VOLXC.] PHILOSOPHICAL TRANSACTIONS. 6Q3 



From what has been said it follows, that any objections that may be alleged, from 

 the supposed agency of heat in other circumstances than in its state of radiance, or 

 heat-making rays, cannot be admitted against my experiments. For, notwithstand- 

 ing 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 endeavour 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 founda- 

 tion 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 6 different kinds, 3 solar, and 3 terrestrial ; but, as the di- 

 visions 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 3 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 ; and as, in the 2d, we may 

 obtain it from terrestrial flames, such as torches, candles, lamps, blue-lights, &c. 

 Our next division comprehends the heat of coloured radiants. This we obtain, in 

 the first place, from the sun, by separating its rays in a prism ; and, in the 2d, by 

 having recourse to culinary fires, openly exposed. The 3d division relates to heat 

 obtained from radiants, where neither light nor colour in the rays can be perceived. 

 This, as I have shown, is to be had, in the first place, directly from the sun, by 

 means of a prism applied to its rays ; and, in the 2d, we may have it from fires in- 

 closed in stoves, and from red-hot iron cooled till it can no longer be seen in the dark. 

 Besides the arrangement in the order of my experiments which would arise from 

 this division, we have another subject to consider. For, since the chief design of 

 this paper is to give a comparative view of the operations that may be performed on 

 the rays that occasion heat, and of those which we already know to have been 

 effected on the rays that occasion light, it will be necessary to take a short review 

 of the latter. I shall merely select such facts as not only are perfectly well known, 

 but especially such as will answer the intention of my comparative view, and arrange 

 them in the following order. 1 . Light, both solar and terrestrial, is a sensation 

 occasioned by rays emanating from luminous bodies, which have a power of illumi- 

 nating objects; and, according to circumstances, of making them appear of various 

 colours. 2. These rays are subject to the laws of reflection. 3. They are also 

 Bubject to the laws of refraction. 4. They are of different refrangibility. 5. They 



