and the Mode of its Communication. 125 



this velocity becoming less as the distance from the 

 sonorous body increases, the sound is weakened in the 

 same proportion. 



There are several circumstances which might lead us 

 to suspect that colour depends on the frequency of those 

 pulsations which have been supposed to constitute 

 light ; and that the heat produced by them is in pro- 

 portion to their force. 



If this supposition should be well founded, a knowl- 

 edge of that important fact might perhaps enable us to 

 explain several very interesting phenomena, the com- 

 bustion of inflammable bodies, for instance, and the great 

 intensity of the heat which is produced by the concen- 

 tration of calorific rays. 



There are several well-known experiments with burn- 

 ing-glasses which show that the intensity of the heat 

 generated by the concentration of the solar rays is not 

 simply as the condensation of those rays, but in a higher 

 proportion ; and that it depends much on their direc- 

 tion^ being greater as the angle is greater at which they 

 meet at the focus of the lens. 



That fact is certainly very remarkable. It has often 

 been the subject of my meditations, and it has contrib- 

 uted not a little to the opinion I have been induced to 

 adopt respecting the nature of light and of heat. I 

 never could reconcile it with the supposition that heat 

 is caused by the accumulation of anything emitted by the 

 sun, or by any other body which sends off calorific 

 radiations. 



Reserving for a future communication an account 

 of the sequel of my inquiries respecting the subject 

 which I have undertaken to investigate, I shall conclude 

 this long paper with some observations concerning the 



