The theory of radiant heat is intimately connected with that of the cooling of bodies; and the first effort to determine the laws relating to this subject was made by Sir Isaac Newton, who, from theoretical considerations, inferred that when a heated body is exposed to a constant cooling cause, as the uniform action of a current of air, it ought to lose at each instant a quantity of heat proportional to the excess of its temperature above that of the surrouuding air, and consequently that its losses of heat in equal intervals of time should form a decreasing geometrical progression. But it is now known that this law holds good only when the differences of temporaturo do not exceed 40 or 50 degrees, and its inaccuracy at high temperatures was first pointed out by Martine (1790).
Front the experiments of MM. Dulong and Petit, it is found that, if it were possible to obtain the absolute loss of heat which a body in vacuo experiences (or that loss which would take place if there were no restoration of heat from surrounding bodies), the velocities of coOing, estimated by the diminutions of temperature indicated by an air thermometer, would increase in a geometrical progression when the temperature of the heated body increases in an arithmetical pro gression ; and further, that the ratio of the former progression ( = 1.0077) would be the same for all bodies, whatever might be the state of their and whether the temperature of the vacuum remained constant or increased in an arithmetical progression. But, on taking account of the quantity of heat sent back at every instant by tho surrounding medium (a quantity which will be constant if the temperature of that medium does not vary), it is found that the velocities of cooling in rune increase, for equal increments of tempe rature, In a geometrical progression whose terms are diminished by a constant quantity, which quantity varies in a geometrical progression when the temperature of the medium varies in an arithmetical pro gression. By direct experiments on the cooling of heated bodies in air and hydrogen gas, Dulong and Petit determined what Leslie had before ascertained by an indirect process, namely, that the loss of heat, when a body is in contact with a gas is independent of the surface of the cooling body, They found also, by experiments on dilated air and carbonic acid at various temperatures, that the velocity with which a body cools from the mere contact with gas (when the excess of temperature of the heated body above that of the surrounding gas is constant) depends on the density and temperature of that gas ; but this dependence is such that the velocity of cooling remains the same if those elehients change in such a way that the elasticity of the gas remains constant. The same chemists have also ascertained that, when the elasticity of air varies in a geometrical progression, its cooling power varies likewise in a geometrical progression, in such a manner, that when the common ratio of the first progression is 2, that of the latter is 1-366. If, instead of common air, hydrogen gas, carbonic
acid, or olefiant gas be in contact with the heated body, the ratio of the first progression being as before, that of the second is And they conclude that the cooling power of each of the last-mentioned gases is nearly proportional to the square root of the elasticity of the gas. (‘ Annales de Chimie,' vii. ; ' Annals of Philos.', xiii.) That the colours of bodies have some effect on the velocity of radiation end on the absorption of heat has been proved by experi ments made by Dr. Stark of Edinburgh (1833). This gentleman surrounded the bulb of a thermometer successively with equal weights of black, red, and white wool, and placed it in a glass tube, which was heated to the temperature of 180° by immersion In hot water; the tube was then cooled down to 50° by immersion in cold water, and the several times of cooling were respectively 21, 26, and 27 minutes. On winding successively black, red, and white wool about the bulb, and raising the temperature from 50° to 170°, the times in which the thermometer so surrounded acquired the latter temperature were respectively 4.1, 54, and 8 minutes. (Turner's ' Elements of Chemistry,' Heat.) Mr. Glaisher (` Phil. Trans.' 1847) found the order of the radiating power of coloured wools exposed to a clear nocturnal sky to be black, green, white, crimson, scarlet, orange, yellow, dark blue, light blue, the difference between light blue and black being 1e-3.
The refrangibility of heat was first examined by Dr. (afterwards Sir William) Herschel, who, having analysed by the prism, as usual, a beam of solar light, and having placed a Fahrenheit's thermometer successively within the fields of the different coloured rays in the spectrum, found that in the violet rays the temperature was 2°, and, gradually increasing towards the other extremity, in the red rays it was 7° above the general temperature of the apartment. He also ascertained that there was a point beyond the limits of the visible red rays at which the excess of temperature was a maximum. Similar observations were made about the same time by Sir Henry Englefield; and it was hence evident, not only that the calorific rays were refrangible, but that the property existed in them in a higher degree than iu light. Dr. Herschel afterwards made a number of observations on small pencils of heat proceeding from a lighted candle, a common fire, iron heated to redness, and also from iron heated to a lower degree ; and he discovered that, in all these cases, the calorific rays were susceptible of refraction. He found however that there was some difference between the heat of the aim and that of terrestrial bodies, the former passing more freely through the glass than the latter. (` Phil Trans.', 1800.) 31. Melloni has subsequently ascertained, by using prisms of rock-salt (a mineral which possesses in a high degree the power of transmitting heat), that heat from different sourceadlire light of different colours, has different degrees of refrangibility.