224 Royal Society :— 



same thing, from 218°, the boiling-point of a saturated solution, 

 down to 64° the solubility increases ; but at 64° the solution under- 

 goes a new molecular modification, and begins to form crystals of the 

 7-atom hydrate (the x salt of Ziz). This salt is much more soluble 

 at ordinary temperatures than the 10-atom hydrate, its maximum so- 

 lubility being at 80°* 6. Thus the sodic sulphate has three maxima 

 of solubility, viz. 93°* 2 when it is under the molecular constitution 

 of the 10-atom hydrate, 78°"8 to 80 o, 6 when it is under the mole- 

 cular constitution of the 7-atom hydrate, and 62°-G to 64°-4 when 

 it is under the molecular constitution of the anhydrous salt. At 

 these three maxima the saturated solutions are about equally rich 

 in salt. The 7-atom hydrate and the anhydrous salt can only main- 

 tain their molecular constitution when in contact with the mother- 

 liquor in closed vessels, in which they are sheltered from the air 

 and from other bodies that act upon them as nuclei. No sooner are 

 they exposed to the air than they become opaque and warm, and as- 

 sume the molecular constitution of the 10-atom hydrate, as well as its 

 solubility. Hence the conclusion is that supersaturated solutions of 

 the sodic sulphate are not really so, since they hold a salt of much 

 greater solubility at ordinary temperatures than the normal 10-atom 

 salt. Lowel extends his inquiry to sodic carbonate and magnesia 

 sulphate, and endeavours to show that in their supersaturated solutions 

 salts of a lower degree of hydration and of greater solubility than the 

 normal salts are formed ; and his general conclusion is 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 crystallization, he regards it as the effect of one of those mys- 

 terious 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 inducing 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 su- 

 persaturated 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 exerted by nuclei in inducing crystallization under certain con- 

 ditions, 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 substances were purified by 

 recrystallizfttion, 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, 



" :; " Comptes Renrtus, vol. lx. p. 833. A similar method was adopted by Sehiff, 

 Ann. derChem. unrt Pharm. vol. cxi. p. G8. 



