Pictet, however, corrected the mistake by means of the apparatus known by the name of conjugate mirrors. A very transparent square of glass was placed between a thermometer and the heat of a lighted candle concentrated by the apparatus; the mercury in some moments rose several degrees; there was a perceptible elevation of temperature also when the candle was removed and a small jar filled with boiling water put in its place.
Some years later Herschel undertook a very extensive series of experiments on the same subject. They are described in the volume of the Philosophical Transactions for 1800. The author employs no artifice to increase the action of the rays of heat, and contents himself with the direct measurement of their effect by placing the thermometer at a very short distance from the diaphanous body.
But doubts were started as to the conclusions drawn from these different results. It was objected that part of the radiant heat was first stopped at the nearer surface of the glass, that it was gradually accumulated there and afterwards propagated from layer to layer, until it reached the further surface whence it began again to radiate on the thermometer. It was maintained even that nearly the whole of the effect was produced by this propagation. In short, some went so far as to deny altogether that the heat emitted by terrestrial bodies can be freely transmitted through any other diaphanous substance than atmospheric air.
M. Prevost, by means of a very ingenious contrivance, demonstrated the erroneousness of this opinion. Having attached to the pipe of a fountain a spout consisting of two parallel plates, he obtained a strip of water about a quarter of a line in thickness. On one side of this he placed an air thermometer and on the other a lighted candle or a hot iron. The thermometer rose, almost always, some fraction of a degree. Now it is quite evident that, in this case, a successive propagation through the several layers of the screen, which was in a state of perpetual change, could not take place. It was admitted, therefore, that other diaphanous media besides atmospheric air sometimes transmit the rays of heat as instantaneously as they always transmit those of light.
M. Prevost's process could not however be applied to solid bodies. It was therefore impossible to determine, by means of it, whether caloric was immediately transmitted through screens of glass. Delaroche completely solved this problem by employing a method invented by Maycock.
- Pictet, Essai sur le Feu, § 52 et seq.
- Journal de Phiysque, de Chimie, d'Histoire Naturelle et des Arts, par M. Delametherie, 1811.—P. Prevost, Mémoire sur la Transmission du Calorique à travers l'Eau et d'autres Substances, § 42 et 43.
- Nicholson, A Journal of Natural Philosophy, Chemistry and the Arts, vol. xxvi. May and June 1810.—J. D. Maycock, Remarks on Professor Leslie's Doctrine of Radiant Heat.