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 SHORT ACCOUNT 
 
 experiments anD 
 
 DEPENDING 
 
 ON THE RELATIONS OF AIR 
 
 HEAT AND MOISTURE. 
 
 BY JOHN LESLIE, F. R. S. E. 
 
 PROFESSOR OF MATHEMATICS IN THE UNIVERSITY OF EDIJJBUKGH. 
 
 EDINBURGH : 
 
 VRIXTED FOR WILLIAM BLACKWOOD, AND J. BALLANTYNE & CO ; 
 
 AND FOR LONGMAN, HURST, REES, ORME, & BROWN ; 
 
 AND JOHN MURRAY, LONDON.. 
 
 1813.
 
 Printed by Abernetliy & Walker, 
 Old Bank Close, Lawninarket. 
 
 A
 
 ADVERTISEMENT. 
 
 J.N the Preface to the ' Experimental In- 
 quiry into the, Nature and Propagation of Heat? 
 I promised to resume that subject, and to ex- 
 tend my researches into the relations subsist- 
 ing between heat and moisture, which eluci- 
 date and confirm all the former deductions. 
 
 As I was anxious to obtain still more accu- 
 rate results, it became requisite to devise new 
 experiments, and to procure apparatus of the 
 largest dimensions, and of the most refined 
 elaborate construction. Some of the obser- 
 vations, it was necessary to repeat at different 
 seasons ; and the circle of my professional 
 avocations, joined to that of the publications 
 which I had connected with them, could not 
 fail to create delay, and to suspend in part my 
 application to experimental research. Several 
 years have thus unfortunately gone by, with- 
 out allowing me sufficient time to perform, in 
 
 1253613
 
 IV ADVERTISEMENT. 
 
 the way which I had proposed, my original 
 engagement. I have at length advanced so 
 far, however, that I may hope to be able very 
 soon to discharge that task. 
 
 In the meantime, it seemed desirable for 
 the advancement of science, to promote the 
 circulation of several instruments founded on 
 my speculations, and to give the public more 
 correct notions of their principle and their 
 mode of operation. With this view, I was in- 
 duced to draw up a concise statement, in as 
 popular a form as the regard to accuracy 
 would admit, but which, from the accumula- 
 tion of materials, has swelled by degrees into 
 a volume. I have only farther to add, that, 
 in order to avoid troublesome calculations, I 
 have satisfied myself with citing round num- 
 bers ; but these, if not absolutely correct, are 
 in general very near approximations to the 
 truth. . 
 
 V 
 
 COLLEGE OF EDINBURGH,) 
 5tk May 1813. J
 
 ON 
 
 THE RELATIONS OF AIR 
 
 HEAT AND MOISTURE. 
 
 r I^HE various phenomena of heat are most 
 easily conceived, by referring them to 
 the operation of a peculiar fluid, possessing 
 extreme activity, and diffused through all 
 bodies. This fluid exists, however, only in 
 a state of combination, and never appears un- 
 der a distinct and separate form. Though 
 subject, from different causes, to partial de- 
 rangement, it constantly endeavours to reco- 
 ver its equilibrium among adjacent bodies. 
 Its accumulation in any substance is invari- 
 ably marked by a corresponding expansion,
 
 ^ RELATIONS OF AIR 
 
 unless an absolute change of constitution has 
 been induced. Thus, a lump of ice intensely 
 cold, if exposed to a mild air, will regularly 
 expand, till it begins to melt ; and during its 
 conversion into water, it will suffer a material 
 contraction, but, after this change, it will 
 again dilate, from the repeated accessions of 
 warmth. In like manner, a bit of clay, though 
 in the furnace it contracts, from the expulsion 
 of part of the water combined with it, will, 
 on being withdrawn and suffered to cool, ex- 
 pand by the application of any lower degree 
 of heat than what it had before sustained. 
 
 When the heat shared among bodies is 
 mutually balanced, they are said to have the 
 same temperature. But such balance or equali- 
 ty of temperature is far more quickly attain- 
 ed in some substances than in others. Silver 
 transmits heat more readily than platina, pla- 
 tina than glass, and glass than loose down. 
 This property, by which bodies are so wide- 
 ly diversified, is called their conducting power; 
 and it has a most extensive influence in the 
 economy of nature.
 
 TO HEAT AND MOISTURE. 3 
 
 Among different substances, too, the rise 
 of temperature is accompanied by very diffe- 
 rent degrees of expansion. Air is found, in 
 like circumstances, to expand 5 times more 
 than alcohol, 20 times more than mercury, 
 160 times more than platina, and even 580 
 times more than glass. 
 
 The thermometer is an instrument contrived 
 to measure its own expansions, and conse- 
 quently fitted, by its nice sensibility, to indi- 
 cate the temperature of surrounding bodies. 
 Still the thermometer can mark only the heat 
 of its own bulb, as affected by external com- 
 munication; and any farther inferences drawn 
 from its different indications are merely the 
 result of some process of reasoning. 
 
 The primary source of heat is the sun, 
 whose genial rays are partly detained in the 
 atmosphere, and partly received at the surface 
 of the land and of the ocean. The incessant 
 addition thus made to the elementary fluid, 
 varying with the latitude and the change of 
 seasons, are speedily dispersed by the vehicle 
 of aerial currents, and gradually absorbed into
 
 RELATIONS OF AIR 
 
 the general mass of our globe. Yet this an- 
 nual influx of heat, however important in its 
 transient and superficial effects, must bear no 
 comparison to the wide accumulated store, 
 since in the course of ages scarcely any sensi- 
 ble increase of temperature over the world 
 has been distinctly ascertained. 
 
 But, even after it has gained its equilibrium, 
 the fluid of heat is not equally dispersed 
 among bodies, or shared out in proportion to 
 their quantities of matter. Had an opposite 
 system obtained, no internal change of tem- 
 perature could ever have taken place; for, 
 amidst all the mutations of form and condi- 
 tion to which bodies are subject, their mass 
 continues unaltered, and would consequent- 
 ly retain the same measure of heat. In the 
 actual constitution of things, a provision is 
 happily made for those grateful alternations 
 and interchange of elements, which enliven 
 the face of nature. Heat combines with dif- 
 ferent substances in proportions widely va- 
 ried, and depending in each on its peculiar 
 and intimate structure. In general, it is more
 
 TO HEAT AND MOISTURE. 5 
 
 copious in liquids than in solids, and in the 
 aeriform fluids than in liquids. But still the 
 allotment among the different bodies, ap- 
 pears to be as various as their distinctive 
 properties. Under similar circumstances, 
 hydrogen gas will hold or absorb ten times 
 as much heat, as an equal mass of atmos- 
 pheric air ; water twice as much as olive oil, 
 and three times as much as concentrated sul- 
 phuric acid ; sulphuric acid, again, twice as 
 much as glass ; and glass itself, twice as much 
 as silver, and five times as much as mercury. If 
 a pound of water heated 30 degrees, be pour- 
 ed into another pound of water, at the tem- 
 perature of the apartment, the surplus heat will 
 become equally shared between the two mas- 
 ses, the infused portion losing 15 of its 
 heat, and the recipient gaining 15. But if a 
 pound of mercury, heated 30 degrees above 
 the standard, be poured into a pound of water; 
 while both of these now acquire the same 
 temperature, the mercury will lose 29 de- 
 grees, and the water gain only one degree. 
 Hence, in the state of quiescence, mercury
 
 6 RELATIONS OF AIR 
 
 contains 29 times less heat than water, and 
 has its temperature 29 times more affect- 
 ed by equal accessions of that elementary 
 fluid. But even the same substance, if its 
 form be mutable, will exhibit similar differ- 
 ences, according to the aspect which it as- 
 sumes. Thus, ice is more easily heated than 
 water, and water than steam. The same ad- 
 dition of heat which would raise the tempera- 
 ture of ice 10 degrees, would only raise that 
 of water 9 degrees, and that of steam 6 de- 
 grees. At each stage of transition, there is 
 hence an apparent pause, attended with a 
 corresponding absorption or evolution of 
 heat. 
 
 Thus, if a vessel filled with ice, be sus- 
 pended over a steady fire, the ice will con- 
 tinue at the freezing point, till, perhaps in an 
 interval f ,of half an hour, it be entirely melt- 
 ed ; it will then grow regularly warmer, till, 
 after 40 minutes, the water begins to boil : 
 nor will the temperature of the liquid now 
 receive any farther increase, the subsequent 
 accessions of heat being wholly expended in
 
 TO HEAT AND MOISTURE. 7 
 
 the formation of the expelled steam, and 
 which would require the space of three hours 
 and a half. In the act of thawing, therefore, 
 and again in the process of ebullition, there 
 is a successive absorption of heat, amounting 
 respectively to a difference of temperature in 
 the water of about 75 and 525 of the centi- 
 grade degrees, or 135 and 945 on Fahren- 
 heit's scale. But the heat thus absorbed is 
 nowise distinguished from the rest, or fitted 
 to perform any different function ; it blends 
 its action and its expansive energies with the 
 general fluid, and merely serves to restore 
 the equilibrium that had been disturbed by 
 the enlarged capacity, or rather the increas- 
 ed attraction, of the mass with which it com- 
 bines. 
 
 A similar expence of heat invariably at- 
 tends the conversion of water into the gase- 
 
 
 
 ous state of vapour, by whatever powers that 
 transformation is effected. Thus, dry air 
 brought in contact with moisture, will at-* 
 trac^ it to some point of saturation, or will ab- 
 sorb a certain portion of the humidity, and as-
 
 8 RELATIONS OF AIK 
 
 similate this with its own substance ; and such 
 action is greatly augmented, either by raising 
 the temperature of the medium, or by re- 
 ducing or withholding the atmospheric pres- 
 sure. The application of warmth invigo- 
 rates the dissolving power of the air, while, 
 by distending the particles of the subjected 
 water, it facilitates the passage into vapour ; 
 and the diminution or removal of the incum- 
 bent weight of the atmosphere produces a si- 
 milar effect, by giving a freer or less restrain- 
 ed play to the repulsion of the liquid par- 
 ticles, and hence assisting indirectly the at- 
 traction of the solvent medium. On this 
 principle, depends the consumption of heat 
 and the consequent reduction of temperature, 
 occasioned by the evaporation of liquids which 
 are exposed to the access of thy air. 
 
 The evolution or absorption of heat, and 
 thence the raising or lowering of the general 
 temperature, must therefore be the result of 
 every change in the constitution of bodies, 
 whether it belongs to the class of physical or 
 of chemical phenomena. In the case of mix-
 
 TO HEAT AND MOISTURE. 9 
 
 ture or solution, the effect is limited and 
 merely transient ; but where a process of de- 
 composition goes forward, the discharge of 
 heat is copious and continued. Thus, if sul- 
 phuric acid be poured upon water, there will 
 appear a remarkable evolution of heat, as the 
 capacity of the dilute acid so formed is less 
 than the mean capacity of both its compo- 
 nents. Again, nitre or sal ammoniac, dis- 
 solved in water, occasions an absorption of 
 heat or a depression of temperature, because 
 the capacity of the saline solution is greater 
 than the intermediate capacity of the water 
 and of the salt. ' Such extrication or abstrac- 
 tion of heat is, however, a single act, and can 
 produce only a momentary change of tem- 
 perature in the mass affected. But when the 
 united streams of oxygen and hydrogen gases 
 are inflamed, or when charcoal is burnt in 
 contact with atmospheric air, there is a con- 
 tinual and profuse emission of heat ; for, in 
 both cases, a decomposition is carried on, and 
 its effects are incessantly renewed, the oxy- 
 gen and hydrogen gases forming steam, which
 
 10 RELATIONS OF AIR 
 
 has less capacity, and the union of the char- 
 coal changes the pure portion of common air 
 into carbonic gas, whose capacity is likewise 
 much lower. On the other hand, a perpetual 
 absorption of heat is caused by the evapora- 
 tion of liquids exposed to the free circulation 
 of dry air, from the increased capacity of the 
 invisible exhalations drawn from them and 
 dissolved in the aerial medium. This de- 
 pression of temperature, though it can be 
 supported for any length of time, is in most 
 cases, however, confined within very mode- 
 rate limits. In general, the means within 
 our reach of procuring the discharge of heat 
 are of a nature far more powerful than those 
 which occasion its absorption, or the seeming 
 production of cold. 
 
 The gaseous substances are so loosely con- 
 stituted, that a difference in their composi- 
 tion is sufficient to alter materially their inti- 
 mate properties. Thus, common air, on being- 
 condensed 30 times, has its capacity for heat 
 reduced to one half; and, if suddenly com- 
 pressed to 20 times its ordinary density, it
 
 TO HEAT AND MOISTURE. 11 
 
 will disengage so much heat as to show an 
 elevation of temperature equal to 900 degrees 
 by Fahrenheit's scale, and sufficient for the 
 inflammation of most bodies. On this pro- 
 perty, is founded a pretty contrivance lately 
 made in France, the stroke of a small con- 
 densing syringe being employed to set on 
 fire a bit of tinder. An opposite effect, when 
 air is suddenly rarefied, takes place ; a certain 
 quantity of heat being now absorbed, or an 
 apparent cold produced. 
 
 The increased capacity of rarefied air is the 
 true cause of the cold which prevails in the 
 higher regions of the atmosphere. From the 
 unequal action of the sun's rays and the vi- 
 cissitudes of day and night, a perpetual and 
 quick circulation is maintained between the 
 lower and the upper strata ; and it is obvious, 
 that, for each portion of air which rises from 
 the surface, an equal and corresponding por- 
 tion must also descend. But that which mounts 
 up, acquiring an enlargement of capacity, 
 has its temperature proportionally diminish- 
 ed j while the correlative mass falling down
 
 12 RELATIONS OF AIR 
 
 carries likewise its heat along with it, and, con- 
 tracting its capacity, seems to diffuse warmth 
 below, A stratum at any given height in the 
 atmosphere is hence alike affected by the 
 passage of air from below, and by the return 
 of air from above, the former absorbing heat, 
 and the latter evolving it. But the mean tem- 
 perature at any height in the atmosphere is 
 still on the whole permanent, and consequent- 
 ly those disturbing causes must be exactly ba- 
 lanced, or the absolute measure of heat is 
 really the same at all elevations, suffering 
 merely some external modification from the 
 difference of capacity in the fluid with which 
 it has combined. That temperature is hence 
 inversely as the capacity of air possessing the 
 rarity due to the given altitude. Having 
 therefore ascertained, by some delicate expe- 
 riments, the law which connects the capacity 
 with the rarity of air, it was not very diffi- 
 cult to trace the gradations of cold in the 
 higher atmosphere, and even to mark the 
 precise limit where the reign of perpetual 
 congelation must commence. Thus, I find
 
 TO HEAT AND MOISTURE. 13 
 
 that, under the equator, the boundary of the 
 frozen region begins at the altitude of 15207 
 feet, in the parallel of 45 at 7671 feet, in 
 the latitude of London at 5950, and in that 
 of Stockholm at 3818, while towards the pole 
 it comes to graze along the surface *. 
 
 It is curious to examine the mode by which 
 heat is conducted through substances differ- 
 ently constituted. In the case of solid bodies, 
 there can be no actual transfer of the heated 
 particles, but these must alternately receive 
 and deliver their successive impressions. If 
 heat be applied, for instance, at one end of 
 a cylinder of metal, wood, or glass, which 
 communicates at its other end with some ex- 
 tended mass, a regular descending gradation 
 of temperature will soon be established along 
 the whole chain of connection. But if each 
 intermediate portion of the conductor were, 
 during its action, to preserve unvaried the 
 temperature which it had once acquired, no 
 transfer of heat whatever could obtain. Each 
 
 * See Elements of Geometry, pp. 495 and 496.
 
 14 RELATIONS OF AIR 
 
 successive space into which the cylinder is 
 distinguished, must therefore undergo a cer- 
 tain limited oscillation, to excess and defect 
 from the state of equilibrium, thus partially 
 dilating and again contracting, and at each 
 alternate act receiving from the anterior por- 
 tion a minute excess of heat, which in the 
 next it delivers to the succeeding one. A 
 series of connected internal vibrations must 
 hence keep shooting along the whole chain of 
 communication, and the rate with which the 
 heat is conveyed by this tremulous excite- 
 ment and successive transfer, will depend 
 chiefly on the extent and the celerity of each 
 elementary vibration. 
 
 But when the conducting medium is a 
 fluid, the mobility of the particles affected 
 will so derange the mode of operation, as al- 
 most entirely to change its nature. The 
 proximate portion of the medium, dilating as 
 it becomes warmed, is gently forced to re- 
 cede ; and being likewise rendered specifi- 
 cally lighter, it rises to the surface. The heat 
 then quickly spreads through the buoyant
 
 TO HEAT AND MOISTURE. 15 
 
 substance in horizontal strata, the hottest 
 particles occupying the highest place, and the 
 rest arranging themselves according to their 
 respective temperatures. Thus, if a hot body 
 be plunged in water, a portion of the heat is 
 gradually absorbed by the surrounding liquid, 
 and transmitted by successive fits through the 
 internal mass, in the same manner as if this 
 had been congealed into solid ice. The re- 
 maining portion is discharged by the slow re- 
 cession of the heated particles, or the per- 
 pendicular motion produced by their mutual 
 distension. The subsequent diffusion of this 
 heat, which is made to descend by the suc- 
 cessive transfer of minute differences from 
 stratum to stratum, is performed very tardily 
 and with extreme difficulty. The lower part 
 of the liquid acquires in general but a very 
 small share of the heat retained near the top. 
 Hence it is, that very deep collections of 
 water exposed to the variable influence of the 
 sun and sky, are always found to have their 
 bottom greatly colder, and their surface 
 warmer, than the mean temperature of the
 
 16 RELATIONS OP AIR 
 
 climate. The superficial portion, dilated by 
 the action of the summer's heat, continues 
 passively to float, without much affecting the 
 mass below ; but, when it has felt the rigours 
 of winter, it undergoes a contraction of vo- 
 lume with a corresponding increase of den- 
 sity, and it sinks downwards in this chilled 
 state. During all the changes of tempera- 
 ture which supervene, and while the inter- 
 nal communication through the mass appears 
 so tardy and imperfect, there hence is an obvi- 
 ous tendency to accumulate warmth at its sur- 
 face, and to precipitate the opposite impres- 
 sions. Accordingly the bottoms of profound 
 lakes are uniformly and intensely cold. The 
 same property is not indeed observed in deep 
 open seas, because the various currents and 
 heaving tides by which these are agitated, so 
 effectually intermix the different portions of 
 water, as in some degree to equalize their 
 temperature. Near extensive banks, how- 
 ever, where the interchange between the 
 higher and the lower strata becomes again 
 impeded, no such equality seems to prevail ;
 
 TO HEAT AND MOISTURE. 17 
 
 and the increasing coldness of the water 
 drawn up from considerable depths in the 
 ocean has lately been proposed as a sure mark 
 of the approach of soundings, if not of the 
 land itself. 
 
 Since water, on being heated, expands 
 in a rapid progression, the portion of heat 
 which it abstracts from a body immersed in 
 it, by means of the recession and incessant 
 change of its contiguous affected particles, 
 must be greatly augmented in the higher 
 temperatures. Near the freezing point, this 
 influence becomes extremely small, and water 
 is there scarcely a better conductor than ice ; 
 but, as it approaches to ebullition, it acquires 
 such an increase of mobility, as to conduct 
 heat five times faster than in its torpid state. 
 In other liquids, the increase of temperature 
 will occasion a similar alteration of the con- 
 ducting powers, though not so marked, as 
 their expansions deviate less from an uniform 
 progression. 
 
 But, through air and other gaseous fluids, 
 the conveyance of heat is still more com- 
 B
 
 18 RELATIONS OF AIR 
 
 plex ; and a close investigation of that pro- 
 cess, by unfolding certain latent properties 
 of matter, has led to some very unexpect- 
 ed and interesting results. A new principle 
 appears to combine its influence, and the rate 
 of dispersion, in aeriform media, is found to 
 depend chiefly on the nature of the mere 
 heated surface. From a polished metallic 
 surface, heat is feebly emitted ; but, from a 
 surface of glass, or still better from one of 
 paper, it is discharged with profusion. If 
 two equal balls of thin bright silver, one 
 of them entirely uncovered, and the other 
 sheathed in a case of cambric, be filled 
 with water slightly warmed, and then sus- 
 pended in a close room, the former will lose 
 only 11 parts of its heat in the same time 
 that the latter will dissipate 20 parts. Of 
 this expenditure, 10 parts from each of the 
 balls is communicated in the ordinary way, by 
 the slow recession of the proximate particles 
 of air, as they come to be successively heat- 
 ed. The rest of the heat, consisting of 1 part 
 from the naked metallic surface, and of 10
 
 TO HEAT AND MOISTURE. 19 
 
 parts from the cased surface, is propagated 
 through the same medium, but with a certain 
 diffusive rapidity, which in a moment shoots 
 its influence to a distance, after a mode en- 
 tirely peculiar to the gaseous fluids. The 
 very superior propellent energy of a surface 
 of glass or paper in comparison of that of a 
 metallic one, lies within the compass even of 
 ordinary observation. If a glass carafFe or a 
 pot of porcelain be filled with boiling water, 
 on bringing towards it the palm of the hand, 
 an agreeable warmth will be felt at the dis- 
 tance, of an inch or two from the heated sur- 
 face ; but if a silver pot be heated in the same 
 way, scarcely any heat is at all perceptible on 
 approaching the surface, till the. fingers have 
 almost touched the metal itself 
 
 It is curious to inquire how such a singular 
 diversity can arise. If the silver ball be co- 
 vered with the thinnest film of gold-beater's 
 skin, and which exceeds not the 3000th part 
 of an inch in thickness, the power of disper- 
 sion will be augmented from 1 to 7 ; if an- 
 other pellicle be added, there will be a farther
 
 20 RELATIONS OF AIR 
 
 increase of this power, from 7 to 9 ; and so 
 repeatedly growing, till after the application 
 of five coats, when the propellent energy will 
 reach its extreme limit, or the measure of 10. 
 In this case, the metallic surface is precluded 
 from all contact with the air, and it must, there- 
 fore,act in consequence of its mere approxima- 
 tion to the external boundary. We may thence 
 infer, that air never comes into actual contact 
 ith any surface, but approaches much nearer 
 to glass or paper than to polished metal, from 
 which it is separated by an interval of at least 
 the 500th part of an inch. A vitreous surface, 
 from its closer proximity to the recipient 
 medium, must hence impart its heat more 
 copiously and energetically, than a surface of 
 metal in the same condition ; and the metal, 
 to a certain extent, can act in reducing the 
 power of the other. When a pellicle was 
 applied, the metallic surface immediately un- 
 der it repelled partially the atmospheric boun- 
 dary, and reduced the darting efflux of heat 
 from 10, which would have been thrown by 
 the skin alone, to about 7, or only 6 more
 
 TO HEAT AND MOISTURE. 21 
 
 than the efficacy of the naked metal. The 
 repelling influence of the metallic plate was 
 sensible even under four coats, or at the dis- 
 tance of the 750th part of an inch from the 
 external surface. 
 
