[MCINTOSH] SUPERSATURATED SOLUTIONS 267 



Times, Heated Remaining Liquid 



1 1 



2 2 



3 5 



4 7 ■ 



5 9 



6 9 



7 10 



8 10 



9 10 

 10 all 



A tube remaining liquid for a reasonable time could be depended 

 on to remain liquid indefinitely. The 11 tubes mentioned had not 

 crystallized after two months. 



Size of Particles Inducing Crystallization 



As far as I am aware, the only investigations of the size of par- 

 ticles necessary to cause precipitation in super-cooled or supersatur- 

 ated solutions are those made by Ostwald. A brief description may 

 not be out of place. 



One gram of the crystals was mixed with 99 grams of an indifferent 

 substance, such as silica, and ground for many hours, usually in a 

 small ball-mill. A gram of this mixture containing 10'- of the 

 crystals was taken, mixed with 99 grams of silica and the grinding 

 repeated, 1 gram contained 10^ of the crystals. By further dilution 

 mixtures containing any mass of crystals could be obtained. If a 

 weighed amount of a mixture dropped into the supersaturated 

 solution brings about precipitation, the mass of the added crystals 

 may be easily calculated. The second method applies only to anhy- 

 drous salts and was used by Ostwald in his investigation of sodium 

 chlorate solutions. A platinized platinum wire was dipped a certain 

 distance into a very dilute solution of the chlorate. The solution ad- 

 hering weighed approximately 10^ gram, so the mass of the solute 

 could be easily calculated. The platinum wire was carefully dried, 

 and used to inoculate the supersaturated solutions. 



The weakness of both methods is clear. It is by no means certain 

 that many crystals were not present in the material used for inocu- 

 lation, and that one of these might have produced crystallization. 

 However, by these methods Ostwald found the size of particles 

 necessary to cause precipitation to be: 



