the Weights of Atoms. 193 



Considering the great difficulty of the experimental 

 investigation, we may regard the agreements of the three 

 results for each separate gas as, on the whole, very satisfactory, 

 both in respect to the accuracy of: Loschmidt's experiments 

 and the correctness of Maxwell's theory. It certainly is a 

 very remarkable achievement of theory and experiment to 

 have found in the four means of the sets of three deter- 

 minations, what must certainly be somewhat close approxi- 

 mations to the absolute values for the four gases, hydrogen, 

 oxygen, carbon-monoxide, and carbon- dioxide, of something 

 seemingly so much outside the range of experimental obser- 

 vation as the inter-diffusivity of the molecules of a separate 

 gas. 



§ 44. Maxwell, in his theoretical writings of different 

 dates, gave two very distinct views of the inner dynamics of 

 viscosity in a single gas, both interesting, and each, no doubt, 

 valid. In one*, viscous action is shown as a subsidence from 

 an "instantaneous rigidity of a gas/'' In the other f, 

 viscosity is shown as a diffusion of momentum : and in p. 347 

 of his article quoted in § 41 above he gives as from " the 

 theory/' but without demonstration, a formula (5), which, 

 taken in conjunction with (1), makes 



- P = D • ( 1); 



p denoting the density, //, the viscosity, and D the molecular 

 diffusivity, of any single gas. On the other hand, in his 

 1866 paper he had given formulas making J 



^=-648D ...... (2). 



P 



6 45. Viewing viscosity as explained by diffusion of mo- 

 mentum we may, it has always seemed to me (§ 34 above) , 

 regard (1) as approximately true for any gas, monatomic, 

 diatomic, or polyatomic, provided only that the mean free 



* Trans. Koy. Sec, May 1866 ; Scientific Papers, vol. ii. p. 70. 



+ " Molecules," a lecture delivered before the Brit. Assoc, at Bradford, 

 Scientific Papers, vol. ii. p. 378. See also O. E. Mever 's ' Kinetic Theory 

 of Gases,' (Baynes' trans. 1899), §§ 74-76. 



% The formula for viscosity (Sci. Papers, vol. ii. p. 68) taken with the 

 formula for molecular diffusivity of a single gas, derived from the formula 



of inter-diff usivity of two gases of equal densities, gives —-* = o-p which 



is equal to "648 according to the values of A x and A 2 shown in p. 42 of 

 vol. ii. Sci. Papers. 



Phil. Mag. S. 6. Vol. 4. No. 20. Aug. 1902. 



