71 MOLECULAR FREE PATHS 165 



The assumption that the sphere of action of a molecule 

 is actually altered neither by pressure nor by heat has much 

 to entice us; for we have become used to consider a molecule 

 as an aggregate of atoms which can certainly be altered by 

 chemical transformations, but not by processes which belong 

 to the narrower region of physics. If this doctrine, which 

 in former days ruled without question, were true, the size of 

 the molecular sphere of action could not be conditioned by 

 the temperature or the pressure of the gas. 



But this view is contested by abundant observations and 

 especially by the phenomena of dissociation. Numerous 

 instances of gases and vapours can be cited wherein the 

 molecules are composed of more atoms at lower temperatures 

 than at higher. The vapours and many gases deviate at low 

 temperatures from the laws of the ideal state of gas, especi- 

 ally from those of Boyle and Gay-Lussac, as has been 

 already described in Chapter IV., these deviations being such 

 that the gases have too great a density and an expansibility 

 which is much greater than that of ideal gases. These and 

 many other irregularities force us to the conclusion that the 

 molecules of those gases form bigger aggregates of molecules 

 at lower temperatures than at higher. By increment of heat 

 the molecules break up into smaller ones, and therefore the 

 mass of the molecule is decreased by rise of temperature ; 

 consequently their extension in space, and therewith the 

 size of their sphere of action, will both become smaller 

 when the temperature rises. From this we should expect 

 that, by reason of dissociation, the molecular free path for 

 vapours and non-perfect gases increases as the temperature 

 rises. 



Moreover, even for gases which undergo no dissociation 

 of their molecules, it is possible to suppose that a diminution 

 of the molecular sphere of action may occur and demand 

 explanation. We have only to remember that the sphere 

 of action need not denote the space which the molecule 

 itself occupies or claims for itself ; but its radius is the least 

 distance to which the centres, or, more generally, the centres 

 of gravity, of two molecules can approach each other during 

 a collision. To assume that this distance is smaller at higher 



