202 Observations vpon the i'^alure of L'/ght. 



are promiscuously arranged, if such an expression may be a!^ 

 lowed ; or that they attract the light they receive with a degree 

 cf force so as not to allow it to pass through. In the first case 

 it would be reflected, iii the latter absorbed; and lastly, it is 

 both transmitted and reflected, by those particles of light being 

 reflected which full upon the molecule; themselves^ and by those 

 passing through which fall in the direction of the canals through 

 which it is transmitted. 



By the term absorption, used as above, will be understood 

 light combining chemically with the substaiice which is exposed 

 to it; and hence, when a bod/ gives out light, it is acted upon by 

 some chemical agent, such as is observed in combustion, &c. 

 An exception to this observation, at first, appears in those ani- 

 inals which are phosphorescent, and have the property of emitting 

 a great quantity of light ; but this is probably owiiig to some 

 chemical change taking place in the animal, in a similar man- 

 ner as we observe carbon to be emitted from the lungs by 

 means of the organs of respiration. From v.hat has been ad* 

 vanced it therefore appears, that, of the two theories, the one 

 adopted by Newton is the more applicable to the phaenomena 

 of light. 



By many philosophers heat and light have been considered 

 only as modifications of each other. A concise history of the 

 opinions upon this subject may be seen in Dr. Frank's Treatise 

 on the Theories of Chemistry*. It has also been advanced, that 

 heat is only the effect of a vibratory motion amongst the parti- 

 cles of the body that is heated, and that light is only heat in a 

 state of radiation. If this were the case, we should expect 

 to find that the point of greatest illumination in the prismatic 

 spectrum should also be the hottest : the contrary of which ia 

 shown by Herschel in his experiments on the prism. Also the 

 focus of light and focus of heat ought to be the same ; but Dr. 

 Herschel has likewise shown that these differ, and his experi- 

 ments are further confirmed by those made by Sir Henry Eiigle- 

 field. Sir Henry observes, in the account he gives of his ex- 

 perimentsf, that "Mr. Gary, optician in the Strand, and Dr. 

 Hunter, were present at these experiments, and repeatedly saw 

 the thermometer sink when carried into the light, and rise again 

 when carried into the dark. Dr. Hunter also received the focus 

 on the palm of his hand, where the heat was sensibly felt ; and 

 shutting his eyes, and pointing with a long pen to where the 

 heat was greatest, he always touched his hand beyond the visible 

 light." As another objection, it may be observed, that phos- 

 phorescent bodies that emit light very freely have a temperature 



* Medical and Pliysical Journal, vol, i. t Journal of the Ruyal 



Institution. 



very 



