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



somewhat retarded. Precisely the same thing is illustrated in 

 the aggregation of groups of molecules (to form masses), as in 

 the aggregation of single molecules to form compound 

 molecules. Thus when a bar of iron is welded by heat, the 

 molecules (though aggregated or combined) do not settle down 

 into their final positions of proximity until the bar cools, the 

 bar being observed to contract on cooling. In this instance 

 also the cooling of the bar is somewhat retarded by the approach 

 of the molecules in the act of cooling. 



8. In the case of the ignition of a solid body, the same con- 

 siderations no doubt apply as in the case of a gas. Thus, for 

 example, the molecules of oxygen are impinging against the 

 surface of a piece of coal, but do not produce ignition. To 

 effect this a certain number of the molecules must be impelled 

 with sufficient energy against the coal so as to carry them over 

 the neutral point (i. e. beyond the initial repulsion). The 

 application of a flame, which consists of matter in a state of 

 violent agitation, suffices to effect this, and, no doubt by 

 loosening some of the molecules of carbon (of the coal) and 

 giving them translatory motion and mixing them with the air, 

 facilitates the process. 



9. As a further illustration of the exact similarity of behaviour 

 of single molecules and groups of molecules (masses) as regards 

 the existence of the above-mentioned neutral point, we may 

 take the case of the substance iodine. This substance gives 

 off a visible vapour at normal temperatures. The single 

 molecules of iodine composing the vapour rebound from each 

 other without uniting ; and this can only be due to the exist- 

 ence of the above-mentioned neutral point, outside which there 

 is a repulsion. If the colliding molecules were to approach 

 within the neutral point, they would unite and form solid 

 iodine. No doubt some of the molecules of the vapour (as 

 their velocities are known to be very diverse) do pass beyond 

 the neutral point; and thus molecules of vapour striking against 

 the fragments of solid iodine in the bottle, will sometimes 

 unite with the solid iodine and form part of it, while, on the 

 other hand, other molecules of the solid which happen to pos- 

 sess excessive vibrating energy are thrown off, this being the 

 known way in which the balance in evaporation is maintained. 

 The masses of iodine have the same neutral point as the 

 single molecules, since two masses of the substance when 

 pressed together will not readily unite ; i. e. the neutral point, 

 where the outer repulsion terminates, must be passed first*. 



* The above effects were described in a little book ( Physics of the 

 Ether' (E. & F. N. Spoil), published by me in 1875 ; but the cause of the 

 reduction of the pressure of the medium, which determines the approach 