 By what process the several portions of heat, 
 thus delivered to the atmosphere, shoot 
 through the fluid mass, it seems more diffi- 
 cult to conceive. They are not transported 
 by the streaming of the heated air, for they 
 suffer no derangement from the most violent 
 agitation of their medium. The air must 
 therefore, without changing its place, disse- 
 minate the impressions that it receives of 
 heat, by a sort of undulatory commotion, or 
 a series of alternating pulsations, like those 
 by which it transmits the impulse of sound. 
 The portion of air next the hot surface, sud- 
 denly acquiring heat from its vicinity, ex- 
 pands proportionally, and begins the chain 
 of pulsations. In again contracting, this 
 aerial shell surrenders its surplus heat to the 
 one immediately before it, and which is now 
 in the act of expansion ; and thus the tide o*f
 
 RELATIONS OF AIR 
 
 heat rolls onwards, and spreads itself on all 
 sides. These vibratory impressions are not 
 strictly darted in radiating lines, but each 
 successive pulse, as in the case too of sound, 
 presses to gain an equal diffusion. Different 
 obstructions may, therefore, cause the undu- 
 lations of heat to deflect considerably from 
 their course. Thus, if successive rings of 
 pasteboard be fashioned into the twisted form 
 of a cornucopia, and its wide mouth present- 
 ed at some distance to the fire, a strong heat 
 will, in spite of the gradual inflection of the 
 tube, be accumulated at its narrow end ; in 
 the same manner probably, as waves, flow- 
 ing from an open bay into a narrow harbour, 
 now contracted and bent aside, yet without 
 being reflected, rise into furious billows. 
 
 But the same pulsatory system will enable 
 the atmosphere to transmit likewise the im- 
 pressions of cold. The shell of air adjacent 
 to a frigid surface, becoming suddenly chilled, 
 suffers a corresponding contraction, and which 
 must excite a concatenated train of pulsations. 
 This contraction is followed by an immediate
 
 TO HEAT AND MOISTURE. 23 
 
 expansion, which withdraws a portion of heat 
 from the next succeeding shell, itself now in 
 the act of contracting ; and the tide of ap- 
 parent cold, or rather of deficient heat, shoots 
 forwards with diffusive sweep. The energy 
 of transmission is subject, in this case also, to 
 the same modifications from the nature of 
 the affected surface. Thus, a goblet filled 
 with pieces of broken ice, or still better with a 
 frigorific mixture composed of snow and salt, 
 will, at a moderate distance, yet seem chill ; 
 but a silver pot, filled with a similar mixture, 
 will not cool the hand, till it has become pro- 
 fusely covered with dew, and therefore now 
 presents a non-metallic surface. 
 
 But the same quality by which a surface 
 propels the hot or cold pulses, equally fits it, 
 under other circumstances, to receive their 
 impressions. If a vitreous surface sends forth 
 its heat the most copiously, it will also, when 
 opposed to the tide, arrest with entire efficacy 
 the affluent wave ; and if, on the other hand, 
 a surface of metal sparingly parts with its 
 heat, it in like manner detains only a small
 
 24 UELAT1UKS OF Alii 
 
 share of each appulse, and reflects all the rest. 
 The power of superficial absorption and that 
 of reflection are therefore exactly contrasted* 
 and the one always supplies the deficiency of 
 the other. The naked bulb of a thermome- 
 ter held near a goblet full of boiling water, 
 will mark a very sensible afflux of heat ; but 
 if it be gilt or covered with tinfoil, it will 
 scarcely seem at all affected. For the same 
 reason, the hand cased in a glove of burnish- 
 ed metal may approach the fire with impuni- 
 ty, since the vehement pulsations of heat are 
 mostly driven back, or turned aside from their 
 attack. A sheet of paper, opposed to the 
 aerial tide, will absorb the whole impression* 
 a pane of glass will repel about one-tenth 
 part, while a plate of polished silver will re^ 
 fleet nine^tenths of the heat, detaining only 
 the remaining tenth. But if the metallic 
 plate be covered with a pellicle of the 3000th 
 of an inch in thickness, out of 10 parts of 
 heat no more than 3 will be reflected^ the 
 rest being now absorbed ; and by applying 
 successively other pellicles, till a coat equal
 
 TO HEAT AND MOISTURE. 25 
 
 to the 500th of an inch in thickness has been 
 formed, the quantity of reflection will gradu- 
 ally become insensible. The power of a me^ 
 tallic speculum in concentrating at its focus 
 the pulses of heat or cold is hence very strik- 
 ing, while the corresponding effects of a glass 
 mirror seem to be extremely feeble. 
 
 The very different powers of a vitreous 
 and of a metallic surface in propagating or ab- 
 sorbing the pulsations of heat, are well con- 
 trasted by an experiment of the simplest and 
 easiest kind. Let a small pane of glass about 
 four inches square have one of its sides half 
 covered with smooth tinfoil ; or, what is more 
 elegant, let a small square of thin mica have 
 one side gilt half over with silver leaf. On 
 holding the partly covered surface of the glass 
 or mica opposite and very near the fire for the 
 space of a few seconds, and then passing the 
 finger lightly over the posterior surface, scarce- 
 ly any warmth is perceptible under the me- 
 tallic sheath, but an intense degree of heat 
 will be felt behind the naked portion of the 
 plate. Again, reversing its position and ex-
 
 RELATIONS OF AIR 
 
 posing the uncovered side to the fire, an op- 
 posite, though less marked effect is observed ; 
 the coat of metal will become sensibly hotter 
 than the adjacent naked space ; because the 
 heat absorbed along the interior surface be- 
 ing afterwards more feebly discharged from 
 the tin or silver leaf, is allowed to accumulate 
 in that part of the screen. In this latter case 3 
 the difference of temperature produced is very 
 nearly the double, and in the former it is no 
 less than tenfold. But effects of the same 
 kind, and which are alike contrasted, though 
 inferior in degree, will be perceived, if a thin 
 pellicle be spread over the compound surface 
 of the glass and tinfoil, or of the mica and sil- 
 ver leaf, the mere proximity of the metallic 
 surface repelling the atmosphere, and conse- 
 quently enfeebling the powers of absorption 
 and emission. 
 
 The very singular and unexpected facts now 
 detailed merit attention, and suggest a variety 
 of improvements in the practical management 
 of heat. A vessel with a bright metallic sur- 
 face is the best fitted to preserve liquors either
 
 TO HEAT AND MOISTURE. 27 
 
 long warm, or as a conservatory to keep them 
 cool. A silver pot will emit scarcely half as 
 much heat as one of porcelain ; and even the 
 very slightest varnishing of gold, platina or 
 silver, which communicates to the ware a cer- 
 tain metallic gloss, renders this new kind 
 of manufacture about one-third part more 
 retentive of heat. The addition of a cover- 
 ing of flannel, though indeed a slow con- 
 ductor, far from checking the dissipation 
 of heat, has directly the contrary tendency ; 
 for it presents to the atmosphere a surface of 
 much greater propulsive energy, which it 
 would require a thickness of not fewer than 
 three folds of this loose substance fully to 
 counterbalance. The cylinder of the steam- 
 engine has lately been most advantageously 
 sheathed with polished copper. 
 
 The progress of cooling is yet more re* 
 tarded, by surrounding the heated vessel on 
 all sides, at the distance of near an inch, 
 with a case of planished tin ; and the addi- 
 tion of other cases, following at like intervals, 
 augments continually the effect. With an
 
 28 RELATIONS OF AIK 
 
 obstraction of one case, the rate of refrigera- 
 tion is 3 times slower, with two cases it is 5 
 times slower, with three cases it is 7 times 
 slower, and so forth* as expressed by the suc- 
 cession of the odd numbers. By multiplying 
 the metallic cases, therefore, and disposing 
 them like a nest at regular intervals, the in- 
 nermost could be made to retain the same 
 temperature with little variation for many 
 hours or even days. Such an apparatus would 
 obviously be well calculated for various culi- 
 nary and domestic purposes. 
 
 In the conveyance of heat by means of 
 steam, the surface of the conducting tubes 
 should have a metallic lustre. On the con- 
 trary, if it be intended by that mode to warm 
 an apartment, they should be coated on the. 
 outside with soft paint, to facilitate their dis- 
 charge of heat. For the same reason, me- 
 tallic pots are more easily heated on the fire, 
 after their bottoms have become tarnished or 
 smoked. If a bright surface of metal be 
 slightly furrowed or divided by fine flutings, 
 it will emit heat sensibly faster, because the
 
 TO HEAT AND MOISTURE. 29 
 
 prominent ridges, thus brought closer to the 
 general atmospheric boundary, will excite the 
 pulsations with augmented energy. 
 
 On the other hand, a plate of metal, how- 
 ever thin, if only burnished on each side, 
 will form the most efficacious screen. A 
 smooth sheet of pasteboard, gilt over both 
 sides, would answer the same purpose. But 
 a complete and elegant screen might be com- 
 posed of two parallel sheets of China paper, 
 placed about an inch asunder, and having 
 their inner surfaces gilt, and their outsides 
 sprinkled with flowers of gold and silver. 
 
 Since, in a still atmosphere, the momentary 
 flow of heat from any vessel, whatever this 
 may contain* depends merely on the condi- 
 tion of its surface, the whole accumulated 
 discharge, during similar descents of tempe- 
 rature, is evidently proportional to the time 
 elapsed. Hence a very simple and accurate 
 method is suggested, for ascertaining the ca- 
 pacity of different liquids or their specific at^ 
 traction to heat. Into a glass ball, two or 
 more inches in diameter and blown extremely
 
 30 RELATIONS OP AIR 
 
 thin, with a narrow short neck, and having a 
 delicate thermometer inserted through it, 
 the liquid to be examined, which had been 
 previously warmed a few degrees, is carefully 
 introduced by means of a funnel. The ball 
 is then made to rest against the tapering 
 points of three slender glass rods at the 
 height of several inches above the table, and 
 sheltered from any irregular agitation of the 
 air of -the apartment by a large receiver 
 passed over it. The number of seconds 
 which the thermometer now takes to sink 
 from one given point to another, or to the 
 middle of its distance from the limiting tem- 
 perature, is noted by help of a stop-watch ; 
 and the ball being thoroughly emptied and 
 again successively filled with other liquids, 
 the like observations are repeated. These 
 several intervals of time, allowing a slight 
 correction for the matter of the shell itself 
 and of the inserted bulb of the thermometer, 
 will consequently express the proportional 
 quantities of heat contained in equal bulks 
 of the successive liquids. But their densities
 
 TO HEAT AND MOISTURE. 31 
 
 being already known, it is hence easy to com- 
 pute their respective capacities, or the quan- 
 tities of heat which equal weights of them 
 are capable of containing. By a process 
 grounded on the same principles, the capaci- 
 ty of a solid, when broken or reduced to a 
 gross powder, may be determined. 
 
 The same regulated mode of cooling will 
 serve to detect with precision the expendi- 
 ture of heat, and to discriminate its various 
 allotment, in the different gases. For this 
 purpose, a ball of about three inches in dia- 
 meter, and formed of bright and very thin 
 silver, is preferable ; and it may be succes- 
 sively covered with a pellicle or with cambric, 
 or painted with a coat of ivory-black. Not to 
 multiply unnecessary details, it will perhaps 
 be deemed sufficient to cite the case of hy- 
 drogen gas, which is by far the most distin- 
 guished. The portion of heat emitted in this 
 energetic species of gas by the system of pul- 
 sations, whether from a vitreous or a metallic 
 surface, if not exactly, is very nearly the same 
 as in atmospheric air ; but that other portion 

 
 32 RELATIONS OF AIR 
 
 which is abstracted by the gradual recession 
 of the nearest heated particles of the fluid, 
 exceeds no less than four times the corre- 
 sponding discharge in the ordinary medium. 
 Why such a striking difference should arise, 
 can hardly be conjectured. Hydrogen gas, 
 though ten times lighter than air, yet con- 
 tains, in the same volume, an equal quantity 
 of heat ; and it is fitted, by its very superior 
 elasticity, to transmit the pulsatory impres- 
 sions more than three times faster. It must, 
 therefore, as a counterbalance, receive those 
 impressions three times slower from the heat- 
 ed surface. But if such influence be confined, 
 as it would seem' most probable, to the 
 mere boundary of the medium or its thinnest 
 conterminous shell, the measure of heat im- 
 bibed at a given rise of temperature from the 
 attenuated expanse, would be diminished be- 
 tween two and three times. This mutual 
 compensation of effect nearly agrees with the 
 actual result. With respect to the quadru- 
 pled increase of that portion of heat which is 
 abstracted by the slow but continued renewal
 
 TO HEAT AND MOISTURE. 33 
 
 of the adjacent stratum of the fluid, we must 
 refer it chiefly to the very great mobility of 
 hydrogen gas, exceeding three times that of 
 common air. If these strata were supposed 
 to have in both cases the same thickness, they 
 would each of them carry off the same share 
 of heat. 
 
 The portions of heat transmittedby pulsation 
 through hydrogen gas, from a painted and a 
 metallic surface, being, as before, expressed re- 
 spectively by 10 to 1, the other portion, which 
 is altogether independent of the nature of the 
 cooling surface, and is dispersed by abduction, 
 or the incessant retreat of the strata of the 
 fluid as they come to be successively affect- 
 ed, will amount* to 40. Under like circum- 
 stances, therefore, the whole expenditure of 
 heat from a painted and a metallic surface, 
 and which in atmospheric air was denoted by 
 20 and 11, will in hydrogen gas be represent- 
 ed by 50 and 41. Those opposite surfaces 
 are thus less contrasted in a medium of hydro- 
 gen gas, their different rates of discharging 
 heat being nearly in the proportion of 5 to 4. 
 
 .
 
 34 RELATIONS OP AIU 
 
 The silver ball cased with cambric, cools 2| 
 times faster, if immersed in hydrogen gas ; 
 but when exposed naked in the same fluid, it 
 loses its heat almost four times as fast as in 
 common air. 
 
 The superior mobility of hydrogen gas ac- 
 celerates remarkably the dispersion of heat, 
 by the process of abduction. But the exposing 
 of a heated body to the action of any cur-r 
 rent of a fluid substance, will occasion a simi- 
 lar expenditure of heat, and which is exact 
 ly proportioned to the celerity of the stream. 
 If a very large bulb of a thermometer be sud- 
 denly plunged into water flowing at the rate of 
 one-third of a mile in the hour, it will be found 
 to lose its heat twice as fast, as when immersed 
 in the stagnant pool ; and a current of two 
 miles in the hour would, therefore, cause 
 through the liquid a dissipation of heat no 
 less than seven times more rapid than usual. 
 A similar acceleration of effect is produced, by 
 the impulse of a stream of air. With a velo- 
 city of about four miles in the hour, the su- 
 peradded influence of a current equals the or-
 
 TO HEAT AND MOISTURE. 35 
 
 dinary power of abduction. Hence the play 
 of a breeze of eight miles an hour will double 
 the rate of cooling from a painted, and will tri- 
 ple that from a metallic, surface ; but a wind 
 sweeping with a velocity of forty miles in the 
 hour, would accelerate the cooling of the paint- 
 ed surface six times, and that of the metallic 
 one no less than eleven times, thus bringing 
 them both near an actual equality of perform- 
 ance. In general, the hourly velocity of wind 
 might be computed, by multiplying eight 
 miles into the proportional surplus effect ex- 
 erted in the refrigeration of a vitreous or 
 painted surface. 
 
 But even in still air, if the body ex- 
 posed to its action have a very consider- 
 able elevation of temperature, the progress 
 of cooling will be sensibly quickened, by the 
 continual ascent of the heated portions of the 
 medium, and which form in fact a stream, 
 varying in force according to the intensi- 
 ty of excitement. Supposing the excess of 
 temperature to be 30 centesimal degrees, or 
 54 by Fahrenheit's scale, this gentle per-
 
 36 RELATIONS OF AIR 
 
 pendicular flow of heated air will conjoin 
 an influence equal to the ordinary abduc- 
 tive dispersion of heat, and therefore cor- 
 responding to that of a current which moves 
 at the rate of four miles an hour. Hence, 
 if the silver ball be 90 centesimal degrees 
 hotter than the encircling air, the effect of 
 the vertical stream is tripled, or the aggre- 
 gate expenditure, from the painted and from 
 the naked surface, will be expressed by 50 and 
 41, the dissipation arising from the increased 
 flow of the medium amounting in each of 
 them to 30. By a singular coincidence, this 
 proportion is precisely the same as what ob- 
 tains near the equilibrium of temperature in 
 an atmosphere of hydrogen gas. But hydro- 
 gen gas betrays in its own constitution still 
 greater modifications. At the same elevation 
 of 90 centesimal degrees of temperature, the 
 combined powers of cooling which it exerts 
 on the contrasted surfaces, are expressed by 
 170 and 161. It would be fatiguing, how- 
 ever, to pursue this intricate analysis much 
 farther.
 
 TO HEAT AND MOISTURE. 37 
 
 The meteorological phenomena on which 
 the nature of climate chiefly depends, are 
 more immediately connected, however, with 
 the relations of air and moisture. The va- 
 pour which exhales from the surface of our 
 globe, and rises into the atmosphere, is again, 
 by the operation of certain disturbing causes, 
 precipitated in the form of clouds, dew and 
 rain, or of snow and hail. To discover the 
 disposition of the air with respect to mois- 
 ture, is therefore a problem of great import- 
 ance ; and an instrument which should at any 
 time indicate with facility and precision the 
 actual state of the medium in regard to hu- 
 midity or dryness^ must be considered as a 
 valuable acquisition to science. But the act 
 of evaporation itself, from which all those va- 
 rious appearances derive their origin, is at- 
 tended by two distinct circumstances that 
 seem to offer the means of constructing such 
 an instrument, the dilatation imparted to 
 the ambient medium, and the depression 
 of temperature produced on the humid sur- 
 face.
 
 38 RELATIONS OP AIR 
 
 1. Steam, in whatever way it is formed, has 
 probably double the elasticity of common air, 
 or it would, under the same pressure, occupy 
 about twice as much space. In uniting with that 
 fluid, it must hence communicate an expansion 
 which is exactly proportioned to the quantity 
 dissolved, or to the portion of moisture requir- 
 ed for the complete saturation of the air. 
 This principle appeared to suggest the means 
 of constructing an accurate Hygrometer* ', to 
 which my researches had been early directed. 
 Inverting a small barrel tumbler, I ground 
 the mouth perfectly flat ; and having drilled 
 a hole through the bottom, I cemented into 
 it, a sort of syphon gage or slender recurved 
 tube with a narrow bore, passing through a 
 perforated cap of lead, and holding a portion 
 of nut oil tinged with alkanet root, a scale 
 being affixed with divisions corresponding to 
 the 10,000 parts of the ordinary elasticity or 
 pressure of the air. Having now spread a few 
 drops of water over the surface of a bit of plate 
 
 * From vygej, moist or humid, and fttr^ov, a measure.
 
 TO HEAT AND MOISTURE. 39 
 
 glass, and slipped the tumbler upon this, the 
 included air quickly dissolved as much mois- 
 ture as was sufficient for its saturation, and 
 marked the expansion thus acquired, by forg- 
 ing the column of oil to rise proportionally. 
 The quantity of effect was often very consi- 
 derable, amounting in fine weather to 100 or 
 120 degrees. This little apparatus satisfied 
 my expectation, but, as it always required a 
 certain portion of address, I soon abandoned 
 it for other instruments, which promised to 
 be more easily and readily managed. With 
 some modification, however, it would prove 
 very serviceable in a variety of delicate phy- 
 sical inquiries, by detecting the minutest al- 
 terations of volume which take place in the 
 union of different substances, and thus point- 
 ing out the curious and extended influence 
 of chemical action. 
 
 2. But the process of evaporation has 
 not been hitherto examined with atten- 
 tion, or its consequences rightly understood. 
 The depression of temperature which al- 
 ways accompanies it, has been hastily sup-
 
 40 RELATIONS OF AIll 
 
 posed to be proportional to the rate with 
 which the moisture is dissipated, and to 
 be therefore augmented by every circum- 
 stance that can accelerate this effect. If wa- 
 ter contained in a porous vessel, expose on 
 all sides its surface to a current of air, it 
 will cool down to a certain point, and there 
 its temperature will remain stationary. The 
 rapidity of the current must no doubt hasten 
 the equilibrium, but the degree of cold thus 
 induced will be still the same. A little re- 
 flection may discover how this takes placa 
 Though the humid surface has now ceased to 
 grow colder, the dispersion of invisible va- 
 pour, and the corresponding abstraction of 
 heat, still continue without intermission. The 
 same medium, therefore, which transports the 
 vapour, must also furnish the portion of heat 
 required for its incessant formation. In fact, 
 after the water has been once cooled down, 
 each portion of the ambient air which comes 
 to touch the evaporating surface must, from 
 its contact with a substance so greatly denser 
 than itself, be likewise cooled down to the
 
 TO HEAT AND MOISTURE. 41 
 
 same standard, and must hence communi- 
 cate to the liquid its surplus heat, or the 
 difference between the prior and the subse- 
 quent state of the solvent, and which is pro- 
 portioned to the diminution of temperature 
 it has suffered. Every shell of air that in 
 succession encircles the humid mass, while 
 it absorbs, along with the moisture which 
 it dissolves, the measure of heat necessary 
 to convert this into steam, does at the same 
 instant thus deposite an equal measure of 
 its own heat, on the chill exhaling sur- 
 face. The abstraction of heat by vaporiza- 
 tion on the one hand, and on the other its 
 deposition at the surface of contact, are, 
 therefore, opposite contemporaneous acts, 
 which soon produce a mutual balance, and 
 thereafter the temperature induced continues 
 without the smallest alteration. A rapid cir- 
 culation of the evaporating medium may 
 quicken the operation of those causes ; but, 
 so long as it possesses the same drying quali- 
 ty, it cannot in any degree derange the re- 
 sulting temperature. The heat deposited by
 
 42 RELATIONS OF AIR 
 
 the air on the humid surface becomes thus 
 an accurate measure of the heat spent in va- 
 porizing the portion of moisture required for 
 the saturation of that solvent at its lowered 
 temperature. The dryness of the air is there- 
 fore, under all circumstances, precisely indi- 
 cated, by the depression of temperature pro- 
 duced on a humid surface which has been ex- 
 posed freely to its action. 
 
 In this investigation, we have only consider- 
 ed the effect arising from the recession or the 
 quickened transfer of the contiguous portions 
 of the ambient medium. But the contermi- 
 nous air must besides communicate heat to 
 the water by pulsation ; and consequently the 
 balance of temperature would be liable to in- 
 cidental variations, if moisture, with its em- 
 bodied heat, were not likewise abstracted by 
 some corresponding process. And such is the 
 harmonious adaptation of these elements. The 
 discharge of vapour appears to be subject 
 precisely to the same conditions as the emis- 
 sion of heat, and in both cases the proxi- 
 mity of a vitreous or a metallic surface pro-
 
 TO HEAT AND MOISTURE. 43 
 
 duces effects which are entirely similar. Let 
 two pieces of thin mirror-glass, or what is 
 called Dutch plate, be selected, about four 
 inches and a half square ; and having applied 
 a smooth coat of tinfoil, four inches square, 
 to one of these, cover them both with a layer 
 of the thinnest goldbeater's skin, which will 
 adhere closely on being wetted, and after it 
 has again become dry, cut it on each into an 
 exact square of four inches and a quarter : 
 Now place the two glass plates horizontally 
 in the opposite scales of a fine balance, and 
 adjust them to an exact counterpoise : then, 
 with a hair pencil, spread two grains of wa- 
 ter over the surface of each pellicle : in a 
 few seconds, the plate which is coated with 
 tinfoil will preponderate, and after the former 
 has lost all its moisture, this will be found 
 to retain still three-tenths of a grain. The 
 proximity of the subjacent metal to the hu- 
 mid surface, therefore, impedes the process of 
 evaporation, in the ratio of 17 to 20; the very 
 same, as in like circumstances, had been the 
 retardation of the efflux of heat. From this
 
 44 KELATION'S OF AIR 
 
 and other experiments, we learn, that some 
 constant portion from a humid surface is al- 
 ways abstracted by the pulsation of the aerial 
 medium. The steam exhaled, in uniting with 
 the air, communicates to this elastic fluid a 
 sudden dilatation, which will continue to pro- 
 pagate itself in successive waves. 
 
