1876.] Supersaturated Saline Solutions. 129 



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

 heating the test-tube locally after crystallizing, it is redissolved, 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 pro- 

 portions, 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 platinum foil, lead foil, bone, gutta percha, or any 

 soft substance. The effect is of coarse 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 unstable 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 supersaturated 

 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. 



3. Even the sulphates are unaffected by the dust of the open air and 

 generally of ordinary rooms. 



4. Anhydrous salts or modified salts, sometimes new, are produced by 

 the spontaneous evaporation of the solutions in drops. 



5. Drops can be rapidly touched on their surface with crystals of the 

 same salt without crystallizing. 



6. G-reasy surfaces, whether films or lenses, have no effect. 



7. The shape of the vessel has sometimes a material influence on the 

 possibility of obtaining a supersaturated solution. 



8. Air and dirty surfaces are active on salts which apparently cannot 

 exist in air. 



9. Scratching a hard surface will cause a particular solution to 

 crystallize. 



The crystal theory, modified as it now must be, seems on the whole 

 the best explanation of the phenomeua. The case of the hyperacid 

 sodium sulphate, however, remains to be explained. If the crystal 



