42 MOLECULAR MOTION AND ITS ENERGY 22 



being struck perpendicularly to the direction of its motion 

 and so that the direction of the blow passes through its 

 centre of mass ; then the striking particle will cede part 

 of its speed to the struck particle, which, as it experiences 

 /no resistance in the direction of the path it has thus far 

 traversed, will retain its own motion undiminished, and, 

 therefore, receiving in addition a further speed from the 

 striking particle, will move more quickly than before the 

 collision, and in changed direction, while the other moves 

 more slowly, since it must lose speed. 



This example shows that in such an aggregation of 

 molecules as we assume in gaseous bodies in our theory 

 the speeds of the individual molecules cannot be equal in 

 the state of equilibrium. Equilibrium can, therefore, only 

 consist in a condition of continuous interchange of speed 

 between every pair of colliding particles, every particle now 

 gaining and now losing, its velocity being now big and now 

 little, and changing as often in direction as in magnitude. 



It is for this continuous change of motion of the particles 

 as they dash about that we have to investigate the law. 



23. The Applicability of the Calculus of 

 Probability to the Kinetic Theory 



The attempt to deduce a law for something that is sub- 

 ject only to chance may seem singular and strange, but this 

 should not deter us from undertaking a research which 

 touches the very core of the kinetic theory. 



This theory, indeed, seeks for the cause of regular phe- 

 nomena and regular properties of gaseous media in irregular 

 tumultuous motions of the molecules. We have here to 

 look at the observed facts, not as direct necessary conse- 

 quences of unchangeable laws as is usual, but as the result 

 of a large number of elementary processes which are subject 

 to no law but that of chance. And yet all phenomena 

 occur in unchangeable regularity. 



This is certainly a very uncommon position, but it is 

 by no means unwarranted, and it is also not in the least 

 limited to this theory only. During every chemical reaction 



