212 On Le Sage's Theory of Gravitation. 



that gravity may be sensibly proportional to mass) that the 

 total volume of free space in a substance, in the form of inter- 

 stices between the molecules or in their structure, must be 

 great compared with the total volume of matter contained in 

 the molecules themselves. Le Sage assumed the molecules of 

 substances to have a sort of open structure in the form of 

 cages with wide interstices. This condition of free interstices 

 would be equally satisfied by assuming the molecules to be 

 small relative to their mean distance, or on the condition of 

 the vortex-ring atom theory, without any necessity for making 

 the above someAvhat fantastic assumption of cage-structure. 



11. It is necessar}^ to assume that the particles producing 

 gravity are in very close proximity compared with molecules, 

 otherwise the particles would be unable by their motion to 

 produce a perfectly equable pressure upon the molecules of 

 matter. It might be thought that, because the particles of the 

 gravific medium are so close, and the molecules of ordinary 

 matter relativelj^ far apart, therefore the quantity of matter in 

 the form of gravific particles enclosed in a given volume of 

 space must be very great compared with the quantity of ordi- 

 nary matter that that same volume of space would contain — 

 or, in other words, that there must be a relatively enormous 

 quantity of matter in the form of gravific particles. This by 

 no means follows ; for although the gravific particles may be 

 very close, 1 he relative quantity of matter in them may be very 

 small, provided the particles themselves are small. Indeed by 

 simply conceiving an extreme degree of subdivision, the par- 

 ticles pervading a given volume of space may by continued 

 subdivision be conceived to be brought into as close proximity 

 as we please ; and though the space itself is large, the total 

 quantity of matter thus used may be conceived as small as we 

 please. No consequence hoAV minute the size (or mass) of a 

 particle may be, the effect produced by its motion remains as 

 great, provided its velocity be adequately augmented. The 

 minute size is the very condition adapted to a high velocity ; 

 and this minute size is at the same time the necessary condi- 

 tion for a long mean path. Thus we may observe that the 

 mechanical conditions of the problem fit into each other. The 

 matter of the gravific medium is in such a finely subdivided 

 state, and its motion so rapid, that its presence necessarily 

 eludes detection. The pressure (termed " gravity ") due to 

 the motion of the particles of the gravific medium is no more 

 difficult of realization than the pressure due to the motion of 

 the molecules of air. If the motion of the molecules of air be 

 unrecognized by the senses, how much more must this be the 

 fact with the minute gravific particles ; indeed it is difficult 



