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liquor will then move quicker under any impression. Each leg of the instrument is from three to six inches in height, and the balls are from two to four inches apart. The lower portion of the syphon is cemented at its middle to a slender wooden pillar inserted into a round or square bottom, and such that the balls stand on a level with the centre of the speculum. A moment's attention to the construction of this instrument will satisfy us that it is affected only by the difference of heat in the corresponding balls, and is calculated to measure such difference with pecus liar nicety. As long as both balls are of the same temperature, whatever this may be, the air contained in the one will have the same elasticity as that in the other, and consequently the intercluded coloured liquor, being thus pressed equally in opposite directions, must remain stationary. But if, for instance, the ball which holds a portion of the liquor be warmer than the other, the superior elasticity of the confined air will drive it forwards, and make it rise in the opposite branch above the zero, to an elevation proportional to the excess of elasticity or of heat. The interval between freezing and boiling water being distinguished into an hundred equal parts, called centigrade, each of these subdivided decimally constitute the degrees which I employ, and which, following up the same system of nomenclature, would be termed milligrade.' pp.9-11.
The canister being placed on a table, a few feet distant from the reflector, with its papered or blackened side directly trouting it, and the place of the corresponding focus being found by a lighter taper; the ball of the differential thermometer, called the focal ball, is inoved to that spot, the plane of the instrument being parallel to the face of the reflector. On filling the canister with boiling water, the coloured liquor of the thermometer was seen to rise; in the space of two or three minutes it had mounted to nearly the top of the scale, and having remained a short time stationary, it slowly descended as the canister cooled. Mr. L. found this effect, in every case, exactly proportionate to the heat of the canister, or the difference of its temperature from that of the room. The experiment was found to succeed equally well with cold as with heal, for the canister being filled with ice, or with a frigorific mixture, the focal ball was chilled, and the coloured liquor consequently sunk: the effect, though in a contrary direction, being still rigorously proportional to the difference of temperature. Hence it appeared unquestionable, as Mr. Leslie observes, that some hot or cold matter actually flowed from the canister towards the reflector, and from the reflector to the focal ball.
The experiment was repeated with the following changes and results. The black side of the canister filled with boiling water being turned to the reflector, the liquor of the differential thermometer rose to 100%; another side covered with paper being thus disposed produced an effect equal to 980; another side covered with a pane of crown-glass, produced an effect only equal to about 90°; and the bright side of the canister being brought to face the reflector, the coloured liquor quickly sunk to 12o. To produce this latter effect, it is only necessary to employ any clean metallic surface. These are the chief differences which respect the canisters.
With regard to the difference of the reflectors:-A concave mirror being employed instead of the tin reflector, the liquor rose through a small but visible space, nor was this effect increased by rubbing off the silvering from the mirror, nor by grinding its hinder surface. By coating it with Indian ink, the effect became altogether invisible, and by covering the face of the mirror with a sheet of tin-foil closely adapted to its surface, an effect was produced which exceeded ten times that of the naked mirror.
Mr. L. now advances with confidence, and seems to anticipate the triumph of his theory.
The facts related in this chapter will be deemed at least very curious; and viewed all together, they are calculated, I think, to affect us with surprize. Nay, they are repugnant to our first notions, and might ex-. perience contradiction, if they were not so easily verified.' .
• The power of absorbing heat, and the power of emitting it, seem always conjoined in the same degree; and this uniform conjunction clearly betrays a common origin, and discovers the evolution of a single fact, which assumes contrary but correlative aspects.-In the reflecting of heat also, we readily perceive that the very different aptitudes exhibited by different surfaces are derived from the same principle. That portion of heat only is reflected which has not been previously absorbed. Thus a coat of china ink affords no reflection perceptible because it is most absorbent of heat.'
Whatever reasonings are employed concerning the operations of Heat, the same must, with equal propriety, apply to those of Cold. . . Do not both of them produce their distant effects by the agency of the same individual fluid, susceptible, like all matter, of every possible degree of temperature ? pp. 23-25.
Various experiments are next described in which different substances were interposed as screens between the canister and reflector, the blackened surface of the canister being presented, -and the screen being placed about two inches from it. A sheet of tinfoil thus disposed appeared completely to intercept the effect upon the focal ball ; and the same power was found to be possessed by gold leaf: manifesting that there is an actual flow or impulsion of some corporeal substance. A pane of crown glass being substituted for the tinfoil, the thermometer rose to 20°, being one fifth of the intensity manifested when nothing was interposed. This experiment impels Mr. Leslie to the consideration-Has the fluid which is thrown from the canister any relation to light? In resolving this question Mr. Leslie reminds us that, although light permeates glass and other diaphanous substances, it yet suffers in its passage a certain degree of diminution or absorption; and that, whether light passes in a
condensed or in a diffused state, it must, in either case, sustain the same proportional loss; because each particle, travelling through the same range of matter, must incur the same risk of impediment. Here, then, Mr. Leslie thinks is a simple criterion by which to decide, whether the fluid, which is emitted from the heated surface, really penetrates through the glass, and thence emerging with diminished quantity, continues its course; since it would experience the same measure of absorption, four fifths of the whole, in whatever part of its transit, from the canister to the reflector, it encountered the screen. The pane of glass was therefore successively carried forwards, the effect on the focal ball proportionally diminishing, and, when it was advanced a foot before the canister, not exceeding the thirtieth part of the full effect. Hence Mr. Leslie concludes that the fluid thrown from the canister is not, like light, capable of permeating glass. A sheet of writing paper being substituted for the glass at the original distance of two inches from the canister, an effect was produced nearly equal to the fourth part of that which was produced without the screen.
The following experiment being considered by Mr. Leslie as particularly illustrative of his opinion, we deliver it at length, together with the inference which he deduces from it..
