37 



MAXWELL'S LAW 



81 



where F denotes the area of the orifice AB, and ft cos s is 

 put for GA ; and the number of molecules we denote by 



nFl cos s. 



We have still the number n to determine. It represents 

 the number of particles which move in unit volume in the 

 direction given by CA and therefore determined by s. This 

 number is easy to calculate for a gas at rest, since in this 

 case no one direction differs from any other, equal numbers 

 of molecules therefore moving in every direction. Consider 

 all the N particles which are in the unit volume to be 



FIG. 4 



brought to one and the same point 'A, and to begin their 

 rectilinear paths from this point ; then at a small area K 

 of a sphere described about A with unit radius there will 

 arrive 



*s 



4-7T 



particles ; for the whole sphere, whose area is 4?r, will be 

 symmetrically met by the N particles. If we take the 

 element K to be the zone CDD'C' obtained by rotating the 

 radii AC, AD about the normal AG as axis, then 



K = 27rrA, 



if we denote by A the breadth CD = C'D' of the zone, and 

 by r the mean radius of the circles which make up this zone 

 and are parallel to its boundaries CC' and DD r . The number, 

 therefore, of particles which meet this zone is 

 1 



4-7T 



KN = 



