Prof. Norton on Molecular' Physics. 281 



spheres now contract instead of expanding; and in the final act 

 of solidification the contiguous molecules assume the positions 

 due to their own special forces. While all this is being accom- 

 plished, the molecular atmospheres contract, and heat is given 

 out. 



The explanation of the process of evaporation will be readily 

 inferred from what has already been stated with regard to the 

 condition of the surface of a liquid (p. 280). The nice equipoise 

 of the surface particles may be disturbed either by a slight ele- 

 vation of temperature, or a diminution of the tension of the 

 vapour resting upon them. The cooling effect of the evaporation 

 is to be attributed to the expansion which the electric atmo- 

 spheres experience on being freed from the compressing forces 

 previously existing*. 



In the process of ebullition, the expansive action of the heat 

 absorbed by the lower layers of the liquid increases until the 

 superincumbent pressure, the cohesive attraction of the vessel 

 for the liquid, and the effective attractions subsisting between 

 the molecules of the liquid (represented by the ordinates be- 

 tween a and b, fig. 1), are overcome. When this point is reached 

 at any part of the liquid stratum, the separated particles will 

 expand rapidly into bubbles of vapour, in opposition to the 

 pressure of the atmosphere, and the attractions denoted by the 

 decreasing ordinates between b and c, fig. 1. The expansion 

 should continue until the distance between the atmospheres of 

 two particles increases to the limit Od, at which the repulsion 

 attains to its maximum value; or rather to a limiting distance 

 somewhat greater than Od, at which the repulsion due to the 

 heat-pulses present in the molecules, plus the molecular repul- 

 sion at that distance, is equal to the external pressure. 



It cannot proceed further than this without a direct expendi- 

 ture of heat-force, which will raise the temperature of the vapour. 

 The heat which becomes latent, as the phrase is, is expended in 

 the act of expansion, and in forcing up the molecular atmo- 

 spheres in opposition to the attractive action of the atoms and all 

 compressing forces. The amount of work thus taken up by the 

 atmospheres manifests itself also as work of expansion, since it 

 is so much work of the atomic attraction and of the compressing 

 forces neutralized. When the heat-pulses are not wholly ex- 



* It is apparently not necessary to suppose, as has been done on p. 280, 

 that the tension of the vapour resting on the surface of a liquid, when at 

 its maximum, is in equilibrium with the outward repulsion experienced by 

 the outer layer of liquid particles. The equilibrium may be a dynamical 

 one, the vapour may be continually rising at certain points of the surface 

 and continually passing back into the liquid condition at other points, 

 the condensation compensating exactly for the evaporation. 



Phil. Mag. S. 4. Vol. 28. No. 189. Oct. 1861. U 



