XIX. Experiments on the solar, and on the terrestrial rays that occasion heat; with a comparative view of the laws to which light and heat, or rather the rays which occasion them, are subject, in order to determine whether they are the same, or different
In the first part of this Paper it has been shewn, that heat derived immediately from the sun, or from candent terrestrial substances, is occasioned by rays emanating from them; and that such heat-making rays are subject to the laws of reflection, and of refraction. The similarity between light and heat, in these points, is so great, that it did not appear necessary to notice some small difference between them, relating to the refraction of rays to a certain focus, which will be mentioned hereafter. But the next three articles of this Paper will require, that while we shew the similarity between light and heat, we should at the same time point out some striking and substantial differences, which will occur in our experiments on the rays which occasion them, and on which hereafter we may proceed to argue, when the question reserved for the conclusion of this Paper, whether light and heat be occasioned by the same or by different rays, comes to be discussed. Article iv.— Different Refrangibility of the Rays of Heat . We might have included this article in the first part of this Paper, as a corollary of the former three; since rays that have been separated by the prism, and have still remained subject to the laws of reflection and refraction, as has been shewn, could not be otherwise than of different refrangibility; but we have something to say on this subject, which will be found much more circumstantial and conclusive than what might have been drawn as a consequence from our former experiments. However, to begin with what has already been shewn, we find that two degrees of heat were obtained from that part of the spectrum which contains the violet rays, while the full red colour, on the opposite side, gave no less than seven degrees; and these facts ascertain the different refrangibility of the rays which occasion heat, as clearly as that of light is ascertained by the dispersion and variety of the colours. For, whether the rays which occasion heat be the same with those which occasion the colours, which is a case that our foregoing experiments have not ascertained, the arguments for their different refrangibility rests on the same foundation, namely, their being dispersed by the prism; and that of the rays of light being admitted, the different refrangibility of the rays of heat follows of course. So far then, a great resemblance again takes place.