September. . . ,. .^^^^ i :||i 206 VARIATION IN DECLINATION. 586 The amplitude of the annual variations on tlic average of ftll hours is small, amounting to less than 3'. It is generally greater for the hours of the day and less for those of the night, being about 6' for 2 P.M., and leas than 2' for 1 A M. The mean annual declination is not the same in consecutive years, but undergoes changes from year to year, called secular variations. 636. At Toronto the mean declination increased from 1° 29' W. in 1846 to 1° 41' W. in 1851. The westerly movement has been subsequently more rapid, the mean value of the declination being at present (1861) about 2° 20' W. NoTB-The amount of this secular variation and its gradual change in direction may bo seen by the following table of its amount in London between the years 1580 and 1850. The plus sign indicates western doclma- tion, and the minus sign eastern. Years 1580. 1622. 1660. 1692. 1780. 1765. 1818. 1880. PecUnatVon. -^6' -6o .-f +13o +20o +2401V +2^^30 Hfttenervear 7' 8' 10' 11' H' 9' «' 6 Uwm be observed that the yeariy increase or decrease differs-being ffreatest near the minimum of variation. 'Twashington the variation was +0-6o in 1800 and ^^ad mcrea-d t^ +2-9° in 1860. At Buriington, Vermont, the variation was +« 8 m UM, +8°30 in 1830, +9«'7 in 1840, and +10°30 in 1860. 537. As regards other parts of Canada while the periodic vari- ations are in their general character approximately the same pro- bably as at Toronto, the mean value about which the periodic os- cillation takes place,diflFers much in different parts of the Province. Lines of equal declination, i. e. lines through places having the same mean annual declination, are inclined at an angle of about 30O to the west of north, this inclination being greater on the lines that are situated more and moro to the eastwa. . On proceeding eastward the declination becomes more puu mo.t westerly. 538. As before remarked there are certain places on the earth's surface where there is no variation. These places are connected to one anothei" ny .wo lines called Agones or lines of no variation, and distinguislied ■-.* the eastern and western agones. The west- ern agone commences in Hudson's Bay, lat. 60^ runs through Lake Erie about its centre, thence in a southerly direction through Virginia, the Great Antilles, touches Caye St. Roque, and cuts the THEORIES OF TERRESTRIAL MAGNETISM. 207 c average of is generally )f the night, [. consecutive illed secular •oml°29'W. lent has been ! declination radual change int in London CBtern doclina- 1818. I860. 4-24°ll' +22^30 6' 6' differs— being ad increased to } +7-8° in 1790, periodic vari- ■ the same pro- tie periodic os- [" the Province. ces having the angle of about greater on the jastwaiu. On lore aad RiOic J J on the earth's are connected of no variation, nes. Thewest- >, runs through rection through ae, and cuts the meridian of Greenwich at lat. 65° S. The principal eastern agone (for there are two) begins in the White Sea and descending in a great semicircle, southward and eastward, to the Sea of Japan ; it then goes westward and southward through China, and India, to Bombay ; thence southward and eastward to the northern coast of Australia ; and thence directly southward along the meridian 130° E. of Greenwich. Note.— These linos and the isogonal lines seem to indicate the existence of two north and two south magnetic polos of tho earth. Ilansteon has con- cludod, froir a great ntuubor of observations made at long intervals and at places widely separated, that these polos have a regular revolution around tho earth, tuo tvv( northern ones from west to oast in an oblique direction, and tho two southern ones from east to west— thoir periods of rovoluiion being us follows : The strongest north polo, 1740 years. The strongest south pole, 4609 years. The weakest north polo, 860 years. Tho weakest south pole, 1804 years. Hanstcen has also pointed out a curious connection between these periods and tho precession of tho equinoxes. Tho shortest time in which all tho four poles can accomplish a cycle and return to tho same state as at present coincides exactly with the period in which tho procession of the equinoxes amounts to a complete circle at the rate of F in 72 years. 