ON THE PHENOMENA AND THEORIES OF SOLUTION. 



467 



A supersaturated solution may be regarded as a mixture of a saturated 

 solution, with an extra quantity of the salt retaining a liquid state ; in 

 fact, in the condition which is commonly spoken of as superfusion. N'ow, 

 when we examine cases of superfusion, such as water, melted phosphorus, 

 melted sulphur, phenol, acetic acid, and so on, we find that the liquid 

 state is preserved so long as the liquid is cooled only a moderate degree 

 below its melting-point.' Water may be cooled to 10° or 12° below 

 freezing ; phosphorus, which melts at 44"2°, can be cooled to the common 

 temperature of the air, say 30° lower ; but sulphur, which melts at 115°, 

 cannot be cooled to the air temperature, which is 100° below its melting- 

 point, save in very small drops (Faraday). Melted sulphur may, how- 

 ever, be kept liquid at the temperature of boiling water, if protected 

 from dust, &c. (Gernez, ' Compt. Rend.' Ixxxiii. 217). Similarly, super- 

 saturated solutions remain liquid at the common temperature of the air, 

 but crystallise at 20° or 30° lower when cooled by a freezing mixture. 

 The fact is supersaturation is a case of superfusion. Gernez supposed 

 (' Compt. Rend.' 1866, p. 218) that he had discovered a difference between 

 the two when he made the observation that superfused phosphorus and 

 sulphur might be made to solidify by rubbing two hard bodies together 

 under the surface of the liquid, as when the inside of the containing vessel 

 is scratched with a wire or a glass rod. But Mr. J. G. Grenfell showed 

 in 1876 (' Proc. R. Soc' xxv. 129) exactly the same phenomenon with 

 a solution of sodium sulphate in sulphtiric acid, and it appears to diflFer 

 in no respect from the well-known effect when solution of platinum per- 

 chloride or of sodium hydrogen tartrate is mixed with a potassium salt and 

 the liquid is vigorously stirred. 



My view of supersaturation, then, is that it is identical with superfusion. 

 The explanation of the one phenomenon is the explanation of the other. 

 The case of thiosulphate of sodium is a very interesting one. This 

 salt melts at 48-5° without addition of any water whatever beyond what 

 it contains in chemical combination. This salt may be kept in a liquid 

 state in an ordinary flask exposed to the air for Aveeks. Advocates of the 

 theory which considers supersaturated solutions (and other solutions) to 

 contain molecules of the anhydrous salt in a free state must regard this 

 liquid as a solution of one molecule of Na2S203 in five molecules of water. 

 I cannot help believing that this liquid is none other than the compound 

 (N'a,So035H20)x in a liquid state, just as liquid water is the compound 



' There is a commonly recognised difference between melting and solidifying 

 points, but this seldom amounts to more than a few degrees. 



H H 2 



