564 JAMES CLERK MAXWELL. 



proportional to the mass of a molecule, i.e. to its combining 

 weight, which is Gay Lussac's law of equivalent volumes. 



If two gases be in communication with one another, the 

 particles of each are found to penetrate the other until the 

 gases become uniformly mixed, unless some external force 

 act to prevent the uniformity of the mixture. This 

 phenomenon, known as gaseous diffusion, has been studied 

 by Graham, Loschmidt, and others, and is a direct con- 

 sequence of the dynamical theory. It is plain that the rate 

 of diffusion of one gas into another will be proportional to 

 the average velocity of the particles in the case of two 

 gases being separated by a diaphragm perforated by very fine 

 holes, so that each mixture may be considered homogeneous 

 up to the diaphragm. Maxwell showed that in this case, 

 when the pressures on the two sides of the diaphragm are 

 equal, " the volumes diffused will be as the square roots of 

 the specific gravities inversely, which is the law of diffusion 

 established by Graham." 



The slowness of gaseous diffusion was for some time 

 regarded as an objection to the kinetic theory of gases. 

 Clausius overcame this difficulty by the introduction of the 

 conception of the mean free path of a molecule, and by 

 showing how short this path is. The mean free path is the 

 average distance through which a particle passes between 

 two successive collisions with other particles. The first 

 estimate of the mean free path in air at ordinary pressure 

 and temperature was made by Maxwell from determinations 

 of viscosity or rate of diffusion of momentum in air. 

 Depending on the length of the mean free path are (1) the 

 diffusion of matter; (2) the diffusion of momentum or 

 viscosity; and (3) the diffusion of energy, or the thermal 

 conductivity of the gas. From determinations of the rate at 

 which these three kinds of diffusion proceed, independent 

 determinations of the mean free path in different gases have 

 been made. For hydrogen, at standard pressure and tem- 

 perature, it is about Y5TTo~o~o~ ^ an mcn - For other gases 

 and air it is somewhat less. The length of the mean path 

 in air given by Maxwell in 1859 was -j-g-gVon ^ an mcn - 



