570 BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 



Herschel, W.: Synopsis of the Writings of — Coutinued. 



A. D. Tol. P. 



they may be acconnted for by one. A beam of radiant heat, eman- 

 ating from the sun, consists of rays that are differently refrangi- 

 ble. The range of their extent, when dispersed by a prism, begins 

 at violet-coloured light, where they are most refracted and have 

 the least efficacy. Wo have traced these calorific rays throughout 

 the whole extent of the prismatic spectrum; and found their power 

 iucreasing, while their refrangibility was lessened, as far as to the 



1800 90 292 coufiuesofrcd-ccloured light. But their diminishing refrangibility 

 and increasing power did not stop here ; for we have pursued them 

 a considerable way beyond the imsmaVic spectrum, into an invisible 

 state, still exerting their iucreasing energy, with a decrease of re- 

 frangibility, up to the maximum of their power; and have also 

 traced them to that state where, though sMU refracted, their energy, 

 on account, we may suppose, of their now failing density, decreased 

 pretty fast; after which the invisible Qiermomelrieal sin-ctrum, if I 

 may so call it, soon vanished. 

 "If this be a true account of solar heat, fur the support of which I 

 appeal to my experiments, it remains only for us to adaiit 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 Icuown to do upon all the 

 other parts of our body, by occasioning a sensation of heat." 

 Explanation of Plate XI, in which is given a view of the apparatus. 

 [Dated] Slough, near Windsor, March 17, lyOO. 



1800 90 293 Ex2}erime)tts on the solar and on the ierrestiiaJ liaijs that occasion Heat ; 

 u-ith a eomparative view of the Latvs 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. 

 Part I. Read May J5, 1800. 

 "The word heat, in its common acceptation, denotes a certain sen- 

 sation well known to every person. The cause of this sensation, 

 to avoid ambiguity, ought to have been distinguished by a name 

 different from that Avhich is used to point out its effect. Various 

 authors, indeed, avIio 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 anpellatious, however, would have com- 

 pletely answered my purpose. I might, as in the x»receding papers, 

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

 celebrated author, and which cei'taiulj' 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 those rays themselves are heat, nor do I in any re- 

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

 "From what has been said it follows that any objections that may 

 be alleged from the supposed agency of heat iu other circum- 

 294 stances than in its state of radiance or heat-making rays cannot 

 be admitted against my experiments. For, notwithstanding I may 

 be inclined to believe that all phenomena iu which heat is con- 

 cerned, such as the expansion of bodies, fluidity, congelation, fer- 