539. The more recent hypothesis with regard to the magnet- ism of the earth, assumes that the crust or surface, and not the interior of the earth, is the seat of magnetic force. The surface of the earth being magnetized, the two fluids are separated, and this separation, though not regular or equal in all latitudes, is most complete at the poles, and least so at the equator. It supposes that this inequality in the degree of separation is due to ''it difterence of temperature, which regulates the coercitive force of the materials of the earth's crust. This view regards the daily and annual variations of temperature and declination as cause and effect, and accounts for the very close relation between the isoclinal lines, and the isothermal lines or lines of equal temperature. 540. IsoGONAL Lines, or lines of equal variation, are lines joining those places on the earth's surface where the declination of the magnetic needle is equal in amount and direction. Tlie 11 208 TERRESTRIAL MAGNETISM. isogonal lines do not by any means coincide with either the meridians or the parallels of latitude, but run in an irregular manner— often crosping one another. 541. Irregular Variations in the magnetic needle are pro- duced by magnetic storms which are simultaneously observed by means of the needle in widely separated countries. They are supposed to be produced by auroras and otb-r natural electrical phenomena. MAGNETIC DIP. 542. If a bar of non-magnetized steel or iron be carefully balanced by its centre it remains horizontal and takes any direc- tion indifferently. Now, if the bar be magnetized, it points towards the north and is no longer horizontal, but one end dips or inclines towards the earth. 543. The dip op the Needle, or, as it is termed, its inclination, is greatest near the poles and decreases towards the equator ; but. the magnetic equator or line of no inclination does not exactly coincide with the terrestrial equator. Of course in the northern magnetic hemisphere the north pole dips, and in the southern, the south pole. 544. Isoclinal Lines, or lines of equal inclination, are those that join such places on the earth's surface as exhibit the same degree dip. Of these the most prominent is the magnetic equator, which in one place departs from the terrestrial equator as much as 20° N., and in another as much as 13o S. Isoclinal lines very nearly, if they do not exactly, coincide with the isothermal lines. 545. IsoDYNAMic Lines, or lines of equal power, are lines joining those localities in which the needle oscillates with equal energy. They are very nearly coincideut with the isoclinal lines. either the n irregular le are pro- Dbserved by They are al electrical ELECTRO-MAGNETISM. 209 ELECTRO- MAGNETISM. 36 carefully }S any direc- d, it points Dne end dips 3 inclination^ equator ; but- not exactly the northern ;he southern, on, are those ibit the same aetic equator, ator as much nal lines very thermal lines. B lines joining equal energy, lines. LECTURE XXXIX. ELECTRO-MAGNETS, GALVANOMETERS, ELECTRO- DYNAMIC HELLICES, ELECTRIC TELEGRAPH, EliECTRO-MAGNRTISM. 546. Electro-Magnetism is that branch of electrical science which treats of the magnetism produced in dia-magnetic bodies during the passage of a current of electricity through them. 547. When a magnetic needle, having freedom of motion around its centre, is brought near a wire of copper, or other con- ducting medium through which a current of electricity is flow- ing, it is instantly deflected and places itself at right angles to the conducting wire, or conjunctive wire ns it is called. From this it appears that the conjunctive wire becomes itself a mag- net, or rather an infinite number of small magnets arranged transversely to the course of the wire, while the current is pass- ing along it. NoTE.-Iron filings adhere to the conjunctive wire while the current i'. traversing it. tn 548. If a current of positive electricity is transmitted from south to north through a conducting wire arranged horizontally in the magnetic meridian, then a free magnetic needle would have its north end deflected to the west when it is placed below the conjunctive wire, and to the east when it is placed above the wire ; the north end is depressed when the needle is placed on the west side, and points upward when it is on the east side. If the cur- rent is sent along the wire from north to south all these move- ments are reversed. Note -In order to impress on the memory the direction in which the needle is thus deflected by the cotijunctive wire, the following formula has been devised by Ampere : ,. ,^ * u Let ami one tdentifn Jivnse(f toith the current or suppose himself to be mmg in "the direction of the positive current so that it enters htsjett ana pass^es out through his head. Then, his face being turned to the needle, f)H' north jwie of the latter is always deflected to the right hand. no ELECTRO^MAQNETISM. GALVANOMETERS. 