366 Mr. Poioellon Terrestrial Light and Heat. [May, 



to absorption again from bodies on which it impinges, in propor- 

 tion to the darkness of their colour, and thus becomes heat, dis- 

 playing its effects as temperature. 



The views to which I have been led as to the distinct nature 

 of the two parts of the total heating eifect, so far tend to disprove 

 this theory, that we here evidently perceive a very considerable 

 portion of the radiant matter not at all converted into light, but 

 merely increased in intensity. 



If, therefore, we still adhere to the supposition that light is 

 onlv heat in a difterent state, we must so far modify the hypo- 

 thesis as to admit that only a part of the igneous fluid undergoes 

 this change. But here we must further ask, whether such an 

 admission can be made in consistency with the other parts of this 

 theory, or even with its fundamental principles. For this pur- 

 pose we must take a brief review of its leading features, and the 

 grounds on which it is built. 



(15.) Prof. Leslie, in his Inquiry into the Nature of Heat, 

 p. 150, maintains the opinion of the materiahty of light, and of 

 its existence in actual combination with bodies. He then 

 examines the phenomenon of its absorption as connected with 

 reflection, &c. ; he attributes to lightin its state of combination 

 the heating property, from which he is led to the conclusion, 

 that " heat is light in a state of combination," p. 162. 



The ground upon which he adopts this theory is this; having 

 come to the experimental conclusion that " heat is an elastic 

 fluid extremely subtle and active," he asks (p. 150), " Is it a new 

 and peculiar kind of fluid, or is it one with which, from its 

 effects, we are already in some manner acquainted? If any such 

 can be discovered that will strictly quadrate with the phenomena, 

 the spirit of true philosophy which strives to reduce the number 

 of ultimate principles, would certainly persuade us to embrace it. 

 But in searching further, we may, perhaps, educe direct proofs of 

 identity ; " and then, from a comparison with the etJects of 

 light before mentioned, he concludes their identity. We may^ 

 however, be permitted to ask, whether, to suppose two exist- 

 ences (in some particulars at least), possessing such very differ- 

 ent properties as light and heat do, to be merely the same sub- 

 stance in diflerent states, is not rather departing from the general 

 simplicity of natural causes; and supposing a new sort of rela- 

 tion between two existences with which in other parts of nature 

 we are unacquainted, should we not be more in the true spirit 

 of inductive philosophy, if, admitting the distinct existence of 

 light and heat we sought to explain, the facts of the one being 

 apparently produced by the other, according to some laws 

 already known to act in the constitution of things. 



Prof. Leshe shows (p. 175), that all rays of light from whatever 

 source must issue from that source with the same identical 

 celerity. " It hence appears," he observes, " that light must 



