[62] These results are obtained from the valuable observations of Robert Were Fox, Esq., made with great care by that gentleman in several of the Cornish mines: Report on some observations on Subterranean Temperature.—British Association Reports, vol. ix. p. 309; Philosophical Magazine, 1837, vol. ii. p. 520.
[63] From his experiments, the following conclusions were arrived at by M. Delaroche:—
1. Invisible radiant heat may, in some circumstances, pass directly through glass.
2. The quantity of radiant heat which passes directly through glass is so much greater, relative to the whole heat emitted in the same direction, as the temperature of the source of heat is more elevated.
3. The calorific rays which have already passed through a screen of glass, experience, in passing through a second glass screen of a similar nature, a much smaller diminution of their intensity than they did in passing through the first screen.
4. The rays emitted by a hot body differ from each other in their faculty to pass through glass.
5. A thick glass, though as much or more permeable to light than a thin glass of worse quality, allows a much smaller quantity of radiant heat to pass. The difference is so much the less as the temperature of the radiating source is more elevated.
6. The quantity of heat which a hot body yields in a given time, by radiation to a cold body situate at a distance, increases, cæteris paribus, in a greater ratio than the excess of temperature of the first body above the second.—Journal de Physique, vol. lxxv.
[64] Sir David Brewster differs from the conclusions arrived at by Delaroche. He thus explains his views:—“The inability of radiant heat to pass through glass, may be considered as a consequence of its refusing to yield to the refractive force; for we can scarcely conceive a particle of radiant matter freely permeating a solid body, without suffering some change in its velocity and direction. The ingenious experiments of M. Prévost, of Geneva, and the more recent ones of M. Delaroche, have been considered as establishing the permeability of glass to radiant heat. M. Prévost employed moveable screens of glass, and renewed them continually, in order that the result which he obtained might not be ascribed to the heating of the screen; but such is the rapidity with which heat is propagated through a thin plate of glass, that it is extremely difficult, if not impossible, to observe the state of the thermometer before it has been affected by the secondary radiation from the screen. The method employed by M. Delaroche, of observing the difference of effect, when a blackened glass screen and a transparent one were made successively to intercept the radiant heat, is liable to an obvious error. The radiant heat would find a quicker passage through the transparent screen; and, therefore, the difference of effect was not due to the transmitted heat, but to the heat radiated from the anterior surface. The truth contained in M. Delaroche’s fifth proposition is almost a demonstration of the fallacy of all those that precede it. He found that ‘a thick plate of glass, though as much or more permeable to light than a thin glass of worse quality, allowed a much smaller quantity of radiant heat to pass.’ If he had employed very thick plates of the purest flint glass, or thick masses of fluid that have the power of transmitting light copiously, he would have found that not a single particle of heat was capable of passing directly through transparent media.”—Sir D. Brewster, On new properties of heat as exhibited in its propagation along plates of glass. Philosophical Transactions, vol. cvi. p. 107.
[65] Proposal of a New Nomenclature for the Science of Calorific Radiations, by M. Melloni. Bibliothèque Universelle de Genève, No. 70. Scientific Memoirs, vol. iii. part 12. Many of the terms, as Diathermasy, or transparency for heat; Adiathermasy, opacity for heat; Thermochroic, coloured for heat, and others, are valuable suggestions of forms of expression which are required in dealing with these physical phenomena.