CHEMICAL RAYS. 249 



And here I would ask you to make familiar to your 

 minds the idea that no chemical action can be produced by 

 a ray that does not involve the destruction of the ray. But 

 the term &quot; ray &quot; is unsatisfactory to us at present, when 

 our desire is to abolish all vagueness, and to affix a definite 

 physical significance to each of our terms. Abandoning 

 the term ray as loose and indefinite, we have to fix our 

 thoughts upon the waves of light ; and to render clear to 

 our minds that those waves which produce chemical action 

 do so by delivering up their own motion to the molecules 

 which they decompose. We have here forestalled to some 

 extent a question of great importance in molecular physics, 

 which, however, is worthy of being fixed more definitely in 

 your mind ; it is this : When the waves of ether are in 

 tercepted by a compound vapor, is the motion of the waves 

 transferred to the molecules of the vapor, or to the atoms 

 of the molecules ? We have thus far leaned to the con 

 clusion that the motion is communicated to the atoms ; for 

 if not to these individually, why should they be shaken 

 asunder ? The question, however, is capable of, and is 

 worthy of, another test, the bearing and significance of 

 which you will immediately appreciate. 



As already explained, the molecules are held in their 

 positions of equilibrium by their mutual repulsion on the 

 one side, and by an external pressure on the other. Their 

 rate of vibration, if they vibrate at all, must depend upon 

 the elastic force which they mutually exert. If this force 

 be changed, the rate of vibration must change along with 

 it; and after the change the molecules could no longer 

 absorb the waves which they absorbed prior to the change. 

 Now, the elastic force between molecule and molecule is 

 utterly altered when a vapor passes to the liquid state. 

 Hence, if the liquid absorbs waves of the same period as its 

 vapor, it is a proof that the absorption is not effected by 

 the molecules. Let us be perfectly clear on this important 



