MEMOIR XXX.] ON BURNING GLASSES AND M1HRORS. 439 



pressing on the ether during cooling a correspondingly 

 prolonged series of motions. And is not this the cause 

 of that remarkable relation between the atomic weights 

 of elementary bodies and their specific heats, discovered 

 by Dulong and Petit ? 



These considerations may lead us to inquire whether 

 the general cause of the decomposition of compound bod- 

 ies by radiations is due to the circumstance that all the 

 atoms of which their molecules are composed take on the 

 vibratory motion with unequal facility. Thus if a cer- 

 tain compound molecule be submitted to the influence of 

 an intense radiation, some of its constituent particles may 

 vibrate consentaneously at once, and others more tardily. 

 Under these circumstances, the continued existence of 

 the group may become impossible, and decomposition en- 

 sue in the necessity of the case. 



In entering upon the experimental analysis of the ac- 

 tion of a ray upon a decomposable body, there are three 

 different points to be considered, so far as the ray itself 

 is concerned : 1. To what extent and in what manner is 

 the result affected by the intensity of the ray, or by the 

 amplitude of the vibrating excursions? 2. How is it af- 

 fected by the frequency of the pulsatory impressions? 

 and, 3. How by the direction in which the vibrations 

 are made, as involved in the idea of polarization? I 

 shall now examine these in succession. 



1. To what extent and in what manner is the decompo- 

 sition of a compound body affected by the INTENSITY of a 

 ray or the amplitude of the vibrating excursions ? 



If the different degrees of facility with which atoms 

 receive the impression of ethereal vibrations be the true 

 cause of decomposition by light, we should expect that 

 many such changes would become possible under the in- 

 fluence of a burning-lens which are not so in the direct 

 rays of the sun. 



