Kinetic Theory of Gases to Gravitation. 119 



do not observe a solid block structure, if I may so express it, 

 but a more or less open structure as consistent with lightness 

 combined with elasticity. So molecular architecture (as size 

 is only relative, and principles apply everywhere the same) 

 may be of an open structure, as consistent with elasticity. 

 This open structure, involving various separated parts, would 

 give the molecule the faculty of taking up various vibrations, 

 as it is known to be capable of doing. 



4. Admitting, therefore, molecules to possess an open struc- 

 ture, the passage of the gravific medium through the mole- 

 cules of matter might be compared (merely for a simile) to 

 the passage of a stream of air through a scaffolding, the air 

 passing in great part through, but exercising a gentle pressure 

 against the solid parts of the scaffolding. So, in analogy, with 

 the gravific medium ; or by this open structure of molecules 

 we have all the conditions for the pressure termed " gravity," 

 together with the permeability essential to make gravity pro- 

 portional to mass. [We make no postulate as to the form of 

 open structure.] 



5. One point may be noticed here in connexion with the 

 inference that the molecules of solids are in contact. The old 

 postulate of perfectly rigid molecules put a difficulty in the 

 way of assuming that the molecules of a solid are in contact, 

 because the " elasticity " (or compressibility within certain 

 limits) of a solid could scarcely be reconciled with this postulate 

 of perfectly rigid molecules. The dynamical theory of molecules 

 put forward by Sir William Thomson, which explains the elas- 

 ticity of a molecule by a simple motion of the matter forming 

 it, enables us to explain the elasticity of a solid (with mole- 

 cules in actual contact) by the elasticity of the molecules them- 

 selves. By this theory also the open structure we have sug- 

 gested becomes a natural consequence. 



6. That matter does possess an open structure, due to some 

 cause, appears to be sufficiently proved by independent facts. 

 How otherwise could waves of light and the magnetic disturb- 

 ance pass so freely through matter? It appears natural to 

 assume that the molecules of a solid are in contact, on account 

 of the resistance they oppose to displacement in all directions. 

 If so, it would appear necessary to look for the interstices in 

 the molecules themselves ; and we think we have shown that 

 this conclusion is not merely warranted by the case of gravity, 

 but that it is in itself rather probable on independent grounds. 



7. It may be observed that, by means of interstices in the 

 molecules themselves, a mass may possess any degree of open- 

 ness and yet be practically closed — i. e. closed to the penetra- 

 tion of all ordinary matter, such as the air, liquids, &c, — as 



