134 ON SL'PERSATTJRATED SOLUTIONS. 



uncovered. In the afternoon it crystallised and set so firmly that 

 on inverting the test tube not a single drop of liquid fell. I think 

 that this experiment proves conclusively that air is not at all times 

 full of sulphate of soda. 



Next, with regard to other solutions than that of the sulphate, I 

 find that sodium acetate is a most remarkable salt. It is extremely 

 difficult to make a solution of this salt crystallise without intro- 

 ducing a crystal of the salt.* In a paper communicated to the 

 Royal Society I described how I rubbed it in vain with dust on a 

 bottle which had been standing for two years untouched in my 

 laboratory, how I put it on the floor and all kinds of surfaces 

 without causing crystallisation, and how drops remained super- 

 saturated for two months on a plate exposed to air in my laboratory, 

 and then crystallised on introducing a crystal. I have obtained 

 similar results with Sodium Carbonate and with the Alums. 

 Hence it is clear that these salts also are not generally present in 

 the air or dust of my laboratory. I could describe experiments by 

 the hundred, all proving the same point that air and dust are not 

 storehouses of crystals of all kinds. 



(2.) The Adhesion Theory. — In one of the experiments described 

 above I laid stress on the fact that Sodium Sulphate remained 

 liquid on films of oil, even when rubbed with them. Professor 

 Tomlinson some years ago brought out the theory that crystallisa- 

 tion as the normal salt was caused by adhesion to a greasy or 

 chemically unclean surface. The salt adheres to the surface while 

 the liquid does not, and hence separation ensues. The main 

 argument in its favour was that while a clean glass rod was inactive 

 in these solutions, passing the rod through the fingers sufficed to 

 make it immediately active. This is undoubtedly true — I explain 

 the fact by supposing that in working with the salts and solutions 

 the fingers take up some of the crystals which are then transferred 



* A plate of drops was handed round for those present to try to make the 

 drops crystalise ; it was returned with the drops liquid. 