549 The Galvanometer is an instrument used for measuring the iniensity of the electric current. It was constructed by Schweigger, and depends in its action upon the deviation of a magnetic needle across the line of the electric current. Fig. 59. The principle on which the galvanometer acts may be understood by a refer- ence to the accompanying figure, in wliich the con- junctive wire is bent in the Ibrm of a rectangle within which the needle is sup- ported on a pivot. The r;ter \ttSot or the arro... all parts o. tl.o ..re tend to ncreLthe deflection of the wire in the same direction : and the amount of that deflection depends upon the intensity of the current, being very small for weak currents and approaching 90° for very strong ones. 550 In galvanometers employed in very delicate investiga- tions two improvements are made on the form represented m Fig 59 The first consists in causing the conjunctive wire instead of making only o^e convolution or turn, to bend several hundred times on itself so that the current may act again and again on the needle and render a very feeble force perceptible ; the second consists in using the so called astatic needle in place of a simple needle. 551 The astatic needle is made by fixing two magnetic nee- dles of equal power parallel to one another on the same axis, with their poles reversed. The effect of the arrangement is to neu- tralize the directive power of the earth, which, in the case of a simple needle, interferes with the indications of the galvano- meter, since it acts in opposition to the deflecting power of the current. In the astatic galvanometer one of the needles is placed within the rectangle and the other above it, so that they are both moved in the same direction by the current which circu- lates through the conjunctive wire. ELECTRO-MAGNETISM. 2U leasuring ucted by ition ot a vire tend to the amount , being very mea. investiga- :esented in ictive wire end several again and •erceptible ; die in place ignetic nee- le axis, with it is to neu- the case of he galvano- ower of the les is placed hat they are vhich circu- Pig. 60. This arrangement will become clear by a reference to Fig. 60, which represents the form of the aatatic galvanometer in common use. The astatic needle is suspend- ed, it will be observed, by a fine thread, so as to have perfect freedom of motion. 552. When electric currents flow in two parallel wires which have freedom of motion, the wires are attracted or re- pelled according to the follow- ing law : Parallel currents repel one _ another when their directions are opposite, but attract one another when they are both moving in the same direction. ' 553. A conjunctive wire may be made in all respects to simu- late a magnet. We have seen that it possesses the power of attraction and repulsion, and we have now to remark that it possesses polarity and the power of induction. 554. If a conjunctive wire be coiled into a spiral form, and its ends carried back through its axis as shown in Fig. 61, it con- stitutes what is called an electro-dynamic helix. Now if this be suspended so as to move freely, horizontally, or vertically, it acts precisely as a magnetized needle, i. e., it points north and south, and exhibits the magnetic dip. .Of course it does this only ^ ^^yy ^ j^y y ^y y, while the electric current is passing along it. OUOOOoOOoOfloO 555. It has been already stated that when a copper wire, insulated by being covered with silk, is coiled into a helix and a current of electricity sent through it, a bar of iron placed within the helix becomes powerfully magnetized. This magnet- ism is induced in the bar of iron or steel, by the circular and parallel currents which pass along the several volutes of the Fig. 6V. H'l 212 ELECTRIC TELEQRAPfl. helix, and is temporary in the case of soft iron, but permanent in the case of steel. 5* m If the conjunctive wire which constitutes the helix is coiled to the right, as in a common •ork-screw, it forms what is called a right-handed heHx,&r\d if in the reverse direction, a Iqft-handed helix. In a right-handed helix, the north pole of the bar is that towards the zinc plate of the battery, in a left'handed helix that towards the copper plate. Faraday has given the following rule to enable the student to understand the polarity of the helix. " Imagine that you are looking down upon the dipping needle or the north magnetic pole of the earth, and think upon the direction in which the hands of a watch move, or of the motion of a direct screw, then currents in that direction loould produce such a mag- net as the dipping needle." 