• Select two panes of crown-glass as flat and smooth as possible, and coat one side of each with tinfoil, by means of a little gum-water. Thus prepared, and the apparatus put in order, join those panes together with their tin surfaces in contact, and attach them to the frame of the screen ; the focal ball will receive an impression equal to about 18 degrees. Invert the panes of glass, placing them with the tin coatings outmost: the liquor of the differential thermometer will now sink back again to the beginning of the scale.
Such is the experimentum crucis. It establishes beautifully and, I think, beyond the power of contradiction, the simple theory to which we have been led by a close train of induction, In both cases the obstacle presented, or the compound screen, is absolutely the same. If the effects in the focus of the reflector were produced by some subtle emanation capable of permeating solid substances, how could such a singular contrast obtain ? It seems impossible to elude the force of this argument.'
pp. 35–36. Admiring, as we do, the excellence and simplicity of Mr. Leslie's apparatus, the felicity of his experiments, and the acuteness of his reasoning, we are yet unable to admit all the points of his theory. Indeed when we reflect on the general adoption which his theory has experienced, and the high authority by which it has been sanctioned, we venture not without reluctance to entertain a suspicion of its solidity. But the sixth and seventh experiments have excited doubts, which we have not been able to remove. Nothing offered by Mr. Leslie satisfactorily explains to us, how such a difference could have arisen, as that which he observed, with a screen of tinfoil, and of crown glass respectively. In the former case, no visible effect was perceived ; but in the latter a fifth part of the full effect was produced. That some substances had been transmitted through the glass was therefore a conclusion which fair reasoning would directly form, and the conjecture that these substances were heat and light seemed consonant with known facts. But Mr. Leslie is of · opinion that it is proved by the experiment, in which the pane of
glass is moved forward to a greater distance from the heated canister, that the fluid thrown from the canister is not, like light capable of permeating glass, since so great an effect is not produced, as when the glass was near to the canister. But in this experiment, when the pane of glass was placed, one foot distant from the canister, and two feel from the reflector, a thirtieth part of the full effect upon the focal ball of the thermometer was then observable; when if, instead of glass, any perfectly opaque substance had been employed, no visible effect would have been produced.
To us it appears highly probable, at least, that light exerts a much more powerful influence, in the production of the phænomena occurring in Mr. L's experiments, than he is disposed to allow, Common observation has taught, that the light of the sun brings with it heat also, and the experiments of Dr. Herschelt, slighted by Mr. Leslie, but confirmed by Sir Henry Englefield and M. Ritter, shew the co-existence of caloric with light, in a sun-beam. Now without attempting to discover the mode of union, by, which light and caloric are combined; or presuming to determine whether light, like other elastic fluids, gives out a quantity of caloric, on its assuming a more solid form, as seems to be proved by the heat made sensible upon the plentiful absorption of light by a dark surface, we will venture to assert, that light, like other elastic Auids, may suffer changes of temperature, from the influence of caloric. No incongruity therefore appears in the suppositions, that the light emitted by any body will possess a degree of temperature, proportioned to the quantity of caloric which that body discharges; and that the fascis of rays proceeding from the surface of that body, when converged to a focus, may manifest the degree of tema perature it has acquired, by its influence on an instrument so delicate as the differential thermometer of Mr. Leslie. We have not observed any of the phænomena produced by Mr. Leslie's experiments which might not be explained, at least as well, on these principles, as on those which he has aimed to establish, His experiments with the pane of glass, (Exp. 6 & 7.) exactly accord, in supporting our opinion, with the well known fact, that if a pane of glass be interposed between the fire and the face, no heat will be perceived; but if the glass be held there, until it has itself derived an increase of temperature, then not only will the light pass through, but a glow of heat will be felt on the face; shewing, either that light and caloric, separately pass through or that the light is united and surcharged with ealoric, derived in part directly from the fire,and, in part, from the heated glass, through which it passes. The value of these observations will be easily appreciated, by making almost any of Mr. Leslie's experiments, in a dark room; the light employed to shew the effect on the thermometer being so confined, as to prevent its falling on either the canister or reflector : should the thermometer under these circumstances, he affected, to the same degree as when the room was not darkened, we shall no longer contend. for the agency of the surrounding light. • The observations on the refrigeration of bodies are exceed. ingly curious and interesting, the results being in several instances very different from what we should have expected.
By the application of the principle on which the differential thermometer had been constructed, Mr. Leslie was enabled to form an instrument of extraordinary delicacy, which he considers as well adapted to the mensuration of light. As in the differential thermometer, so in this instrument named by its inventor, the Photometer, its extraordinary susceptibility of impression depends on the influence of two counteracting balls: one of which in this instrument is left naked and pellucid, and the other is rendered black and opaque. The accession of heat, during any given time, Mr. Leslie considers, is evidently as the number of lucid particles that are absorbed by the black ball.
It is evident that; agreeable to the generally received opinions respecting light and caloric, this instrument should rather be considered merely as a calorimeter, or an instrument for measuring the caloric which is blended with light. This instrument indeed, seems to furnish very strong proof of the propriety of the inferences made by Dr Herschell from his well known experi. ments. But Mr. Leslie, contending that light and heat are only different states of the same substance, supposes this instrua ment to yield a correct measure of the quantity of light itself; heat, according to his theory, being only light in the state of combination with bodies, and light being the same subtile matter projected with extreme celerity. The Photometer gives the measure of the quantity of heat formed by the light combining with the darkened ball of the instrument, whilst the differential thermometer is supposed to measure the same subtile matter, projected with that celerity which is assumed by our author to constitute light. But it should be considered, that in no one of the experiments with the heated canisters and refiectors, is the emission of light from the heated body made manifest, although this subtile matter is then supposed to undergo that rapid projection on which its appearance under the form