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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 feet 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. Herschell, 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 fluids, 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 tem 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 caloric, 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 exceedingly 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 experiments. But Mr. Leslie, contending that light and heat are only different states of the same substance, supposes this instrument 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 reflectors, 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

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of light is said to depend. A fluid is indeed propelled, which possessing reflexibility is made to converge: but we cannot conceive this to be light, liberated from a state of combination, or to depend on certain oscillations or vibratory impressions excited in the ambient air: we should rather suspect it to be the light of the room, heated at the surface of the canister, and determined to a focus by the reflector.

The Photometer by no means appears to be an accurate measure of light. As a proof of this, we need only observe, that the light of a wax candle, at two inches distance, produced an effect of 6 degrees; the dull reddish light of a coal fire produced 8 degrees. Now it is sufficiently evident that, in the intensity and quantity of light emitted, the latter is far superior to the former. Again, the light of the moon, we know perfectly well, is at least as good, for the purposes of reading or writing, as that of a dull coal fire. But the action of the moon's rays on the Photometer was absolutely imperceptible, even when concentrated in the focus of a large burning glass. It is for Mr, Leslie to say how these facts demonstrate the identity of heat and light. He endeavours to account for the debility of the moon's rays, by estimating the quantity of light she reflects, which he considers as 150,000 times weaker than that of the sun at the same altitude; and by reviving a curious conjecture, to be found in Riccioli's Almagestum Novum, respecting the phosphorescent property of the lunar surface. On the contrary, the moon, we think, may have the power of reflecting a great deal of light, while it absorbs the heat with which it was charged; and, if the moon be habitable, something of this sort might be expected to take place.

In the twentieth chapter, Mr. L. observes that his Photometer is calculated to give us correct notions on a variety of interesting subjects, and to assist in the prosecution of several philosophical inquiries.

We shall mention a few of the results obtained by this elegant contrivance. In lat. 56°, the direct impression of the sun at noon in summer amounts to 90 degrees: at the altitude of 17, it is reduced to the half, and at 3° above the horizon the whole effect exceeds not one degree. The greatest force in the depth of winter measures only 25.-It requires about 2 degrees of light to enable one to read or write with pleasure. The Photometer. placed, in a winter day, on newly-ploughed ground, on russet grass, on sand, and on snow; indicated 24, 30, 33, 44. respectively. Thus snow nearly doubles the impression of the incident light, by a copious reflection; and hence is disproved, according to Mr. L., the idea of its possessing a phosphorescent quality. The photometer ascertains the various degrees of transparency, as well as the amount of reflection. Of 100 parts of incident

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light, cambric admits 80, if wetted, 93: vellum paper 49, thin post 62; if oiled 80, and 86.We could wish that some trials were made on diaphanous substances previously heated to a determinate degree. We have no doubt that the effect of lunar light would be increased, and that of solar light diminished, by the intervention of hot and cold media respectively.

We do not perceive that any experiments have been made on the effect of plain reflectors on the Photometer. We should apprehend, that a difference would appear between the first appulse of light on the photometer, (whether from a reflecting, or through a transparent, substance,) and its influence after these substances had become saturated with heat. In our preceding remarks we have confined our references to Mr. L's own experiments to have followed them practically would have required as much time from us as they did from him; and even to refer to all the known experiments which have a relation to the question would have rendered this article very voluminous. One familiar experiment already alluded to, we think it right again to notice. A pane of glass, placed between the face and the fire, completely intercepts the heat, though it transmits the light undiminished; in a short time the glass becomes heated, and the heat passing through it sensibly affects the face. In the first instance, the heated light appears to lose its temperature in consequence of parting with its caloric to the colder glass; afterwards, the glass having acquired a similar temperature, it passes through with little diminution of heat.

The following fact is also well known. A glass mirror, held before the fire, reflects no heat; while a metallic mirror reflects it in considerable quantity. Is not the caloric of the heated light detained during the transmission of the light through the glass of the mirror in its passage to and from the reflecting surface and does not the heated light, in the other case, preserve its heat in consequence of not passing through any cold dense medium? In confirmation of this supposition we must add, that the glass mirror at length becomes heated, while the temperature of the metallic mirror, which is but little absorbent, is scarcely at all affected. A little attention to these experiments will, we trust, render the identity of heat and light very problematical; and also shew, perhaps that several of Mr. L's experiments will bear very different explanations from those which he has adopted.

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We cannot close this article without expressing our sense of obligation which Mr. Leslie has conferred on the scientific world, by his labours and ingenuity; and our confident hope, that instruments so excellently adapted to the objects of inquiry will shortly prove a clue to the most important and interesting discoveries.

A warm

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* * A warm contest has arisen at Edinburgh in consequence of a note inserted at the end of Mr. L.'s volume, which applauds, without qualification, Mr. Hume's Essay on Necessary Connection. On this ground Mr. L,'s election to the Mathematical Chair at Edinburgh was strongly opposed by the clergy. Mr. D. Stewart and Mr. Playfair, particularly, have accused the ministers of Edinburgh of conspiring to engross to themselves the various Professorships; and a similar charge has been retorted upon the accusers. It appears that the various laws requiring subscription to the national confession of faith, have been, for half a century, disregarded by the professors; the Presbytery are now determined to enforce these regulations. The question, therefore, assumes a very serious aspect; the parties too it are at issue on positive facts, and the paper war, we perceive, rages with unremitted ardour. We shall not attempt to furnish a view of this controversy, till the pleadings on both sides are brought to a termination.

Art. X. A History of the College of Arms, and the Lives of all the Kings, Heralds, and Pursuivants, from the reign of Richard III. Founder of the College, until the present Time. With a Preliminary Dissertation relative to the different Orders in England, particularly the Gentry, since the Norman Conquest. By the Rev. Mark Noble, F.A.S. of L. and E. Rector of Barming, in Kent, and Domestic Chaplain to George, Earl of Leicester. 4to. pp. 449. Price 11. 11s. 6d. Debrett and Egerton. 1804.

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ITERARY research has lately included the most prominent subjects in every Science. Hence, it may excite surprise, that the History of the College of Arms was not added, at an earlier date, to the number of topographic productions. The delay may, perhaps, partly be ascribed to detached accounts of it, which appear in congenial works of great celebrity. It may, also, have been occasioned in some degree, by the length of time since the present production was announced, as it would naturally preclude others from undertaking the work, when a writer of such acknowledged talents as Mr. Noble was known to be engaged in it. His labours are at last before us; and in a general view, we consider him as being entitled to the acknowledgements of the learned, for the persevering assiduity displayed in his collection and arrangement.

The work is (with permission) inscribed, very properly, to his Majesty. In a preface the author states his plan, and explains the utility of his production; acknowledging, also, his obligations to various gentlemen from whom he has derived assistance, especially to the present officers of the College of Arms, to whose curious and valuable library he had free access. A list of authorities next appears, including MS. and topographical works.

The history is introduced by a dissertation of 44 pages on the state of society, and the rise of families, since the Norman con

quest;

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