280 Prof. Norton on Molecular Physics. 



of the particles at the different orders of distance come succes- 

 sively into action, being less intercepted by intervening particles. 

 At the same time, the individual molecular atmospheres expand, 

 or recede from their central atoms, under the action of the heat- 

 pulses that penetrate to these atoms ; and so the energy of the 

 attractive force of each of these molecules declines. The sur- 

 face particles will thus continue to recede at the same time that 

 they are restrained by the attractions of those below them. 

 This effect will extend from the surface downward; and as a 

 final result, a certain number of layers are brought into the 

 liquid condition, in which, as we have seen (p. 278), the particles 

 mutually repel each other, in consequence of the exertion of a 

 compressing force at the surface. In the case of a liquid that 

 emits vapour at the temperature of liquefaction, we must con- 

 clude that the particles at the very surface become ultimately 

 subject to an effective repulsion from the united action of those 

 below it, which is in equilibrium with the tension of the vapour 

 resting on the surface; and that this effective repulsion extends 

 to all points above the surface. 



The heat of fluidity is consumed in forcing up the molecular 

 atmospheres. As a final result of the liquefaction, these atmo- 

 spheres remain in an expanded condition. The effect of this 

 expansion is to diminish the values of v given by equation (1) 

 (see p. 200), and increase the distance 0«, fig. 1. The actual 

 distance between two contiguous atmospheres is less than the 

 increased distance Oa, by reason of the compressing force that 

 takes effect throughout the liquid mass. But the ultimate com- 

 pression imparted to the individual atmospheres will depend 

 in a great degree upon the final value of the attractive action 

 v between the molecules, and may therefore still be less than 

 that which obtained in the solid state. In this diminished value 

 of v we have, at the same time, the explanation of the dimi- 

 nished force of cohesion attendant upon the liquid state. The 

 comparative densities of the liquid and solid also depend upon 

 v. For we have just seen that the distance between the con- 

 tiguous atmospheres of two particles of the liquid is less than 

 the increased value of 0«, but this distance may, according to 

 the intensity of the attractive force v, be either greater or less 

 than the original value of Oa } which was the distance between 

 the atmospheres of the same particles in the solid condition. 

 Accordingly the liquid may be either more or less dense than 

 the solid from which it is derived. 



The passage from the liquid to the solid state is essentially 

 the inverse of that which has just been under consideration, and 

 in the general survey we are now taking need not be considered 

 in detail. The mass of molecules and their individual atmo- 



