Laws of Molecular Force . 251 



we must reject the idea of a statical pressure, aud replace it 

 by its kinetic equivalent of a to and fro transfer of momentum ; 

 this may take place as a quite indiscriminate traffic of indi- 

 vidual molecules across the plane, or as such a traffic modified 

 by the existence of streams of molecules iu opposite directions. 

 If streams, or motions of molecules in swarms, actually exist 

 in fluids, then our interpretation of the equation of the virial 

 would have to take account of their existence. The kinetic 

 energy of the motion of a swarm as a whole would not count 

 as heat but as mechanical energy, and for the amount of it 

 we should have approximately the kinetic energy of the swarm 

 motion equal to the virial of the forces between the swarms. 

 Therefore we require to divide the energy into two parts, 

 that of molecular motion inside the swarm constituting heat 

 and that of the swarm ; in the same way the internal virial is 

 divided into two parts, one within each swarm, the other be- 

 tween the swarms. But the swarms could on the average be 

 regarded as equivalent to spheres of radius L, where L must 

 be supposed nearly independent of mass and liable to the same 

 variations with temperature and pressure as the linear dimen- 

 sions of any quantity of the liquid, so that L is proportional 

 to a, and our expression (theoretical) WA7rp(31og2L/a — 4) 

 for the internal virial becomes purely proportional to the 

 mass and purely proportional to the density, as 3WZ/4v our 

 experimental internal virial for a mass W of a compound 

 liquid is. 



This hypothesis would affect somewhat the rigorousness of 

 certain thermodynamical relations as usually interpreted, such 

 as JX=(v 3 — ■v-JTdp/dT, since it provides a supply of internal 

 mechanical energy not taken account of ; but if this supply 

 is only slightly variable with pressure and temperature it 

 would make little difference in most parts of thermodynamics. 



With the addition of this hypothesis of molecular swarms, 

 which will be used only in calculating molecular distances, 

 and will not affect at all the rest of our work, the law of the 

 inverse fourth power is brought into strict harmony with the 

 behaviour of compound liquids and of elements both as liquids 

 and gases. We must therefore inquire what experimental 

 evidence there is for the existence in liquids of a motion of 

 swarms of molecules, possessing the remarkable property of 

 not being degraded to heat as ordinary visible motions are. 

 In the motion long familiar to microscopists as the Brownian 

 movement we have such evidence. Gouy has recently 

 (Compt. Rend. cix. p. 103) recalled the attention of physicists 

 to this remarkable ceaseless motion of granules in liquids. 

 He states that it occurs with all sorts of granules, and with an 



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