Kinetic Theory of Gases, 791 



be that if s<4 the attractions o£ distant molecules in the 

 gas will then be o£ equal or greater importance than those 

 of the neighbouring molecules. In such a case the recorded 

 pressure at any point would be affected or even dominated 

 by the distant attractions and would depend on the general 

 shape of the whole body of gas. This is of course directly 

 contrary to experience, and no such attractions of sensible 

 magnitude* can be admitted. 



Finally, it is interesting to observe that we can obtain 

 the case 5 = 4 as a limiting case in which the intensity of 

 the intermolecular attraction between any pair of molecules 

 tends to zero as s~>4 from above. In this limiting case 

 and this only, we do in fact obtain a value of a which is a 

 true constant independent of the temperature. This, however, 

 can only be achieved at the price of a zero value for II (<j), 

 and therefore for Sutherland's constant ; this is inadmissible 

 in the case of any actual gas. 



§(6). The ivork done in bringing a molecule from the 

 interior to the boundary of a gas. — The molecular model that 

 we consider is that of a bard elastic sphere of diameter a, 

 surrounded by a field of attractive force. Two molecules 

 whose centres are distant r apart act on each other with 

 an attraction </>(r) along their line of centres. It is this 

 molecular model which is so successful in explaining the 

 " transport phenomena ,J in gases and their variation with 

 temperature at ordinary temperatures — in particular the 

 coefficient of viscosity. When such fields of force are 

 postulated, the distribution of molecules in the gas is not on 

 the average uniform with reference to any selected molecule. 

 If v is the general molecular density in the gas, the molecular 

 density at a distance r from the selected molecule is, not v, 

 butt 



(• oo 



2j\ ^ <fi(x)dx 



ve , . . . . . (5a) 



* Gravitational attractions are in this category,, and are of course 

 insensibly small. Electrostatic attractions are in the same category, 

 but are so large that they entirely dominate the distribution of charged 

 molecules or ions. 



f See, e. g., Boltzmann, Vorlesimgen tiler Gastheorie, vol. ii. p. 150 

 or Jeans, he. cit. p. 132. Jeans' suggested correction of this formula is 

 here insensible. Owing to the standardization of the use of h for Planck's 

 constant, I believe that the time has come for the use of some letter 

 other than h for the constant in Maxwell's Law for the distribution of 

 velocities, and I propose to use j for this purpose. I believe, further, 

 that it is desirable to use a notation distinguishing systematically 

 between "the gas-constant for one gramme-molecule " and " the gas- 

 constant for one molecule" (the constant k in 2j=l/kT) and to conform 

 to the continental notation of B, and k respectively. 



