Mr. W. R. Browne on Action at a Distance. 441 



lionth part only is occupied by the really solid part (i. e. the 

 part which would stop the particles of the gravity-gas) of the 

 molecules composing that surface. Then it is obvious that a 

 layer of such molecules a few millions thick would be practi- 

 cally certain to stop the whole of the gravity-particles impin- 

 ging upon it. No arrangement of the molecules one behind 

 the other will get over this, because the gravity-particles are 

 assumed to come in all directions at once. Now such a layer 

 would certainly be no more than a small fraction of an inch 

 in thickness. And yet it is absolutely necessary for the 

 theory (in order to explain how gravity varies as mass) to sup- 

 pose that these gravity-particles pass through the T67OOO miles 

 of the earth's diameter, under the enormous density, pressure, 

 and temperature which must exist in the interior, without 

 having more than a very small proportion of their number 

 stopped in the passage. The difficulty is rendered the greater 

 when we remember that, ex hyp., these attenuated molecules 

 cannot act on each other at a distance, in producing the various 

 phenomena of solid bodies, but only in one of the three modes 

 of direct impact enumerated above. 



(3) Another difficulty arises from the fact that the heavenly 

 bodies are not found to experience any perceptible resistance 

 whatever in passing through this gravity-gas. It is clear that 

 if a body be in motion in the midst of a shower of such par- 

 ticles coming equally from all directions, it will receive a 

 greater number of blows on its front surface, and a less num- 

 ber on its rear surface, than if it were at rest ; and conse- 

 quently its motion will be retarded. The only way of sur- 

 mounting this difficulty is to suppose that the heavenly bodies, 

 in relation to the gravity-gas, are practically at rest; in other 

 words, that the velocity of the gravity-particles is practically 

 indefinite compared w 7 ith that of the heavenly bodies. Since 

 in the case of Mercury, for instance, this latter velocity is 

 about thirty miles per second, it is clear that the velocity of 

 the gravity-particles must be something altogether beyond cal- 

 culation; and then, since the effect of the collisions is, after 

 all, very limited, the mass of the particles must be assumed 

 correspondingly small. Hence our conception of the gravity- 

 gas must practically be that of an indefinite number of inde- 

 finitely small particles moving in all directions with indefi- 

 nitely high velocities — a conception from which it hardly seems 

 safe to draw any definite conclusion whatever. 



(4) The last-mentioned difficulty leads to another, viz. to fix 

 the relations between the gravity-gas and the luminiferous 

 ajther. They cannot be the same ; for Mr. Tolver Preston and 

 Prof. Maxwell have shown * that the velocity of propagation 



* S. Tolver Preston, Phil. Mag. June 1877. 



