Spontaneous Crystallization of Supercooled Liquids. 91 



Taking p as 1 to 2, and a as 10 3 , this gives a value for r o£ 

 the order of 2 x 10~ 5 cm., which is equivalent to about three 

 hundred molecular diameters. 



Consisting of a foreign substance which does not dissolve, 

 the particle acts as a centre upon which successive layers of 

 molecules may be deposited. If it is 30 per cent, too small 

 to bring about immediate crystallization, successive layers 

 are deposited, but continually re-dissolve until by chance 

 a succession of depositions carry it past the critical size. 

 This appears to happen after an average life of about an 

 hour in the case of p.-toluidine, where this 30 per cent, 

 increment seems to represent about 100 layers of molecules. 



Theoretically a particle of sufficient size should relieve 

 supercooling, even in quite close proximity to the melting- 

 point, but a limit is set by the well-known fact that colloidal 

 particles become less active when their size increases beyond 

 a certain value. The reason for this is not clearly understood, 

 but the magnitude of this limit is probably as specific as all 

 other catalytic phenomena. 



It would be rash to assume that the relationships described 

 in this paper are entirely general, but a well-defined class 

 of cases seem to come within the scope of the principles 

 suggested. 



We hope to carry investigations further by ultra-micro- 

 scopic observation, as this seems to be the direction in which 

 results of some interest in connexion with the problems of 

 surface energy might be found. 



Summary . 



The statistical investigation of the spontaneous crystal- 

 lization of several supercooled organic liquids leads to the 

 following conclusions : — 



1. In the case #f these substances crystallization is 

 provoked by the colloidal organic dust particles from the 

 air. 



2. The activity of these particles diminishes in general 

 as the result of heating or ageing. 



3. Their effectiveness depends upon their radius. If this 

 is equal to the radius of a small particle of the solid which 

 should tbermodynamically be in equilibrium with the super- 

 cooled liquid, then crystallization occurs at once. If the radius 

 is less than this, the supercooled liquid has an average life 

 depending on the discrepancy between the equilibrium 

 radius and the radius of the particles present. 



The magnitudes of these quantities are discussed. 



