Dynamical Theories of Gravitation, 153 



a medium is wanted in other respects, for instance for the 

 elucidation of magnetic and electric phenomena. 



It may be mentioned, further, that the explanation of gravi- 

 tation carries with it the great advantage of rendering super- 

 fluous the idea of the existence of two (inherently different) 

 kinds of matter, " ponderable " and " imponderable." The 

 smaller atoms in space do not gravitate, only because the 

 mechanism of gravitation cannot itself be subject to the con- 

 ditions for producing gravitation. So, therefore, disappear 

 the almost contradictory properties, " ponderable M and "im- 

 ponderable," which have been arbitrarily attributed to matter: 

 and we have therefore no reason for believing that the atoms 

 diffused in space differ essentially from gross molecules, 

 excepting in their dimensions. To the abandonment of the 

 idea of two inherently different kinds of matter, the abandon- 

 ment of two supposed different kinds of energy is analogous — 

 viz., energy with motion, and energy without motion. Accord- 

 ingly there would remain only one kind of energy, namely, 

 that which a moving body possesses. 



Another important quantitative relation is so conditioned 

 that the " shelter " is evidently proportional to the surface 

 exposed to the moving atoms ; the gravitational effect, on the 

 other hand, is proportional to the mass, as experiment shows. 

 This result can only be achieved by supposing gross matter to 

 possess a very porous structure. In that way, the gross 

 molecules inside a body are reached or affected by the 

 penetrating aether atoms almost with the same facility as the 

 external molecules of the body. If we assume that the 

 quantity of material contained in the substance of any mole- 

 cule is very small compared with the vacant space contained 

 in that same molecule, and if one does not suppose any super- 

 fluous material in the structure of the molecule ; the pro- 

 portionality existing between gravitation and mass can be 

 satisfied as closely as observation requires. 



Some Remarks on Crystal Structure. 



Even Le Sage recognized that for the elucidation of the 

 gravitational effects the assumption of a porous or open struc- 

 ture in matter is necessary. Lord Kelvin draws a curious 

 inference from this. In the Philosophical Magazine, May 

 1873, postscript p. 331, Lord Kelvin supposes that it might 

 be probable that bi-axial crystals would not be penetrable 

 with equal facility in all directions by the aether atoms. If 

 that were so, such crystals would possess a (even if very small) 

 difference of weight, according as the one or the other axis is 



