Page:Scientific Memoirs, Vol. 1 (1837).djvu/72

60 Let us now proceed to consider the transmission of heat through heterogeneous screens. The calorific rays emerging from each plate exposed to the action of the same source produce a particular elevation of temperature when they fall on the thermoscopic body of our apparatus. Whence we have inferred that the quantity of heat which passes through a given screen varies according to the quality and thickness of the substance. But, it may be asked, is this the only difference between the rays immediately transmitted through bodies of different kinds?

For the purpose of answering this question we have made the following experiments.

If the rays from a Locatelli lamp be brought to act on a thermoelectric pile after having previously passed through a screen of diaphanous matter (such as citric acid) but in a slight degree permeable to radiant heat, the effect obtained in the ordinary case, in which the whole action is equivalent to 30° of the thermomultiplier, will be very inconsiderable; but it may be increased by bringing the source of heat nearer, or by concentrating its rays on the plate with the help of metallic mirrors or lenses of rock salt. I suppose then that a deviation of 25° or 30° of the galvanometer has been produced through a plate of citric acid. I now interpose a plate of alum in such a manner that the rays emerging from the citric acid may be forced to pass through it before they can reach the thermoscopic body; the magnetic needle descends only about 3 or 4 degrees.

I now recommence the operation on any other diaphanous and colourless substance different from the citric acid; that is to say, I vary the distance from the lamp to the pile until I obtain the same galvanometric deviation of 25° or 30° by the action of the radiant heat on this new substance also. I then interpose the plate of alum, and the magnetic index, as in the case of the citric acid, descends again not more than about 3 or 4 degrees, but it approaches nearer to zero, and the