Page:Proceedings of the Royal Society of London Vol 1.djvu/43

Rh their decreasing refrangibility and increasing power, these having been traced far beyond the prismatic spectrum in an invisible state; that as their density gradually decreases, their energy at last vanishes, till at length the thermometrical spectrum, as the Doctor is willing to call it, becomes wholly imperceptible. Hitherto the effects of these heating rays have been observed as far as one inch and a half from the conﬁnes of the red ray.

If this be a true account of solar heat, (says our author at the close of his paper.) it remains only for us to admit, that such of the rays of the sun as have the refrangibility of those which are contained in the prismatic spectrum, by the construction of the organs of sight, are admitted under the appearance of light and colours; and that the rest, being stopped in the coats and humours of the eye, act upon them, as they are known to do upon all the parts of our body, by occasioning a sensation of heat.

queriments 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. By William Herschel, LL.D. F.R.S. Read May 15, 1800. [Phil. Trans. 1800, p. 293.]

In the prefatory part of this paper, the author found it necessary to limit the sense he aﬁixes to the word heat,- and after excluding the late terminology of latent, absolute, speciﬁc, sensible heat, the matter of heat, caloric, and even radiant heat, which last, however, comes nearest to the expression he has adopted, he desires to be understood, that, in speaking of rays which occasion heat, he does not mean that those rays themselves are heat, hut that he here considers heat merely as the effect of a cause, the nature of which is no part of his present inquiry.

Having thus determined the subject of his investigation, the Doctor distinguishes heat into six different kinds; whereof three are solar, and three terrestrial. These, however, are reducible into three general divisions, each of the solar and terrestrial kinds resembling each other respectively. The ﬁrst is the heat produced by luminous bodies, whether by the sun or by terrestrial ﬂames. The second comprehends the heat of coloured radiants, such as that of the sun separated by a prism, and that of culinary ﬁres openly exposed. And the third relates to heat from radiants, where neither light nor colour can be perceived; such as the heat of invisible solar rays, refracted by a prism, which have been the subject of a former paper; and the terrestrial heat from ﬁres inclosed in stoves, and from metals heated short of the lowest degree of incandescence.

The chief object of the present inquiry being to give a comparative view of the operations that may be performed on the rays that occasion heat, and of those which we know to have been effected on the rays that occasion light, a short detail is given of the principal facts