the Weights of Atoms. 195 



In these formulas, as originally investigated by Maxwell for 

 the ease of an ideal gas composed of hard spherical atoms, 

 s is definitely the diameter of the atom, and is the same at 

 all temperatures and densities of the gas. When we apply 

 the formulas to diatomic or polyatomic gases, or to a mon- 

 atomic gas consisting of spherical atoms whose spheres of 

 action may overlap more or less in collision according to the 

 severity of the impact, s may be defined as the diameter which 

 an ideal hard spherical atom, equal in mass to the actual 

 molecule, must have to give the same viscosity as the real gas, 

 at any particular temperature. This being the rigorous 

 definition of s, we may call it the proper mean shortest 

 distance of inertial centres of the molecules in collision to 

 give the true viscosity ; a name or expression which helps us 

 to understand the thing; defined. 



§ 47. For the ideal gas of hard spherical atoms, remem- 

 bering that V is independent of the density and varies as t* 

 {t denoting absolute temperature), § 46 (2) proves that the 

 viscosity is independent of the density and varies approxi- 

 mately as t*. Rayleigh's experimental determinations of the 

 viscosity of argon at different temperatures show that for this 

 monatomic gas the viscosity varies as t' 815 ; hence § 46 (2) 

 shows that s' 2 varies as t~" SVo , and therefore s varies as t~ * 16 . 

 Experimental determinations by Obermayer* of viscosities 

 and their rates of variation with temperature for carbonic 

 acid, ethylene, ethylene-chloride, and nitrous oxide, show 

 that for these the viscosity is somewhat nearly in simple 

 proportion to the absolute temperature : hence for them s' 2 

 varies nearly as t~ ' 5 . His determinations for the five 

 molecularly simpler gases, air, hydrogen, carbonic oxide, 

 nitrogen, and oxygen show that the increases of fi, and 

 therefore of s~ 2 , with temperature are, as might be expected, 

 considerably smaller than for the more complex of the gases 

 on which he experimented. Taking his viscosities atO° Cent., 

 for carbonic acid and for the four other simple gases named 

 above, and Rayleigh's for argon, with the known densities of 

 all the six gases at 0° C. and standard atmospheric pressure, 

 we have the following table (p. 196) of the values concerned 

 in § 46 (1). 



§ 48. The meaning of " s" the diameter, as defined in 

 § 46, is simpler for the monatomic gas, argon, than for any 

 of the others ; and happily we know for argon the density, 



* Oberaiaver, Wien. Akad. 1870, Mar. lGtb, vol. 73, p. 433. 

 02 



