196 PHENOMENA DEPENDENT ON MOLECULAR PATHS 79 



coefficient of transpiration, in consequence of which they 

 must also have the same free path and collision-frequency. 

 This rule, however, does not hold in general, but only for 

 such substances whose molecules are made up of the same 

 number of atoms. Thus, for instance, CO, N 2 , and NO, 

 which all three have two atoms in the molecule, have nearly 

 equal free paths and collision-frequencies in addition to 

 nearly equal molecular weights, while the molecule C 2 H 4 , 

 consisting of six atoms, shows a much smaller free path 

 and a much larger collision-frequency, although it has the 

 same molecular weight. Further, the molecules C0 2 and 

 N 2 0, composed of three atoms, agree in respect of weight, 

 viscosity, free path, and collision-frequency, while the only 

 slightly heavier molecule C 2 H 6 0, consisting of nine atoms, 

 has a much smaller viscosity and free path, and a much 

 larger collision-frequency. In both these cases, therefore, 

 the collision-frequency is the greater the more atoms in the 

 molecule. 



If now we arrange the substances enumerated according 

 to the number of atoms in the molecule, we first obtain the 

 following two series, whose regularity is obvious : 



The numbers here tabulated confirm our expectation so 

 far as to show that the values of the coefficient of friction, 

 free path, and collision-frequency within each group now 

 rise and now fall in a periodic way while the molecular 

 weight increases. 



