Spontaneous Crystallization of Supercooled Liquids. 79 



merely represents a stage, in principle somewhat ill-defined, 

 at which the probability of crystallization becomes very 

 oreat. De Coppet found * that after the lapse of varying 

 intervals of time, supercooled liquids crystallized within 

 the region which the metastable limit was supposed to 

 bound. The experiments just quoted, however, show that 

 in practice the probability of crystallization must vary very 

 rapidly in the neighbourhood of this point. 



The object of the experiments to be described in this 

 paper was to determine the mode of variation with 

 temperature of the probability of crystallization, in the 

 hope of throwing some light on the nature of the causes 

 which lead to the formation of a crystal nucleus in a 

 supercooled liquid. Tammann t has measured the way in 

 which the linear velocity of crystallization varies with 

 temperature, and also the number of nuclei formed in a 

 mass frozen suddenly at different temperatures, but not 

 the probability of crystallization — that is, the probability 

 of the formation of a single nucleus from an initially 

 homogeneous liquid. 



It is clear that to obtain results of quantitative signi- 

 ficance a statistical method must be adopted. A large 

 number of similar tubes must be filled with equal volumes 

 of the liquid under investigation, and the number counted 

 whose contents have crystallized at the end of various 

 intervals of time at a constant known temperature. 



A number of organic substances were investigated — 

 ... 

 salol, phenol, p.-toluidine, diphenylamine, and o.-nitro- 



phenol, as these melt at convenient temperatures. 



The procedure was to fill about a hundred tubes, to 

 seal them, and tie them to a board ; or in some experiments 

 where very light and small tubes were employed they 

 were sewn to a piece of nickel gauze. The contents were 

 then melted by immersion in a bath of water at a known 

 temperature above the melting-point, sufficient time being- 

 allowed to ensure that all crystalline nuclei dissolved. 

 The tubes were then transferred to a thermostat, and the 

 number of crystallizations observed from time to time. When 

 one experiment was completed, those tubes in which crystal- 

 lization had not taken place were induced to crvstallize 

 as far as possible by immersion in cold water. Then the 

 hole set were melted up again and a fresh experiment 



begun. 



* Loc. cit. 



t ' Krystallisieren imd Schmelzen,' Leipzig, 1903, p. 131. 



