Mr. S. T. Preston on the Kinetic Theory of Gravitation. 303 



produced by a flame. In the case of solid bodies, where the 

 molecules are fixed or under control, a forcible pressure or 

 concussion may serve to bring the molecules over the neutral 

 point (and thus effect combination), as illustrated by the effect 

 of the blow struck in " percussion " powders. It would not 

 appear that matter in the gaseous state could ever be exploded 

 by pressure (so long as the gaseous state was retained) ; for 

 the molecules of gases cannot be pressed against each other by 

 any amount of pressure, since, the molecules being in free trans- 

 latory motion among themselves, the only effect of pressure 

 would evidently be to put a greater number of molecules into 

 unit of volume, without thereby causing the molecules in their 

 encounters to approach nearer to each other than before. The 

 degree of approach of the molecules (in their encounters) 

 depends evidently on their momentum or velocity ; and this 

 remains the same whatever the pressure. 



7. Heat could not apparently be said to augment the energy 

 of chemical combination, since, in general, heat is known to 

 possess the exactly opposite effect, or to disintegrate matter. 

 The part played by heat in effecting chemical combination 

 would seem to consist simply in producing a molecular dis- 

 turbance, whereby unavoidably some molecules are urged to- 

 wards each other so as to pass the outer neutral point, which 

 is the necessary preliminary to combination. No doubt, when 

 heated elements combine, the original heat adds itself to the 

 work thus to be derived, as the heat cannot be destroyed, 

 though it cannot increase the work of combination. Heat 

 may (as is known) entirely prevent chemical combination, and 

 even dissociate combined elements. The action of heat in 

 preventing chemical combination and producing dissociation 

 would on the above principles consist in the fact that, when 

 the vibratory motion of the molecules becomes excessive, this 

 vibratory motion generates such a pressure in the intervening 

 layer of aether on the approach of the molecules as to prevent 

 them from passing the neutral point : or, indeed, no neutral 

 point may exist, provided the pressure or repulsion thus 

 generated be such as to outweigh the action of the gravific 

 medium, as appears actually to take place in the dissociation 

 of matter by excessive heat. Thus it would appear probable 

 from this, that when combination ensues in the case of a mix- 

 ture of gases previously considerably heated (but not so much 

 so as to produce dissociation), the molecules on combination 

 do not at once settle down into that full proximity (which 

 belongs to a lower temperature), but they do so gradually as 

 the temperature falls. Thus the work of combination is pro- 

 longed over the falling temperature, and the cooling thereby 



