218 PHENOMENA DEPENDENT ON MOLECULAK PATHS 85 



ture, while the quadratic factor is to express the change of 

 the second quantity occurring in the formula for the co- 

 efficient of viscosity, viz. the molecular free path. The 

 second power is here chosen, because from the formula 



L = \ 3 /V2 



we may assume that the free path can depend on the 

 temperature only in so far as the central section of the 

 sphere of action is variable ; hence 7 might be looked on as 

 the measure of the diminution which s the radius of the 

 sphere of action undergoes with rise of temperature. The 

 new formula excellently satisfies the observations which 

 Schumann made by Maxwell's oscillation method, yet 

 it seems to correspond less well to the transpiration experi- 

 ments which Bar us 1 has carried out within a far larger 

 range of temperature. 



The reasons for the assumption that the section of the 

 sphere of action diminishes as the temperature rises have 

 been given already in 71. These depend for the most 

 part on conceptions which would be as well fitted to explain 

 the deviations from the laws of perfect gases, so that they 

 might be applied to vapours. But in the question of actual 

 gases, those attempts at explanation will certainly meet with 

 most acceptance which do not assume a real diminution 

 of the molecules or their spheres of action, but only an ap- 

 parent alteration. From this point of view Sutherland's 

 view deserves to be preferred to all others. According to 

 him we have not to deal with the real sphere of action, but 

 with an apparent sphere of action which is larger than the 

 real one. 



The attractive forces which are recognised by us in 

 liquids and solids as cohesion and hardness are, according 

 to Joule and Lord Kelvin's experiments, not absent 

 from the molecules of gases too. Certainly they only come 

 into play when two molecules come very closely together ; 

 but they may then cause two molecules to collide which 

 without such attraction would have passed by each other. 



1 Bull U. S. Geological Surv. No. 54, p. 278 ; Wied. Ann. 1889, xxxvi. 

 p. 386. 



