23 



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. 



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. 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 affixes 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 un- 

 derstood, that, in speaking of rays which occasion heat, he does not 

 mean that those rays themselves are heat, but 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 gene- 

 ral 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 re- 

 lates 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 ter- 

 restrial 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 occa- 

 sion 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 



