172 THE FORMS OF WATER IN 



of the liquid either into the solid or the vaporous 

 condition. 



436. But if into the over-chilled water you throw a 

 particle of ice, the cohesion is ruptured, and congelation 

 immediately sets in. And if into the superheated 

 water you introduce a bubble of air or of steam, cohe- 

 sion is likewise ruptured, and ebullition immediately 

 commences. 



437. Faraday concluded that in the interior of any 

 body, whether solid or liquid, where every particle is 

 grasped so to speak by the surrounding particles, and 

 grasps them in turn, the bond of cohesion is so strong 

 as to require a higher temperature to change the state 

 of aggregation than is necessary at the surface. At the 

 surface of a piece of ice, for example, the molecules are 

 free on one side from the control of other molecules ; 

 and they therefore yield to heat more readily than in 

 the interior. The bubble of air or steam in overheated 

 water also frees the molecules on one side ; hence the 

 ebullition consequent upon its introduction. Prac- 

 tically speaking, then, the point of liquefaction of the 

 interior ice is higher than that of the superficial ice. 

 Faraday also refers to the special solidifying power 

 which bodies exert upon their own molecules. Cam- 

 phor in a glass bottle fills the bottle with an atmos- 

 phere of camphor. In such an atmosphere large crystals 

 of the substance may grow by the incessant deposition 

 of camphor molecules upon camphor, at a temperature 