556. Electro-magnets are masses of soft iron wound with coils t>f closely packed insulated copper wire, the size and length of which varies according to the power required in the electro- magnet. They have been constructed capable of raising several tons, and from their enormous poVer and the complete and instantaneous paralysis and reversal of that power by reversing the poles of the battery, it has by many been thought possible to apply them economically to the working of machinery. Note.— From a series of experiments made by Hunt, with respect to the' applicability of electro-magnetism as a motive power, it appears that: A grain of coals burnt beneath the boiler of a Cornish engine, lifted 143 lbs. 1 toot high. A grain of zinc consumed in a battery to move an electro-magnetic engine, lifted but 80 lbs. 1 foot high. A cwt. of coal cost ^^• A cwt. of zinc cost 216d. Hence to do an equal amount of work, the electro-magnetic engine is more expensive than the steam engine in the proportion of 50 to 1. -J' ^!' - I ELECTRIC TELEGRAPH. 557. Of all the practical applications of electro-magnetism, by far the most important is the electric telegraph. All the varie- ties of electro-telegraphic communications may be reduced to one or other of two methods, viz : the electro-mechanical and the electro'chemical. ELECTRIC TELEGRAPH. 213 Dermanent otho right, ul heliXftind nded helix, battery, in understand 2 Q 3 D R 4 E- S 5 F T — 6 G U 7 H V 8 I-- W 9 ._ J X 0-—— K Y L Z M & N-- MAGNtlfO-ELtiCl'IlICtTV. 215 3 point s is sn the lever , b, rings to a I are the >n the lever the distant the keeper a ie upon the combination t being com- 559. -Electro-Chemical Telegraphs depend upon the produc- tion of visible and permanent marks or characters by electro- chemical decompositions. In the best form a paper ribbon, saturated in ferrocyanide of potassium or other salt of iron, is carried by a clock-work movement, similar to that of Morse, over a cylinder of metal which constitutes one pole of the circuit. The other pole terminates in a steel or copper pen which is in contact with the paper. The least passage of electricity pro- duces a stain on the moistened paper, which is red if the pen is copper, and blue if the pen is steel. MAGNETO-ELECTRICITY. LECTURE XL. MAGNETO-ELECTRICITY, THERMO-ELECTRICITY, ANIMAL ELECTRICITY. MAGNETO-ELECTRICITY. 560. When a current of electricity is transmitted through the wire of a helix it induces magnetism in a bar of iron or steel placed within it. Now the converse of this is equally true ; when a permanent magnet is alternately thrust into and removed from a helix, it induces currents of electricity in the wire composing the latter, as is easily evidenced by connection with a galvanometer. The peculiarity of these currents is their momentary duration, and hence their name momentary currents, or, from their discoverer. Far adian-cur rents. 561. The Magneto-Electric Machine, Fig. 63, is an instrument by means of which magneto-electricity, induced in the manner alluded to, may be obtained in sufficient intensity and quantity to decompose acidulated water, give powerful physiological shocks, and exhibit the electric spark and other effects of ordinary elec- tricity. It consists of two horse-shoe magnets, C D, placed one over the other, with an armature on which a double inducing coil is wound. This is made to revolve between the poles of the magnets by means of the multiplying wheel W. The two ends 216 MAdNETO-ELECl^RlCIT\^. i:m^ m^ M m ■jvl ■' % 'i of the copper wire Pig. 63 forming the coils are soldered to a ferrule or break- piece on each side of the axis, and a- gainst this two me- tallic springs press, having also a me- tallic connection with the two cups o, 6. As the arma- ture revolves, the iron centre-pieces are brought between the poles of the horse-shoe magnets, and are rendered magnetic by induction ; and this polarity is alternately made, annihilated, reversed, and so on. At each change of polarity a current of electricity is induced in the wire, and flows through it to the cups in a continuous stream, if the wheel revolves with sufficient rapidity. The physiological efl'ects are experienced, and the spark seen, only when the current is interrupted. In order to break the current, an elastic steel spring, connected with one of the cups, a or 6, presses against the pins of a little crown wheel attached to the axis. When the spnng presses against the pin the circuit is closed, and the current is unbroken ; but while it passes from one pin to another the current is broken, and a secondary current ot still greater power induced. 