194 



Lord Kelvin on 



path is large in comparison with the sum of the durations of 

 the collisions. Unfortunately for this view, however, com- 

 parisons of Loschmidt's excellent experimental determinations 

 o£ diffusivity with undoubtedly accurate determinations of 

 viscosity from Graham's original experiments on transpiration, 

 and more recent experiments of Obermeyer and other 

 accurate observers, show large deviations from (1) and are 

 much more nearly in agreement with (2). Thus taking 

 •0000900, -001430, '001234, -001974 as the standard densities 

 of the four gases, hydrogen, oxygen, carbon-monoxide, and 

 carbon-dioxide, and multiplying these respectively by the 

 diffusivities from Loschmidt's experiments and Maxwell's 

 theory, we have the following comparison with Obermeyer 's 

 viscosities at 0° C. and standard pressure, which shows the 

 discrepance from experiment and seeming theory referred to 

 in § 34. 



Col. 1. 



Col. 2. 



Col. 3. 



Col. 4. 



Gas. 



Viscosity calculated 



by Maxwell's theory 



from Loschmidt's 



diffusivities 



Viscosities according 

 to Obermeyer. 



Eatio of values in 



Col. 3 to thoae 



in Col. 2. 



H 2 



o 2 



CO 



co 2 



•000119 

 •000269 

 •000212 

 •000218 



•0000822 

 •0001873 

 •0001630 

 •0001414 



•691 

 •695 

 •769 

 •649 



§ 46. Leaving this discrepance unexplained, and elimi- 

 nating B between (1) of § 44 and (3) of § 41, we find as 

 Maxwell's latest expression of the theoretical relation between 

 number of molecules per cubic centimetre, diameter of the 

 molecules, molecular velocity, density, and viscosity of a 

 single gas, 



y 9 2 = i_ 





1629 



Vp 



(1). 



The number of grammes and the number of molecules in a 

 cubic centimetre being respectively p and iV, p/N is the mass 

 of one molecule in grammes ; and therefore, denoting this 

 by ??2, we have 



^ 



m = 2v/37r£s 2 = 6'140-£s 



V 



V 



(2)- 



