An experiment, which may be regarded as the reverse of the above, was afterwards tried. Here the thermnrue ter was made to receive heat from the canister, its bulb being first left in its usual state, and afterwards covered with tin-foil carefully applied, so as to admit of a degree of polish. It was then found, that the coated thermome ter, analagous to the polished side of the canister, received only about VI as much heat as when the instrument was without the metallic covering. The power of surfaces in absorbing heat, seems therefore to be intimately connected with that of radiating it, materially influenced by the nature of the surface, much less, when consisting of polished metal, and greatly augmented when covered with paper, varnish, or any other not-metallic substance. These ex periments seemed to indicate, that the power of radiating and of absorbing heat was in an inverse ratio to that by which heat is reflected from surfaces ; and this point was afterwards made the subject of direct experiment by Pro fessor Leslie. He had already found, that the polished surface of the canister was a bad radiator, and that the coated thermometer, in consequence of its polish, was equally unfitted for absorbing heat. The next object was to examine, whether these surfaces, which were the least fitted to radiate and absorb it, were not as much superior in their reflecting, as they were defective in their radiating, power. The tin reflector was removed, and in its place was substituted one of glass, while the canister of boiling water, and the differential thermometer, were placed as be fore in the respective foci. In order to produce the great est action upon the mirror, the varnished side of the vessel was opposed to it ; but the effect produced by reflection in this case was not considerable. No important alteration was produced, by removing the metallic coating from the back part of the mirror, or by roughening it with emery ; but when the anterior surface of the glass was covered with Indian ink, no heat was sent off from it. On the con trary, when the front of the mirror was coated with tin-foil, the thermometer rose ten times as much as it did from the effect of the naked glass. These experiments, when com pared with each other, lead to the following conclusions : That when the rays of heat strike against polished glass, a large portion of them is absorbed, and tends to raise the temperature of the glass itself; but when they impinge against polished metal, few of them enter the metal, and nearly the whole are reflected. Inquiry, p. 21.
Professor Leslie having now fully established the con nection between the radiating and the absorbing power of different kinds of surfaces, afterwards proceeded to vary the effects in different ways, and to compare them with each other under different-modifications. We have men tioned above, that the polished side of the canister, when opposed to the mirror, produced in the differential ther mometer a certain rise, which we have called equal to 12, that of the varnished surface being estimated, as a standard of comparison, at 100. When the tin was rubbed with a small quantity of mercury, an effect was produced equal to 14, and when completely coated with it equal to 20. As the brilliancy in this case appears not to have been less than, from the pure tin, we must conclude that there is either a different radiating power attached to different metals, inde pendently of their mechanical properties, or that an amal gam of tin and mercury was formed of a soft consistence. It was afterwards found, that when a metal loses its bril liancy by oxidation, the radiating power is increased in the same proportion: a fresh surface of lead raised the ther mometer only i-th as much as the lead when covered with a layer of minium. This effect of oxidation might perhaps have been predicted from the preceding experiments ; but the alteration caused by simply scratching or roughening the metal, seems more remarkable. If the perfectly smooth surface produced on the thermometer an effect equal to 12, by rubbing it with a file or with sand, so as quite to destroy its polish, an elevation took place equal to 26, or rather more than twice as much as the former. It was afterwards found, that when the tin canister was coated with an ani mal substance, such as jelly, if it was spread upon it only to the thickness of a fine film, it raised the thermometer to 38 ; but when laid on so as to form a thick coating, the ef fect was about twice as great, or near 80. The effect in
this case, was not found, however, to bear an exact ratio to the thickness of the coating ; nor was a difference in the thickness of the coating observed to have any effect, except when an animal substance was used, or something of a similar nature, for metallic coatings seemed to act merely from their surface. Some experiments were made upon the effect of colour, in modifying the radiating power of surfaces ; but the results are not sufficiently uniform to prove any thing very decisive on this point. Although the most considerable effects were produced by lamp-black, yet, as has been mentioned above, writing paper was found to be nearly as powerful. We have already had occasion, more than once, to refer to the relation which exists be tween the radiating and the reflecting power of bodies, a fact which, although contrary to what might have been ex pected upon a transient view of the subject, is yet estab lished by numerous and decisive experiments. Thus scratching the surface of the mirror, diminishes its reflect ing power as remarkably as it augments its radiation ; and also a layer of animal matter spread over the face of the canister, diminished its reflecting power to about 1-3d, as was determined by the thermometer. Inquiry, p. 76. et seq.
Professor Leslie was afterwards led to confirm, and very much extend the views which Scheele originally suggested respecting the power of certain substances to retain the ca loric which falls upon them by radiation, when they are such as not to send it off again by reflection. In this case, the body receiving the heat, experiences an elevation of temperature, until after some time it becomes itself a source or centre of heat, which emits it to other bodies. It was in this way that heat was separated from light, as we have already related, the light passing without interruption through a plate of glass, by which the transmission of heat is, for a certain time at least, entirely obstructed. When the heat is emitted or radiates from its new source, its pro gress is then found to be varied from that which it original ly possessed, and to be entirely directed by the condition of the surface from which it last escapes. This property of heat was illustrated by a series of experiments, in which screens of different kinds were interposed between the mir ror and the canister, to which we have so often referred : (Inquiry, p.26, and 17.) A sheet of tin was found entirely to intercept the heat, and a plate of glass a considerable portion of it ; but what constitutes a curious difference be tween the effect of heat and light is, that the quantity of heat intercepted varies greatly, according to the vicinity of the glass to the radiating body ; more heat being transmu ted when the glass is near the canister, than when it is more distant from it. But, perhaps, sonic of the most curious of all Professor Leslie's experiments were those in which he employed two screens of tin, one side of each of which was covered with the black varnish, the other being left uncovered. When these tin plates were laid together, with both their painted sides in contact, and of course the bright sides both external, little heat passed through them, be cause the side nearest the canister was not adapted to re ceive caloric, nor the other to radiate the little which might have been received ; but when the varnished sides were placed externally, the quantity of heat that passed through them was considerable, because here the varnish enabled the one plate to absorb, and the other to radiate caloric with facility. 'When only one plate was used, intermediate ef fects were produced. Several circumstances that were noticed by Professor Leslie, led hint to conclude that the rays of heat, like those or light, where they proceed from a near object, are sent off' in lines that have a sensible diver gence. The radiation of heat seemed to be entirely sus pended, by having the heated body immersed in water ; and it appears probable that it cannot exist in any medium ex cept air. Inquiry, p. 92.