1898.] Mr Wilson, On Velocity of Solidification. 29 



experiment of passing a loaded wire through a block of ice. If 

 O — is one degree P for water is roughly 



80 x 42 x 10 6 , a , _. dynes 



-— = 12 x 10 6 -*— - , 



273 cm 2 



or about 12 atmospheres. If the water is also supposed com- 

 pressed the corresponding value of P is about 150 atmospheres 

 and of course depends entirely on the change of volume during 

 solidification. The effect when the water is unconstrained is 

 therefore more than ten times greater than the effect usually only 

 considered and applies equally well to all substances whether there 

 is a change of volume on solidification or not. 



The pressure P may be supposed to drive the molecules from 

 the liquid into the solid just as the osmotic pressure of a salt 

 solution causes it to diffuse. If A is the force required to give 

 unit velocity to one gram of the liquid diffusing through itself, 

 and if it is supposed that the pressure P acts on a layer of 

 molecules at the surface of separation of thickness a, then the 

 velocity of solidification (&>) will be 



_P_ _F_ o -0 



Asa Aa ' O 



Only the orders of magnitude of A and a can be estimated at 

 present. The force required to give unit velocity to a gram 

 molecule of HC1 diffusing in water is about 10 15 dynes (see 

 Ostwald's Solutions, translated by P. Muir, p. 146), and for other 

 substances it is of the same order of magnitude. Hence A may 

 be taken as about 10 15 . a is probably of the same order of magni- 

 tude as the mean distance between the molecules of the liquid, 

 so we may take a as 10 -8 cm. Hence if Q — 6 = 1 degree 



_ 80 x 42 x 10 6 _ cms 

 03 ~ 10 15 x 10- 8 x 273 sec ' 



which is of the right order of magnitude. 



Thus for pure phenol I have found that the velocity increases 



by about 03 — - for each degree of supercooling, and the velocity 



cm 

 calculated as above, using the same values of A and a, is 0"4 . 



Since only the orders of magnitude of A and a are known, an 



accurate comparison between the calculated and observed velocities 



is impossible. Since A increases rapidly as the temperature falls, 

 rr a a 



the expression -r- . -^ — indicates that as the supercooling is 



Act 0o 



increased the velocity of solidification will attain a maximum 



