1820.] SohhUity of the Salts in Water. 5 



that has been just said may be appHed to them. We may obtain 

 these solutions by evaporating very slowly a portion of the sol- 

 vent ; but it answers better to cool slowly solutions already satu- 

 rated ; for some salts, as the sulphate and carbonate of soda, 

 the cooling may be considerable b«fore the crystallization begins ; 

 but in general it ought to be very little. The general cause 

 producing supersaturation being evidently the same for each 

 salt, it will be sufficient to observe the effects in those which 

 show them with the greatest intensity. I shall take, as an 

 example, the carbonate of soda. 



A supersaturated solution of this salt crystallizes, just like 

 water remaining liquid below the point of congelation, either by 

 agitation, or the immersion of a crystal of carbonate of soda, or 

 of a foreign body ; and, as is the case with water, we cannot 

 assign the limit at which the supersaturation stops short. This 

 limit, in each experiment, is entirely accidental. It depends 

 upon the nature of the vessel, on its smoothness, its conduction, 

 the agitation of the air. But as we determine crystallization in 

 a supersaturated solution of carbonate of soda by a slight agita- 

 tion, it is obvious that this supersaturation does not depend on 

 affinity, but on a force purely mechanical ; for motion cannot of 

 itself produce chemical effects. 



There are a great many saline solutions which crystallize as 

 soon as they lose a portion of the solvent, or their temperature is 

 somewhat lowered ; and it would be difficult by the means 

 pointed out above to know whether they can be supersaturated ; 

 but the phenomena of crystallization leave no doubt on that 

 head. 



If we consider a saline solution in which some scattered crys- 

 tals have formed, if it be exposed to evaporation, these crystals 

 will increase without the formation of any new ones, or at least 

 this will often happen. But evaporation taking place only at the 

 surface of the liquid, it follows of necessity that the saline mole- 

 cules, which have lost a portion of their solvent, still continue in 

 solution till they approach the crystals which seize upon them ; 

 for if they precipitated as soon as they lose their solvent, we 

 could not conceive the regular increase of the crystals. This is 

 what actually happens, when the evaporation is too rapid, rela- 

 tively to the supersaturation which the solution is capable of 

 assuming. In that case, a multitude of small crystals are depo- 

 sited on the sides of the vessel containing the sahne solution. 



Supersaturation does not appear to depend on the affinity of 

 the salt for the solvent, for it is very far from being proportional 

 to it. This is a further proof that it is owing to a particular 

 disposition of the saline molecules, in consequence of which they 

 resist more or less a change of state. 



I have chosen water as an example to explain by analogy the 

 supersaturation of saline solutions, because the permanence of 



