DECOMPOSITION BY LIGHT 111 



molecule as a group of atoms, held together by their 

 mutual forces, but still capable of motion among them- 

 selves. The vapor of the nitrite of amyl is to be regarded 

 as an assemblage of such molecules. The question now 

 before us is this : In the act of absorption, is it the 

 molecules that are effective, or is it their constituent 

 atoms? Is the vis viva of the' intercepted light- waves 

 transferred to the molecule as a whole, or to its con- 

 stituent parts? 



The molecule, as a whole, can only vibrate in virtue 

 of the forces exerted between it and its neighbor mole- 

 cules. The intensity of these forces, and consequently the 

 rate of vibration, would, in this case, be a function of the 

 distance between the molecules. Now the identical ab- 

 sorption of the liquid and of the vaporous nitrite of amyl 

 indicates an identical vibrating period on the part of 

 liquid and vapor, and this, to my mind, amounts to an 

 experimental proof that the absorption occurs in the main 

 within the molecule. For it can hardly be supposed, if 

 the absorption were the act of the molecule as a whole, 

 that it could continue to affect waves of the same period 

 after the substance had passed from the vaporous to the 

 liquid state. 



In point of fact, the decomposition of the nitrite of 

 amyl is itself to some extent an illustration of this in- 

 ternal molecular absorption; for were the absorption the 

 act of the molecule as a whole, the relative motions of its 

 constituent atoms would remain unchanged, and there 

 would be no mechanical cause for their separation. It is 

 probably the synchronism of the vibrations of one portion 

 of the molecule with the incident waves that enables the 

 amplitude of those vibrations to augment, until the chain 



