85 VISCOSITY OF GASES 219 



The forces of cohesion have therefore the same result as 

 an increase of the section of the sphere, which with 

 Glaus ius we have called the sphere of action, would have. 

 The amount of this enlargement depends on the speed with 

 which the particles move, and, therefore, on the temperature 

 of the gas ; it is the greater the less the speed or the lower 

 the temperature. The sphere of action will therefore appear 

 to be less enlarged at higher temperatures than at lower, 

 and this produces the same result as if it becomes smaller 

 when the temperature rises. The consequence of this be- 

 haviour is an increase of the molecular free path with the 

 temperature. 



We might a priori take it as very probable that this 

 view is right, because the gases for which tolerably large 

 or, indeed, variable values of n have been found belong to 

 those which have strong cohesion, and can therefore be 

 condensed to liquids with proportionate ease. 



In bringing forward this hypothesis to explain the 

 phenomena Sutherland 1 had really the greatest success. 

 According to his theory, explained in 71, the free path L 

 at the absolute temperature <B) is connected with its value L 

 at the freezing-point by the relation 



In the constant magnitude C which here occurs we have 

 to see a measure of the strength of the attractive forces 

 which act between the gaseous molecules, and, therefore, a 

 measure of the cohesion of the gaseous medium ; a is the 

 coefficient of expansion. If we introduce this value of L 

 and also the value 



into the formula for the coefficient of viscosity 



77 = 0-30967 

 we obtain the equation 



Phil Mag. 1893 [5] xxxvi. p. 507. 



