SUSPENDED CHANGES IN NATURE 



25 



vestigator must take his solution into the open air, but here again he 

 must be careful, for his clothes and hair ma} 7 carry enough powered ma- 

 terial to inoculate the solutions. 



The amount of material required to inocu- 

 late a solution is very, very small, far beyond 

 the sensitiveness of the most delicate balance. 

 Xowhere in science is the importance of the 

 fact that " the very small is as real as the very 

 great" better illustrated than in the case of 

 undercooled liquids; a human hair lightly 

 brushed against a crystal of thymol will collect 

 enough of this material to inoculate a flask of 

 the undercooled liquid thymol. A titbe of 

 undercooled sodium acetate may be divided by 

 a piece of parchment paper into two parts 

 (Fig. 3). The inoculation of the solution in 

 A causes the separation of crystals which ulti- 

 mately appear in B via the parchment. The 

 pores in the parchment, which are of micro- 

 scopic size, become filled with the undercooled 

 solution, and as the crystals forming in the 

 pores can not be larger than the pores it is 

 evident that these crystals are of microscopic 

 dimensions only. 



How far can a liquid be cooled below the 

 temperature at which crystals ought to sepa- 

 rate? This depends entirely upon the sub- 

 stance used. "With some liquids if the under- 

 cooling is greater than a degree or two, crys- 

 tals at once separate spontaneously from the 

 solution. In the case of water the under- 

 cooling has been carried to as low as twenty degrees below zero before 

 crystals of ice separated. 



Sodium acetate on being strongly undercooled shows an interesting 

 property, for as the temperature becomes lower and lower the liquid be- 

 comes less mobile until at fifty degrees below zero just before spontane- 

 ous crystallization the liquid assumes a viscous glassy appearance. Its 



similarity to glass is more than 

 superficial. When molten glass 

 is cooled it gradually becomes 

 more and more viscous until 

 finally it has all the appearances 

 of a solid. But at no definite 

 temperature does it suddenly harden, as would be the case in cooling 

 mercury or molten lead. The glass differs from the undercooled sodium 



Fig. 2. Crystals Grow- 

 ing ix a Tube of Under- 

 cooled Sodium Acetate 

 Solution. 







Fig. 3. 