 Nor is the mode of transmission, by the 
 play of alternate pulses, confined to heat and 
 moisture alone; smells would appear to be 
 conveyed through the atmosphere by a simi- 
 lar agency. It is well known that, though 
 the wind disperses widely the odorous parti- 
 cles, yet their scent will, to a certain way, pe- 
 netrate even against the current. Nay, the 
 action of smell may, by means of a tapering 
 tube, be concentrated on the nostril, nearly 
 after the same manner as the impressions of 
 sound are collected and heightened in the ear- 
 trumpet. Having spread musk over a circular 
 bit of pasteboard, I placed jt a few inches be- 
 fore a small metallic reflector, in the focus of 
 which was suspended a glass bulb covered with 
 cambric, and another similar bulb set an inch
 
 TO HEAT AND MOISTURE. 45 
 
 aside from it : In a few minutes, these were 
 both examined, and the one which had oc- 
 cupied the focus smelt more strongly of musk 
 than the other. The experiment was next 
 varied, by stretching across the speculum a 
 sort of chaplet of pea-blossoms, marked with 
 slight incisions ; on presenting a small disk 
 of pasteboard covered with dry ammonia, the * 
 blossoms became of a greenish hue where- 
 ever the juices transuded, but the colour was 
 evidently the most intense near the focus. In 
 these cases, the odorous substance must, in 
 the act of dissolving in the air, have excited a 
 sort of pulsatory impression like that which is 
 caused by the production of sound. The only 
 difference appears to lie in this, that smell and 
 moisture, consisting of matter sensibly ponde- 
 rable, somewhat resemble wrecks floating on 
 the waves, and are consequently not carried 
 forward with the same accuracy, or to such a 
 great distance, as heat, which possesses the 
 inherent and extreme subtilty of light itself. 
 Having therefore ascertained the great law 
 of evaporation, and shown that the cold-
 
 46 DELATIONS OF AIR 
 
 ness occasioned by it, is^not, in any degree, 
 affected by agitation or other extraneous 
 jnfluence, nothing seemed wanting to con- 
 struct an Hygrometer on just principles, but 
 to contrive a thermometer that should mark 
 the smallest alterations of temperature. At 
 first I employed a very delicate thermometer 
 with a short range, open at the top, where 
 a small cap of glass or ivory was attached. A 
 cup made of thin porous earthen-ware, nearly 
 of the shape of a lady's thimble, but some- 
 what larger, and filled with water, was expos- 
 ed to the air, while the thermometer lay be- 
 side it in an horizontal position. (See Jig. 4. ) 
 After a few minutes, the thermometer was 
 lifted up and plunged vertically into the cup ; 
 and the thread of quicksilver, which had ex- 
 tended through the whole length of the bore, 
 being, by this change of position, cut off at 
 the top of the tube, immediately contracted, 
 and marked, by the space of its descent, the 
 diminution of temperature in the liquid. 
 . On a larger scale, but altogether unconsci- 
 ous of the true principle, the natives of the
 
 TO HEAT AND MOISTURE. 47 
 
 more sultry climates have long practised the 
 method of artificially cooling their liquors by 
 means of evaporation. The rude species of t 
 pottery so common among such people, could 
 not fail to suggest the application. The Moors 
 introduced into Spain those unglazed earthen 
 jugs, named bucaros or alcarrazas, which, be- 
 ing filled with water, present to the atmo- 
 sphere a surface constantly humid, and fur- 
 nish by evaporation, during the dry and hot 
 weather, a refreshing beverage. The same 
 practice has been adopted by degrees in va- 
 rious parts of the south of Europe. In In- 
 dia, during certain months, the apartments 
 are kept comparatively cool, by dashing wa- 
 ter against the matting of reeds or bamboos, 
 which line the doors and the outside of the 
 walls. Even the more luxurious mariners, 
 in their voyages between the tropics, are ac- 
 customed to cool their wines, by lapping 
 the bottle with wet flannel, and suspending 
 it from the yard or under the cabin-windows. 
 In all such cases, the effect is accelerated, 
 though not augmented, by the swiftness of
 
 48 RELATIONS OF AIR 
 
 the current of air. What have been called 
 Egyptian coolers., and lately produced by our 
 .potters, are less perfect in their operation. 
 Being very thick, they require only to be 
 soaked in water, and the evaporation from 
 their surface cools the adjacent air. On the 
 inside, however, where the bottle is placed^ 
 the action, in consequence of the confined hu- 
 midity, must be enfeebled. In damp weather, 
 those vessels are entirely useless. 
 
 The mode above described, for discovering 
 the dryness of the air, though susceptible 
 of considerable precision, yet, as it always re- 
 quired a sort of manipulation, was not quite 
 satisfactory ; and, in meditating farther on the 
 subject, I was led to the invention of the Dif- 
 ferential Thermometer, an instrument extreme- 
 ly simple in its construction, and of singular 
 utility in various delicate physical researches. 
 It is indeed only a modification, though a 
 most essential one, of the air thermometer, 
 consisting of a recurved or double stem like 
 the letter U, terminated by two hollow 
 balls which contain air, and holding an inter-
 
 TO HEAT AND MOISTURE. 49 
 
 mediate portion of sulphuric acid tinged with 
 carmine, the only substance apparently that 
 will, under the peculiar circumstances, strike 
 a fine and permanent colour. When these 
 balls are of the same temperature, the liquor 
 will remain stationary ; but if one of the 
 balls be warmer than the other, the liquor, 
 urged by the increased elasticity of the air, 
 will descend proportionally on that side. The 
 reason why I prefer such a fixed and ponde- 
 rous liquid as sulphurig acid, is that, in the 
 subsequent changes of temperature, it may 
 not affect the dryness, and therefore elastici- 
 ty, of the included masses of air which press 
 against it. Alcohol is lighter, and, in its mo- 
 tions, nimbler ; but then, by mixing its va- 
 pour with the differently heated portions of 
 air, it would dilate them unequally, and disturb 
 the accuracy of the indication. To measure 
 the difference of heat between the two balls, 
 I have adopted a milligiwde scale, or I dis- 
 tinguish the whole interval between freezing 
 and boiling water into 1000 equal parts. (See 
 
 .fig- i.; 
 
 D
 
 50 RELATIONS OF AIR 
 
 If the ball of the differential thermometer 
 that serves as a reservoir of the liquid, be gilt 
 completely with thick silver leaf, this modi- 
 fication will form the Pyroscope* ; an instru- 
 ment which is adapted to measure the pulsa- 
 tory commotion of the air, or the intensity 
 of the heat darting continually from the fire 
 into a room. The hot pulses are mostly 
 thrown back from the bright metallic surface, 
 but, upon the naked ball of glass, they pro- 
 duce their full impression, and consequently 
 make the coloured acid to sink proportion- 
 ally in the stem. In this construction, the 
 light of the fire, which is always comparative- 
 ly feeble, passing through the diaphanous 
 ball, and being reflected from the brilliant 
 convexity of the other, has no disturbing in- 
 fluence. The calorific action marked by the 
 pyroscope diminishes, on receding from the 
 fire, in the ratio of the square of its dis- 
 tance ; yet such is the sensibility of this in- 
 strument, as to be still affected at the re- 
 
 From -Trtfyjlre, and o4;>r?*ft#<, / examine.
 
 TO HEAT AND MOISTURE. 51 
 
 motest part of the room. Placed therefore 
 at the same given distance from any grate, it 
 will indicate the absolute force of the fire, 
 or will measure the tide of heat which flows 
 into the apartment. The facility of making 
 observations of this kind may, it is evident, 
 be of considerable use in various practical 
 concerns. To insure the accuracy of the in- 
 strument, however, the metallic coat should 
 be very carefully applied, and no external 
 trace left of the size which is employed to 
 fix the silver leaf. The ordinary modes of 
 gilding and enamelling are insufficient. 
 
 But the pyroscope will mark likewise the 
 pulsations from a cold surface. In a warm 
 room, it is visibly affected at a few inches 
 from an earthen jug filled with water just 
 drawn from the well. A sheet of wet paper 
 stretched before the counteracting balls, will 
 also, when the atmosphere is tolerably dry, 
 produce, from its chilled surface, a certain 
 corresponding impression. 
 
 This instrument serves besides to confirm 
 the existence of the hot or cold pulses excited
 
 52 RELATIONS OF AIR 
 
 in the air. Having procured a cone of pla- 
 nished tin, with the top cut off, near 6 inches 
 wide at the mouth, and about 14 inches long, 
 it was divided, in the direction of its axis, 
 into two equal portions, the inside of each of 
 them being painted with lamp black. Turn- 
 ing one of these semi-cones towards the fire, 
 and setting in its narrow neck the naked or 
 sentient ball of the pyroscope, the impression 
 was increased from 20, its direct and unaid- 
 ed effect, to 25 ; but, on adapting likewise 
 the other half of the cone, it rose to no less 
 than 70. Now if such augmentation of heat 
 were occasioned by any internal reflections, 
 the effect would only be doubled in the com- 
 plete cone, or carried from 25 to 30. This 
 great accumulation must, therefore, be re- 
 ferred to some other source; and what can 
 appear more probable as the cause, than the 
 gradual concentration of the aerial pulsations, 
 in their advance to the ball of the pyroscope? 
 J[f the differential thermometer have one 
 of its balls diaphanous, and the other coated 
 with China ink, or rather blown of deep black
 
 TO HEAT AND MOISTURE. 53 
 
 enamel, it will become a Photometer*, and in- 
 dicate the comparative force of the light to 
 which it is exposed. The rays which fall on 
 the clear ball pass through it, without suffer- 
 ing obstruction ; but those which strike the 
 dark ball are stopt and absorbed at its sur- 
 face, where, assuming a latent form, they act 
 as heat. This heat will continue to accumu- 
 late, till its farther increase comes to be coun- 
 teracted by an opposite dispersion, caused by 
 the rise of temperature which the ball has ac- 
 quired. At the point of equilibrium, there- 
 fore, the constant accessions of heat derived 
 from the action of the incident light, are ex- 
 actly equalled by the corresponding portions 
 of it, again abstracted in the subsequent pro- 
 cess of cooling. But, in still air, the rate of 
 cooling is, within moderate limits, propor- 
 tioned to the excess of the temperature of a 
 given surface above that of the surrounding 
 medium. Hence the space through which 
 the coloured liquid sinks in the stem, will 
 
 From <}>*$, light, and /uj?g*, a measure.
 
 54 RELATIONS OF AIR 
 
 measure the momentary impressions of light 
 or its actual intensity. To prevent any ex- 
 traneous agitation of the air from accelera- 
 ting the discharge of heat at the surface of the 
 black ball, and thereby diminishing the quan- 
 tity of aggregate effect, the instrument is al- 
 ways sheltered, and more especially out of 
 doors, by a thin glass case. The addition of 
 this translucid case is quite indispensable. It 
 not only precludes all irregular action, but 
 maintains, around the sentient part of the in- 
 strument, an atmosphere of perpetual calm. 
 Under the same force of incident light, the 
 temperature of the black ball must still rise 
 to the same height above that of its encir- 
 cling medium. The case will evidently have 
 some influence to confine the heat actually 
 received, and hence to warm up the internal 
 air. Wherefore, corresponding to this excess, 
 the black ball will acquire a farther eleva- 
 tion of temperature ; but the clear ball, being 
 immersed in the same fluid, must experience 
 a similar effect, and;which will exactly coun- 
 
 T. 
 
 terbalance the former. The difference of
 
 TO HEAT AND MOISTURE. 55 
 
 temperature between the opposite balls thus 
 continues unaltered; and neither has the size 
 or the shape of the case, nor the variable state 
 of the exterior atmosphere with respect to 
 rest or agitation, any influence whatever to 
 derange or modify the results exhibited by 
 this delicate instrument. The photometer 
 exhibits distinctly the progress of illumina- 
 tion from the morning's dawn to the full vi- 
 gour of noon, and thence its gradual decline 
 till evening has spread her sober mantle ; 
 it marks the growth of light from the winter 
 solstice to the height of summer, and its sub- 
 sequent decay through the dusky shades of 
 autumn ; and it enables us to compare, with 
 numerical accuracy, the brightness of diffe- 
 rent countries, the brilliant sky of Italy, for 
 instance, with the murky air of Holland. 
 
 But, with respect to photometrical obser- 
 vations, I have not yet had opportunities for 
 collecting a sufficient body of facts. In the 
 latitude of Edinburgh, the direct impression 
 of the sun at noon, during the summer sol- 
 stice, amounts to 90 degrees ; but it regular-
 
 56 HELATIONS OF v AIli 
 
 ly declines, as his rays become more oblique. 
 At the altitude of 17, it is already reduced 
 to the one half; and at 3 above the horizon 
 the whole effect exceeds not one degree. In 
 the same parallel of latitude, the greatest 
 force of the solar beams, in the depth of 
 winter, measures only 25 degrees. Their 
 diminished vigour is evidently caused by 
 the dispersion and absorption which they 
 must suffer in their protracted slanting pas- 
 sage through the atmosphere. Between a 
 fourth and a fifth part of the whole light of the 
 sun is lost in a vertical descent to the surface 
 of the earth ; but, in our insular sky, a thin 
 haze, even during the finest weather, gene- 
 rally floats near the horizon, and the succes- 
 sive waste corresponding to each equal num- 
 ber of aerial particles which a very oblique 
 ray encounters in its track, will often amount 
 to the third. 
 
 Of the quantity of indirect light which is 
 reflected from the sky, we are apt to form a 
 false estimate, in consequence of its being so 
 much attenuated by diffusion. But, though
 
 TO HEAT AND MOISTURE. 57 
 
 extremely fluctuating, it is often -very consi- 
 derable. In this climate, it may amount to 
 30 or 40 degrees in summer, and to 10 or 15 
 in winter. This secondary light is most 
 powerful when the sky is overspread with 
 thin fleecy clouds ; it is feeblest in two very 
 different conditions, either when the rays are 
 obstructed by a mass of thick congregated va- 
 pours, or when the atmosphere is quite clear 
 and of a pure azure tint. In mists and low fogs, 
 the diminution of the light is comparatively 
 small, it being then affected more from indis- 
 tinctness than through any want of intensity. 
 When the sky is obscured by a dense body 
 of clouds, the darkness seems to be much in- 
 creased in proportion to the obliquity of the 
 solar rays. In summer, the photometer, 
 placed in the open air at noon, seldom or ne- 
 ver marks less than 10 degrees ; but, in some 
 of those sable-shrouded days, which, in this 
 remote region, deform the winter, I have re- 
 peatedly observed, that the whole effect, un- 
 der similar circumstances, did hardly exceed 
 even one degree.
 
 58 RELATIONS OF Alii 
 
 In the higher regions of the atmosphere, 
 the rays of the sun, not being impaired by 
 such a length of passage, are more vigorous 
 than at the surface of the earth ; but the dif- 
 fuse indirect light of the sky, as it is reflected 
 from a rarer mass of air, is therefore propor- 
 tionally feebler. It would be most interest- 
 ing, to make observations of this kind on the 
 lofty summits of the Alps or the Andes. 
 The traveller who visits those elevated tracts, 
 is struck with the dark hue of the azure ex- 
 panse, through which his keen eye may, even 
 during the day time, discern the brighter pla- 
 nets and some stars of the first magnitude. 
 
 When the photometer stands quite detach- 
 ed out of doors, it must evidently receive the 
 rays which come from all sides. But set in 
 the inside of the window, it can only feel the 
 impression caused by a part of the sky ; and 
 unless it chance to front the quarter from 
 which the sun shines, it will there seldom in- 
 dicate more than 15 degrees. On drawing 
 the instrument back into the room, the effect 
 will rapidly decrease ; for the intensity of ac-
 
 TO HEAT AND MOISTURE. 59 
 
 tion is obviously proportional to the visual 
 space included by the window. Near two 
 degrees of light are required, to enable one 
 to read or write with pleasure ; a greater por- 
 tion of it offends by its excessive glare, and a 
 much smaller quantity tires and strains the 
 eyes. 
 
 Placed in open air, the photometer is not 
 only affected by the light sent from the sky, 
 but also, in some measure, by what is reflect- 
 ed from the ground This derangement, how- 
 ever, is generally very small, and may easily be 
 excluded altogether, by fixing a black screen 
 or circular horizontal rim about the glass-case 
 near the top of the scale. Hie reflection from 
 a green field perhaps exceeds not the twen- 
 tieth part of the whole incidence ; but it in- 
 creases considerably as the colour inclines 
 to whiteness. From a smooth sandy beach, 
 the reflected light will amount to the third 
 part of what is received from the sky ; and 
 from a wide surface of snow, it will reach to 
 five-sixths of the direct impression ; the nu- 
 merous facets of the bright snowv flakes.
 
 60 RELATIONS OF AIR 
 
 which are presented in every possible po- 
 sition, detaining only one-sixth of the inci- 
 dent rays, and scattering the rest in all di- 
 rections. 
 
 The photometer affords a ready mode of 
 ascertaining the various degrees of transpa- 
 rency. Of 100 parts of the whole incident 
 light, cambric transmits 80, and, when wetted, 
 93. Fine paper suffers 49 parts to pass 
 through it ; but, soaked in oil, it will allow 
 the passage of 80 parts. The wide dispersion 
 which the rays suffer in traversing paper and 
 other like substances, clearly shows that they 
 are not sent directly through the supposed 
 vacuities or pores, but escape by some intricate 
 tracks, and experience in their progress various 
 deflections occasioned by the repulsive and 
 attractive energies of the proximate matter. 
 The addition of water or oil to the cambric or 
 paper, forms a real chemical union, and be- 
 stows on the compound an intermediate cha- 
 racter, more nearly inclined however to the 
 nature of a fluid. 
 
 By help of the photometer, we can esti-
 
 TO HEAT AND MOISTURE. 61 
 
 mate the relative density of various artificial 
 lights, and even compare their power of illu- 
 mination with that of the solar rays. But 
 into this inquiry, however practically useful, 
 I have scarcely entered at ah 1 . On placing 
 the balls of the photometer two inches from 
 the flame of an ordinary wax candle, an im- 
 pression was received of six degrees; and, on 
 gradually drawing back the instrument, this 
 effect diminished, as, we might expect, in the 
 ratio of the square of the distance. Conse- 
 quently, at the distance of four feet, where the 
 flame would present to the photometer the 
 same visual magnitude as the sun himself, its 
 action would be reduced to the 96th part of a 
 degree. But the full impression of the solar 
 rays, if not enfeebled by their passage through 
 the atmosphere, would amount to 125 de- 
 grees. Wherefore the light emitted by the 
 sun is 96 times 125, or 12,000 times more 
 powerful than that of a wax candle ; or, if a 
 portion of the luminous solar matter, rather 
 less than half an inch in diameter, were,trans- 
 ported to our planet, it would throw forth a
 
 62 IlELATIONS OF All: 
 
 blaze of light equal to the effect of twelve 
 thousand candles. 
 
 Since the operation of the photometer de- 
 pends, on the mutual balance of the action of 
 light, with its subsequent effort to diffuse it- 
 self in the latent form of heat, this instrument 
 is hence fitted to discover, with delicate pre- 
 cision, the relative conducting powers, not 
 only of different gases, but of the same gas 
 in its various states of modification. While 
 the absorption of the incident rays continues 
 the same, the change of temperature which 
 they produce on the sentient ball must be in 
 the inverse ratio of the energy of the refrige- 
 rating process. Under an equal degree of ab- 
 sorption, a metallic surface is twice as much 
 affected as one of glass, because, by its con- 
 stitution, it cools twice as slow. If the sur- 
 rounding medium conduct the accumulated 
 heat more tardily than before, the impression 
 made on the opaque ball will be augmented ; 
 or if, on the contrary, it performs the disper- 
 sion with more rapidity, the effect will be 
 proportionally diminished.
 
 TO HEAT AND MOISTURE. 63 
 
 For the investigation of the conducting 
 powers of different media, I prefer a photome- 
 ter of the simplest kind, being no other than a 
 differential thermometer with straight upright 
 stems, and having its sentient ball blown of 
 black glass, or coated with a leaf of coloured 
 metal. Set on the transferrer of an air-pump, 
 it is then covered by a narrow receiver, of a 
 clear uniform substance, and swelling regu- 
 larly near the top. The included air can thus 
 be rarefied to any required degree, and other 
 gases occasionally introduced in its place. In 
 this state, the instrument is exposed to a 
 bright sun, while another photometer, of the 
 ordinary construction, and placed in a simi- 
 lar situation, serves to measure the correspon- 
 ding effects. 
 
 While the comparative photometer indi- 
 cated 100 degrees, the impression made in air 
 which had been rarefied 256 times, amounted 
 to 185 on the ball of black glass, and rose 
 even to 620 on a coat of coloured metal. In 
 such a thin medium, therefore, the heat is 
 conducted nearly twice as slow from a vitre-
 
 64 RELATIONS OF AIR 
 
 ous surface, and more than three times slower 
 from a metallic one. But, in hydrogen gas of 
 the common density, the effect, compared 
 with the same standard, was only 44 degrees 
 on a black ball and 56 on a gilt ball. This 
 gas hence conducts more than twice as fast 
 as atmospheric air from a surface of glass, 
 and almost four times faster from one of me- 
 tal. The same gas being rarefied 256 times, 
 its impression on the black ball was 96 de- 
 grees, and on the gilt one 156, being more 
 than doubled in the former, and almost tri- 
 pled in the latter. If the rarefaction be there-r 
 fore pushed to a certain length, hydrogen gas 
 will have its conducting power reduced to 
 nearly the same as that of common air. 
 
 The photometer has two general forms ; 
 the one portable, in which the black ball is 
 about an inch higher than the other, and bent 
 forward to the same vertical line or the axis 
 of the translucid cylindrical case, (see Jig. %.); 
 and the other stationary, having both its balls 
 of the same height, and reclining in opposite 
 ways ; the case being composed of a wide cy-
 
 TO HEAT AND MOISTURE. 65 
 
 Under surmounted by the larger segment of a 
 hollow glass sphere. (SeeJtg.S.). The portable 
 photometer admits likewise an outer case of 
 ebony or mahogany, which not only protects 
 it from risk or injury, but occasionally serves 
 when out of doors for holding it in an erect ' 
 position. The other form of the instrument, 
 however, though in some respects less com- 
 modious, is yet on the whole better adapted 
 for nice observations, since besides receiving ^^ 
 the light more regularly, its balls, from being / ^g * 
 on the same level, are not liable to be any \h,A 
 how disturbed in their indications by the dif- 
 ferent strata of unequally heated air. 
 
