562 JAMES CLERK MAXWELL. 



their motion is changed in accordance with the laws of im- 

 pact of perfectly elastic bodies. 



It is probable that the investigations connected with 

 Saturn's rings, which led Maxwell to contemplate the con- 

 dition of the air in the midst of " a flight of brickbats," first 

 led him to take up the subject of the kinetic theory of gases. 

 At any rate, these investigations led him to the adoption of 

 what he aptly termed " the statistical method," which in his 

 hands became so fruitful in its applications to molecular 

 science. This method consists in the separation of all the 

 things considered into classes, which fulfil certain conditions, 

 and the determination of the number of individuals which 

 at any instant are to be found in each class, without reference 

 to the behaviour of any particular individual. In " the his- 

 torical method," on the other hand, the life history of each 

 individual is traced, and if classes are considered at all it is 

 only in their relation to the particular individual we are 

 contemplating. The first method deals with the interests of 

 the community at large and disregards the fate of the indi- 

 vidual, except as affecting the average condition of the com- 

 munity; the second deals with the interests of the individual 

 alone. It is obvious that when the individuals are to be 

 counted by millions the only way in which they can be 

 successfully treated by a finite mind is by the statistical 

 method, and this was the method which Maxwell adopted. 



In the earliest papers of Clausius he treated all the 

 molecules of a particular gas at the same temperature as 

 moving with the same velocity. Maxwell showed that this 

 could not be the case, and that, even if all the molecules were 

 started with the same velocities, their mutual collisions would 

 increase the velocity of some and diminish that of others, 

 until at length the velocities would be distributed " according 

 to the law of errors." This law will be best understood from 

 Maxwell's own illustration. He prepared a diagram illustrat- 

 ing the distribution of bullet marks on a target on the 

 hypothesis that all the shots are aimed at the centre of " the 

 bull " by the same marksman, while the probability of an 

 error of any given magnitude occurring is less the greater 



