CHAITKR II 



SUPERCOOLING AND THE SUPERCOOLED STATE 



A liquid is said to ])(' sii])orcoolod (or snbcooled, or 

 luidorcooied) wiicii it is still in the liquid slate at a tem- 

 perature lower tliaii the fi'eezing point. 



The fiindameiital character of the snbcooled state is to 

 be unstable. The circumstances which control the passage 

 of a snbcooled liquid into the crystalline state are very 

 incompletely known. The following are often mentioned 

 in textbooks of ])hysics: stirring, shaking, jarring, touch- 

 ing with a solid body, keeping in presence of impurities, 

 maintaining in contact with the air, mostly with dust-con- 

 taining air, etc. On the other hand, crystallization is said 

 to be prevented when the liquid is heated before being- 

 cooled, when it is reduced to small droplets, when it is 

 enclosed in capillary tubes, etc. It is difficult to say to 

 what extent some of these various ways of inducing or 

 preventing crystallization, wiiich are sometimes effective 

 and at other times not, have really been observed, and how 

 frequently they entered the physical literature through the 

 back door of a too conservative teaching of insufficiently 

 controlled observations. A consideration of their mecha- 

 nism of action might help one to correlate the data 

 concerning their efficacy. 



It is generally thought that the first crystallization 

 nuclei originate from the collisions of the molecules and 

 that any factor liable to bring the molecules in closer prox- 

 imity should facilitate congelation while any factor render- 

 ing more difficult the collision of the molecules should delay 

 congelation. Initiation of crystallization should, then, 

 depend on the number of molecules present, on the time 

 allowed for the molecules to collide and on the speed of 

 motion, that is, on temperature. The presence of all- 

 formed crystals, which need only to grow, is expected 

 to induce congelation. A mechanical disturbance might, 

 by a sudden change in the orientation of the molecules, 

 increase the chances of successful collisions. On the 



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