The nature of the apparatus, and the experiments made by Leslie on the radiation of heat, are described under HEAT. From these it appeared that the amount of radiation from a polished surface was only about one-eighth of that which took place from a surface without polish ; and hence it was evident that the velocity of radiation depends more on the surface than on the nature of the radiating body. In making experiments respecting the absorbing power of substances, Leslie found that when the heat from the canister was suffered to fall on the glass bulb of the differential thermometer, the quantity received by the latter, if covered with tin-foil having a polish, was only one fifth of that which it received in its ordinary state.
Leslie also repeated the experiments of Pictet for determining what was called the radiation of cold ; and filling his canister with ice or snow, he found that the cold apparently emitted from the varnished side was the greatest, and that from the polished side the least ; he observed also that the cold, like the radiant heat, varied with changes in the absorbent power of the thermometer and of the surface of the mirror. The mercury in the thermometer fell more when the mirror was of polished metal than when of glass ; it also fell more when the bulb was in its ordinary state than when covered with a polished tin foil. (` Inquiry into the Nature of Heat,' 1804.) These circumstances, which seemed at one time to favour the opinion that cold had a .material existence like heat, have received an explanation from 31. Prevost of Geneva. (`Journal de Physique,' xxxviii.) This philo sopher observes that a mass of ice or snow may send out radiant heat less in abundance than that which is emitted from ether bodies in its vicinity ; and this comparatively small quantity of heat, when reflected from one mirror to the other, and from the surface of the latter to the focus, may there produce the effect of absolute cold, by causing the thermometer to send out radiant heat to the ice, and thus render the mercury in it lower than it would otherwise be. This interchange of caloric between the thermometer and the ice may be conceived to go on, the former on the whole losing, and tho latter acquiring it, till an equilibrium is established between their temperatures.
From the result, of Leslie's experiments, it is apparent that the power by which bodies absorb heat is directly proportional to that by which they allow it to radiate from them ; and that both are inversely so to the power of reflecting it. When the rays of heat fall on polished glass, a large portion of them is absorbed; these raise tho temperature of the glass, and from the surface of tho latter that heat is afterwards radiated in abundance ; whereas, when the rays strike the surface of polished metal, nearly all of them are reflected. It was found how
ever that different metals. independently of their polish, have (Efferent degrees of reflecting and radiating power. A mirror of tin being rubbed with mercury, the reflection was increased in the ratio of 12 to 20, though the degree of polish was the same ; and a metal having lost some of its brilliancy by oxidation, the radiating power was propor tionally augmented.
Lealio'a canister was found, moreover, to produce different effects, according to the thickness of the covering material on its sides. One coat of jelly on the tin side produced an elevation of the thermometer equal to 38', and four such coats produced an effect equal to 54° ; but after a certain thickness the difference was insensible. A plate of metallic leaf rendered however the radiating power as great as a thick plate of the name metal.
Experiments made by Count Rumford, soon after the publication of Leslie's work, produced nearly the same results as the latter hal ob tained; and the Count draws from them several useful conclusions. l le observes (' Phil. Trans.', 1804) that when we would confine heated substances, solid or fluid, in a vessel, the surface of the latter should be highly polished ; on the other hand, if the object be to cool the substances, the surface should be painted or varnished, or be covered with a soft coating which is not metallic. Also, in warming apart ments by steam, the intention being to promote radiation as much as possible, the tubes conveying the steam should be unpolished or painted.
In pursuing the views of Scheele concerning the transmission of heat through screens, Leslie found that a sheet of tin interposed between the canister and the mirror entirely intercepted the heat, and a plate of glass nearly so. Employing two sheets of tin, each of which had one side covered with black varnieh, while the other side was polished, the following were the results : when the :varnished aides were in contact, little heat passed through, evidently because one of the exterior sides was not well adapted to receive the caloric, nor the other to radiate the little which might have been acquired ; but when the varnished sides were placed exteriorly, the quantity which passed through was considerable, the varnish enabling one plate easily to absorb and the other as easily to radiate the heat.