524 



Mr. Charles Tomlinson on 



[Dec. 20, 



while hot and allowed to cool with it, they are inactive. Lowel (1850-57) 

 denies that air, whether wet or dry, has any nuclear action ; but he 

 admits that solids exposed to the air become active, and that alcohol is 

 always active. Selmi and G-oskynski, in 1851, assert that dry air is 

 nuclear, and acts by getting rid of water at the surface, and producing 

 small crystals there which continue the action. This seems to be a 

 revival of Gay Lussac's theory, namely, that air is absorbed at the surface 

 of the solution and precipitates a portion of the salt in the same way 

 that one salt may precipitate another, and this precipitate continues the 

 crystallization. Lieben, in 1854, states that soot is a nucleus, also 

 platinum black whether ignited or not ; that pounded glass heated in 

 sulphuric acid produces sudden crystallization, but that platinum 

 sponge and precipitated sulphate of baryta after being heated have no 

 action. Schroder, in 1859, remarks that it is always a matter of chance 

 whether such or such a substance produces crystallization. " Such facts," 

 he says, " singularly increase the difficulty of interpreting theoretically 

 the phenomena of supersaturation." He concludes that the only general 

 rule that can be admitted in the presence of so many opposed and con- 

 tradictory results is that bodies act on supersaturated solutions only 

 after having been exposed to the air. In 1866 Grernez and Tiollette and 

 in 1868 Schiff are satisfied that there is only one nucleus for a super- 

 saturated solution, and that is a salt of the same kind as the one in solu- 

 tion or one isomeric therewith. In 1866 Jeannel opposes this theory of 

 pancrystallography, as he calls it, on the ground that it cannot be sup- 

 posed that crystals, often of rare salts, are to be found waiting in the 

 atmosphere, ready to enter our flasks as soon as they are uncovered. 

 Pellogio* also, in 1875, "gives proofs that the phenomena of supersatu- 

 ration are not so simple as the Trench physicists would imply, namely, 

 that the only nucleus is a salt of the same kind," seeing that some super- 

 saturated solutions, such as those of hyposulphite of soda, acetate of 

 lead, acetate of soda, &c, may be exposed to the air, in places where the 

 air is any thing but still, for fifteen or twenty days without the formation 

 of crystals. He states further that porous bodies are active, such as 

 common sponge, platinum black, iron reduced by hydrogen, and carbon. 

 For example, carbon was raised to a red heat, quenched under mercury, 

 and introduced into a solution of 100 sodic sulphate to 102 of water : it 

 fell to the bottom and disengaged gas for some time ; crystallization then 

 set in and spread all through the mass. Tiollette, on the contrary, finds 

 that bodies greedy of water and capable of being hydrated, such as the 

 fused sulphates of copper and of iron, and porous bodies recently cal- 

 cined, such as carbon, have no action on supersaturated saline solutions. 



In the midst of all this conflict of testimony, my own results were not 

 likely to escape censure. Grernez and others opposed me ; and Professor 



* Rendiconti R. 1st. Lomb., 1 Luglio. References to most of the other authorities 

 are given in previous papers. 



