1877.] 



Supersaturated Saline Solutions. 



531 



3. Potash alum was also singularly active, but more so on glass than 



on paper. 



4. Zinc acetate crystallized almost immediately. 



5. Magnesic sulphate crystallized almost immediately. 



6. Zinc sulphate crystallized after about half an hour. 



7. Sodic carbonate flattened and crystallized by evaporation, first on 



paper and then on glass. 



8. Sodic tartrate similar to 7. 



9. Lead acetate became solid after about three hours. 



10. Sodic acetate was liquid after forty-eight hours, when the experi- 

 ment was terminated. 



On the table in my study, in which this last experiment was conducted, 

 was a glass plate on which drops of sodic acetate had been deposited 

 fifteen days before — one compound drop surrounded by six single drops, 

 all of which, except two single drops, had crystallized. These two 

 had apparently retained their surface-tension, while the other drops 

 had become more or less deformed. There was heavy rain on the 27th 

 August, and on the morning of the 28th all the drops, which had pre- 

 viously crystallized, were quite liquid. Sodic arseniate solution of the same 

 strength (2 to 1) behaves in a similar manner, absorbing moisture from 

 the air in damp weather, and remaining liquid the while ; whereas drops 

 of sodic phosphate solution (2 to 1) become solid after a few hours even 

 in damp weather. 



An extreme case of persistence in the liquid form during damp weather 

 is afforded by sodic acetate that has undergone the watery fusion ; that 

 is, the salt newly crystallized is melted in a test-tube over a spirit-lamp 

 in its own water of crystallization and so boiled. Drops of this on a 

 glass plate will remain liquid during many hours, and even days, if the 

 wind is in a westerly or southerly quarter. Meanwhile they absorb 

 moisture from the atmosphere, as was proved by balancing 44 grains of 

 this solution ; it gained 11 grains in the course of a couple of days, and 

 then crystallized slowly with large crystals and interposed water, as 

 in an evaporating-dish, and without that loss of weight which accom- 

 panies the sudden crystallization of a supersaturated saline solution. 

 There is only one exception to this loss of weight that I am aware 

 of, and that is in the case of a salt under the aqueous fusion. This, 

 in suddenly solidifying, becomes very hot; but there is no loss of 

 water from evaporation, because the whole of the water present becomes 

 constitutional water in the salt. For example, 45 1 grains of recently 

 fused sodic acetate were poured into a balanced watch-glass while still 

 warm, and the solution formed a resplendent convex lens. In the course 

 of an hour it had gained one grain in weight from the absorption of 

 moisture. "While watching it, its bright surface became suddenly clouded : 

 from a point in the circumference crystalline lines diverged, so as quickly 



