"Inactive" Condition of Solids. 137 



had been exposed to the air immediately produced crystallization ; 

 but if driven through the cork so as to touch the supersaturated 

 solution, it did not induce crystallization. A knitting-needle is 

 given as a convenient illustration of this phenomenon. The most 

 efficient nucleus in inducing crystallization is a crystal of the salt 

 itself; but the efflorescence from a solution into the neck of a 

 phial contained in a large flask in which the air was dried by 

 means of caustic potash, was repeatedly redissolved by agitating 

 the vessel without inducing crystallization. An open vessel con- 

 taining a supersaturated solution will remain liquid if lightly 

 covered, as with a watch-glass. Ziz distinctly recognized the ex- 

 istence of two varieties of sulphate of soda with different fusing- 

 points. Indeed the paper is in advance of the time at which it 

 was written, and its merit has not, I think, been sufficiently re- 

 cognized. 



M. Henri Leowel has published a number of memoirs on su- 

 persaturated solutions*, in which he recognizes the distinction 

 between active and inactive solids, or, as he terms them, dynamic 

 and adynamic, in inducing crystallization ; and he is of opinion 

 that the cause of the difference is not mechanical, but catalytic. 

 He found that if a glass rod be heated to 30° or 40° C, it could 

 be used for stirring up a supersaturated solution without indu- 

 cing crystallization. Rods of copper, iron^ and zinc were passed 

 through the corks used for closing flasks containing boiling so- 

 lutions. As the solutions cooled, vapour condensed on the 

 metal rods, which became dry in two or three days, the iron 

 being oxidized ; but they were all inactive. Metal rods heated to 

 100° C. in boiling water or otherwise were also inactive ; if heated 

 to from 150 c to 200° C. and left to cool under cover they became 

 inactive ; but if exposed to air for some time they resumed the 

 active condition in a quarter of an hour, or from that to an hour. 

 Rods of glass, &c, kept in cold water become partially or wholly 

 inactive, but regained their activity by exposure to the air so as 

 to evaporate the water. The author does not pretend to explain 

 these phenomena, but speaks of " that mysterious action M which 

 the air and other bodies exert in inducing crystallization, 



I think it would not be difficult to show that this so-called 

 " inactive " condition of solid matter is simply a question of ad- 

 hesion. The bits of iron wire, flint, glass, small coins, &c. in 

 their dry state induce crystallization because there is adhesion 

 between them and the supersaturated solution ; that is, they are 

 wetted by the solution ; but if previously wetted with water they 

 are not wetted by the solution when thrown into it, and conse- 

 quently cannot act as nuclei, because the solution does not really 



* The first memoir, " Sur la Sursatnration des Dissolutions Salines," is 

 contained in the Annates de Chimie et de Physique for 1850. 