562. The production of induced currents is not exclusively confined to magnetic agency. A current of voltaic electricity from any source whatever, while flowing through a coil of insu- lated wire, induces a secondary current in a contiguous coil. The most powerful of all artificial means of producing electricity of high tension (the induction coil)^ depends in its action upon the secondary current induced in a very long insulated wire (60,000 or 70,000 feet), by which means voltaic electricity is converted, as it were, into statical electricity. ANIMAL ELECTRICITY. 217 s, and art) Itcrnately jhange oi wire, and the wheel [lafk seen, break the the cups, I attached p, circuit is 3 from one current ot ixclusively electricity lil of insu- ;uous coil, electricity ction upon lated wire jctricity is TIIEIIMO-ELECTUICITY. 563. If a bar of bismuth and one of antimony be soldered to- gether, as in Fig. 64, and a weak current of electricity transmitted through the arrangement from the antimony to the bismuth, the temperature of the junction c will be raised considerably ; but if the current be sent from the bismuth to the antimony, the tem- perature of the junction c is so much lowered that a small quan- tity of water, placed there, may be frozen. Fig. 64. Now if, in place of transmitting a current of electricity through the combination, we change the temperature of the part where the bars are soldered together, we shall by that means produce a current of electricity, which flows from the antimony to the bis- muth, if we cool the junction, but from the bismuth to the antimony, if we heat it. These currents are termed thermo-electric currents. NOTE l.-Thermo.electric currents are exceedingly weak probably owing to their originating in good conductor.. The thinnest him ot oxide is sufficient to prevent the passage of the current into a wire. NOTE 2.-Melloni'8 thormo-multiplicr consists of a number of bars ot bismuth and antimony soldered together at ^^^^'Tl^ ^lu^^^' ing the opposite members connected, by means of wires, to a K^jj^^^^j;^ The least difTerence of temperature between the opposite faces of the Lrangementproduccs a thermo-electric current which deflects the needle of the galvanometer. NOTE 3 -Although bismuth and antimony are the metal^ employed in Jrmolctric batt'eries, other metals and even -n-metal ic^^^^^^^^^^^^^ be substituted. In the following list the combination ot the metals at the two extemes produces the most powerful thermo-electrical arrangement -the effects of the intermediate metals diminishing as they approach. 1. Galena. 2. Bismuth. 3. Mercury. 4. Platinum. 5. Manganese 6. Tin. , , 7. Lead. ^■' * 8. Brass. - - 9. Gold. ' 10. Copper. ' 11. Silver. . - 12. Zinc. /• 13. Iron. 14. Arsenic. 15. Antimony. ANIMAL ELECTRICITY. 564 It has been shown by Matteucci, of Pisa, that electricity is, in some mysterious maimer, intimately connected with vital . t ' i 218 MISCELLANEOUS PROBLEMS. power. He has demonstrated that n current of positive electri- city is always flowing from the interior to the exterior of a muscle. T?y using delicate galvanometers, Du Bois Raymond, of Vienna, has proved the existence of electric currents in his own person. KoTK.— The irritable muacles of the frog's log form an olectroscopo about G0,000 times more sensitive tlian tlio most delicate gold-loaf olcctro- Bcope. 565. Certain marine and fresh- water fish (about eight genera) possess a special apparatus by means of which they can produce, at pleasure, powerful discharges of statical electricity. This power is doubtless designed either as a means of defence or for the purpose of securing their prey. The special apparatus con- sists of an alternate arrangement of cellular tissue and nervous matter, the latter being contained in hexagonal cells, and the whole constituting a perpetually charged electric battery, which is discharged, and a violent shock produced, by touching the opposite extremities of the animal. The most remarkable of these electrical animals are : I. The Gymnotus, or Electric Eel of Surinam ; and II. The Torpedo, or C ramp-Fish of the North American coast. Note.— The shock received from an electric eel is sufficient to disable a man or even a horse. Faraday has estimated the discharge to be equal to that from a fully charged Leydon battery of 16 jars containing a coated surface of 3500 square inches. The same philosopher succeeded in deflect- ing the galvanometer, evolving light and heat, magnetising steel bars and eifecting chemical decompositions by the electricity obtained from the gymnotus. MISCELLANEOUS PROBLEMS. 