314 Prof. J. J. Thomson on Effect of Electrification 



greater than that over a plane surface, the equilibrium vapour- 

 pressure increasing as the curvature of the drop increases : 

 hence, as Maxwell points out in the ' Theory of Heat,' the 

 water will tend to evaporate from a very small drop, which 

 will therefore grow smaller, and the tendency to evaporate 

 will get still greater. Thus the condensation of steam into 

 drops is attended by the difficulty that, even if a small drop 

 happens to get formed, it will, unless the steam around it is 

 excessively supersaturated, begin at once to evaporate. In 

 consequence of the excessive infantile mortality among the 

 drops, the production of fog from water-vapour free from 

 foreign substances is attended by almost insuperable diffi- 

 culties. This point is illustrated in a very striking way by 

 Aitken's experiments, which show how greatly the deposition 

 of fog is facilitated by the presence of dust. The particles of 

 dust furnish surfaces on which the moisture can be deposited 

 in layers having much smaller curvatures than any layers 

 which could be formed if the drop had no nucleus over which 

 to spread ; the dust, in fact, almost annuls the effect of surface- 

 tension by enabling the drop to come into existence without 

 passing through the stages in which surface-tension plays the 

 greatest havoc. 



We can apply a more general method of reasoning to the 

 effect of surface-tension, and one which has the merit of being 

 immediately applicable to the consideration of the effect of 

 electrification on the condensation. 



The condensation of steam into drops of water is, in conse- 

 quence of the surface-tension of the water, accompanied by an 

 increase in the potential energy of the system proportional to 

 the area of the water-drop : the ratio of this potential energy 

 to the heat given out by the steam when it condenses to form 

 the drop increases indefinitely as the size of the drop dimin- 

 ishes. Thus the existence of surface-tension causes an in- 

 crease in the potential energy when the change from steam 

 to water-drop takes place, and will therefore (see J. J. Thom- 

 son, ' Application of Dynamics to Physics and Chemistry/ 

 p. 162) tend to retard this change. On the other hand, any 

 circumstance which would cause a diminution in the potential 

 energy when the change from steam to water-drop takes place 

 will facilitate this change. Now an effect of this kind takes 

 place when the water-drops are deposited in an electric field. 

 Water has, as Cohn and Kosa's experiments have shown, a 

 specific inductive capacity of about 76. This value is so large 

 that the diminution due to the water-drop in the potential 

 energy in the electric field is very much the same as if a con- 

 ductor of the same size were substituted for the water-drop. 





