6q'1 philosophical transactions. [anno 1800. 



allowed, by the rules of philosophizing, to admit 1 different causes to explain cer- 

 tain effects, if they may be accounted for by 1 . A beam of radiant heat, ema- 

 nating from the sun, consists of rays that are differently refrangible. The range 

 of their extent, when dispersed by a prism, begins at violet-coloured light, where 

 they are most refracted, and they have the least efficacy. We have traced these 

 calorific rays throughout the whole extent of the prismatic spectrum ; and found 

 their power increasing, while their refrangibility was lessened, as far as to the con- 

 fines of red-coloured light. But their diminishing refrangibility, and increasing 

 power, did not stop here; for we have pursued them a considerable way beyond the 

 prismatic spectrum, into an invisible state, still exerting their increasing energy, 

 with a decrease of refrangibility up to the maximum of their power; and have also 

 traced them to that state where, though still less refracted, their energy, on ac- 

 count, we may suppose, of their now failing density, decreased pretty fast; after 

 which, the invisible thermometrical spectrum, if I may so call it, soon vanished. 

 If this be a true account of solar heat, for the support of which I appeal to my 

 experiments, 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 on them, as they are known to do on all the other parts of our body, 

 by occasioning a sensation of heat. 



In the view of the apparatus, fig. 14, pi. 11, ab is the small stand; 1, 2, 3, the thermometers on 

 it j cd the prism at the window; e the spectrum thrown on the table, so as to bring the last quarter of 

 an inch of the red colour on the stand. 



XV. 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 Wm. Herschel, LL.D., F.R.S. p. 293. 



The word heat, in its common acceptation, denotes a certain sensation well- 

 known to every person. The cause of this sensation, to avoid ambiguity, ought 

 to have been distinguished by a name different from that which is used to point out 

 its effect. Various authors indeed, who have treated on the subject of heat, have 

 occasionally added certain terms to distinguish their conceptions, such as, latent, 

 absolute, specific, sensible heat ; while others have adopted the new expressions of 

 caloric, and the matter of heat. None of these descriptive appellations however 

 would have completely answered my purpose. I might, as in the preceding papers, 

 have used the name radiant heat, which has been introduced by a celebrated author, 

 and which certainly is not very different from the expressions I have now adopted; 

 but, by calling the subject of my researches, the rays that occasion heat, I cannot 

 be misunderstood as meaning that these rays themselves are heat; nor do I in any 

 respect engage myself to show in what manner they produce heat. 