1. How many degrees Reaumur are equivalent to 14-6° F.? How many F. are equivalent to — G*8* C. ? 2. What time is required for light to come from the moon to the earth, the distance being 240000 miles? 3. How many images will be seen in a kaleidoscope when the two mirrors forming it are placed at an angle of IS'' ? MISCELLANEOUS PROBLEMS. 219 ive eleotri- Lterior of a Raymond, -ents in his olectroscopo l-lcaf olectro- ght genera) an produce, icity. This fence or for laratus con- md nervous lis, and the ttery, which ouching the markable of and erican coast. it to disable a to be equal to ning a coated ded in deflect- steel bars and ined from tUo to 14-6° F.? the moon to jpe when the 4 Two simihir balls are chaigcd with electricity, and it is found that one repels the needle of Coulomb's torsion electro- meter through 30^ while the other repels it through tO^ Cora- pare the intensity of the charges. 5 A concave mirror collects the rays of solar light to a focus 8 inches in front of it. What will be the distance of the image cast by an object 12 feet before the mirror ? 6 The north star is estimated to be 281295000000000 miles from the earth-suppose that by the fiat of Omnipotence it were destroyed, what length of time would elapse after its annihilation before we should lose sight of it?-in other words, how long would the last rays that leave it occupy in reaching our earth? 7. If 10 lbs. of lead at the temperature 490° F. melt U lbs. of ice ; what is the specific heat of the lead? 8 A railroad is constructed in winter when the average tem- perature is 30^ F.-how far apart must the ends of the iron rails be laid in order to allow sufficient room for expansion at the tem- perature of 110^ F., assuming each rail to be 20 feet long ? 9 The speculum of a Gregorian telescope is 30 inches in diameter, taking the diameter of the pupil of the eye as ^ of an inch ; compare the relative amounts of light received by each. lo' An incipient red heat is equal to97r F., a cherry-red heat 1832^ F and a da/zling white heat 9732° F. Express these temperatures in degrees Reaumur and degrees Centigrade. 11 What temperature will be produced by mixing together equal weights of water at 212^ F., and mercury at 32° K? What temperature, by mixing equal volumes of mercury at 212 F., and of water at 32° F.? • 12 Calculate the illuminating powers of Herschel's great telescope and also of Rosse's telescope-assuming in each case that 1^1 of the incident light is reflected by the speculum. 13. Allowing gases to decrease in volume ^ of their bulk at 32° F for "every degree their temperature is lowered— at what^emperature would all gases cease to exist, and hence what may we regard as the absolute zero? '0 \< i 220 »ll«CELLANE0U8 PROBLEMS. -I. :U 14. To what height above the centre of the roof sliould u| single lightning-rod extend in order to protect the whole build- ing — taking its dimensions as 40 feet by 30 feet? 15. How many units of heat are absorbed by 11 lbs. of tin in passing from the solid to the liquid form — the unit of heat being the amount of heat required to raise 1 lb. of water from 32'' F. to33T.? 16. The radius of a concave reflector is 9 inches, the focus of incident rays 24 inches — what is the focus for reflected rays? 11. An object t inches in diameter, is placed 40 inches before a convex mirror, whose radius of curvature is 60 inches — tlie upper edge of the object being in the principal axis of the mirror, determine the position, distance, and size of the image. 18. The temperature of a common fire is equal to 616'lo C, of human blood 36§° C, and the estimated temperature of plan- etary space is 50° C. Express these temperatures in degrees F. and R. 19. The greatest degree of cold ever measured, is, according to Faraday, 88" R., and according to Natterer 112° R. Reduce these temperatures to degrees F. and C. 20. How must an object be placed before a double equi-con- vex lens so that the image shall be double the size of the object, and erect? 21. Equal weights of water at 32° F, and ice at 32° F. are mixed together. What is the resulting temperature ? What is the temperature produced by placing together equal parts of boiling water and melting mercury ? 22. What is the focal length, for solar rays, of a meniscus whose radii are 19 and 21 inches ? 23. What is the focal length of a double convex lens whose radii of curvature are 11 and T inches, for rays of light ema- nating from a point 30 inches before the lens ? 24. In order to determine the melting point of lead, 100 ounces of the melted metal were poured into 900 ounces of water at 2^ R., and the resulting temperature was 20- r C. Required the heat of fusion of lead in degrees F. MISCELLANEOUS PROBLEMS. 