184 Lord Kelvin on 



perceptible volume to be exceedingly great. We shall see 

 that in gases which at ordinary pressures and temperatures 

 approximate most closely, in respect to compressibility,, 

 expansion by heat, and specific heats, to the ideal perfect 

 gas, as, for example, hydrogen, oxygen, nitrogen, carbon- 

 monoxide, the free path is probably not more than about one 

 hundred times the distance between centres during col- 

 lisions, and is little short of 10~ 5 cm. in absolute magnitude. 

 Although these moderate proportions suffice for the well- 

 known exceedingly close agreement with the ideal gaseous 

 laws presented by those real gases, we shall see that large 

 deviations, from the gaseous laws are presented with con- 

 densations sufficient to reduce the free paths to two or three 

 times the diameter of the molecule, or to annul the free paths 

 altogether. 



§ 32. It is by experimental determinations of diffusivity 

 that the kinetic theory of gases affords its best means for 

 estimating the sizes of atoms or molecules and the number of 

 molecules in a cubic centimetre of gas at any stated density. 

 Let us therefore now consider carefully the kinetic theory of 

 these actions, and with them also, the properties of thermal 

 conductivity and viscosity closely related to them, as first 

 discovered and splendidly developed by Clausius and Clerk 

 Maxwell. 



§ 33. According to their beautiful theory, we have three 

 kinds of diffusion; diffusion of molecules, diffusion of energy, 

 and diffusion of momentum. Each in solids, such as gold and 

 lead, Roberts-Austen has discovered molecular diffusion of 

 gold into lead and lead into gold between two pieces of the 

 metals when pressed together. But the rate of diffusion 

 shown by this admirable discovery is so excessively slow that 

 for most purposes, scientific and practical, we may disregard 

 wandering of any molecule in any ordinary solid to places 

 beyond direct influence of its immediate neighbours. In an 

 elastic solid we have diffusion of momentum by wave motion, 

 and diffusion of energy constituting the conduction of heat 

 through it. These diffusions are effected solely by the com- 

 munication of energy from molecule to molecule and are 

 practically not helped at all by the diffusion of molecules. 

 In liquids also, although there is thorough molecular diffus- 

 ivity, it is excessively slow in comparison with the two other 

 diffusivities, so slow that the conduction of heat and the 

 diffusion of momentum according to viscosity are not prac- 

 tically helped by molecular diffusion. Thus, for example, the 