 But the sensibility of the photometer may 
 be very considerably augmented, by help of 
 a judicious combination of cases. If the black 
 ball be encircled by a series of concentric 
 shells of glass, though they freely admit the 
 influx of light, yet they will greatly retard 
 its subsequent dispersion in the form of heat, 
 and therefore promote a high degree of ac- 
 cumulation. Nor is the impression thus ex- 
 cited at all disturbed or diminished, by any
 
 66 11ELATIONS OF A1U 
 
 counteracting efforts of the clear ball, which 
 being situate without theinclosure and in open 
 space, maintains the temperature "of the atmo- 
 sphere. These spherical shells, each compos- 
 ed of two adapted segments, are chosen as 
 thin and clear as possible, their diameters ris- 
 ing in regular succession, by a difference of 
 at least half an inch. Every additional case 
 would nearly double the impression which 
 is made on the instrument. With six cases, 
 therefore, it would become ten times more 
 sensible, and consequently fitted to measure 
 the dilute shadings of light. The extent of 
 the scale must necessarily be contracted in 
 proportion to the enlargement of the degrees. 
 There is still another photometrical combi- 
 nation, but which is calculated only for mea- 
 suring parallel rays. It consists of a small 
 reflector made of copper plated with silver 
 and nicely hammered into a deep parabolic 
 figure, the black ball of a wide differential 
 thermometer being fixed in the focus, and 
 the front of the speculum covered with a circle 
 of clear glass having a slight degree of con-
 
 TO HEAT AND MOISTURE. 67 
 
 vexity. This compound instrument, placed 
 at the remote side of a room and directly 
 facing the window, will indicate, with great 
 precision, the very minute and variable quan- 
 tities of light which, during all the changes 
 of the seasons and fluctuations of the sky, will 
 penetrate across an apartment. 
 
 The ball of the differential thermometer 
 which contains the supply of coloured liquid, 
 being covered with several coats of cambric 
 or tissue paper, and wetted with pure water, 
 the instrument now forms a complete Hygro- 
 meter ; for it will mark, by the descent of the 
 column in the opposite stem, the constant div 
 minution of temperature which is caused by 
 evaporation from that humid surface, and it 
 must consequently express the relative dryness 
 of the ambient air. In a very short space, sel- 
 dom indeed exceeding two minutes, the full 
 effect is produced ; and under the same cir- 
 cumstances, it will continue unaltered, till the 
 whole of the moisture has exhaled. To ex- 
 clude entirely the mixture of photometrical 
 influence, or prevent any derangement which
 
 68 RELATIONS OP AIll 
 
 the action of light might otherwise occasion, 
 the opposite balls are made to exhibit nearly 
 the same colour and opacity, the naked one 
 being blown of green or blue glass, and the 
 papered one besides covered with a bit of 
 thin silk, of rather a light shade, so as to 
 take a deeper tint when moistened. 
 
 The hygrometer has, like the photometer, 
 two different forms ; the one portable, and 
 the other stationary. The former, having its 
 balls in the same perpendicular line, is pro- 
 tected by a case of wood or ivory, and fitted 
 for carrying in the pocket ; two or three drops 
 of pure water from the tip of a quill or a hair 
 pencil being applied to the surface of the co- 
 vered ball, and the instrument held in a ver- 
 tical position as often as it is used. (Seejig.6.). 
 The latter form is calculated for somewhat 
 greater accuracy than the other, since its balls, 
 though bent opposite ways, are on the same 
 level. In this construction of the instrument, 
 the covered ball, after being once wetted, is 
 kept constantly moist, by means of some 
 fibres of floss-silk passing close over it, and
 
 TO HEAT AND MOISTURE. 69 
 
 immersed, at the distance of a few inches in 
 a tall glass decanter full of water, with a stop- 
 per which leaves open a small projecting lip. 
 (See Jig. 5.). The capillary attraction of these 
 filaments conveys the liquid to the surface of 
 the humid ball, as fast as it wastes by evapo- 
 ration ; but to insure their regular action, the 
 silk should be previously soaked in hot water, 
 to extract any gum that may adhere to it, 
 and the mouth of the decanter should stand 
 a little higher than the balls of the hygrome- 
 ter. When thus arranged, the hygrometer 
 will, without any help, perform accurately 
 for weeks or even months ; and after the 
 silky filaments have become choked with 
 dust, their activity may be again restored by 
 washing them carefully with a fine wet 
 brush. 
 
 The condition of the atmosphere with re- 
 spect to dryness is extremely variable. In 
 our climate, the hygrometer will, during win- 
 ter, mark from 5 to 25 degrees ; but, in the 
 summer months, it will generally range be- 
 tween 15 and 55 degrees, and may even rise,
 
 70 DELATIONS OF AlR 
 
 on some particular days as high as 80 or 90 
 degrees. In thick fogs, the instrument stands 
 almost at the beginning of the scale ; it com- 
 monly falls before rain, and remains low du- 
 ring wet weather ; but it mounts powerfully 
 in continued tracts of clear and warm wea- 
 ther. The greatest dryness yet noticed, was 
 at Paris in the month of September, when it 
 reached to 120 degrees. But for want of 
 observations, we are totally unacquainted 
 with the real state of the air in the remote 
 and tropical climates. 
 
 When the indication of the hygrometer 
 does not exceed 15 degrees, we are directed 
 by our feelings to call the- air damp; from 30 
 to 40 degrees we begin to reckon it dry ; from 
 50 to 60 degrees we should account it very 
 dry, and from 70 degrees upwards we might 
 consider it as intensely dry. A room is not 
 comfortable, or perhaps wholesome, if it has 
 less than 30 degrees of dryness ; but the at- 
 mosphere of a warm occupied apartment will 
 commonly produce an effect of upwards of 
 50 degrees.
 
 TO HEAT AND MOISTURE. 71 
 
 But this hygrometer will perform its office 
 even if it be exposed to frost. The moisture 
 spread over the surface, and imbibed into the 
 coat of the papered ball, will first cool a few 
 degrees below the freezing point, and then 
 congeal quickly into a solid compound mass. 
 The moment in which congelation begins, a 
 portion of heat liberated in that act brings the 
 ball back to the temperature of freezing, and 
 the coloured liquor, in proportion to the cold- 
 ness of the external air, starts up in the oppo- 
 site stem, where it remains at the same height, 
 till the process of consolidation is completed. 
 After the icy crust has been formed, evapora- 
 tion again goes regularly forward ; and if new 
 portions of water be applied, the ice will, from 
 the union of those repeated films, acquire a 
 thickness sufficient to last for several days. The 
 temperature of the frozen coat becomes lower- 
 ed in proportion to the dryness of the atmo- 
 sphere. The measure of heat deposited on 
 the chill surface by .the contact of the ambi- 
 ent air is then counterbalanced by the two 
 distinct, though conjoined measures of heat,
 
 72 RELATIONS OF AIR 
 
 abstracted in the successive acts of converting 
 
 O 
 
 the exterior film of ice into water and this 
 water into steam ; which transformations that 
 minute portion must undergo, before it can 
 unite with its gaseous solvent. But the heat 
 required for the melting of ice being about 
 the seventh part of what is consumed in the 
 vaporization of water, it follows that the 
 hygrometer, when the surface of its sen- 
 tient ball has become frozen, will, in like 
 circumstances, sink more than before by 
 one degree in seven. This inference is en- 
 tirely confirmed by observation. Suppose, 
 in frosty weather, the hygrometer, placed on 
 the outside of the window, to stand at 28 
 degrees ; it may continue for some consider- 
 able time at that. point, until the congelation 
 of its humidity commences : but after this 
 change has been effected, and the equilibrium 
 again restored, the instrument will now mark 
 32 degrees. 
 
 The theory of this hygrometer will enable 
 us to determine not only the relative, but 
 even the absolute dryness of the air or the
 
 TO HEAT AND MOISTURE. 73 
 
 quantity of moisture which it can absorb, by 
 comparing the capacity of that solvent with 
 the measure of heat required to convert a 
 given portion of water into steam. To dis- 
 cover the capacity of air, is however a pro- 
 blem of great difficulty, and it has not been 
 yet ascertained with much precision. It is 
 generally estimated, I am convinced, by far 
 too high ; and from several concurring ob- 
 servations, I should reckon the capacity of 
 air to be only three-eighth parts of that of 
 water. But 600 centigrade degrees, or 6000 
 on the millesimal scale, being consumed in 
 the vaporization of water, this measure of 
 heat would prove sufficient to raise an equal 
 mass of air 16,000 millesimal degrees, or 
 those 6000 degrees augmented in the ratio 
 of 8 to 3. Now, at the state of equipoise, 
 the quantity of heat that each portion of the 
 aerial medium deposites in touching the chill 
 exhaling surface, or what answers to the de- 
 pression of temperature which it suffers from 
 this contact, must, as we have seen, be ex- 
 actly equal to the opposite measure of heat
 
 74 RELATIONS OP Alll 
 
 abstracted by it in dissolving its correspond- 
 ing share of moisture. Wherefore, at the 
 temperature of the wet ball, atmospheric air 
 would take up moisture amounting to the 
 16,000th part of its weight, for each degree 
 marked by the hygrometer. Thus, supposing 
 the hygrometer to mark 50 degrees, the air 
 would then require humidity equal to the 
 320th part of its weight for saturation at its 
 reduced temperature. When the papered 
 ball of the hygrometer is frozen, the degrees 
 on this instrument must have their value in- 
 creased by one-seventh, so that each of them 
 will now correspond to an absorption of mois- 
 ture equal to the 14,000th part of the weight 
 of the air. 
 
 But the value of those degrees becomes 
 augmented in a much higher proportion, if 
 the hygrometer be immersed in hydrogen gas. 
 Since this very dilute medium has ten times 
 the capacity of common air, the quantity 
 of heat which, under similar circumstances, 
 it will deposite on the evaporating surface, 
 must likewise, from the same principle of
 
 TO HEAT AND MOISTURE. 75 
 
 mutual balance, be tenfold greater, and conse- 
 quently each hygrometric degree will indicate 
 an absorption of moisture equal in weight to 
 the 600th part of the solvent. The energy of 
 hydrogen gas is therefore not less remark- 
 able in dissolving moisture than in contain- 
 ing heat. 
 
 If a large receiver, having a delicate hy- 
 grometer suspended within it, be placed on 
 a brass plate and over a metal cup containing 
 some water ; the included air will, from the 
 solution of the moisture, become gradually 
 damper, and this progressive change is mark- 
 ed by the instrument. Yet the mass of air 
 will never reach its term of absolute humidi- 
 ty, and before the hygrometer points at 5 de- 
 grees, the inside of the receiver appears co- 
 vered with dew. While the humifying pro- 
 cess therefore still goes on, the close attrac- 
 tion of the glass continually robs the conti- 
 guous air of a portion of its moisture ; so 
 that, a kind of perpetual distillation is main- 
 tained through the aerial medium ; the va- 
 pour successively formed, being again con-
 
 76 RELATIONS OF AIll 
 
 densed on the vitreous surface. But if, in- 
 stead of the receiver, there be substituted a 
 vessel formed of polished metal, the confined 
 air will pass through every possible degree of 
 humidity, and the hygrometer will, after some 
 interval, arrive at the beginning of its scale. 
 
 The contrasted properties of a vitreous and 
 a metallic surface in attracting and repelling 
 moisture, may be shown still more easily. In 
 clear, calm weather, let a drinking glass and 
 a silver cup be placed empty near the ground, 
 on the approach of evening ; and as the damp- 
 ness begins to prevail, the glass will become 
 insensibly obscured, and next wetted with 
 profuse dew, before the metal has yet betray- 
 ed any traces of humidity. 
 
 Glass and the metals, particularly silver, 
 thus again manifest similar dispositions with 
 respect to heat and moisture, and their distinc- 
 tive powers would seem to proceed from the 
 same cause, or from the different approxima- 
 tions of the atmospheric boundary. But this 
 conclusion is at once put beyond all doubt, 
 by an experiment made with a modification
 
 TO HEAT AND MOISTURE. 77 
 
 of the hygrometer. Let each ball of a pyro- 
 scope be covered with a coat of the thinnest 
 gold-beater's skin and moistened equally, any 
 excess of humidity being taken away by touch- 
 ing the necks of those balls with a folded bit 
 of cambric. In this state, supposing the dry- 
 ness of the room to be 60 degrees, the co- 
 loured liquor will, for the space of two or 
 three minutes, remain stationary at the com- 
 mencement of the scale ; but it will then be- 
 gin to descend, and will in five minutes sink 
 to 20 degrees, where it will continue for a short 
 time, and afterwards slowly mount again till it 
 recovers its position of rest. It is evident, 
 therefore, since the opposite effects are at 
 first completely balanced, that the same 
 change of temperature must have been effect- 
 ed by evaporation on both the balls, or that 
 the encircling air dissolved a quantity of 
 moisture at each surface exactly proportioned 
 to the heat which it there deposited. The 
 efficacy of air in warming up such chilled 
 surfaces, was already shown to be modified, 
 by the proximity of the subjacent glass and
 
 78 RELATIONS OF AIR 
 
 metal, in the ratio of 20 to 17. Consequent- 
 ly, if the evaporation had been as copious 
 from the gilt as from the vitreous surface, the 
 effect produced on the former would have 
 amounted to 60 degrees, and on the latter to 
 only 51 degrees ; or the liquor, instead of re- 
 maining perfectly balanced, would have sub- 
 sided 9 degrees in the stem. It follows, there- 
 fore, that the naked ball must spend 20 parts 
 of moisture, while the gilt one loses only 17 ; 
 and hence the former will become sooner dry 
 than the latter, or the covered pyroscope will 
 soon act partially as a hygrometer, indicating 
 the difference between the states of its oppo- 
 site balls. On applying another pellicle to each 
 ball, the effects of evaporation were balanced 
 for nearly five minutes, when the liquor under 
 the vitreous surface began to sink, and in 
 about ten minutes it fell to 12 degrees, from 
 which it afterwards slowly remounted. More 
 coats being added, similar appearances, though 
 less striking, were observed, till they had ac- 
 quired a thickness equal to the 500th part of 
 an inch; after which, no alteration took place,
 
 TO HEAT AND MOISTURE. 79 
 
 and the column of liquor was held in steady 
 equipoise during the whole time that any 
 moisture remained on both the balls. 
 
 If the papered ball of an hygrometer be 
 suffered to become dry, the instrument, even 
 in that state, will mark, though for a short 
 time only, the different condition of the media 
 into which it is transported. Thus, the air of 
 a room being supposed to have 50 degrees of 
 dryness, on carrying the quiescent hygrome- 
 ter into another apartment of 70 degrees, the 
 column of liquor would fall near 20 degrees, 
 from the renewed evaporation of that portion 
 of moisture which had still adhered to the 
 coats of paper. But if the same instrument 
 were carried into an apartment of only 30 
 degrees of dryness, the coloured liquor would 
 actually rise near 20 degrees above the be- 
 ginning of the scale, the paper now attracting 
 the excess of humidity from the air. This va- 
 pour, in combining with it, passes into the 
 state of water, and therefore evolves a corre- 
 sponding share of heat. The equilibrium, 
 however, unless the coats of paper have a
 
 80 RELATIONS OF AIR 
 
 considerable thickness, is again restored in a 
 very few minutes. 
 
 Those changes are most readily perceived, 
 on immersing the quiescent hygrometer al- 
 ternately in two receivers containing air 
 drier and damper than that of the room. 
 If a pyroscope, having both its balls cover- 
 ed with gold-beater's skin, be treated in the 
 same way, it will indicate an effect, though 
 momentary indeed, of a similar kind : For, 
 in air which is drier, the pellicle of the naked 
 ball will throw off its moisture more freely 
 than that of the gilt ball ; and in damper air 
 it will, on the contrary, imbibe the surplus 
 humidity with greater eagerness ; thus losing 
 some portion of heat in the one process, and 
 gaining a minute accession in the other. The 
 quantity of moisture concerned in producing 
 such fleeting alterations, may not exceed the 
 thousandth part of a grain. 
 
 The separate and distinct effects of evapo- 
 ration, the coldness it occasions, and the 
 quantity of moisture it abstracts, are obvi- 
 ously seen in the hygrometer, as contrasted
 
 TO HEAT AND MOISTURE. 81 
 
 with an instrument which I have lately formed 
 to measure the quantity of exhalation from a 
 humid surface in a given time, and which I 
 therefore call an Atmometer *. (See Jig. 8.J 
 This instrument consists of a thin ball of po- 
 rous earthen-ware, two or three inches in dia- 
 meter, with a small neck, to which is firmly 
 cemented a long and rather wide tube, bear- 
 ing divisions, each of them corresponding to 
 an internal annular section, equal to a film 
 of liquid that would cover the outer surface 
 of the ball to the thickness of the thousandth 
 jjprt of an inch. These divisions are ascer- 
 tjined by a simple calculation, and number- 
 ed downwards to the extent of 100 or 200 ; 
 to the top of the tube is fitted a brass cap, 
 having a collar of leather, and which, after 
 the cavity has been filled with distilled or 
 boiled water, is screwed tight. The outside 
 of the ball being now wiped dry, the instru- 
 ment is suspended out of doors, and exposed 
 to the free action of the air. 
 
 From ctlfttf, exhalation or vapour^ and fttl^n f a measure. 
 F
 
 82 RELATIONS OP AIR 
 
 Evaporation is always proportioned to the 
 extent of the humid surface. If a sheet of 
 wet paper be applied to a plate of glass, it 
 will, in a close room, lose its weight exactly 
 at the same rate, whether it be held vertical- 
 ly or horizontally, and whether it occupies 
 the upper or the under side of the plate. 
 The quantity of evaporation from a wet ball 
 is the same as from an equal plane surface, 
 or from a circle having twice the diameter of 
 the sphere. In the atmometer, the humidi- 
 ty transudes through the porous substance, 
 just as fast as it evaporates from the ex 
 ternal surface ; and this waste is measurexj* 
 by the corresponding descent of the water in 
 the stem. At the same time, the tightness 
 of the collar, taking off the pressure of the 
 column of liquid, prevents it from oosing so 
 profusely as to drop from the ball ; an incon- 
 venience which, in the case of very feeble 
 evaporation, might otherwise take place. As 
 the process goes on, a corresponding portion 
 of air is likewise imbibed by the moisture on 
 the outside, and being introduced into the
 
 TO HEAT AND MOISTURE. 83 
 
 ball, rises in a small stream, to occupy the 
 space deserted by the subsiding of the water 
 in the tube. The rate of evaporation is no- 
 wise affected by the quality of the porous 
 ball, and continues exactly the same when 
 the exhaling surface appears almost dry, as 
 when it glistens with abundant moisture. 
 The exterior watery film attracts moisture 
 from the internal mass with a force inversely 
 as its thickness, and will therefore accommo- 
 date the supply precisely to any given degree 
 of expenditure. When this consumption is 
 excessive, the water may be allowed to per- 
 colate, by unscrewing the cap, avoiding how- 
 ever the risk of letting it drop from the ball. 
 In still air, the indications of the hygrome- 
 ter, and those of the atmometer, bear the same 
 proportion ; and the quantity of evaporation 
 for every hour is expressed, in thousandths 
 of an inch in depth, by the twentieth part of 
 the hygrometric degrees. For example, in 
 this climate the medium dry ness in winter 
 being reckoned 15, and in summer about 
 40, the daily exhalation from a sheltered
 
 84 RELATIONS OF A1U 
 
 spot will amount in winter to a thickness of 
 .018, and in summer to .048 decimal parts of 
 an inch. If we reckon the mean daily eva- 
 poration from the ground while screened at 
 .030, the waste during the whole year will 
 amount to near 1 1 inches, being scarcely the 
 half perhaps of what, under the circulation 
 of the atmosphere, actually obtains. The 
 dissipation of moisture indeed is vastly ac- 
 celerated by the action of sweeping winds, 
 the effect being sometimes augmented 5 or 
 10 times. In general, this augmentation is 
 proportional, as in the case of cooling, to the 
 swiftness of the wind, the action of still air 
 itself being reckoned equal to that produced 
 by a celerity of 8 miles each hour. Hence 
 the velocity of wind is easily computed, from 
 a comparison of the indications of an hygro- 
 meter with an atmometer, or of a sheltered, 
 with those of an exposed, atmometer. Thus, 
 suppose the hygrometer to mark 40 degrees, 
 or the column in a sheltered atmometer to 
 subside at the rate of 2 divisions each hour, 
 while in one exposed to the current, the de-
 
 TO HEAT AND MOISTURE, 85 
 
 scent is 12 divisions ; then, as 2 is to 10, the 
 superadded effect of the wind, so is 8 to 40 
 miles, its velocity during the hour. 
 
 It is curious to remark, what a small propor- 
 tion of any stream of air can acquire heat or 
 moisture, by flowing over a warm or a humid 
 surface. Supposing the air to have 20 degrees 
 of dryness, the ordinary evaporation would 
 every hour equal a film of the thousandth 
 part of an inch thick. But this portion of 
 moisture would be sufficient, we have seen, 
 to saturate 800 times its weight of air at such 
 a low state of dryness ; or reckoning the air 
 850 times lighter than water, this weight 
 would correspond to that of a cylinder of air 
 57y feet high, and having its base equal to 
 the surface of the humid ball, -or to a cylinder 
 230 feet high, and of the same diameter as 
 that ball. Now, since the ordinary evapora- 
 tion at 20 degrees of the hygrometer, is equal 
 to the increased effect occasioned by a Cur- 
 rent of air, moving with the velocity of 8 miles 
 in the hour, and forming therefore against 
 the ball a cylinder of 42,240 feet in height ;
 
 86 RELATIONS OP AIR 
 
 it hence follows, that not more than the 
 184th part of this advancing column can be 
 humified, by its streaming over the surface of 
 the ball. Such communication of moisture 
 is no doubt confined within the narrow limits 
 of physical contact. Each minute portion of 
 air which comes to graze along the humid 
 surface has its velocity retarded, and acquir- 
 ing new elasticity from the moisture which it 
 dissolves, it is quickly thrown back into the 
 current. On the rapidity of these successive 
 contacts, will depend the absolute quantity 
 of evaporation. 
 
 But, in perfectly calm air, the power of 
 evaporation, if it be very considerable, will 
 yet, as in the case of a heated surface, create 
 an artificial stream, which mingles its influence 
 with the ordinary dissipation of moisture. 
 When the hygrometer marks 75 degrees, this 
 current will have a corresponding velocity 
 of one mile every hour, and must there- 
 fore augment the regular effect of evapora- 
 tion by an eighth part. In general, to find 
 the correct hourly evaporation in a medium
 
 TO HEAT AND MOISTURE. 87 
 
 of still air, as expressed in atmometric divi- 
 sions, or the thousandths of an inch of super* 
 ficial thickness, after having divided by 20, 
 the number of degrees indicated by the hy- 
 grometer, let the quotient be increased in the 
 ratio of that number to 600. Thus, if the 
 hygrometer were to mark 30 degrees, then 
 1.5 is the approximate measure of evapora- 
 tion ; and since 30 is contained 20 times in 
 600, the correction to be added to 1.5 is like- 
 wise its twentieth part, or .075 ; so that the 
 hourly evaporation, estimated in f 'atmometric 
 divisions, amounts to 1.575, and the daily to 
 18.9. This correction is however in most in- 
 stances so small, that it may, without material 
 inaccuracy, be entirely overlooked. But, in con- 
 fined hydrogen gas, at the same state of dry- 
 ness, the atmometer is as much affected as if 
 it were exposed in open air to a wind having 
 the velocity of 12 miles an hour ; and conse- 
 quently the dispersion of moisture in such a 
 powerful medium is, like that of heat under 
 such circumstances, two and a half times more 
 profuse than in atmospheric air.
 
