232 PHENOMENA DEPENDENT ON MOLECULAR PATHS 89 



on multiplying by N, the whole number of the particles, and 

 putting 



B = Nm, D = Nm lt 



so that 8 represents the density of the vapour in its actual 

 state in which it is partly dissociated, and D the density 

 which it would attain when all the molecules were broken 

 up and the vapour therefore completely dissociated. Since, 

 further, N l + N 2 = -^ we mav express JYj and N 2 by the 

 densities 8 and D ; for the foregoing formulae give 



Hence it follows that the coefficient of viscosity rj of the 

 partially dissociated vapour is represented by 



7? = ^ 1 8*JD{2D - 8 + 2*fo 1 / 7; 2 )t(8 - D) }-* 



in which rj l is the coefficient of viscosity of the vapour 

 when completely dissociated into simple molecules ; 7? 2 , on 

 the contrary, denotes the value of the coefficient of viscosity 

 for this substance if no dissociation at all has taken place, 

 but all the molecules are combined together in pairs. There 

 further come into the formula the magnitudes 8 and Z), 

 which represent respectively the density of the actual vapour 

 and the value which the density would attain at the same 

 pressure and temperature if all the molecules were dis- 

 sociated into simple molecules by dissociation. The co- 

 efficient of viscosity therefore of a vapour appears to be 

 variable with its density, in contradistinction to the beha- 

 viour of perfect gases. 



It follows that the formula is not to be used for such 

 vapours as exhibit no dependence of their viscosity on the 

 pressure ; and we may conclude that in all cases in which 

 the coefficient of friction has been found to be independent 

 of the pressure the dissociation of the molecules has been of 

 no material influence. This occurs in most of the vapours 

 hitherto experimented on ; especially is it shown in the 

 experiments of Puluj l on the friction of ether vapour, and 



1 Wiener Sitzungsber. 1878, Ixxviii. Abth. 2, p. 279. 



