87 VISCOSITY OF GASES 225 



0-003665, as for gases. Mercury vapour therefore alters its 

 viscosity in a rather larger ratio than can be explained by 

 Sutherland's theory. 1 



We meet with similar difficulties in regard to the ob- 

 servations made by 0. Schumann 2 on the viscosity of 

 the vapour of benzol and of different esters ; for the func- 

 tion of the temperature that represents the behaviour of 

 benzol is 



(1 + 0-00185 3) V(l + 0-0043), 



in which each term increases more rapidly with the tem- 

 perature than the theory can explain. For many of the 

 esters, certainly, the law of alteration with temperature that 

 was found lies between the theoretical limits ; but the 

 function 



(1 + 0-00164 -9)V(1 + 0-004 3), 



which Schumann has deduced as the most probable mean 

 of all his observations on esters, increases nearly as rapidly 

 as the extreme limiting case admissible under Sutherland's 

 theory. 



Let us now examine whether Schumann's observa- 

 tions satisfy this limiting value, and for this let us express 

 the coefficient of viscosity by 



7, = 



where TJ Q is its value at C., and a is taken for all vapours 

 equal to 0*00367 ; or more simply let us put 



7) = HW, 



where if is a constant and the absolute temperature ; we 

 then find a tolerably good agreement between the theoreti- 

 cal formula and the results of experiment. The following 

 tables contain the mean values of the magnitudes measured 

 by Schumann and the values of H calculated from 

 them. 



1 Compare 92. 



2 0. Schumann, Tiibinger Habilitationsschrift, 1884; Wied. Ann. 1884, 

 xxiii. p. 353. 