 88 RELATIONS OF AlK 
 
 The atmometer is an instrument evidently 
 of extensive application and of great utility 
 in practice. To ascertain with accuracy and 
 readiness the quantity of evaporation from 
 any surface in a given time, is an important 
 acquisition, not only in meteorology, but in 
 agriculture, and the various arts and manu- 
 factures. The rate of exhalation from the sur- 
 face of the ground is scarcely of less conse- 
 quence than the fall of rain, and a knowledge 
 of it might often direct the farmer advanta- 
 geously in his operations. On the rapid dis- 
 persion of moisture, depends the efficacy of 
 drying houses, which are too frequently con- 
 structed most unskilfully, or on very mistaken 
 principles. But the purposes to which the 
 atmometer so aptly applies, were hitherto 
 supplied in a rude and imperfect manner. 
 The loss that water sustains in a given time 
 from evaporation, has commonly been esti- 
 mated by weight or measure. If a piece of 
 flannel, stretched by a slender frame, be wet- 
 ted and suspended in the free air, its dissipa- 
 tion of moisture, after a certain interval, is
 
 TO HEAT AND MOISTURE. 89 
 
 found by help of accurate scales ; or if wa- 
 ter in a shallow pan be exposed in a simi- 
 lar situation, its daily waste is detected by 
 the application of a finely divided rod or 
 gage. But these methods are extremely 
 troublesome, and are subject besides, especi- 
 ally the latter one, to great inaccuracy. Both 
 the flannel and the sheet of water require to 
 be sheltered against the wind and rain, and 
 consequently they will not exhibit, like the 
 atmometer, the real exhalation which takes 
 place from the. ground. The bottom and 
 sides of the pan must also, from their extent 
 of dry surface, affect the temperature of the 
 water, and consequently modify the quantity 
 of evaporation. 
 
 An atmometer suspended in still air might 
 therefore, on taking into account the time 
 intervened, answer nearly the purpose of the 
 hygrometer ; and this mode can be employ- 
 ed with advantage, in discovering the mean 
 dryness of an apartment after the lapse of 
 hours or days. But the delicacy of the hy- 
 grometer indicates directly, and almost spon-
 
 90 RELATIONS OF AIR 
 
 taneously, the actual dryness of the medium. 
 This instrument is hence indispensable in all 
 meteorological observations, and may contri- 
 bute essentially towards laying the founda- 
 tion of a juster and more comprehensive 
 knowledge of the various modifications which 
 take place in the lower regions of our atmo- 
 sphere. Heat and moisture are the chief 
 agents which nature employs in producing 
 those incessant changes ; and if the inven- 
 tion of the thermometer has tended so much 
 to correct and enlarge the views of physical 
 science, may not the introduction of an accu- 
 rate hygrometer be expected to confer a si- 
 milar benefit, and to direct our researches 
 into many departments that are still unex- 
 plored ? To possess the means, for instance, 
 of comparing distant climates, must be deem- 
 ed highly important. We have at length ac- 
 quired tolerably complete notions of the gra- 
 dation of warmth, in the whole extent from 
 the frigid to the torrid zone ; but, concern- 
 ing the degrees of dryness which actually 
 prevail in remote parts of the globe, we are
 
 TO HEAT AND MOISTURE. 91 
 
 left merely to draw conjectural inferences 
 from a few loose reports, depending on the 
 imperfect and often illusive impressions re- 
 ceived by the organs of sense. It would be 
 most interesting to ascertain the dampness 
 which has been supposed to pervade the 
 American Continent and its adjacent Archipe- 
 lago, and to detect the dryness which scorches 
 the sandy deserts of the interior of Africa* 
 and parches the wide and lofty plains of Up- 
 per Asia. Nor would it be less curious to 
 discover the real qualities of those singular 
 winds, which, under the names of sirocco, har- 
 mattan, and simoom, infest the hotter climates, 
 and are described by travellers as working 
 such strange and formidable effects on the 
 animal frame. Even in this island, the seve- 
 ral winds have their distinct characters. If it 
 blows from the northern quarter, x the hygro- 
 meter generally inclines to dryness ; but a 
 southerly wind, along with warmth, invari- 
 ably brings an excess of humidity. In clear 
 and calm weather, the air is always drier near 
 the surface during the day than at a certain
 
 92 RELATIONS 0V AIll 
 
 height above the ground, but it becomes 
 damped on the approach of evening, while, 
 at some elevation, it retains a moderate de- 
 gree of dryness through the whole of the 
 night. If the sky be clouded, less alteration 
 is betrayed in the state of the air, both du- 
 ring the progress of the day and at different 
 distances from the ground ; and if wind pre- 
 vail, the lower strata of the atmosphere, thus 
 agitated and intermingled, will be reduced to 
 a still nearer equality of condition. 
 
 In the regulating of many processes of art, 
 and in directing the purchase and selection of 
 various articles of produce, the application of 
 the hygrometer would render material service. 
 Most warehouses, for instance, require to be 
 kept at a certain point of dryness, and which 
 is higher or lower according to the purposes 
 for which they are designed^ The printing 
 of linen and cotton is carried on in very dry 
 rooms, but the operations of spinning and 
 weaving succeed best in air which rather in- 
 clines to dampness. The manufacturer is 
 at present entirely guided by observing the ef-
 
 TO HEAT AND MOISTURE. 93 
 
 fects produced, and hence the goods are often 
 shrivelled, or otherwise injured, before he can 
 discover any alteration in the state of the 
 medium. But were an hygrometer, even 
 of the most ordinary construction, placed in 
 the room, it would exhibit every successive 
 change in the condition of the air, and imme- 
 diately suggest the proper correction. The 
 same means could be employed most bene- 
 ficially, in attempering the atmosphere of pu- 
 blic hospitals. 
 
 That wool and corn have their weight con- 
 siderably augmented by the presence of mois- 
 ture, is a fact well known. Without sup- 
 posing that any fraudulent practices are used, 
 this difference, owing merely to the variable 
 state of the air in which the substances are 
 kept, may yet in extreme cases amount to 
 10 or even 15 per cent. Grain or paper 
 preserved in a damp place, will be found to 
 swell nearly after the same proportion. But 
 the real condition of such commodities might 
 easily be detected, by placing the hygrometer 
 within a small wired cage, and heaping over
 
 94 RELATIONS OF AIR 
 
 this, for a few minutes, a quantity of the 
 wool or grain which is to be examined. 
 
 The hygrometer will enable us to compare 
 with accuracy the various absorbent powers 
 of different substances. These powers are 
 totally distinct from the force of capillary ac- 
 tion, and appear to be as much diversified 
 as the other intimate properties of bodies. 
 When water ascends the narrow bore of a 
 glass tube, or spreads itself through the 
 interstices of sand, it occasions no change 
 whatever in their internal constitution, and 
 the glass and the sand retain precisely the 
 same place and condition as before. But 
 when wood or ivory, linen or paper, im- 
 bibe moisture, this act of absorption is inva- 
 riably attended in them, by a corresponding 
 diminution of the aggregate volume, and 
 some disengagement of heat. The instru^ 
 ment already described, for measuring the 
 expansion which air acquires on being damp- 
 ed, shows a very visible contraction of bulk 
 in the union of a bit of linen with a few drops 
 of water. The heat evolved in this union is
 
 TO HEAT AND MOISTURE. 95 
 
 not less remarkable, and it is always propor- 
 tioned to the dryness of the previous state of 
 the absorbing substance. Let a large piece of 
 cambric or linen be dried intensely before 
 the fire, and then lap it about the bulb of a 
 delicate thermometer, and introduce them 
 both into a tall narrow glass, which is placed 
 in a close room, with a stopped phial con- 
 taining water set beside it. After the space 
 of an hour or two, when this apparatus has 
 acquired the same temperature, pour a little 
 water from the phial upon the cambric, and 
 the bulb of the thermometer will become in- 
 stantly affected, and indicate an extrication 
 of heat, amounting perhaps to three or four 
 degrees on Fahrenheit's scale. With oak 
 
 O 
 
 saw-dust, which has been previously parched, 
 the effect is still more striking ; and the well- 
 known fact, that biscuit recently baked feels 
 hot in the mouth, must evidently be referred 
 to the same principle. But the hygrometer 
 itself, in its quiescent state, displays conspi- 
 cuously, although on a smaller scale, proper- 
 ties which are entirely similar. If the paper-
 
 96 KELATIONS OF AIR 
 
 ed ball be suffered to grow dry, and two or 
 three drops of water at the ordinary tempe- 
 rature be applied to it, the coloured liquor 
 will quickly rise in the opposite stem five or 
 perhaps ten millesimal degrees, according to 
 the thickness of the coat and its previous in- 
 tensity of dryness. This impression however 
 is only momentary, and soon succeeded by 
 the proper hygrometric action. Heat was 
 therefore at first disengaged, during the act 
 of combination of the water with the bibu- 
 lous paper. 
 
 But those absorbent substances, besides 
 assimilating to their essence a portion of the 
 liquid which touches them, are likewise dis- 
 posed to attract* though with various energy, 
 the humidity from the atmosphere. The 
 more solid, as well as the softer, materials ex- 
 ert this power* and which is exactly analo- 
 gous to that of the concentrated acids and 
 the deliquescent salts. In their several affi- 
 nities to moisture, the earthy bodies discover 
 the most essential differences of constitution. 
 To examine these properties, let the sub-
 
 TO HEAT AND MOISTURE. 97 
 
 stance be dried thoroughly and almost roast- 
 ed before a strong fire, and introduced im- 
 mediately into a phial with a close stopper ; 
 the powder, having undergone that sort of pre- 
 paration, is at any time afterwards thrown par- 
 tially into a very large wide-shaped bottle, and 
 shut up till it has attracted its share of humi- 
 dity from the confined air; and a delicate hy- 
 grometer, being now let down into the bottle, 
 indicates the measure of the effect produced 
 by absorption. In this way, it was found that, 
 about the temperature of 60 on Fahrenheit's 
 scale, alumine causes a dryness of 84 degrees 
 in the air included with it ; the carbonate 
 of magnesia, 75 ; the carbonate of lime, 70 ; 
 silica, 40 ; the carbonate of barytes, 32 ; and 
 that of strontites, only 23. These simple bo- 
 dies also betray other differences in their in- 
 ternal constitution. The absorbing powers 
 of silica, barytes and strontites, are not only 
 feeble, but of small extent, and these earths 
 become very soon saturated with humidi- 
 ty. Alumine and magnesia, on the other 
 hand, have their energy scarcely impaired
 
 98 KELATIONS OF AIll 
 
 by repeated absorptions. It is singular, 
 that marble and quicklime should produce 
 exactly the same effect, and that in general 
 no sensible difference in that respect can 
 be perceived between the pure earths and 
 their carbonates. The great absorbent power 
 of the argillaceous, compared with the sili- 
 ceous bodies, likewise deserves particular no- 
 tice. But many of the compound earths and 
 stones possess the power of attracting mois- 
 ture from the air, in a still higher degree. 
 These substances need only be pounded or 
 broken into small fragments, and then expos- 
 ed, for a certain time, near a strong fire. 
 Pipe clay, though it contains a large pro- 
 portion of silica, occasioned a dryness of 
 85 degrees ; whinstone or trap, though 
 very compound, produced an effect equal 
 to 80 degrees ; and sea-sand, having an ad- 
 mixture of shells, gave a dryness of 70 de- 
 grees. But torrefaction seems remarkably to 
 diminish the faculty of the earthy substances 
 to attract moisture. Clay, roasted in a strong 
 fire, causes a dryness of only 35 degrees ; and 
 urged in a blacksmith's forge, it affords no
 
 TO HEAT AND MOISTURE. 99 
 
 more than 8, while whinstone or trap, by 
 the same treatment, has its energy reduced 
 to 23 degrees. The absorbent power of 
 earths depends, therefore, as much on their 
 mechanical condition, as on the species of 
 matter of which they are composed. What- 
 ever tends to harden them, diminishes the 
 measure of effect ; and hence apparently the 
 reason, why the action of fire impairs their 
 desiccating quality. Quartz or silica, which 
 in a blacksmith's forge had suffered a reduc- 
 tion to 19 degrees, after b.eing soaked in wa- 
 ter for the space of a week and again dried, 
 showed an effect equal to 35 degrees, and 
 would probably in time have recovered the 
 whole of its original power. The process by 
 which nature gradually divides, softens, and 
 disposes stony bodies to absorb moisture, is 
 beautifully illustrated in the case of our whin- 
 stone or trap. A piece of solid trap produced, \^~ 
 we have seen, a dryness of 80 degrees by 
 the hygrometer ; another piece, decayed and 
 crumbling, gave 86 degrees; but another piece 
 of the same rock, already reduced to mould,
 
 100 HELAT10XS OF AIR 
 
 afforded 92 degrees. The ameliorating in- 
 fluence of culture is exemplified in sea-sand : 
 fine sand caused a dryness of 70 degrees ; 
 sand collected from the paths of a sheep- 
 walk near the beach, 78 degrees ; and the 
 same sand, lately brought into cultivation, 
 85 degrees. Still these effects are inferior to 
 that of garden mould, which amounts to 95 
 degrees, and to which decomposed trap ap- 
 proaches the nearest. This material, in its 
 mouldered state, constitutes the basis of 
 our richest agricultural districts. Other 
 cultivated soils exert a similar power of ab- 
 sorption, and which appears always propor- 
 tioned to their respective goodness. Nor are 
 such increased energies to be ascribed to the 
 amelioration from manure, since this ingre- 
 dient separately has less influence than the 
 earths- themselves. It therefore seems highly 
 probable, that the fertility of soils depends 
 chiefly on their disposition to imbibe mois- 
 ture. 
 
 I have just glanced at a theory which, if it 
 were confirmed by farther experience, would
 
 TO HEAT AND MOISTURE. 101 
 
 greatly enlarge the boundaries of useful sci- 
 ence. The art of chemical analysis, however 
 much it has been refined, yet fails totally in 
 detecting the intimate constitution of bodies. 
 Stones, the most opposite in their aspect and 
 general characters, are sometimes composed 
 of the same elements, united too in like pro- 
 portions. On the nature of soils, the improve- 
 ments in chemistry have as yet thrown scarcely 
 any light whatever; and notwithstanding the 
 mighty promises held forth to us, agriculture 
 is still obliged, it must be confessed, to grope 
 her way, by the glimmering of rude experi- 
 ence. The various absorbent powers of earths 
 offer something like a principle of distinction 
 among them. The operations of tillage serve 
 to open and mellow the soil ; while the process 
 of paring and burning, if it goes farther than 
 the mere destruction of the noxious roots and 
 fibres, bakes and hardens the surface,rendering 
 it unfit for the purposes of vegetation, till, after 
 some lapse of time, it again recovers, its pro- 
 per constitution. In volcanic countries, like- 
 wise, the fields of lava require the influence
 
 102 RELAf IOXS OF AIR 
 
 of many centuries, to fit them for cultivation. 
 For comparing the absorbent powers of the 
 various kinds of soil, the hygrometer might 
 easily be adapted. The subject now started 
 is at least of such a promising nature as to 
 claim our serious attention. 
 
 The variations both in weight and bulk 
 which absorbent bodies undergo, have been 
 employed to indicate the disposition of the 
 air with respect to moisture. For this rea- 
 son, such substances are likewise termed 
 hygroscopic * ; since they are always affected 
 by the state of the ambient medium, though 
 they may not precisely measure its degrees 
 of humidity or dry ness. But neither heat nor 
 moisture is passively diffused, or yet shared 
 among different bodies in equal proportions. 
 Under the same change of circumstances, 
 100 grains of ivory will attract from the at- 
 mosphere 7 grains of humidity ; the same 
 weight of boxwood, 14 grains ; of down, 16; 
 of wool, 18; and of beech, 28. Other sub- 
 
 * From iy^ef, humid or moist t and oaiFlcfuu, I examine.
 
 TO HEAT AND MOISTURE. 103 
 
 stances will, in their respective measures of 
 absorption, discover still wider differences. 
 
 The dry or humid state of the air is there- 
 fore discovered, from the variable weight of 
 certain bodies exposed to its influence. Rock 
 salt has been applied to that purpose; but 
 potash, the muriate of lirrie, sulphuric acid, 
 and most of the deliquescent substances, 
 whether in a solid or a liquid form, act the 
 most powerfully. Other materials of a firm 
 or adhesive consistence manifest the same 
 properties, though in a lower degree. Plates 
 of slate-clay or of unglazed earthen-ware, the 
 shavings of box or horn, paper or parchment, 
 wool or down, all act as hygrometers. But 
 these substances, especially the harder kinds 
 of them, unless they be extremely thin, receive 
 their impressions very slowly, and hence they 
 cannot mark with any precision the fleeting 
 and momentary state of the ambient medium. 
 Nor is the weight which they gain by exposure, 
 proportioned to the real dampness of the 
 air ; for the measures of their successive ab- 
 sorption increase in a most rapid progression,
 
 104 RELATIONS OF AIR 
 
 as they approach to the point of absolute hu- 
 midity. 
 
 But to weigh the substances with the 
 accuracy befitting such experiments, is a 
 very delicate and troublesome operation. 
 Those thin bodies are liable, besides, to 
 become in time covered with dust, which, 
 while it must evidently augment their weight, 
 cannot be detached from them without in- 
 juring their slender texture. The increase 
 of bulk which they acquire from the por- 
 tion of moisture attracted into their sub- 
 stance, furnishes therefore a more certain 
 and convenient indication of the state of the 
 atmosphere. The solid vegetable and animal 
 fibres are connected by a fine soft netting, 
 in which the power of absorption appears 
 chiefly to reside. Hence the presence of 
 moisture always enlarges the breadth of such 
 substances, without affecting in any sensible 
 degree their length. This effect is visible in 
 the swelling of a door by external dampness, 
 and in the shrinking of a pannel from the op- 
 posite cause. "'But the substances, such as
 
 TO HEAT AND MOISTURE. 105 
 
 paper or parchment, which have a diffuse or 
 interlaced texture, are extended, by the ab- 
 sorption of humidity, almost equally in every 
 direction. On the contrary, twisted cord or 
 gut, being swelled by moisture, suffer a cor- 
 responding longitudinal contraction, accom- 
 panied likewise, if not confined, by some un- 
 coiling of their fibres. 
 
 All these properties have severally been 
 employed in the construction of hygroscopes. 
 The expansion of the thin cross sections of 
 box or other hard wood, the elongation of the 
 human hair or of a slice of whale-bone, and 
 the untwisting of the wild oat, of catgut, of a 
 cord or linen thread, and of a species of grass 
 ^brought from India, have at different times 
 been used with various success. But the 
 instruments so formed are either extremely 
 dull in their motions, or if they acquire 
 greater sensibility from the attenuation of 
 their substance, they are likewise rendered 
 the more subject to accidental injury and 
 derangement ; and all of them appear to 
 lose in time insensibly their tone and proper 
 action,
 
 106 RELATIONS OF AIR 
 
 I have lately revived a method of measur- 
 ing the expansion of absorbent cohesive sub- 
 stances, by their enlargement of capacity 
 when disposed into a shell ; and have, by suc- 
 cessive steps, carried the hygroscope thus 
 formed to as high a state of improvement, as 
 perhaps such an imperfect instrument will 
 admit. A piece of fine grained ivory, about 
 an inch and quarter in length, is turned into 
 an elongated spheroid, as thin as possible, 
 weighing only 8 or 10 grains, but capable of 
 containing, at its greatest expansion, about 
 300 grains of mercury ; and the upper end, 
 which is adapted to the body by means of a 
 delicate screw, has a slender tube inserted, 
 6 or 8 inches long, and with a bore of nearly 
 the 15th part of an inch in diameter. (See 
 Jig. 7.J. The instrument being now fitted 
 together, its elliptical shell is dipped into 
 distilled water, or lapped round with a wet 
 bit of cambric, and after a considerable inter- 
 val of time, filled with mercury to some con- 
 venient point near the bottom of the tubej 
 where is fixed the beginning of the scale. 
 The divisions themselves are ascertained, by
 
 TO HEAT AND MOISTURE. 107 
 
 distinguishing the tube into spaces which 
 correspond each of them to the thousandth 
 part of the entire cavity, and equal to the 
 measure of about three-tenths of a grain of 
 mercury. The ordinary range of the scale 
 will include 70 of these divisions. To the 
 upper end of the tube, is adapted a small 
 ivory cap, which allows the penetration of 
 air, but prevents the escape of the mercury, 
 and thereby renders the instrument quite 
 portable. 
 
 This hygroscope is largely, though rather 
 slowly, affected by any change in the humi- 
 dity of the ambient medium. As the air be- 
 comes drier, it attracts a portion of moisture 
 from the shell or bulb of ivory, which, suf- 
 fering in consequence a contraction, squeezes 
 its contained mercury so much higher in the 
 tube. But if, on the contrary, the air should 
 incline more to dampness, the thin bulb will 
 imbibe moisture and swell proportionally, al- 
 lowing the quicksilver to subside towards its 
 enlarged cavity. These variations, however, 
 are very far from corresponding with the real
 
 108 RELATIONS OF AIR 
 
 measures of atmospheric dryness or humidity. 
 Near the point of extreme dampness, the altera- 
 tions of the hygroscope are much augmented; 
 but they diminish rapidly, as the mercury 
 approaches the upper part of the scale. The 
 contraction of the ivory answering to an 
 equal rise in the dryness of the air, is six 
 times greater at the beginning of the scale 
 than at the 70th hygroscopic division ; and 
 seems in general to be inversely as the num- 
 ber of hygrometric degrees, reckoning from 
 20 below. I have therefore placed another 
 scale along the opposite side of the tube, the 
 space between and 70 of the hygroscope 
 being distinguished into 100 degrees, and 
 corresponding to the unequal portions from 
 the number 20 to 120 on a logarithmic line. 
 This very singular property is more easily 
 conceived from the inspection of the figure. 
 The scale might be safely extended farther, 
 by continuing the logarithmic divisions. Thus, 
 320 degrees by the hygrometer would answer 
 to 108 of the hygroscope, or to a contraction 
 of 108 parts in a thousand in the capacity of
 
 TO HEAT AND MOISTURE. 109 
 
 the bulb. But at the dryness of 300, I have 
 never found the contraction of the ivory to 
 exceed 105. 
 
 Boxwood, I likewise formed into a hygro- 
 scope, of the same shape and dimensions ; 
 but this absorbent material swells twice as 
 much with moisture as ivory does, and there- 
 fore requires its inserted tube to be propor- 
 tionally longer or wider. The contractions 
 of box are still more unequal than those of 
 ivory. Near the point of extreme humidity, 
 those alterations in the capacity of the bulb 
 seem to be more than twenty times greater 
 than, under like changes in the condition of 
 the atmosphere, take place towards the up- 
 per part of the scale. The space included 
 between the commencement and the 140th 
 millesimal division of the scale, might hence 
 be marked with 100 hygrometric degrees, 
 corresponding to the decreasing portions of 
 a logarithmic line from 5 to 105. 
 
 In noticing the rapidly declining contrac- 
 tions which ivory and box undergo, I would 
 not be understood, however, to state the
 
 110 RELATIONS OF AIll 
 
 quantities with rigorous precision ; I mere- 
 ly consider the numbers given above as 
 very near approximations to the truth. I 
 should be ashamed to confess how much .time 
 I have already consumed, in tracing out the 
 law of those contractions. Such experiments 
 are rendered the more tedious, from the pro- 
 tracted action of the hygroscope, whicli often 
 continues travelling slowly for the space of a 
 quarter or even half an hour. This tardiness 
 is indeed the great defect of all instruments 
 of that nature, and utterly disqualifies them 
 from every sort of delicate observation. 
 
 The very large expansions which the hy- 
 groscope shows on its approach to extreme 
 humidity, explains in a satisfactory manner 
 the injury which furniture and pieces of ca- 
 binet-work sustain from the prevalence of 
 dampness. On the other hand, the slight al- 
 teration which the instrument undergoes in a 
 medium of highly dry atmosphere, seems to 
 have led most philosophers to believe that 
 there is an absolute term of dryness, qrt the 
 distance of which, from the *point of extreme
 
 TO HEAT AND MOISTURE. Ill 
 
 moisture, they have generally founded the 
 graduation of the different hygroscopes pro-r 
 posed by them. This opinion, however, is 
 far from being correct, a.nd might give occa- 
 sion to most erroneous conclusions. No 
 bounds can be set to the actual dryness of 
 the air, or the quantity of moisture which 
 it is capable of holding, and which, by the 
 joint application of heat and rarefaction to 
 the solvent, may be pushed to almost inde- 
 finite extent. 
 
