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THE POPULAR SCIENCE MONTHLY 



extent by melted phosphorus, carbolic acid, and many organic sub- 

 stances like thymol and betol that are seldom encountered outside a 

 chemical laboratory. 



Although many undercooled liquids are similar to water in that 

 they may be changed to the stable state by agitation, this property is by 

 no means general. An infallible test for undercooling is the addition of 

 a fragment of the stable substance. Thus a bit of ice causes water to 



Fig. 1. Crystals Growing in Undercooked Thymol. 



solidify, similarly undercooled thymol becomes a crystalline mass upon 

 the addition of a crystal of thymol, while undercooled sodium acetate 

 solutions at once separate needle-like crystals of sodium acetate when 

 a crystal of that substances is added. The addition of a solid fragment 

 for the purpose of causing subsequent crystallization of the liquid is 

 called inoculation or vaccination. The name is particularly fortunate, 

 for the growth and spread of the crystals resembles a bacterial growth. 

 Undercooled liquids are as sensitive to the presence of a crystal of 

 the solid material as milk is to the presence of certain bacteria, and just 

 as much care must be exercised in their preservation. In working with 

 undercooled sodium acetate under no circumstances may an open flask 

 of this substance be brought into a room in which sodium acetate has 

 been ground in a mortar, for the dust in the air carries enough finely 

 divided sodium acetate to inoculate the solution. Frequently the in- 



the solution, without such a separation taking place. Such solutions are usually 

 said to be supersaturated. 



