104 FRAGMENTS OF SCIENCE. 



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 con- 

 stituent atoms? Is the vis viva of the intercepted 

 light-waves transferred to the molecule as a whole, or to 

 its constituent parts ? 



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

 of the forces exerted between it and its neighbour 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 iden- 

 tical absorption of the liquid and of the vaporous nitrite 

 of amyl indicates an identical vibrating period on the 

 part of liquid and vapour, 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 which binds the parts of the molecule 

 together is snapped asunder. 



I anticipate wide, if not entire, generality for the 

 fact that a liquid and its vapour absorb the same rays. 

 A cell of liquid chlorine would, I imagine, deprive light 

 more effectually of its power of causing chlorine and 



