18 MORRIS LOEB 



C will combine with B than would originally combine with it. 

 This is due to the fact that chemical action is due to exchange 

 of atoms when molecules meet. In spite of some reluctance 

 to combine between B and C, the greater number of mole- 

 cules of B present causes more frequent meetings between 

 B and C atoms than between A and C atoms. Throughout 

 chemical reactions it is found to be true that the ultimate 

 result depends thus upon the relative concentrations of re- 

 agents quite as much as upon their affinity. This then is a 

 second aspect of the science of energetics which applies to 

 chemical action. 



Formerly the chemist measured or weighed the substances 

 which he put together; and then measured and weighed the 

 ultimate results, wrote an equation and heaved a sigh of 

 satisfaction at having mastered the mechanism of the reac- 

 tion. Since the ideas above related have been introduced, we 

 strive to follow the reaction while it is going on, to detect 

 the molecules at work. We must often devise special methods 

 for such a purpose and many difficulties have hitherto seemed 

 unsurmountable. But when we do obtain such glimpses, we 

 become more and more convinced of the fewness and sim- 

 plicity of the fundamental ideas which we require for a basis 

 of the science. 



In any simple reaction, there is a perceptible interval of 

 time between the mixture of the ingredients and the com- 

 pletion of the ultimate result. Why is this? Because a 

 measurable interval elapses before all the molecular changes 

 have taken place. This will depend, as pointed out above, 

 first upon the readiness with which the exchange takes place 

 (a question of affinity), secondly upon the frequency of meet- 

 ing of the reacting molecules. Molecules will meet more 

 freely if there are more of them in a given space, and if they 

 move faster through added energy; and colliding molecules of 



