Dynamical Theories of Gravitation. 151 



in the intervening space between these masses, by the collisions 

 of the aether atoms among themselves, the normal proportion- 

 ality between translatory and internal motion of the atoms 

 would be nearly restored (by encounters), and therefore the 

 mutual shelter of the two masses would nearly be nullified. 



The range of gravitation (its sphere of action) is therefore 

 conditioned by the mean length of path of the atoms, and 

 this may be regarded as an interesting deduction from the 

 theory. Accordingly, on the assumption that the mean dis- 

 tances of the stars (excepting, of course, the relatively approxi- 

 mated double stars) are large in proportion to the mean length 

 of path of the atoms, the inference would follow that the stars 

 do not gravitate towards each other — and apparently in that 

 way the universe would rather gain than lose in stability. 

 One sees then that the mean length of path of the yjther atoms 

 must be great in comparison with those distances across which 

 Newton's law has been demonstrated to apply with exactness. 



In an article in the Encyc. Brit. 1875 (or Scientific Papers, 

 vol. ii. p. 476) Maxwell raises the objection that by the atomic 

 encounters gross matter would be raised to a white, heat ; he 

 grounds this inference on the theorem that for thermal equili- 

 brium between atoms or molecules the mean energy of 

 translatory motion must be equal. Now the pressure (to take 

 some symbol) is equal to the product of the mean energy of 

 translatory motion L of an atom into the number N of atoms 

 contained in the unit of volume. If, therefore, the mean 

 energy of translatory motion of an aether atom be equal to 

 that of a molecule of gross matter, which we can calculate in 

 the case of ordinary gases, then N for the aether must have an 

 enormous value, in order to be able to account for the gravita- 

 tion pressure. Now Maxwell says : we are tolerably certain 

 that N for the aether is small compared with the value of 1ST for 

 gross matter. From this he concludes that in order to explain 

 the gravitation pressure, it is necessary to assume L enor- 

 mously great. And according to the theorem that for thermal 

 equilibrium L must be the same for all atoms (or molecules), 

 it would follow that L also for the molecules of gross matter 

 must finally assume a value which is much greater than 

 that which we find in the case of gases. In other words, that 

 all gross matter must be raised to a white heat by the collisions 

 of the aether atoms. But, independently of the fact that the 

 above-named theorem, relating to thermal equilibrium, for 

 molecules or atoms of very different size is still contested, it 

 seems to me that no cogent reason exists for the assumption 

 that N is smaller for the aether than for gross matter. One 

 can, in fact, imagine the aether atoms as small as one pleases ; 