221 oof should tt whole build lbs. of tin in of heat being r from S'i'' F. !, the focus of cted rays? inches before 3 inches — the 1 axis of the f the image, to 616-10 C, ature of plan- 33 in degrees is, according ° R. Reduce ible equi-con- of the object, at 32° F. are re ? What is iqual parts of if a meniscus ex lens whose of light ema- id, 100 ounces !S of water at C. Required 25. What are the linear and superficial magnifying powers of a convex lens whose focal IcLgth is 2 of an inch-taking the limit of distinct vision as 6 inches? What taking the limit of distinct vision as 8 inches or 4 inches ? 26. How long would electricity, light, and saund respectively require to travel from the sun to the earth ? 21. Compare together the retarding powers of two copper wires for a current of electricity—one being 100 feet in length, and jU of an inch in diameter, the other 250 feet in length and ^ of an inch in diameter. 28. In a magic lantern the object has a diameter of 2 inches and is placed 2 inches behind the magnifying lens, the screen being 10 feet before the same lens. What is the diameter of the image ? 29. The eye-piece of a microscope has a focal length of I of an inch— the object being placed § of an inch from the.objective, and the distance between the latter and the focus of the eye-glass being 12 inches— what are the linear and superficial magnifying powers of the instrument to a person whose limit of distinct vision is 4i inches ? What to a person whose limit of distinct vision is 9 inches ? 30. How much latent heat is there in the vapour of water at the temperature of 350° F ? How much at the temperature of 96® F. ? How much at 200«> C. ? 31. The mean annual temperature of Canada is about 44^ F. At what depths beneath the surface of the earth would the ther- mometer severally indicate a temperature of 10° F. ; of 100«> F. j of 2120 F.; of 400OF.? 32. Two lights are compared with one another in Ritchie'a Photometer, and it is found that in order to equally illuminate the different sides of the wedge one has to be placed at the dis- tance of 6 inches, and the other at the distance of 10 inches. What are their comparative illuminating powers ? 33. Compare together the brightness of the picture cast by a magic lantern on a screen at the several distances of 4 feet^ t feet, and 12 feet, from the magnifying lens ? «^, 222 MISOELLANEOUS PROBLEMS. 34. In a refracting telescope the focal length of the object glass is 100 feet, the focal length of the eye-piece is 10 inches, the object glass has a diameter of 4 inches, and the total number of lenses employed is four — assuming the pupil of the eye to be ■j'jy of an inch in diameter — what is the magnifying power, the illuminating power, the penetrating power, and the visual power of the instrument ? 36. "What are the illuminating and penetrating powers of a night-glass having a convex lens of 12 inches in diameter — the pupil of the eye being i of an inch in diameter ? 36. A Pouillet's pyrometer holds 20 cubic inches of air at 32<* P., and it was placed in a furnace, by which means -^g of the air is expelled. Express in degrees P., C, and R. the temperature to which it was exposed. ' 3Y. In a refracting telescope the object glass is 11 inches in diameter and has a focal length of 200 feet, the eye glass has a focal length of 4 inches, the whole number of lenses through which the light has to pass before entering the eye, is five. Re- quired the penetrating, illuminating, visual, and magnifying powers of the instrument. 38. A mass of platinum weighing 5 lbs. is exposed to the full heat of a furnace, and then plunged into 13 lbs. of water of the temperature of 65° P., when* it is observed that the tempe- rature of the water is raised to 65° R. Required the tempera- ture of the furnace in degrees F. and 0. 39. How many units of heat are absorbed by 17 lbs. of sul- phur in passing from the solid to the liquid state ? (See Pro- blem 15). 40. A dealer purchases 1000 gallons of alcohal in mid-winter, how much may he expect it to measure in mid-summer, the temperature of the former being 40° P., and of the latter 80° P. ? THE END or PABT II. AS^ ^ 7326 i n of the object I is 10 inches, total number the eye to be ig power, the 1 visual power ; powers of a iiameter — the I of air at 32<» J -^g of the air > temperature i 11 inches in re glass has a inses through B, is five. Re- i magnifying :posed to the s. of water of at the tempe- the tempera- [1 lbs. of sul- !? (SeePro- n mid-winter, 1-summer, the latter 80O P.?