 The ivory hygroscope, after being for se- 
 veral hours immersed in air of 150 or 200 
 degrees of dryness, was apt of a sudden to 
 split longitudinally. But if the bulb endured 
 such a range of contraction, it appeared in 
 some instances to take at least another set, 
 or to accommodate its constitution, by im- 
 perceptible gradations, to the state of the 
 surrounding medium. If this remark were 
 confirmed, it would elucidate finely the ato- 
 mical system of bodies, and establish the 
 successive limits of corpuscular attraction and 
 repulsion.
 
 112 RELATIONS OP AIR 
 
 But though the bulbous hygroscope is, in 
 extreme cases, liable to much uncertainty and 
 some risk, it may yet be used with visible 
 advantage, in a variety of situations, as an 
 auxiliary merely to the hygrometer. The 
 very sluggishness of the instrument, when 
 the value of its divisions has been once ascer- 
 tained, fits it so much the better for indicat- 
 ing the mean results. After being long ex- 
 posed in situations hardly accessible, it may 
 be conveniently transported for inspection 
 before it can suffer any sensible change. The 
 hygroscope could be, therefore, employed 
 with success to discover the degree of humi- 
 dity which prevails at certain considerable 
 elevations in the atmosphere. It might be 
 likewise used for ascertaining readily the pre- 
 cise condition of various goods and commo- 
 dities. Thus, if the bulb were introduced, 
 for the space perhaps of half an hour, into a 
 bag of wool, a sack of corn, or a bale of pa- 
 per, it would, on being withdrawn from their 
 contact, mark the dry ness or humidity of 
 those very absorbent substances.
 
 TO HEAT AND MOISTURE. 113 
 
 The softer absorbent substances are not 
 only themselves affected by the state of the 
 ambient medium, but are capable, when they 
 expose a broad attractive surface, of assimi- 
 lating to their previous condition, air and 
 other gaseous fluids confined over them. 
 Flannel, for instance, which has been intense- 
 ly dried before a strong fire, will support a re- 
 markable degree of dryness in a close recei- 
 ver ; yet after a few repeated applications, it 
 soon becomes saturated with humidity, and 
 loses its power of absorption. Muriate of 
 lime has a vigorous and extended energy ; 
 but the substance which answers best on the 
 whole as an absorbent* and which continues 
 for a long time to attract moisture with al- 
 most undiminished force, is the concentrated 
 sulphuric acid. By the action of this material, 
 I am enabled to maintain, for weeks or even 
 months together, a magazine of dry air, which 
 affords the means of accurately graduating the 
 differential thermometer and its several mo- 
 difications. But, by exposing, at the same 
 time, under the receiver a surface of water 
 H
 
 114 11ELATIONS OF AIR 
 
 in given proportion to that of the acid, the 
 confined air may easily be reduced to any in- 
 ferior state of dryness. 
 
 The presence of heat likewise augments 
 very considerably all those absorbent powers. 
 Thus, while in winter the introduction of 
 sulphuric 'acid under a receiver and in a 
 room without fire, scarcely sinks the hygro- 
 meter 40 degrees, it will, even in our fee- 
 ble summers, occasion a dryness of 100 
 or 120 degrees. On this principle, water 
 may be rendered cool in the sultriest cli- 
 mates, and in every state of the atmo- 
 sphere ; for nothing more is required than, 
 to expose it to evaporate from a porous ves- 
 sel, in a medium of confined air, and near the 
 action of a large surface of sulphuric acid. 
 Nay, with that arrangement, artificial conge- 
 lation is produced, if the external tempera- 
 ture should come but as low as 38 or 40 
 degrees on Fahrenheit's scale. Wine also 
 could be cooled, by casing the sides of the 
 bottle with wet flannel, and shutting it up in 
 a wide shallow box, which is lined with lead
 
 TO HEAT AND MOISTURE. 115 
 
 or composed of glazed earthen-ware, and has 
 its bottom covered, perhaps to the thickness 
 of half an inch, with a stratum of the acid. 
 If this box, with its contents, were placed in a 
 cellar, the wine would, at all times in this island, 
 have its temperature reduced, in the space 
 of four or five hours, to 40 or 42 degrees by 
 Fahrenheit's scale. By the same very simple 
 contrivance, wine or water might, in the tro- 
 pical countries, be cooled, from 80 degrees 
 on the same scale, to 55, or even lower. 
 Nor is the desiccating efficacy of the acid 
 sensibly impaired, till it has absorbed an 
 equal bulk of moisture, and has consequent- 
 ly on successive days occasioned the mode- 
 rate refrigeration of more than fifty times its 
 weight of wine or water. 
 
 The application of heat constantly increas- 
 es the dryness of the air, or its disposition to 
 dissolve moisture. This property is so ge- 
 ^nerally known, that the evaporating power 
 of the medium is very seldom in practice 
 referred to any other cause. Dryjng houses, 
 for example, are commonly constructed as if
 
 116 RELATIONS OF Alii 
 
 heat were to produce the whole effect, no 
 means being employed for aiding the escape 
 of the air, after it has become charged with 
 humidity, and consequently rendered unfit 
 for performing any longer the process of eva- 
 poration. 
 
 The influence of warmth in augmenting 
 the dryness of the air, or its disposition to 
 imbibe moisture, explains most easily a sin- 
 gular fact remarked by some accurate obser- 
 vers. If two equal surfaces of water be ex- 
 posed in the same situation, the one in a shal- 
 low, and the other in a deep vessel of metal 
 or porcelain ; the latter is always found, after 
 a certain interval of time, to have suffered* 
 contrary to what we might expect^ more 
 waste by evaporation than the former. This 
 observation was once made the ground of a 
 very absurd theory, although the real ex- 
 plication of it appears abundantly simple. 
 Amidst all the changes that happen in the 
 condition of the ambient medium^ the shal- 
 low pan must necessarily receive more com- 
 pletely than the deeper vessel, the chilling
 
 TO HEAT AND MOISTURE. 117 
 
 impressions of evaporation, since it exposes a 
 smaller extent of dry surface to be partly heat- 
 ed up again by the contact of the air. The 
 larger mass being, therefore, kept invariably 
 warmer than the other, must in consequence 
 support a more copious exhalation. 
 
 If heat augments the dryness of the air, its 
 absence produces an opposite effect. When 
 the external air is colder than that of an apart- 
 ment which is rather disposed to humidity, 
 the windows come to be covered with dew on 
 the insides of the panes. The massy walls 
 of cellars and untenanted buildings, being re- 
 latively cool during the summer months, ap- 
 pear then generally dripping with moisture. 
 A caraffe, filled with water fresh drawn from 
 the well, and set on the table in a hot room, 
 becomes soon covered over with dew; and 
 this effect takes place more speedily, when the 
 surrounding air inclines to dampness. Nor 
 will the deposition of moisture cease, until 
 the glass decanter, with the water contained 
 in it, has approached to the heat of the room. 
 On this principle, it has been proposed to
 
 118 RELATIONS OF AIR 
 
 construct a sort of rude hygrometer, by ob- 
 serving the depressed temperature at which 
 the dew again begins to disappear from the 
 surface of a small globlet filled with cold wa- 
 ter. The point of atmospheric saturation will 
 evidently be lower in proportion to the dry- 
 ness of the room. 
 
 But air must become drier, from ha- 
 ving deposited a part of the moisture which 
 it held in solution. A cold body intro- 
 duced under a large receiver, may there- 
 fore occasion a very considerable dryness in 
 the confined medium. Thus, when the tem- 
 perature of the room is about 70 degrees by 
 Fahrenheit, a goblet filled with water at 50 
 degrees, being placed within a receiver of 
 perhaps thrice its surface, a delicate hygro- 
 meter suspended near the middle of the va- 
 cant space, marked 45 degrees. A wide 
 saucer of glass or porcelain, containing wa- 
 ter equally cold, produces almost the same 
 effect. The air included between the cold 
 mass and the receiver, losing its heat on the 
 one side, and again recovering this partly on
 
 TO HEAT AND MOISTURE. 119 
 
 the other, assumes a certain intermediate 
 temperature ; but each portion of it which 
 successively comes to touch the glass, the 
 porcelain, or the water, being chilled by such 
 contact, surrenders its surplus humidity, and 
 then mingling with the rest of the air, re- 
 gains the mean state of warmth, and conse- 
 quently has, in proportion to that change, its 
 dryness or power of solution renewed. The 
 medium thus rendered drier, being therefore 
 capable of performing evaporation, may, by 
 acting on a humid surface, produce a degree 
 of cold little inferior to that of the mass itself, 
 from the introduction of which under the re- 
 ceiver the whole chain of effects originated. 
 If a freezing mixture be formed with salt and 
 snow or pounded ice in a deep and wide 
 saucer, and placed under a lar^e receiver, 
 about an interval of three or four inches from 
 a small cup of porous earthen-ware filled with 
 water ; this water will soon become frozen, 
 . and remain so for several hours, till the whole 
 of the ice or snow has melted down into a 
 liquid state. The sides of the saucer, from
 
 
 120 . KELATIONS OF AIR 
 
 the deposition of moisture conveyed by the 
 air from the cup, become incrusted with hoar 
 frost, which keeps constantly increasing in 
 thickness as long as the action of the frigo- 
 rific mixture lasts. If a covered dish were 
 substituted for the saucer, the icy crust, form- 
 ed over the whole of its external surface, 
 would serve to measure the comparatively 
 small quantity of exhalation from the cup, 
 which was required to freeze the water, and 
 afterwards to support congelation. When 
 the cup containing the water is placed below, 
 instead of above, the magazine of cold, espe- 
 cially under a tall receiver, the evaporating 
 process suffers indeed some reduction, but 
 still the frigorific operation is on the whole 
 rendered more powerful ; for the chilled air 
 falling downwards, though it acquires Httle 
 dryness, yet communicates to the water the 
 impressions of most intense cold with scarce- 
 ly any diminution. 
 
 To discover the precise law by which equal 
 additions of heat augment the dryness of air 
 or its power to retain moisture, is a problem
 
 TO HEAT AND MOISTURE. 121 
 
 of great delicacy and importance. Two dif- 
 ferent modes were employed in that investi- 
 gation, but which led to the same results. 
 The one was, in a large close room, to bring 
 an hygrometer, conjoined with a thermome- 
 ter, successively nearer to a stove intensely 
 heated, and to note the simultaneous indica- 
 tions of both instruments ; or to employ two 
 nice thermometers, placed beside each other, 
 and having their bulbs covered respectively 
 with dry and with wet cambric. By taking 
 the mean of numerous observations, and in- 
 terpolating the intermediate quantities, the 
 law of aqueous solution in air was laborious- 
 ly traced. But the other method of investi- 
 gation appeared better adapted for the high- 
 er temperatures. A thin hollow ball of tin, 
 four inches in diameter, and having a very 
 small neck, was neatly covered with linen ; 
 and, being filled with water nearly boiling, 
 and a thermometer inserted, it was hung 
 likewise in a spacious close room, and the 
 rate of its cooling carefully marked. The 
 experiment was next repeated, by suspend-
 
 122 RELATIONS OF Mil 
 
 ing it to the end of a fine beam, and wet- 
 ting with a hair pencil the surface of linen, 
 till brought in exact equipoise to some gi- 
 ven weight in the opposite scale ; ten grains 
 being now taken out, the humid ball was 
 allowed to rest against the point of a ta- 
 pered glass tube, and the interval of time, 
 with the corresponding diminution of tempe- 
 rature, observed, when it rose again to the 
 position of equilibrium. The same opera- 
 tion was successively renewed ; but, as the 
 rapidity of the evaporation declined, five, 
 and afterwards two, grains only were, at each 
 trial, withdrawn from the scale. From such 
 a series of facts, it was easy to estimate the 
 quantities of moisture which the same air 
 will dissolve at different temperatures, and 
 also the corresponding measures of heat ex- 
 pended in the process of solution. 
 
 By connecting the range of observations, 
 it would appear, that air has its dryness 
 doubled at each rise of temperature, answer- 
 ing to 15 centesimal degrees. Thus, at the 
 freezing point, air is capable of holding a
 
 TO HEAT AND MOISTURE. 123 
 
 portion of moisture represented by 100 de- 
 grees of the hygrometer ; at the temperature 
 of 15 centigrade, it could contain 200 such 
 parts ; at that of 30, it might dissolve 400 ; 
 and, at 45 on the same scale, 800. Or, if we 
 reckon by Fahrenheit's divisions, air absolute- 
 ly humid holds, at the limit of congelation, 
 the hundred and sixtieth part of its weight of 
 moisture ; at the temperature of 59 degrees, 
 the eightieth part ; at that of 86 degrees, the 
 fortieth part ; at that of 113 degrees, the 
 twentieth part ; and at that of 140 degrees, 
 the tenth sport. While the temperature, 
 therefore, advances uniformly in arithmetical 
 progression, the dissolving power which this 
 communicates to the air mounts with the ac- 
 celerating rapidity of a geometrical series. 
 
 It hence follows, that, whatever be the actual 
 condition of a mass of air, there must always 
 exist some temperature at which it would be- 
 come perfectly damp. Nor is it difficult, 
 from what has been already stated, to deter- 
 mine this point in any given case. Thus, 
 suppose the hygrometer to mark 52, while its
 
 124 DELATIONS OF AIK 
 
 wet ball has a temperature of 20 centesimal 
 degrees or 68 by Fahrenheit ; the dissolving 
 power of air at this temperature being 252, 
 its distance from absolute humidity will 
 therefore be 200, which is the measure of so- 
 lution answering to 15 centesimal degrees or 
 59 by Fahrenheit. The same air would con- 
 sequently, at the depressed temperature of 
 59 degrees, shrink into a state of absolute 
 saturation ; and if cooled lower, it would 
 even deposite a portion of its combined mois- 
 ture, losing the eightieth part of its weight at 
 the verge of freezing. 
 
 The hygrometer, it was shown, does not indi- 
 cate the actual dryness of the air, but the dry- 
 ness which it retains, after being reduced to the 
 temperature of the humid ball. The real condi- 
 tion of the medium, however, could easily be 
 determined, from the gradations already ascer- 
 tained in the power of solution. Suppose, 
 for example, that the hygrometer should 
 mark 42 degrees, while the thermometer 
 stands at 16 centigrade; the moist surface has 
 therefore the temperature of 11.8 centigrade,
 
 TO HEAT AND MOISTURE. 125 
 
 at which the dissolving energy is less by 37 
 degrees than at 16 centigrade ; and hence 
 the total dryness of the air, at its former 
 temperature, amounted to 79 degrees. A 
 small table of the solubility of moisture, 
 would greatly facilitate such reductions. 
 
 That the quantity of moisture which air 
 can hold increases in a much faster ratio 
 than its temperature, is a great principle in 
 the economy of nature. It seems first to 
 have been traced from indirect experience by 
 the sagacity of the late Dr James Hutton, who 
 made it the foundation of his most ingeni- 
 ous and solid theory of rain. Air in cooling 
 becomes ready to part with its moisture. 
 But how is it ever cooled in the free atmo- 
 sphere ? Only by the contact or commixture 
 surely with a colder portion of the same fluid. 
 But that portion of air which is chilled must 
 equally in its turn warm the other. If, 
 in consequence of this mutual change of 
 condition, the former be disposed to resign 
 its moisture, the latter is more inclined to 
 retain it ; and consequently, if such opposite
 
 1 C 26 RELATIONS OF AIR 
 
 effects were balanced, there could, on the 
 whole, be no precipitation of humidity what- 
 ever. The separation of moisture on the 
 mixing of two masses of damp air at different 
 temperatures, would therefore prove, that the 
 dissolving power of air suffers more diminu- 
 tion from losing part of the combined heat 
 than it acquires of augmentation from gaining 
 an equal measure of it ; and consequently 
 this power must, under equal accessions of 
 heat, increase more slowly at first than it 
 does afterwards, thus advancing always with 
 accumulated celerity. 
 
 The result of this induction agrees entire- 
 ly with the law of solution in air, which has 
 been already pointed out. It is the simplest 
 of the accelerating kind ; and the relation of 
 an arithmetical to a geometrical progression, 
 seems applicable to other properties of the 
 gaseous or elastic fluids. An example will" 
 elucidate the nature and extent of the aque- 
 ous deposition caused by the commixture of 
 opposite portions of air. Suppose equal bulks 
 of this fluid, in a state of saturation, and at
 
 TO HEAT AND MOISTURE. 127 
 
 % 
 
 the different temperatures of 15 and of 45 
 centesimal degrees, to be intermingled ; the 
 compound arising from such union will evi- 
 dently have the mean temperature of 30 de- 
 grees. But, since at those temperatures the 
 one portion held 200 parts of humidity, and 
 the other 800, the aggregate must contain 
 1000 parts, or either half of it 500; at the 
 mean or resulting temperature, however, this 
 portion could only suspend 400 parts of humi- 
 dity, and consequently the difference, or 100 
 parts, amounting to a hundred and sixtieth 
 of the whole weight of the air, must be pre- 
 cipitated from the compound mass. Nor was 
 it requisite that such portions of air should 
 be perfectly damp. For instance, if the cold- 
 er air had a dryness of 40, and the warmer, 
 that of 110 hygrometric degrees, the former 
 consequently holding 160 parts of moisture, 
 and the latter 690 ; after commixture, they 
 would each contain 425 parts, and therefore 
 of those 25 parts, or a measure equal to the 
 Six hundred and fortieth of the general mass, 
 would still be deposited.
 
 128 RELATIONS OF AIR 
 
 I have now selected an extreme case, and 
 yet the quantity of moisture set loose might 
 appear to be remarkably small. The diffi- 
 culty is, therefore, to reconcile the measure of 
 such deposits as deduced from theory, with 
 the actual precipitations from the atmosphere. 
 The lower mass of air which surrounds our 
 globe may have a mean temperature of about 
 50 degrees on Fahrenheit's scale \ and were 
 it consequently forced, by some internal 
 change of constitution, to discharge the whole 
 of its watery store, this would not exceed the 
 80th part of its weight, or form a sheet of 
 more than five inches deep. Our atmospherer 
 must hence, in the course of a year, deposite 
 five or ten times as much humidity as it can 
 at once hold in solution. But since only a~ 
 very minute portion of the combined mois- 
 ture is in fact, at any given time, separated 
 from the air, it follows that the relative pro- 
 cesses of evaporation and precipitation must 
 rapidly succeed each other, promoted no 
 doubt by the incessant circulation which is 
 maintained between the higher and the lower
 
 TO HEAT AND MOISTURE. 129 
 
 strata. Every humid discharge from the at- 
 mosphere must, therefore, of necessity be lo- 
 cal and temporary. 
 
 The mixture of diiferently heated masses of 
 air is hence insufficient alone to form any very 
 sensible deposit. To explain the actual pheno- 
 mena, we must have recourse to the mutual 
 operation of a chill and of awarm current, driv- 
 ing swiftly in opposite directions, and contin,u T 
 ally mixing and changing their conterminous 
 surfaces. By this rapidity, a large volume of 
 the fluid is brought into contact in a given 
 time. Suppose, for instance, the one current 
 to have a temperature of 50, and the other that 
 of 70 degrees, by Fahrenheit's scale; theblend- 
 ed surfaces will, therefore, assume the mean 
 temperature of 60 degrees. Consequently the 
 two streams throw together 200 'and 334.2 
 parts of moisture, making 567.1 parts for the 
 compound, which at its actual temperature 
 can only hold 258.6 parts ; the difference, or 
 8.6 parts, forms the measure of precipitation, 
 corresponding to the 1860th of the whole 
 weight of the commixed air. It would 
 i
 
 130 11ELATIONS OF AIll 
 
 thus require a column of air 25 miles in 
 length, to furnish, over a given spot and in 
 the space of an hour, a deposit of moisture 
 equal to the height of an inch. If the sum 
 of the opposite velocities amounted to 50 
 miles an hour, and the intermingling influ- 
 ence extended but to a quarter of an inch at 
 the grazing surfaces, there would still, on this 
 supposition, be produced in the same time, a 
 fall of rain reaching to half an inch in alti- 
 tude. These quantities come within the li- 
 mits of probability, and agree sufficiently 
 with experience and observation. But in 
 higher temperatures, though the difference 
 of heat between the opposite strata of air 
 should remain the same, the measure of 
 aqueous precipitation is greatly increased. 
 Thus, while the deposit of moisture from 
 the mixing of equal masses of air at the tem- 
 peratures of 40 and 60 degrees, is only 6.6, 
 that from the like mixture at 80 and 100 de- 
 grees, amounts to 19. This result is entirely 
 conformable to observation, for showers are 
 the most copious during hot weather and in the
 
 TO HEAT AND MOISTURE. 131 
 
 tropical climates. The dew which falls, about 
 the close of evening, in some sultry regions, 
 such as Egypt, is alone sufficient to drench 
 the surface of the earth. 
 
 Dr Hutton's account of the production of 
 rain, though it may want still farther expan- 
 sion, is grounded therefore on correct gene- 
 ral principles. A more precise and complete 
 acquaintance with meteorological facts, which 
 is only to be derived from multiplied obser- 
 vations with the hygrometer, would enable 
 us to pursue and illustrate the details of this 
 beautiful theory. The measure of precipita- 
 tion from the atmosphere depends on a varie- 
 ty of circumstances, on the previous damp- 
 ness of the commixed portions of the fluid, 
 their difference of heat, the elevation of 
 their mean temperature, and the extent of 
 combination which takes place. When this 
 deposition is slow, the very minute aqueous 
 globules remain suspended and form clouds ; 
 but if it is rapid and copious, those particles 
 conglomerate, and produce, according to 
 the state of the medium with regard to heat,
 
 132 RELATIONS OF AIll 
 
 rain, hail or snow. In fine calm weather, 
 after the rays of a declining sun have ceased 
 to warm the surface of the ground, the de- 
 scent of the higher mass of air gradually 
 chills the undermost stratum, and disposes it 
 to dampness, till their continued intermixture 
 produces a fog, or low cloud. Such fogs are 
 towards the evening often observed gather- 
 ing in narrow vales, or along the course of 
 sluggish rivers, and generally hovering within 
 a few inches of the surface. But in all situa- 
 tions, these watery deposits, either to a greater 
 or a less degree, occur in the same disposition 
 of the atmosphere. The minute suspended 
 globules, attaching themselves to the project- 
 ing points of the herbage, form dew in mild 
 weather, or shoot into hoar frost when cold 
 predominates. They collect most readily on 
 glass, but seem to be repelled by a bright sur- 
 face of metal. 
 
 But the profuse precipitation of humidity 
 is caused by a rapid commixture of opposite 
 strata. The action of swift contending cur- 
 rents in the atmosphere brings quickly i
 
 TO HEAT AND MOISTURE. 133 
 
 mutual contact vast fields of air over a given 
 spot. The separation of moisture is hence 
 proportionally copious. In temperate wea- 
 ther, this deposition forms rain ; but, in the 
 cold season, the aqueous globules, freezing 
 in the mid air into icy spiculse, which collect 
 in their slow descent, become converted into 
 flakes of snow. Hail is formed under diffe- 
 rent circumstances, and generally in the sud- 
 den alternations of the fine season, the drops 
 of rain being congealed during their fall, by 
 passing through a lower stratum of dry and 
 cold air. 
 
