204 Dr. Herschel's Experiments on the solar, and 



circumstances than in its state of radiance, or heat-making 

 rays, cannot be admitted against my experiments. For, not- 

 withstanding I may be inclined to believe that all phenomena 

 in which heat is concerned, such as the expansion of bodies, 

 fluidity, congelation, fermentation, friction, &c. as well as heat 

 in its various states of being latent, specific, absolute, or sensible, 

 may be explained on the principle of heat-making rays, and 

 vibrations occasioned by them in the parts of bodies; yet this is 

 not intended, at present, to be any part of what I shall endea- 

 vour to establish. 



I must also remark, that in using the word rays, I do not 

 mean to oppose, much less to countenance, the opinion of those 

 philosophers who still believe that light itself comes to us from 

 the sun, not by rays, but by the supposed vibrations of an elastic 

 ether, every where diffused throughout space ; I only claim the 

 same privilege for the rays that occasion heat, which they are 

 willing to allow to those that illuminate objects. For, in what 

 manner soever this radiance may be effected, it will be fully 

 proved hereafter, that the evidence, either for rays, or for 

 vibrations which occasion heat, stands on the same foundation 

 on which the radiance of the illuminating principle, light, is 

 built. 



In order to enter on our subject with some regularity, it will 

 be necessary to distinguish heat into six different kinds, three 

 whereof are solar, and three terrestrial ; but, as the divisions of 

 terrestrial heat strictly resemble those of solar, it will not be 

 necessary to treat of them separately; our subject, therefore, may 

 be reduced to the three following general heads. 



We shall begin with the heat of luminous bodies in general, 

 such as, in the first place, we have it directly from the sun; 



