rd 
508 On the two great powers, [Ocr. 
ing as this, some philosophers have been induced to consider them 
as modifications of the same matter ; or that light, by its actions on 
bodies, produces the phenomena of heat. But of late years sufficient 
evidence has been brought of their being separate substances. The ex- 
periments of Herscuex, and Sir H. Eneieriexp, shewing that heat is 
not quite so much refracted in the prismatic spectrum as light, whence 
that much heat is found within the red ray, are a strong proof of this. 
Nor do the later experiments of Bsrarp (supposing them more correct), 
which would prove, that the intensity of the heat within the red ray 
is less than was represented by these philosophers, at all invalidate 
the argument. For it is only necessary to shew, that any heat may 
travel from the sun, independent of light, to prove a difference between 
them. 
HerscueEz has also shewn*, that if the red ray be thrown on red 
glass, the light is transmitted, but nearly ;% of the heat are detained ; 
and hence, that this appears incompatible with the supposition that 
the ray is homogeneous ; for were it so, the heat transmitted should 
have corresponded with the light. 
The rays from a fire being differently transmitted by glass (those of 
light being transmitted, but those of heat being most of them detained) 
is an argument of a similar nature. 
Heat is radiated without light by many bodies below certain tem- 
peratures, and others, as phosphori, radiate light without heat. 
The analogy between light and heat is so striking, that since the 
former is material, it is almost necessary to consider the latter as such, 
and yet there is sufficient evidence of a distinction between them. 
TItis evident from his writings, that Sir Isaac Newron was of opini- 
on, that the phenomena of heat arise from the action of light on bodies, 
causing vibrations in a ‘‘ subtle medium” in them. But it is equally 
plain that by heat he meant, those phenomena only which are appar- 
ent to the senses and commonly called heat. From the very imperfect 
state of chemical philosophy in his day, the doctrine of calorific repul- 
sion was scarcely taught; and most of the experiments, in proof of the 
materiality of heat, have been since performed. 
This great man has by several passages, especially by some in the 
18th query in his Treatise on Optics, suggested the existence of a high- 
ly elastic subtle fluid, so nearly allied to the matter of calorific repul- 
sion of the present day, that part of this query, with but the smallest 
modification, is an accurate description of the latter. ‘‘ If,’’ observes 
* Philosophical Transactions for 1800. 
+ Optics, Query 18. 
