Transition Layer between Two Adjacent Phases. 509 



Calculation of the Heat-effect per unit surface area due to the 

 surface change in density. 



It is evident from Table II. that the surface-density 

 approximates more nearly to the bulk density the higher the 

 temperature. It follows therefore according to the principle 

 of Le Chatelier that the density increase is accompanied by 

 an evolution of heat. 



The internal latent heat of vaporization is the amount 

 required to remove one gram of the liquid from the surface- 

 layer, and taking into account Stefan's law, the same 

 amount of work accompanies the passage of one gram 

 from the bulk of the liquid into the surface when new 

 surface is formed thereby. In water at 0° C. this heat- 

 effect is about 570 cals. Water suffers a density increase of 

 1*189 gram/c.c. as we pass from bulk to surface. If we 

 take the thickness of the transition layer to be 5xl0" 6 cm. 

 (Quincke's value for the average range of molecular action), 

 then each unit of surface area corresponds to a surface slab 

 of liquid 5 x 10 ~ 6 c.c. possessing a mass of 2*189 x 5 x 10" 6 

 gram. If there had been no surface-density changes the 

 same slab would have had a mass of 5 X 10" 6 gram. Each 

 additional sq. cm. of surface is therefore associated with the 

 transfer of 1*189 X 5 X 10~ 6 gram of liquid. The heat-effect 

 due to this quantity transferred would be 0*0034 cal. In this 

 connexion one might draw attention to the value obtained 

 experimentally by J. G. Parks (Phil. Mag. iv. p. 240, 

 1902) for the heat given out on moistening powders 

 (such as silica) with water. The cause of the heat-effect 

 is ascribed to surface-density changes in the water where 

 solid and liquid meet. The heat evolution per sq. cm. of 

 powder is 0*00105 cal. — a number which is of quite the 

 same order of magnitude as that calculated for the case 

 water/vapour. 



The general structure of the Surface-layer, 



Willard Gribbs in his celebrated memoir was one of the 

 first to point out that a surface-layer is in general hetero- 

 geneous, as, for example, oil in contact with water. The 

 density changes of each phase do not consist in a steady and 

 rapid fall or rise as we traverse the layer from one side to 

 the other, but if the considerations put forward in this paper 

 are correct the density of each phase will pass through a 



