ON SUPERSATURATED SOLUTIONS, I4I 



Sodium Carbonate adheres to a pin in the same sort of way, but 

 the sulphate is very readily got rid of by cold water. 



J£ffect of Scratching. — In a paper communicated to the Royal 

 Society, I described how a solution of sodium sulphate in about 

 six parts of sulphuric acid, can be made to crystallize by scratching 

 on a hard surface. On finding the remarkable adhesiveness of the 

 acetate mentioned above, I thought, perhaps, my plates were not 

 i^uite clear, especially as I had already found that a scratch on 

 copper which had been active in one drop, proved active at once 

 to a fresh drop, although it had been washed and scrubbed with a 

 brush. I therefore tried all kinds of ways of cleaning the plates ; 

 boiling them with ammonia and barium chloride and drying them 

 over calcium chloride. Yet on removing the plate from the 

 calcium chloride in the open air, and putting drops on the plates, 

 I found that a glass rod, which had also been boiled with barium 

 chloride, was invariably active on scratching. In my previous 

 paper I had attributed the result to vibration upsetting the state of 

 unstable equilibrium In which these solutions exist. I am not 

 sure that it Is not simply a case of absorption. I have already 

 described how shavings of wood were active In this solution j it 

 seems possible that small particles of organic matter adhering to 

 the dry plate and there set up crystallization in the depressions of 

 the plate. As In the case of the boiled pins these might grow 

 very slowly, while scratching brings them in contact with the 

 mass of the solution and sudden crystallization follows. 



