1868.] Mr. C. TomlinsoD on Svpersaturated Saline Solutions. 405 



that all cases of supersaturation are in appearance only, and not in fact. 

 As to the function of nuclei and the inner sides of the flasks in determi- 

 ning crystaUization, he regards it as the effect of one of those mysterious 

 contact actions known as catalytic, of which science has not yet been able 

 to give a satisfactory explanation. Bodies that appear to be active in indu- 

 cing crystallization are designated as catalytic or dynamic, while bodies 

 that are apparently inactive are termed non-catalytic or adynamic. It 

 appears certain," he says, "that but for the mysterious action which the 

 air and other bodies exert on supersaturated solutions, we should obtain 

 sulphate of soda only in the modified state ; that is, crystallized with seven 

 equivalents of water, and possessing at ordinary temperatures of the air a 

 much greater solubility than that of the normal 10-atom salt." 



Later inquirers have endeavoured to explain the nature of the force ex- 

 erted by nuclei in inducing crystallization under certain conditions, and 

 their passivity under others. Gernez* tried no less than 220 solids, and of 

 these he selected 39 that were active in inducing crystallization: 18 were 

 insoluble ; these were carefully washed in distilled water, and dried out of 

 contact with air. When dry they were found to be without action on the 

 solutions they had previously caused to crystallize. The 21 soluble sub- 

 stances were purified by recrystallization, and they all became inactive. 

 Hence it is concluded that sulphate of soda is the only nucleus for solutions 

 of the same salt. That is to say, whenever a glass rod or other body acts 

 as a nucleus, it is contaminated with minute portions of the salt itself, which 

 M. Gernez believes to exist in the air, not only of towns, but in the country. 

 According to this view, the supersaturated solution of any other salt can only 

 be crystalhzed by a saline nucleus of its own kind. But, as M. Jeannelf has 

 pointed out, if this theory be true, we must have floating in the air speci- 

 mens of all kinds of salts that form supersaturated solutions, and crystaUize 

 by the introduction of a solid nucleus ; whereas there are some such salts 

 which cannot exist in the presence of the oxygen or of the ammonia of the 

 air. M. Jeannel shows that a few drops of an ordinary solution of a salt 

 will induce crystallization in a supersaturated solution of the same salt 

 without contact of air. 



3. On the Action of Nuclei. — With respect to the action of nuclei on 

 saline supersaturated solutions generally, the author refers to a theory of 

 his X which seems to account for the liberation of gases from their super- 

 saturated solutions (soda-water, seltzer-water, champagne, &c) when a 

 solid nucleus that had been exposed to the air is immersed in them ; while 

 such nucleus becomes inactive if kept long in water, or passed through 

 flame &c., and dried or cooled out of contact with air. Tlie action of 

 nuclei is referred to adhesion. Nuclei are active in inducing crystaUization, 



^ Comptes Rendus, vol. Ix. p. 833. A similar method was adopted by SchifF, Ann. 

 derChem. und Pbarra. vol. cxi. p. G8. 

 t Ann. de Chim. et de Phys. 4th ser. vol. vi. p. 1G6 ; and Comptes Rendus, Jan. 2, 1866. 

 X Phil. Mag. July and August 1867. 



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