Page:Scientific Memoirs, Vol. 1 (1837).djvu/36

24 Whence it is concluded that the losses still decrease at a distance of about 100 millimetres.

To comprehend at a single glance the law of the propagation of caloric radiating through diaphanous media we have only to reduce the results contained in the first two columns of the Tables A. and B. to a linear construction.

The mere inspection of the curves thus constructed shows that the rays lose very considerably when they are entering the first layers of the medium. But in proportion to their distance from the surface we see that the loss decreases and that at a certain distance it is almost imperceptible, and the rays seem to continue their progress, retaining all their intensity; so that in glass and in oil of colza, and probably in all other diaphanous media, the portion of heat which has forced its passage through the first layers must penetrate to very great depths.

Delaroche had found that the heat which has passed through one plate of glass becomes less subject to absorption when it is passing through a second. The identity of this fact with the law of resistance in continuous media shows that the solution of the continuity and the interposition of the atmosphere between the two screens do not alter the nature of the modifications which the rays undergo in the first plate of glass. It is therefore exceedingly probable that the proposition of Delaroche is true with respect to a very numerous series of thin screens; for we have just seen that in the same medium the losses still diminish to the depth of 80 or 100 millimetres. In reference to this point, the following is the result of the experiments I have made with four plates of the same glass that had been employed in the first attempts to investigate the law of propagation through continuous media The common thickness of these plates was 2mm·068.

It is scarcely necessary to observe that the common radiation to which the screens had been exposed was always 30 degrees, answering to a force or temperature of 35·3. If we represent this radiation by 1000, as we have done in all the foregoing cases, we have:

Whence we have