AND MODERN PHYSICS. 125 



single molecule. Hence the amount of heat required 

 to raise the temperature of a single molecule of all 

 gases 1 is the same. Thus the quantity em is 

 the samo for all gases ; or, in other words, the 

 specific heat of a gas is inversely proportional to the 

 mass of its individual molecules. The density of a 

 gas since the number of molecules per unit volume 

 at a given pressure and temperature is the same for 

 all gases is also proportional to the mass of each in- 

 dividual molecule. Thus the specific heats of all gases 

 are inversely proportional to their densities. This 

 is the law discovered experimentally by Dulong and 

 Petit to be approximately true for a large number of 

 substances. 



In the next part of the paper Maxwell proceeded 

 to determine the average number of collisions in a 

 given time, and hence, knowing the velocities, to 

 determine, in terms of the size of the particles and 

 their numbers, the mean free path of a particle; the 

 result so found differed somewhat from that already 

 obtained by Clausius. 



Having done this he showed how, by means of 

 experiments on the viscosity of gases, the length of 

 the mean free path could be determined. 



An illustration due to Professor Halfour Stewart 

 will perhaps make this clear. Let us suppose we 

 have two trains running with uniform speed in 

 opposite directions on parallel lines, and, further, that 

 the engine? continue to work at the same rate, 

 developing just sufficient energy to overcome the 

 resistance of the line, etc., and to maintain the speed 



