ATOM. 459 



altered by encounters with other molecules. If the molecules in different parts 

 of the medium are of different kinds, their progress from one part of the 

 medium to another can be easily traced by analysing portions of the medium taken 

 from different places. The rate of diffusion thus found furnishes one method 

 of estimating the length of the free path of a molecule. This kind of diffusion 

 goes on not only between the molecules of different gases, but among the 

 molecules of the same gas, only in the latter case the results of the diffusion 

 cannot be traced by analysis. But the diffusing molecules carry with them 

 in their free paths the momentum and the energy which they happen at a 

 given instant to have. The diffusion of momentum tends to equalise the 

 apparent motion of different parts of the medium, and constitutes the pheno- 

 menon called the internal friction or viscosity of gases. The diffusion of energy 

 tends to equalise the temperature of different parts of the medium, and 

 constitutes the phenomenon of the conduction of heat in gases. 



These three phenomena the diffusion of matter, of motion, and of heat 

 in gases have been experimentally investigated, the diffusion of matter by 

 Graham and Loschmidt, the diffusion of motion by Oscar Meyer and Clerk 

 Maxwell, and that of heat by Stefan. 



These three kinds of experiments give results which in the present im- 

 perfect state of the theory and the extreme difficulty of the experiments, 

 especially those on the conduction of heat, may be regarded as tolerably con- 

 sistent with each other. At the pressure of our atmosphere, and at the 

 temperature of melting ice, the mean path of a molecule of hydrogen is about 

 the 10,000th of a millimetre, or about the fifth part of a wave-length of 

 green light. The mean path of the molecules of other gases is shorter than 

 that of hydrogen. 



The determination of the molecular volume of a gas is subject as yet to 

 considerable uncertainty. The most obvious method is that of compressing the 

 gas till it assumes the liquid form. It seems probable, from the great resist- 

 ance of liquids to compression, that their molecules are about the same 

 distance from each other as that at which two molecules of the same substance 

 in the gaseous form act on each other during an encounter. If this is the 

 case, the molecular volume of a gas is somewhat less than the volume of the 

 liquid into which it would be condensed by pressure, or, in other words, the 

 density of the molecules is somewhat greater than that of the liquid. 



Now, we know the relative weights of different molecules with great 



KQ O 



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