the Weights of Atoms. 185 



thermal diffnsivity * of water (*002, according to J. T. Bot- 

 tomley's first investigation, or about '0015 f according to 

 later experimenters) is several hundred times, and the diffus- 

 ivitv for momentum is from one to two thousand times, the 

 difYusivity of water for common salt, and other salts such as 

 sulphates, chlorides, bromides, and iodides. 



§ 34. We may regard the two motional diffusivities of a 



liquid as being each almost entirely due to communication of 

 motion from one molecule to another. This is because every 

 molecule is always under the influence of its neighbours and 

 has no free path. When a liquid is rarefied, either gradually 

 as in Andrews' experiments showing the continuity of the 

 liquid and gaseous states, or suddenly as in evaporation, the 

 molecules become less crowded and each molecule gains more 

 and more of freedom. When the density is so small that 

 the straight free paths are great in comparison with the 

 diameters of molecules, the two motional diffusivities are 

 certainly due, one of them to carriage of energy, and the 

 other to carriage of momentum, chiefly by the free rectilinear 

 motion of the molecules between collisions. Interchange of 

 energy or of momentum between two molecules during 

 collision will undoubtedly to some degree modify the results 

 of mere transport ; and we might expect on this account the 

 motional diffusivities to be approximately equal to, but each 

 somewhat greater than, the molecular diffnsivity. If this 

 view were correct, it would follow that, in a homogeneous 

 gas when the free paths are long in comparison with the 

 diameters of molecules, the viscosity is equal to the molecular 

 diffusivity multiplied by the density, and the thermal con- 

 ductivity is equal to the molecular diffusivity multiplied by the 

 thermal capacity per unit bulk, pressure constant : and that 

 whatever deviation from exactness of these equalities there may 

 be, would be in the direction of the motional diffusivities being- 

 somewhat greater than the molecular diffusivity. But alas, 

 we shall see, § 45 below, that hitherto experiment does not 

 confirm these conclusions : on the contrary the laminar 

 diffusivities (or diffusivities of momentum) of the only four 

 gases o£ which molecular diffusivities have been determined 

 by experiment, instead of being greater than, or at least 

 equal to, the density multiplied by the molecular diffusivity, 

 are each somewhat less than three-fourths of the amount thus 

 calculated. 



* ' Matb. and Phys. Papers,' vol. iii. p. 226. For explanation re- 

 garding diffusivity and viscosity see same volume, pp. 428-435. 

 t See a paper by Milner and Chattock, Phil. Mag. vol. xlviii. 1899. 



