Mr. C. Tomlinson on Supersaturated Saline Solutions. 221) 



that «i modified salt may be deposited, denies that it is due to any 

 molecular change that takes place in the solution, either from reduc- 

 tion of temperature or any catalytic property of the sides of the 

 vessel. His theory is that, when these modified salts are formed, it 

 is the anhydrous salt that is held in solution, a portion of which is 

 thrown down as the temperature falls ; and this anhydrous deposit, 

 entering again into solution, forms a dense substratum containing less 

 water than the upper portions, so that when the modified salt forms 

 in ir, it is out of the reach of sufficient water to form the normal 

 salt. When, on the contrary, under the influence of a -nucleus, 

 crystallization sets in from the surface, the normal salt is formed, and 

 the crystals carry down sufficient water to convert the whole into the 

 ordinary hydrated salt. 



As to the action of nuclei or the sides of the vessel, when che- 

 mically clean the solution adheres to them as a whole, and there is 

 no separation of the salt ; when not chemically clean there is a stronger 

 adhesion between the salt and the nucleus than between the salt and 

 the solvent, and there is a separation of salt ; and the action of sepa- 

 ration once begun, may be rapidly propagated throughout the whole 

 solution. Boiling saturated solutions may be cooled down in chemi- 

 cally clean vessels and kept for any length of time, not because they 

 undergo any molecular change or hold a salt cf greater solubility 

 than the normal salt in solution, but they retain their fluid form 

 simply from the absence of a nucleus. 



The salts examined in this memoir are arranged into five groups 

 according to their behaviour. 



1. Salts of which the supersaturated solutions remain liquid at 

 low temperatures. 



Examples : — 



Sodic sulphate. 

 Sodic acetate. 

 Sodic arseniate. 

 Sodic succinate. 

 Sodic borate. 

 Sodio-potassic tartrate. 

 II. Salts of which the supersaturated solutions suddenly solidify 

 at low temperatures. 

 Examples :- 



Potash alum. 

 Magnesia sulphate. 

 Baric acetate. 

 Calcic chloride. 

 Cupric sulphate. 



Sodic carbonate. 

 Sodic phosphate. 

 Plumbic acetate. 



Sodic hyposulphite. 

 Strontic chloride. 



III. Salts of which the supersaturated solutions deposit their excess 

 of salt at low temperatures or under the action of a nucleus, leaving the 

 mother-liquor saturated. 



Examples : — 



Zinco-acetate. 

 Cupric sulphate. 

 Baric chloride. 



Potassic arseniate. 

 Antimonio-potassic tartrate. 

 Citric acid. 



IV. Salts of which the supersaturated solutions form modified 

 salts of a lower degree of hydration. 