 From a variety of nice experiments, we 
 have seen, that the different effects of heat and 
 of steam are yet modified by the same laws. 
 A conformity may, therefore, be expected in 
 the relations of air to those active fluids. As 
 rarefaction enlarges the capacity of air, it like- 
 wise augments the disposition to hold mois- 
 ture ; at the same time, that the removal of 
 the ordinary pressure facilitates the expansion 
 of the liquid matter, and its conversion into 
 a" gaseous form. Accordingly, if the hygro-
 
 134: RELATIONS OF AIR 
 
 meter be suspended within a large receiver 
 from which a certain portion of air is quickly 
 abstracted, it will sink with rapidity. In sum- 
 mer, the additional dryness thus produced a- 
 mounts to about 50 hygrometric degrees each 
 time the air has its rarefaction doubled ; so 
 that, supposing the operation of exhausting 
 to be performed with expedition, and the re- 
 siduum reduced to a sixty-fourth part, the hy- 
 grometer would mark in all the descent of 
 300. But this effect is only momentary, for 
 the thin air very soon becomes charged with 
 moisture, and consequently ceases to act on 
 the wet ball of the hygrometer. The cold, 
 however, excited on the surface of that ball, 
 by such intense evaporation, will have pre- 
 viously frozen the coating. 
 
 It hence appears, that the loftier regions 
 of the atmosphere are comparatively drier 
 than those below, or that, at the same tem- 
 perature, the attenuated fluid is capable of 
 holding a larger share of humidity. Thus, 
 on the slope of Chimborazo, an hygrometer, if 
 placed in a tent where the air has been warm-
 
 TO HEAT AND MOISTURE. 135 
 
 ed to the moderate condition that prevails 
 on the shores of Lima, would stand about 40 
 degrees higher than at the level of the sea. 
 Without such a constitution of the elements, 
 indeed, our globe must have been shrouded 
 in perpetual darkness ; for the cold which 
 reigns in the upper strata, would have pre- 
 vented the humidity from ascending to a great 
 elevation, and have precipitated it in fogs or 
 clouds. In the actual state of things, the 
 diminution of temperature which we en- 
 counter in ascending, predominates at first 
 over the augmented power of aqueous solu- 
 tion ; and the air becomes successively damp- 
 er, till we reach a certain height in the at- 
 mosphere, at which the opposite effects of 
 cold and rarefaction are balanced, and where 
 the extreme of humidity must of course ob- 
 tain. Above that limit, which is the proper 
 region of the clouds, the influence of the ra- 
 rity of the medium must transcend the ef- 
 fects of its chillness, and the air will now be- 
 come gradually drier, till it insensibly melts
 
 136 RELATIONS OF AIll 
 
 away into the clear ethereal expanse *. From 
 the modified relations of heat and pressure 
 is thus deduced the seat of the clouds, or the 
 boundary beyond which they can seldom rise, 
 which will be found to run generally about two 
 miles above the line of perpetual congelation, 
 being most elevated under the equator, and 
 bending downwards near the poles. 
 
 Since air, by being rarefied, has its scale of 
 watery solution so much extended, the ap- 
 plication of heat must hence occasion an ef- 
 fect proportionally greater upon a thin me- 
 dium. If a cold body be, therefore, conveyed 
 under a receiver from which the air is partly 
 extracted, it will produce a very intense dry- 
 ness. Thus, at the ordinary temperature, a 
 frigorific mass of salt and pounded ice, when 
 its encircling air is from fifty to an hundred 
 limes attenuated, will cause an hygrometer, 
 
 Apparet divum numen, sedesque quietse : 
 Quas neque concutiunt venti, neque nubila nimbis 
 Adspergunt, neque nix acri concreta pruina 
 Cana cadens violat : semperque innubilus aether 
 Integit, et large! diffuse lumine ridet.
 
 TO HEAT AND MOISTURE. 137 
 
 placed at a short distance from it, to sink at 
 least 200 degrees, and still more if the room 
 be warm. Having therefore mixed common 
 salt with three times its weight of ice in a 
 large covered dish of glass or porcelain, near- 
 ly equal to the width of a tall receiver passed 
 over it, and having introduced, a very few in- 
 ches either above or below it, a porous earthen 
 cup about one half or a third part of the same 
 diameter, and filled with water j on rarefying 
 the confined air to the utmost, the small 
 body of water exposed to vigorous evapora- 
 tion will, at all seasons, speedily congeal, and 
 remain in this frozen state till, after the space 
 perhaps of an hour, the compound mass has 
 melted into brine and lost entirely its power 
 of cooling. The quantity of icy crust gradually 
 formed on the external surface of the dish con- 
 taining the frigorific mixture, indicates the 
 measure of exhalation from the cup, which is 
 required to produce and support the congela- 
 tion during the period of its existence. On a 
 principle analogous to what is now stated, 
 JDr Wollaston, to whose acuteness and refined
 
 138 RELATIONS OF Alii 
 
 ingenuity, the physical sciences are so deep- 
 ly indebted, has recently proposed a very 
 simple and portable apparatus, by which a 
 small portion of water, at a short distance 
 from a freezing mixture, may at any time be 
 visibly converted into ice. 
 
 The increased power of aqueous solution 
 which air acquires as it grows thinner, being 
 ascertained and carefully investigated, my 
 next object was to combine the action 
 of a powerful absorbent with the transient 
 dryness produced within a receiver by rare- 
 faction. Having introduced a surface of sul- 
 phuric acid, I perceived with pleasure that 
 this substance only superadded its peculiar 
 attraction for moisture, to the ordinary effects 
 resulting from the progress of exhaustion ; 
 and what was still more important, that it 
 continued to support, with undiminished ener- 
 gy, the dryness thus created. The attenuated 
 air was not suffered, as before, to grow char- 
 ged with humidity ; but each portion of that 
 medium, as fast as it became saturated by 
 touching the wet ball of the hygrometer,
 
 TO HEAT AND MOISTURE. 139 
 
 transported its vapour to the acid, and was 
 thence sent back denuded of the load, and 
 fitted again to renew its attack with fresh 
 vigour. By this perpetual circulation, there- 
 fore, between the exhaling and the absorbing 
 surface, the diffuse residuum of air is main- 
 tained constantly at the same state of dryness. 
 But the powers, both of vaporization and of 
 absorption being greatly augmented in the 
 higher temperatures, the same limit of cold 
 nearly is in all cases attained by a certain 
 measure of exhaustion. When the air has 
 been rarefied 250 times, the utmost that, un- 
 der such circumstances, can perhaps be ef- 
 fected, the surface of evaporation is cooled 
 down 120 degrees of Fahrenheit in winter, 
 and would probably sink near 200 in summer. 
 Nay, a far less tenuity of the medium, still 
 combined with the action of sulphuric acid, 
 is capable of producing and supporting a very 
 intense cold. If the air be rarefied only 5Q 
 times, a depression of temperature will be 
 produced amounting to 80 or even 100 de- 
 grees of Fahrenheit's scale.
 
 140 RELATIONS OF AIR 
 
 We are thus enabled, in the hottest wea- 
 ther, and in every climate of the globe, 
 to freeze a mass of water, and to keep 
 it frozen, till it gradually wastes away by 
 a continued but invisible process of evapo- 
 ration. The only thing required is, that the 
 surface of the acid should approach near to 
 that of the water, and should have a great- 
 er extent, for otherwise the moisture would 
 exhale more copiously than it could be trans- 
 ferred and absorbed, and consequently the 
 dryness of the rarefied medium would become 
 reduced, and its evaporating energy essentially 
 impaired. The acid should be poured to the 
 depth perhaps of half an inch in a broad flat 
 dish, which is covered by a receiver of a form 
 nearly hemispherical ; the water exposed to 
 congelation may be contained in a shallow 
 cup, about half the width of the dish, and 
 having its rim supported by a narrow me- 
 tallic ring upheld above the surface of the 
 acid by three slender glass feet of two or 
 three inches in height. (See Jig. 9.) It is 
 of consequence that the water should be as
 
 TO HEAT AND MOISTURE. 141 
 
 much insulated as possible, or should pre- 
 sent only a humid surface to the contact of 
 the surrounding medium, for the dry sides 
 of the cup might receive, from commu- 
 nication with the external air, such acces- 
 sions of heat, as greatly to diminish, if not to 
 counteract the refrigerating effects of eva- 
 poration. This inconvenience, however, is 
 in a great measure obviated, by investing the 
 cup with an outer case at the interval of about 
 half an inch. If both the cup and its case consist 
 of glass, the process of congelation is viewed 
 most completely ; yet when they are formed 
 of a bright metal, the effect appears altogether 
 more striking. But the preferable mode, and 
 that which prevents any waste of the powers 
 of refrigeration, is to expose the water in a 
 pan of porous earthen-ware. If common wa- 
 ter be used, it will evolve air bubbles very 
 copiously as the exhaustion proceeds ; in a 
 few minutes, and long before the limit of 
 rarefaction has been reached, the icy spiculae 
 will shoot beautifully through the liquid mass, 
 and entwine it with a reticulated contexture.
 
 142 RELATIONS OF AIR 
 
 As the process of congelation goes forward, 
 a new discharge of air from the substance of 
 the water takes place, and marks the regular 
 advances of consolidation. But after the wa- 
 ter has all become solid ice, which, unless it 
 exceed the depth of an inch, may generally 
 be effected in less than half an hour, the cir- 
 cle of evaporation and subsequent absorption 
 is still maintained. A minute film of ice, ab- 
 stracting from the internal mass a redoubled 
 share of heat, passes, by invisible transitions, 
 successively into the state of water and of 
 steam, which, dissolving in the thin ambient 
 air, is conveyed to the acid, where it again 
 assumes the liquid form, and, in the act of 
 combination, likewise surrenders its heat. 
 
 In performing this experiment, I generally 
 seek at first to push the rarefaction as far as the 
 circumstances will admit. But the disposition 
 of the water to fill the receiver with vapour, 
 being only in part subdued by the acti'on of 
 the sulphuric acid, a limit is soon opposed to 
 the progress of exhaustion, and the includ- 
 ed air can seldom be rarefied above a him-
 
 TO HEAT AND MOISTURE. 143 
 
 dred times, or till its elasticity can support 
 no more than a column of mercury about 
 three-tenths of an inch in height. But a 
 much smaller rarefaction, perhaps from ten 
 times to twenty times, will be found sufficient 
 to support congelation after it has once taken 
 place. The ice then becomes rounded by de- 
 grees at the edges, and wastes away insensibly, 
 its surface being incessantly corroded by the 
 play of the ambient air, and the minute exhala- 
 tions conveyed by an invisible process to the 
 sulphuric acid, which, from its absorption of 
 the vapour, is all the time maintained above 
 the temperature of the apartment. The ice, 
 kept in this way, suffers a very slow consump- 
 tion; for a lump of it, about a pound in weight 
 and two inches thick, is sometimes not en- 
 tirely gone in the space of eight or ten days. 
 During the whole progress of its wasting, the 
 ice still commonly retains an uniform trans- 
 parent consistence ; but, in a more advanced 
 stage, it occasionally betrays a sort of honey- 
 combed appearance, owing to the minute 
 cavities formed by globules - air, set loose
 
 144 RELATIONS OP All? 
 
 in the act of freezing, yet entangled in the 
 mass, and which are afterwards enlarged by 
 the erosion of the solvent medium. 
 
 The most elegant and instructive mode of 
 effecting artificial congelation, is to perform 
 the process under the transferrer of an air 
 pump. A thick but clear glass cup being se- 
 lected, of about two or three inches in dia- 
 meter, has its lips ground flat, and Covered 
 occasionally, though not absolutely shut, with 
 a broader circular lid of plate glass, which is 
 suspended horizontally from a rod passing 
 through a collar of leather. (See Jig. 10. J. 
 This cup is nearly filled with fresh distilled 
 water, and supported by a slender metallic 
 ring, with glass feet, about an inch above the 
 surface of a body of sulphuric acid, per- 
 haps three quarters of an inch in thickness, 
 and occupying the bottom of a deep glass ba- 
 son that has a diameter of nearly seven inches. 
 In this state, the receiver being adapted, and 
 the lid pressed down to cover the mouth of 
 the cup, the transferrer is screwed to the air 
 pump, and tM -^refaction, under those cir-
 
 TO HEAT AND MOISTURE. 145 
 
 cumstances, pushed so far as to leave only 
 about the hundred and fiftieth part of a resi- 
 duum ; and the cock being turned to secure 
 that exhaustion, the compound apparatus is 
 then detached from the pump, and removed 
 into some convenient apartment. As long as 
 the cup is covered, the water will remain quite 
 unaltered ; but on drawing up the rod an inch 
 or more, to admit the play of the rare medium, 
 a bundle of spicular ice will, in the space per- 
 haps of five minutes, dart suddenly through 
 the whole of the liquid mass ; and the consoli- 
 dation will afterwards descend regularly, thick- 
 ening the horizontal stratum by insensible gra- 
 dations, and forming in its progress a beautiful 
 transparent cake. On letting down the cover 
 again, the process of evaporation being now 
 checked or almost entirely stopped, the ice re* 
 turns slowly into its former condition. In this 
 way, the same portion of water may, even at 
 distant intervals of time, be repeatedly con- 
 gealed and thawed successively twenty or 
 thirty times. During the first operations of 
 freezing, some air is liberated ; but this cx~ 
 L
 
 146 RELATIONS OF AIK 
 
 trication diminishes at each subsequent act, 
 and the ice, free from the smallest specks, re- 
 sembles a piece of the purest crystal. 
 
 This artificial freezing of water in a cup of 
 glass or metal, affords the best opportunity 
 for examining the progress of crystallization. 
 The appearance presented, however, is ex- 
 tremely various. When the frigorific action 
 is most intense, the congelation sweeps at 
 once over the whole surface of the water, 
 obscuring it like a cloud. But in general the 
 process advances more slowly ; bundles of 
 spiculae, from different points, sometimes 
 from the centre, though commonly from the 
 sides of the cup, stretching out and spreading 
 by degrees with a sort of feathered texture. By 
 this combined operation, the surface of the 
 water soon becomes an uniform sheet of ice. 
 Yet the effect is at times singularly varied ; 
 the spicular shoots, advancing in different di- 
 rections, come to inclose, near the middle of 
 the cup, a rectilineal space, which, by unequal 
 though continued encroachment, is reduced 
 to a triangle ; and the mass below being part-
 
 TO HEAT AND MOISTURE. 147 
 
 ly frozen and therefore expanded, the water 
 is gradually squeezed up through the orifice, 
 and forms by congelation a regular pyramid, 
 rising by successive steps ; or if the projecting 
 force be greater, and the hole more contracted, 
 it will dart off like a pillar. The radiating or 
 feathered lines which at first mark the frozen 
 surface, are only the edges of very thin plates 
 of ice, implanted at determinate angles, but 
 each parcel composed of parallel planes. This 
 internal formation appears very conspicuous 
 in the congealed mass which has been remov- 
 ed from a metallic cup, before it is entirely 
 consolidated. 
 
 When cups of glass or metal are used, the 
 cold excited at the open surface of the liquid 
 extends its influence gradually downwards. 
 But if the water be exposed in a porous ves- 
 sel, the process of evaporation, then taking 
 effect on all sides, proceeds with a nearly re- 
 gular consolidation towards the centre of the 
 mass, thickening rather faster at the bottom 
 from its proximity to the action of the absor- 
 bent, and leaving sometimes a reticulated
 
 148 UELATIOXS OF AIR 
 
 space near the middle of the upper surface, 
 through which the air, disengaged by the 
 progress of congelation, makes its escape. 
 When very feeble powers of refrigeration are 
 employed, a most singular and beautiful ap- 
 pearance is in course of time slowly pro- 
 duced. If a pan of porous earthen-ware, 
 from four to six inches wide, be filled to the 
 utmost with common water till it rise above 
 the lipsj and now planted above a dish of ten 
 or twelve inches diameter, containing a body 
 of sulphuric acid, and having a round broad 
 receiver passed over it ; on reducing the in- 
 cluded air to some limit between the twenti-^ 
 eth and the fifth part of its usual density, ac- 
 cording to the coldness of the apartment, the 
 liquid mass will, in the space of an hour or 
 two, become entwined with icy shoots^. which 
 gradually enlarge and acquire more solidity, 
 but always leave the fabric loose and un- 
 frozen below. The icy crust which covers 
 the rim, now receiving continual accessions 
 from beneath, rises perpendicularly by insen- 
 sible degrees. From each point on the rough
 
 TO HEAT AND MOISTURE. 
 
 149 
 
 surface of the vessel, filaments of ice, like 
 bundles of spun-glass, are protruded, fed by 
 the humidity conveyed through its substance, 
 and forming in their aggregation a fine sil- 
 very surface, analogous to that of fibrous 
 gypsum or satin-spar. At the same time, 
 another similar growth, though of less extent, 
 takes place on the under side of the pan, so 
 that continuous icy threads might appear 
 vertically to transpierce the ware. The whole 
 of the bottom becomes likewise covered over 
 with elegant icy foliations. Twenty or thirty 
 hours may be required to produce those sin- 
 gular effects ; but the upper body of ice con- 
 tinues to" rise for the space of several days, 
 till it forms a circular wall of near three inches 
 in height, leaving an interior grotto lined 
 with fantastic groupes of icicles. In the mean- 
 while, the exfoliations have disappeared from 
 the under side, and the outer incrustation is 
 reduced, by the absorbing process, to a narrow 
 ring. The icy wall now suffers a regular 
 waste from external erosion, and its fibrous 
 structure becomes rounded and less apparent.
 
 150 RELATIONS OF AIR 
 
 Of its altitude, however, it loses but little for 
 some time; and even a deposition of congealed 
 films along its coping or upper edge, seems to 
 take place, at a certain stage of the process. 
 This curious effect is owing to a circumstance, 
 which, as it serves to explain some of the 
 grand productions of nature, merits particu- 
 lar attention. The circular margin of the 
 ice, being nearer the action of the sulphu- 
 ric acid than its inner cavity, must suffer, by 
 direct evaporation, a greater loss of heat j 
 and consequently each portion of thin air 
 that rises from the low cavity, being chilled 
 in passing over the colder ledge, must depo- 
 site a minute corresponding share of its mois- 
 ture, which instantly attaches itself and in- 
 crusts the ring. Whatever inequalities ex- 
 isted at first in the surface of the ice, will 
 hence continually increase. 
 
 This explication seems to throw some light 
 on the origin of those vast bodies of ice which 
 occur within the Arctic Circle, and which, 
 towering like clustered peaks above the sur- 
 face of the ocean, have received the name of
 
 TO HEAT AND MOISTURE. 151 
 
 icebergs. They frequently project above an 
 hundred feet, and must therefore have ten 
 times as much depth concealed under wa- 
 ter. To suppose them to have been de- 
 tached from a solid field of such tremendous 
 thickness, would seem utterly improbable. 
 It may indeed be doubted, whether any part 
 of the ocean be ever naturally frozen. The 
 ice which I have formed from salt water by 
 the frigorific process, was always incompact, 
 inclosing brine within its interstices, and re- 
 sembling the aspect of what is called water- 
 ice, or dilute syrup congealed. Perhaps an 
 extremely slow congelation, descending re- 
 gularly from the surface, may press down the 
 saline particles, which are' never absolutely 
 detached from the water, and thus force them 
 to combine more largely with the mass below. 
 But even admitting this idea, it would be still 
 required to account for the great elevation 
 of those icy cliffs. The most satisfactory mode 
 probably of explaining the phenomenon, is to 
 refer it to the operation of a general principle, 
 by which the inequalities on the surface of
 
 152 RELATIONS OP Alll 
 
 a field of ice must be constantly increased. 
 The lower parts of the field, being nearer the 
 tempered mass of the ocean, are not so cold 
 as those which project into the atmosphere, 
 and consequently the air which ascends, be- 
 coming chilled in sweeping over the eminen- 
 ces, there deposites some of its moisture, 
 forming an icy coat. But this continued in- 
 crustation, in the lapse of ages, produces a 
 vast accumulation, till the shapeless mass is 
 at length precipitated by its own weight. 
 
 .Other natural phenomena will receive il- 
 lustration from the facts disclosed by the re<- 
 frigerating process. In the rigorous climes 
 of the north, the alternations of the seasons 
 are most rapid. On the approach of spring, 
 the thick fields of ice which, in Russia or 
 Canada, cover the Neva or the St Lawrence, 
 break up with overwhelming fury, accompani- 
 ^d too by tremendous explosions. Nor is this 
 iioise to be ascribed to the mere crash of the 
 falling fragments. In those frightful climates, 
 the winter at once sets in with most intense 
 frost, which probably envelopes : the globules
 
 TO HEAT AND MOISTURE. 153 
 
 of air separated from the water in the act of 
 congelation, and, invading them on all sides, 
 reduces them to a state of high condensation. 
 When the mild weather begins, therefore, to 
 prevail, the body of ice, penetrated by the 
 warmth, becomes soft and friable ; and the 
 minute but numerously interspersed globules 
 of imprisoned air, exerting together their con- 
 centrated elasticity, produce the most violent 
 explosive disruptions. 
 
 If we examine the structure of a hail-stone, 
 we shall perceive a snowy kernel incased by 
 a harder crust. It has very nearly the appear- 
 ance of a drop of water suddenly frozen, the 
 particles of air being driven from the surface 
 towards the centre, where they form a spongy 
 texture. This circumstance suggests the pro- 
 bable origin of hail, which is perhaps occa- 
 sioned by rain falling through a dry and very 
 cold stratum of air. As the congealed drops 
 arrive impregnated with cold at the milci 
 atmosphere near the ground, they besides 
 acquire, from external deposition, a sort 
 of snowy invest lire. An experiment might
 
 154 RELATIONS OF AIR 
 
 even be devised for illustrating this formation. 
 Suppose a capillary funnel, through which a 
 little water would percolate so slowly as to 
 drop only every two or three minutes, were 
 placed at the top of a very tall and narrow 
 receiver that has its air made extremely dry 
 by the joint influence of rarefaction and ab- 
 sorption ; the successive drops would, in their 
 descent, be frozen into globules resembling 
 hail. 
 
 In rarefied hydrogen gas, the process of 
 artificial congelation is performed with still 
 greater rapidity than in atmospheric air. No 
 material advantage, however, can be derived 
 
 l ; 
 
 from this property ; for, in the former me- 
 dium, the cold actually excited is yet, under 
 like circumstances, about a tenth part less. 
 In performing this experiment, the apparatus 
 is disposed in the usual mode and the air ex- 
 tracted, ard hydrogen gas being immediately 
 introduced from a gazometer, the rarefaction 
 is again renewed. If this subtil gas dissolve 
 a larger portion of moisture, it likewise com- 
 municates to the exhaling surface a corre-
 
 TO HEAT AND MOISTURE. 155 
 
 spending augmented share of heat. Ice, con- 
 fined under hydrogen gas, at every degree of 
 density, wastes more than twice as fast by 
 evaporation, as under common air of the 
 same elasticity. 
 
 But the combined powers of rarefaction and 
 absorption are capable of generating much 
 greater effects than the mere freezing of water. 
 I have in winter, when the density of air in- 
 cluded within a receiver was reduced to about 
 the five hundredth part, excited a cold of 
 125 degrees by Fahrenheit's scale below the 
 temperature of the room. In the fine season, 
 therefore, and with a better arrangement, a 
 still more intense refrigeration might no douot 
 be produced. Such frigorific energy, however, 
 is at all times sufficient for effecting the con- 
 gelation of mercury. Accordingly, if mer- 
 cury, contained in a hollow pear-shaped piece 
 of ice, be suspended by cross threads near 
 a broad surface of sulphuric acid under a 
 receiver ; on urging the rarefaction, it will 
 become frozen, and may remain in that 
 solid state for the space of several hours.
 
