Mr. J. Gr. Grrenfell on Supersaturated Saline Solutions. 309 



same time. The clear crystals are inactive in the 6 to 1 solution, 

 while the opaque is active : and this is a clear proof of their identity. 

 Solutions of intermediate strength between 2 to 1 and 6 to 1 often 

 deposit in flasks the whole excess in clear crystals, which are 

 sometimes inactive in the 6 to 1 in a test-tube. It is almost im- 

 possible to obtain these solutions supersaturated in flasks, though 

 it may be done with the utmost facility in test-tubes. Out of 

 many trials with oue flask I only succeeded once by leaving it to 

 cool on the sand-bath. In a test-tube they give the same forms 

 as the 6 to 1. The variety of the forms in which these solutions 

 crystallize is truly astonishing, according to the proportion of 

 acid and salt, amount of water, and the temperature. A flask once 

 gave the most exquisite little, flat, open flakes closely resembling 

 snow-flakes ; but I have not been able to reproduce them. In 

 short the relations of these two substances to each other want 

 working out thoroughly. A certain amount of acid added to the 

 salt which is in excess gives a thin liquid, which will not crystal- 

 lize, and a little fine white powder, the anhydrous salt. Two 

 drops of acid in a test-tube half-full of solution cause drops to eva- 

 porate on a plate in octahedra ; and when the anhydrous is thrown 

 down on heating the test-tube locally after crystallizing, it is redis- 

 solved, leaving, however, well-marked octahedra just before it all 

 disappears. 



The most curious property, however, of the 6 to 1 solution is 

 this : — On a clean glass plate it can be spread out into a thin 

 covering of the plate with the handle of a tooth-brush ; then with 

 the end of a glass rod scratch a letter hard on the plate, and the 

 letter will come out at once in slowly growing crystals. The effect 

 is certain with the right proportions, and is most striking, as 

 a plate of any size can be used. Scratching has the same effect 

 when the solution is placed on gold or copper, but not on plati- 

 num foil, lead foil, bone, gutta percha, or any soft substance. The 

 effect is of course analogous to that of scratching on the ammonio- 

 magnesic phosphate and on soda water in a clean tumbler. Mr. 

 Tomlinson explains these by supposing that a partial vacuum is 

 formed into which the salt and gas separate. I confess it seems 

 to me more probable that the result is due to vibration. With 

 the same solution of sodium sulphate in acid, but of different 

 strength, scratching is inactive, and I have tried it in vain on 

 many aqueous solutions. 



I cannot see why the vacuum should not act equally on all ; but 

 it is easy to understand how the molecular vibrations of one un- 

 stable system should be affected by a particular set of vibrations, 

 whilst those of another system should not. The results obtained 

 thus far, then, are : — 



1. Exposure to air and dust has no effect on some supersa- 

 turated solutions. 



2. The sulphates are the most sensitive. Exposure of a clean 

 glass plate for half an hour to the air of my laboratory caused 

 nearly all the drops of sodium sulphate put upon it to crystallize 

 at once, whilst the same plate recently cleaned is quite inactive. 



