200 PHENOMENA DEPENDENT ON MOLECULAE PATHS 79 



heavy by reason of their mass and extent. With this the 

 run of the numbers agrees ; the collision-frequency first 

 diminishes, and then increases, as the molecular weight 

 increases. 



An exactly similar explanation serves to account for the 

 observation that gaseous chemical compounds which are 

 polyatomic behave nearly alike as regards viscosity, and 

 indeed we see this just as before from the mathematical 

 expression for the collision-frequency. By the entrance of 

 more atoms into the molecule the section irs 2 will in most 

 cases increase, while the molecular speed will diminish ; the 

 interval T between successive collisions may therefore be 

 constant. In this case the coefficient of viscosity must also 

 be constant ; for the formula which gives it, 



T? = 0-30967 pLtt, 

 leads to the relation 



= 0-30967 pO 2 



between the viscosity and the collision-interval T, and by 

 substitution of the pressure by means of the formula 

 p = ^7Tpl 2 we can bring this into the form 



7j IT = 0-03871 TTP = 0-121607^. 



If therefore T is independent of the molecular weight, so too 

 is ?;. Hence all gaseous substances which contain a large 

 number of atoms in their molecule have nearly equal co- 

 efficients of viscosity. 



This law is important in the examination of the behaviour 

 of vapours, which we have still to investigate ( 86). But we 

 must first discuss some other relations which there come 

 into consideration. 



80. Viscosity of Gaseous Mixtures 



Both theory and experiment agree in showing that the 

 principal laws of viscosity hold not only for chemically pure 

 gases, but also for mixtures of different gases. Indeed, it is a 

 mixed gas, atmospheric air to wit, which has been employed 



