440 Mr. W. R. Browne on Action at a Distance. 



showers of " extramundane particles " are sweeping through 

 space equally in all directions, and that a fraction of these, 

 being intercepted by A and B, urge those two bodies towards 

 each other. This hypothesis is encumbered with a large 

 number of arbitrary assumptions; and the latest supporter 

 of the theory, Mr. S. Tolver Preston*, presents it under a 

 greatly modified form. He supposes the solar system to be 

 immersed in an impalpable gas, the particles of which have a 

 mean length of free path greater than the distance through 

 which gravity has been observed to hold (greater, therefore, 

 than the distance between the Sun and Neptune), and which 

 tend to bring together, by the resultant of their impacts upon 

 them, any two bodies within that range. It is not proposed 

 in this paper to attempt an exhaustive discussion of this theory; 

 but were it left as an unquestioned explanation of gravitation, 

 it might be thought a strong presumption that all other actions 

 were to be explained on the same principle. It may therefore 

 be remarked that it is encumbered by very serious difficulties. 

 In addition to those put forward by Dr. Croll and others, the 

 following may be suggested : — 



(1) Mr. Tolver Preston founds his theory on the late Prof. 

 Maxwell's proof f, that " a self-acting adjustment goes on 

 among a system of bodies or particles in free collision, such 

 that the particles are caused to move equally in all directions, 

 this being the condition requisite to produce equilibrium of pres- 

 sure" %. JSTow this equilibrium of pressure, and the theory 

 based upon it, may be perfectly true for all known gases. But 

 all such gases are under certain conditions, which need not hold 

 universally; in especial they are bounded in some way. The 

 atmosphere, which is the freest gas we can observe directly, 

 is bounded by the earth on one side and space on the other, 

 and is prevented from passing into space by the action of 

 gravity. But we have no right whatever to assume such a 

 boundary for interstellar space, or to assume that a gas filling 

 such a space would have equilibrium of pressure. The proba- 

 bility would seem to be the other way; for any disturbance 

 in such a gas would tend to propagate itself in all directions 

 for ever. In any case, Maxwell's results must be proved, not 

 assumed, to hold for this gravity-gas, as it may be termed. 



(2) Another difficulty in the theory is the enormous degree 

 of porosity which it postulates for solid bodies. To fix our 

 ideas, suppose that, in any unit of surface of a solid, one mil- 



* Phil. Mag. Sept. 1877, Nov. 1877, Jan. 1878, 



t S. Tolver Preston, Phil. Mag. Sept. 1877. 



% I have, unfortunately, failed to verify the reference to this paper of 

 Prof. Maxwell's, given by Mr. Tolver Preston, and therefore can speak 

 of it only from his description. 