 156 UELATIONS OF AIR 
 
 But this very striking experiment, is easi- 
 ly performed without any foreign aid. Ha- 
 ving introduced mercury into the large bulb 
 of a thermometer, and attached the tube to 
 the rod of a transferrer, let this be placed 
 over the wide dish containing sulphuric acid, 
 in the midst of which is planted a very small 
 tumbler nearly filled with water : After the 
 included air has been rarefied about fifty 
 times, let the bulb be dipped repeatedly into 
 the very cold but unfrozen water-, and again 
 drawn up about an inch ; in this way, it will 
 become incrusted with successive coats of ice, 
 the thickness perhaps of the twentieth part 
 i inch : The water being now removed, 
 the pendant icicle cut away from the bulb, 
 and its surface smoothed by the touch of a 
 warm finger, the transferrer is again replaced, 
 the bulb let down within half an inch of the a- 
 cid, and the exhaustion pushed to the utmost : 
 When the syphon-gage has come, to stand un- 
 der the tenth of an inch, the icy crust starts 
 into divided fissures, and the mercury* having 
 gradually descended in the tube till it reach 
 
 "
 
 TO HEAT AND MOISTURE. 157 
 
 its point of congelation, or 39 degrees below 
 nothing, sinks by a sudden contraction almost 
 into the cavity of the bulb ; and the apparatus 
 being then removed and the ball broken, the 
 metal appears a solid shining mass* that will 
 bear the stroke of a hammer. 
 
 Such enormous powers of refrigeration 
 seem to open a wide prospect of future dis- 
 covery. If the machinery of the air pump 
 were improved, if a fluid were selected more 
 evaporable than water, and if an absorbent 
 substratum were employed of greater energy 
 than sulphuric acid, we might expect to 
 produce effects that quite transcend the ordi* 
 nary limits. Alcohol, for instance, will, r JL 
 der like circumstances, cause by its evapora- 
 tion at least ten degrees on Fahrenheit's 
 scale more cold than water ; its fumes being 
 constantly withdrawn from the thin air, by 
 the power of the same absorbent. But the 
 muriate of lime, though generally feebler in 
 its action, yet appears, at a certain state of 
 preparation, to attract humidity more greedi- 
 ly than sulphuric acid. Other deliquescent
 
 158 DELATIONS OF AIll 
 
 substances will no doubt be found to possess 
 similar, and perhaps still higher, powers of 
 absorption. By those joint energies, exerted 
 in vaporizing and again condensing, an ex- 
 treme cold may be procured, capable of per- 
 haps effecting new combinations among bo- 
 dies. Some liquids that have hitherto resist- 
 ed congelation, will hence be frozen, and cer- 
 tain gaseous fluids converted into liquids. 
 
 But almost every practical object is attain- 
 ed, through far inferior powers of refrigera- 
 tion. Water is the most easily frozen, by 
 leaving it, perhaps for the space of an hour, 
 to the slow action of air that has been rare- 
 only in a very moderate degree. This 
 process meets with less impediment, and the 
 ice formed by it appears likewise more com- 
 pact, when the water has been already purg- 
 ed of the greater part of its combined air, ei- 
 ther by distillation or by long continued boil- 
 ing. The water which has undergone such : 
 operation, should be introduced as quickly as ' 
 ,ossible into a decanter, and filled up close to 
 the stopper, else it will attract air most gjeedi-
 
 TO HEAT AND MOISTURE. 159 
 
 ly, and return nearly to its former state in the 
 course of a few hours. 
 
 Artificial congelation is always most com- 
 modiously performed on a large scale. Since 
 the extreme of rarefaction is not wanted, the 
 air pump employed in the process admits of 
 being considerably simplified, and rendered 
 vastly more expeditious in its operation : 
 Two or three minutes at most will be suffi- 
 cient for procuring the degree of exhaustion 
 required, and the combined powers of eva- 
 poration and absorption will afterwards gra- 
 dually produce their capital effect. In general, 
 I prefer plates of about a foot in diameter, 
 and which can be connected at pleasure wi^fe 
 the main body of the pump. The dish hoi 
 ing the sulphuric acid is nearly as wide as 
 the flat receiver ; and a set of evaporating 
 pans belongs to it, of different sizes, from 
 seven to three inches in diameter, which are 
 severally to be used according to circum- 
 stances. The largest pan is employed in the 
 cold season, and the smaller ones may 1 
 successively taken as the season becomes sul-
 
 160 RELATIONS OF AIR 
 
 try. On the whole, it is better not to over- 
 strain the operation, and rather to divide the 
 water under different receivers, if unusual 
 powers of refrigeration should be required. 
 As soon as the air is partly extracted from 
 one receiver, the communication is immedi- 
 ately stopped with the barrel of the pump, 
 and the process of exhaustion is repeated on 
 another. In this way, any number of re- 
 ceivers, it is evident, may be connected with 
 the same machine. If we suppose but six of 
 these to be used, the labour of a quarter of 
 an hour will set as many evaporating pans in 
 full action, and may therefore in less than an 
 ur afterwards produce nearly six pounds of 
 lid ice. The waste which the water sus- 
 tains during this conversion is extremely 
 small, seldom indeed amounting to the fif- 
 tieth part of the whole. Nor, till after multi- 
 plied repetitions* is the action of the sul- 
 phuric acid considerably enfeebled by its 
 aqueous absorption. At first that diminu- 
 tion is hardly perceptible, no,t being the 
 hundredth part when the acid has acquired
 
 TO HEAT AND MOISTURE. 161 
 
 as much as the tenth of its weight of water. 
 But such influence gains rapidly, and rises 
 with accelerated progression. When the 
 quantity of moisture absorbed amounts to 
 the fourth part by weight of the acid, the 
 power of supporting cold is diminished by 
 a twentieth ; and after the weights of both 
 these come to be equal, the refrigerating ener- 
 gy is reduced to less than the half. Sulphu- 
 ric acid is hence capable of effecting the con- 
 gelation of more than twenty times its weight 
 of water, before it has imbibed near an equal 
 bulk of that liquid, or has lost about the 
 eighth part of its refrigerating power. The 
 acid should then be removed, and concentrSPfc 
 ed anew by slow distillation. 
 
 But though the freezing of water is always 
 performed to the best advantage in a large 
 apparatus, a limit will soon occur to the scale 
 of magnitude. Since the efficacy of the process 
 depends on the quick circulation maintained 
 between the opposite exhaling and absorbing 
 surfaces, and consequently on their close 
 proximity* the very extent of those surfaces 
 
 M
 
 RELATIONS OF AIll 
 
 must have a tendency to retard and enfeeble 
 their operation. Accordingly, when the 
 earthen pan is unusually wide, the central 
 portion of its water, being most distant from 
 the acid, seldom becomes firmly congealed. 
 The measure of cold produced, at the same 
 temperature, and with the same rarefaction, 
 is determined, by the mutual proportion of 
 the humid and of the absorbent substance. If 
 the communication between them were quite 
 instantaneous, the effect resulting from such 
 contending powers would be an exact mean. 
 But the humifying influence of the water and 
 the desiccating action of the acid, are each of 
 flpm greater in their immediate vicinity. Nei- 
 ther does the evaporation, therefore, nor the 
 subsequent absorption, take place to the full 
 extent. In most cases, it will be sufficiently 
 near the truth, to estimate the effect on a 
 supposition that the surface of the acid were* 
 doubled, or that of the acid were reduced to 
 the one half. Thus, if the differential ther- 
 mometer, having its sentient ball.- incrusted 
 with *ce, in the rare medium, beside the acid,
 
 TO HEAT AND MOISTURE. 163 
 
 alone marked 300 degrees of dryness ; as- 
 suming the humid surface to be only the 
 fourth part of that of the absorbent, the action 
 would be shared in the ratio of one to a half, 
 or the dryness of the thin fluid encircling the 
 water would reach but to 200 degrees, and 
 occasion a depression of temperature of no 
 more than 36 degrees on Fahrenheit's scale ; 
 and if both the surfaces were equal, the ef- 
 fective dryness would be reduced to 100, and 
 the cold resulting from the operation dimi- 
 nished from 54 to 18 degrees. 
 
 When the exhaling and absorbing surfaces 
 are rightly disposed and apportioned, thf 
 moderate rarefaction, from twenty to for 
 times, which is adequate to the freezing of 
 water, may be readily procuredAjy the conden- 
 sation of steam. In all manufactures where 
 the steam-engine is employed, ice may, there- 
 fore, at all times be formed in any quantity, 
 and with very little additional expence. It is 
 only required, to bring a narrow pipe from 
 the condensing vessel, and to direct it along 
 a range of receivers, under each of whicl j the
 
 164 RELATIONS OF Mil 
 
 water and the acid are severally placed. 
 These receivers, with which the pipe com- 
 municates by distinct aper$res, may, for the 
 sake of economy, be constructed of cast-iron, 
 and adapted with hinges to the rim of a 
 broad shallow dish of the same metal, but 
 lined with lead to hold the acid. Other im- 
 provements, of a practical nature, might easi- 
 ly be devised, to suit particular objects. 
 
 But the excessive dryness of the rare me- 
 dium, exposed to the action of an absorbent, 
 is capable of being usefully turned to other 
 purposes, besides that of congelation. Birds 
 nnd other small objects in natural history, if 
 Introduced within a receiver beside a body 
 of sulphuric acid, would by rarefaction be- 
 come, in a v#-t time, most effectually dried, 
 without suffering any derangement or injury 
 of their parts, which the violent heat of an 
 oven is apt to occasion. Some anatomical 
 preparations might be managed in the same 
 way. This process, indeed, exactly resem- 
 bles what nature is said to perform on the 
 bodies of those unfortunate pilgrims, who are
 
 TO HEAT AND MOISTURE. 165 
 
 suddenly overwhelmed by tornadoes amidst 
 the sandy and arid plains of Arabia. 
 
 In a similar way, could gunpowder be con- 
 veniently brought to any required degree of 
 dryness, without incurring the risk of those 
 terrible explosions which so frequently hap- 
 pen in the ordinary mode of operation. The 
 powder and the acid being exposed in dis- 
 tinct but communicating chambers, and the 
 included air only slightly rarefied, might 
 easily, by means of a simple machinery, be 
 driven alternately from the one to the other, 
 and thus made to transport and deposite its 
 watery store. In some cases, without em- 
 ploying rarefaction at all, it may be sufficient , 
 merely to force the air which has been dried 
 by being confined with a bod;.~>f sulphuric 
 acid, to pass over the surface oi the gunpow- 
 der. But it would be premature to enlarge 
 on a subject, which, from its general import- 
 ance, seems to claim the consideration of our 
 public Boards. 
 
 The process which has been so fully de- 
 scribed, for producing dryness, and thence the
 
 166 RELATIONS OF AIR 
 
 cooling and congelation of water, may be 
 justly deemed an object of extensive utility. 
 In this island, the use of ice at table is con- 
 sidered as a mere luxury ; but, in the hot 
 climates, that practice is viewed in a very 
 different light, as an indispensable necessary 
 of life, and highly grateful and salubrious. 
 The ancients were accustomed to cool their 
 liquors during summer, by the infusion of 
 pure snow carried down on purpose from the 
 mountains. But, about the middle of the fif- 
 teenth century, the method was discovered in 
 Italy, of freezing water anew, by application 
 of the superior cold produced from the mix- 
 ture of snow with salt or nitre. In the East 
 Indies, great expence and attention are be- 
 stowed, in procuring the refreshment of cOol 
 drinks. On the coast of Malabar, the air is 
 generally too humid, for producing any con- 
 siderable effect by evaporation ; but, in the 
 interior of the province of Bengal, during 
 the continuance of the dry season, when a 
 piercing wind generally blows from the lofty 
 mountains of Thibet, films of ice about the
 
 TO HEAT AND MOISTURE. 167 
 
 thickness of a shilling or half-a-crown are ga- 
 thered in the mornings from the surface of 
 shallow earthen pans set close to the ground, 
 in pits lined with the bamboo reed, and 
 which have been filled the preceding evening 
 with water previously boiled and again suf- 
 fered to cool. These thin sheets of ice are 
 immediately rammed together into a compact 
 mass, and sent down, packed in wool, to Cal- 
 cutta, But during at least four months of 
 the year, this resource totally fails ; and the 
 settlers, deprived of their usual indulgence, 
 then suffer from excessive languor. 
 
 In the West Indies again, and the adjacent 
 shores of America, our colonists are, at all 
 times, left without any such relief, to assuage 
 the burning heats of a pestilent climate. 
 Even at home, ice-houses often fail in their 
 construction, and are seldom found to an- 
 swer in the southern and western districts, 
 such as Devonshire and Cornwall. To pro- 
 cure ice, therefore, independent altogether of 
 the disposition of the sky, is a benefit of no 
 small importance. A minute fragment of ice
 
 168 RELATIONS OF AJH 
 
 will yet be sufficient in melting to chill a very 
 large body of water. But the cold affusion has 
 been employed with the happiest success at 
 certain stages of fever. Cooling draughts 
 may, in many cases, too, be administered with 
 obvious advantage ; and though medical wri- 
 ters are not always consistent either with 
 themselves or with nature, the uniform be- 
 lief and experience of whole nations suffi- 
 ciently establish the utility of the application 
 of cold, both externally and internally, in a 
 wide variety of disorders which assail the hu- 
 man frame. It may not therefore be too 
 much, perhaps, to expect, that an enlighten- 
 ed and provident government will be dispos- 
 ed to encourage the introduction of refrige- 
 rating machines into our military hospitals in 
 the tropical climates, and likewise on board 
 Indiamen and the larger ships of war.
 
 TO HEAT AND MOISTURE. 169 
 
 I must now close this lengthened train of 
 investigation. Without entering much into 
 detail, I have endeavoured to exhibit the prin- 
 cipal facts and reasonings in a compressed 
 form. It will therefore not be judged super- 
 fluous, perhaps, in conclusion, to take a con- 
 cise retrospect of the whole. 
 
 The various effects of heat are deducible 
 from the operation of a peculiar subtle fluid, 
 probably the same as light, which combines 
 with bodies in very different proportions. 
 Hydrogen gas contains the largest share of it, 
 and mercury comparatively the smallest. This 
 heat is primarily derived from the incidence 
 of the solar rays, but distributed on the earth 
 by a very unequal allotment, according to the 
 obliquity which the surface of our globe pre- 
 sents to the sun in the different latitudes. The 
 original inequality, however, is greatly redu- 
 ced, by that perpetual, though indirect commu- 
 nication, which is maintained by the motion 
 of the atmosph re between the poles and the 
 equator. The currents from the south con- 
 vey warmth into the northern, regions, and
 
 170 HELATIONS OF AIR 
 
 the opposite currents, in their turn, transport 
 ing cold, serve to mitigate the burning heats 
 within the tropics. From this fact, combined 
 with the supposition, that such a mutual trans- 
 fer is, by the agency of oblique winds, effect- 
 ed completely twice every year between the 
 poles and the equator, the mean temperature 
 of any place, at the level of the sea, jnay be 
 derived by calculation, agreeing remarkably 
 nearly with the results of actual observation. 
 Again, the cold which prevails in the higher 
 spaces of the atmosphere is occasioned by 
 a similar but far more speedy communication, 
 supported between the lower and upper strata. 
 Thin air, by its constitution, holds a compa- 
 ratively larger share of heat. Any portion of 
 the ambient medium that ascends from the 
 surface must have its temperature diminish- 
 ed, and the corresponding portion which falls 
 down will, as to sense, become warmer. <In 
 consequence of the rapid and complete inter- 
 change which obtains, the same absolute 
 measure of heat exists at all heights in the 
 atmosphere, and its apparent decrease pro-
 
 TO HEAT AND MOISTURE. 171 
 
 ceeds wholly from that enlargement of capa- 
 city which the attenuated fluid acquires. 
 Theory here corresponds exactly with obser- 
 vation. 
 
 Heat is not only distributed among bodies 
 in various proportions, but it spreads through 
 them with very different degrees of celeril 
 Through dry wood or eider-down it works it 
 way with extreme difficulty ; yet, through the 
 dense substance of silver, it travels with a 
 rapid progress. In solid bodies, heat is con- 
 ducted by means of a repeated transfer, each 
 portion in the train of communication being 
 successively affected, and always delivering 
 its surplus warmth to the next portion in ad- 
 vance. But, in liquids, the mode of diffusion 
 '" Is more complex ; and besides the successive 
 transfer of heat along the extended mass, a 
 much larger expenditure is produced, by the 
 continual migration of the heated parts of the 
 medium. In the gaseous or elastic fluids, 
 again, there > combined another and most 
 unexpected element of dispersion. A part 
 of the heat discharged from a warm body is
 
 172 RELATIONS OF AIR 
 
 always darted to a distance by a sort of rapid 
 vib 1 lory commotion, exerted, like the im- 
 pression of sound, on the ambient air. This 
 portion, emitted from a surface of glass or 
 paper, will in general amount to half of the 
 whole expenditure ; but, from a bright me- 
 ic surface, it is about ten times less. A 
 reous surface, on the other hand, will, if 
 opposed to the tide of heat, receive nearly 
 the whole impression, while one of polished 
 metal will detain only the tenth part, reflect- 
 ing back all the rest. To this curious and 
 interesting bfanch of the subject ^f heat, I 
 have particularly directed my researches ; 
 and the application of the differential the^ 
 Udometer, which was contrived to indicate the 
 smallest changes in the temperature of its 
 communicating balls, has proved a safe and 
 easy guide in tracing out some of the more 
 recondite properties of matter. A variation of 
 that instrument, called the pyroscope, and 
 having one of its balls gilt, is especially fit- 
 ted for such examination. The peculiar ef- 
 fects exhibited, seem to proceed from those
 
 TO HEAT AND MOISTURE. 173 
 
 different degrees of contact which must ob- 
 tain between the air and the surface of ^ 3s 
 or of metal, the approximation, and conse- 
 quently the impression received or commu- 
 nicated, being much greater in the former 
 than in the latter. 
 
 Different bodies riot only contain 
 rious proportions, but undergo mutations 
 that respectateverychangewhich occur in their 
 constitution or internal arrangement Water 
 is subject to striking alterations in its affinities 
 to heat. In the form of ice, it holds a tenth 
 part less L?* f han before ; but'in the state of 
 steam or vapour, it contains above a half more 
 tKan it tficT in its liquid condition. Hence 135 
 degrees of heat by Fahrenheit's scale are e- 
 volved in the act of congelation, and 945 ab- 
 sorbed in the conversion of water into steam. 
 Evaporation from a humid surface, therefore, 
 makes it colder. But to this depression of 
 temperature there is an evident limit, when the 
 encircling air communicates, by its contact, 
 exactly as much heat as it absorbs, in unit- 
 ing with the quantity of vaporized moisture,
 
 174 RELATIONS OF AIR 
 
 sufficient for its saturation. The cold thus 
 induced, hence marks the dryness of the am- 
 bient medium. On this principle, I have 
 constructed the hygrometer, which promises 
 to become an instrument not only useful to- 
 ds the extension [of philosophy, but of 
 s. service in the improvement of the arts 
 manufactures. It derives elucidation 
 from comparison with the atmometer, which, 
 though grounded on simpler data, is yet sus- 
 ceptible of considerable delicacy. 
 
 Heat and moisture, in all their relations, 
 appear to Jtffcw the same laws. If rarefac- 
 tion enlargeVthe capacity of air for heat, it 
 likewise communicates an increased power 
 of containing humidity. The higher regions 
 of the atmosphere are hence comparatively 
 drier than the lower, or, at the same tempe- 
 rature, they would have a greater effect on the 
 hygrometer. In ascending, therefore, those 
 elevations, the continual diminution of heat 
 augments at first the dampness of the air, till 
 we reach its extreme limit, or the ordinary 
 seat of the clouds, above which the effect of
 
 TO HEAT AND MOISTURE. 175 
 
 attenuation begins to predominate over the 
 cold, and the air grows progressively drier 
 and clearer, till at length it melts away in the 
 resplendent fields of ether. 
 
 The power of air to contain or dissolve 
 moisture is augmented by heat in a rapi 
 progression, doubling for each rise of tern 
 rature corresponding to 27 degrees on Fahre 
 heit's scale. When two parcels of humid air 
 at different temperatures are mingled to- 
 gether, the mean temperature of the com- 
 pound being, therefore, unable to hold their 
 aggregate moisture, disencumblpfctself of 
 the surplus load. If this commrJBR be pro- 
 uced by the meeting of opposite currents in 
 the atmosphere, a very considerable deposi- 
 tion may take place* When the humidity is 
 slowly parted with, it remains suspended in 
 the shape of fogs or clouds ; but when it is 
 more copiously detached, it forms rain or hail. 
 Snow is caused by the freezing of the minute 
 aqueous globules at the instant of their prer- 
 cipitation, but hail seems to derive its origin 
 from the congelation of the large accumu-
 
 : 
 j 
 
 176 RELATIONS OF AIR 
 
 lated drops in their subsequent descent 
 through a cold and dry stratum of air. 
 
 Most of the softer substances are disposed 
 to attract moisture with various degrees of 
 force. On this property, is founded the con- 
 ction of all the hygroscopes which have 
 at different times proposed. Even the 
 ;hy bodies, if previously heated before a 
 strong fire, and kept stopped in glass decanters, 
 have a power to abstract moisture from the 
 confined air, and therefore to render it artifi- 
 cially dry. Cultivated soils possess eminent- 
 ly the sfl ^disposition, and which appears 
 the mofWBRspicuous in proportion to their 
 fertility. 
 
 The concentrated sulphuric acid and the 
 powdered muriate of lime, particularly the 
 former, are on the whole the most energetic 
 and lasting absorbents. But air which has 
 been desiccated by their operation, is fitted, 
 through its invigorated powers of evaporation, 
 to excite greater cold. Hence water or other 
 liquors may at all times be considerably cool- 
 ed, and with very little trouble or expence.
 
 TO HEAT AND MOISTURE. 177 
 
 But far more intense cold is created, by 
 combining the power of an absorbent with 
 that of rarefaction. If water in a small porous 
 pan be set above a wide surface of sulphuric 
 acid, and covered by a low receiver from 
 which the air is mostly extracted, it will 
 congeal, and will remain in that frozen 
 for a very long time, till the ice become in- 
 sensibly wasted away by a continued process 
 of evaporation. A sort of invisible aerial dis- 
 tillation is carried on under the glass ; for 
 the thinness of the medium aujj^jj^Josence 
 of atmospheric pressure, occasJ K&ly the 
 same effect on the humid substance, as the 
 application A fire alone. The steam exhaled 
 from the surface of .the pan is conveyed to 
 the acid, where it condenses and gives out its 
 component heat. After the ice has been 
 formed, the same circulation is still maintain- 
 ed. Heat is indeed merely transferred from 
 the exhaling to the absorbing surface; and 
 the solid cake remains considerably colder, 
 while the sulphuric acid is kept always warm- 
 er, than the atmosphere of the room. A tern- 
 
 V f^-Aj
 
 178 RELATIONS OF AIR, &C. 
 
 perature many degrees below the . noint of 
 freezing water can thus be procured at all sea- 
 sons. The congelation of mercury has already 
 been effected by this process ; and after re- 
 ceiving farther extension and improvemei^c, 
 i|Fmay no doubt come in time to be applied 
 W a variety of the most useful and beneficial 
 purposes in life. 
 
 FINIS. 
 
 c3* The different Instruments and Machines 
 described in this Tract, are to be had, of the 
 most accurate and perfect construction, from 
 Mr CARY, Opticia$,,ondon, and from Messrs 
 MILLER fy ADIE, Edinburgh.
 
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