Experiments on the Rays that occajion Heat. 1"J 



the parts of bodies ; yet this is not intended, at prefent, to 

 be any part of what I (hall endeavour to eftabliih. 



I muft alfo remark that, in ufing the word rays, I do not 

 mean to oppofe, much lefs to countenance, the opinion of 

 thofe philofophers who ftill believe that light itfelf comes to 

 us from the fun, not by rays, but by the fuppofed vibrations 

 of an elaftic ether, every where diffufed throughout fpace ; I 

 only claim the fame privilege for the rays that occafion heat, 

 which they are willing to allow to thofe that illuminate 

 objects. For> in what manner foever this radiance may be 

 effected, it will be fully proved hereafter that the evidence 

 either for rays, or for vibrations which occafion heat, ftands 

 on the fame foundation on which the radiance of the illu- 

 minating principle, light, is built. 



In order to enter on our fubjeel; with fome regularity, it 

 will be neceflary to diftiilguiih heat into fix different kinds, 

 three whereof are folar, and three lerreftrial; but, as the di- 

 vifions of terreftrial heat ftrictly rofemble thofe of folar, it will 

 not be neceflary to treat of them feparately; our fubject, there- 

 fore, may be reduced to the three following general heads: 



We (hall begin with the heat of luminous bodies in ge- 

 neral, fuch as, in the firft place, we have it directly from 

 the fun ; and as, in the fecond, we may obtain it from 

 terreftrial flames, fuch as torches, candles, lamps, blue- 

 lights, &c. 



Our next divifion comprehends the heat of coloured ra- 

 diants. This we obtain, in the firft place, from the fun, by 

 feparating its rays in a prifm j and, in the fecond, by having 

 recourfe to culinary fires openly expofed. 



The third divifion relates to heat obtained from radiants, 

 where neither light nor colour in the rays can be perceived. 

 This, as I have fliown, is to be had, in the firft place, di- 

 rectly from the fun, by means of a prifm applied to its rays ; 

 and, in the fecond, we may have it from fires inclofed in 

 (loves, and from red-hot iron cooled till it can no lodger be 

 feen in the dark. 



Befides the arrangement in the order of my experiments 

 which would arife from this divifion, we have another fubjeft 

 to confider. For, fince the chief defign of this paper is to 



Vol. VIII. D give 



