4 M. Gay-Lussac on the [Jan. 



the point at which the solvent, always in contact with the salt, 

 can neither take up any more, nor let go any more. This point 

 is the only one which should be adopted, because it is deter- 

 mined by chemical forces, and because it remains constant as 

 long as these forces remain constant. According to this defini- 

 tion, every sahne solution which can let go salt without any 

 change of temperature is of necessity supersaturated. I shall 

 now show that in general supersaturation is not a fixed point, 

 and that the cause which produces it is the same as that which 

 keeps water liquid below the temperature at which it congeals. 



When a liquid, or even an elastic fluid, is to become solid, 

 the change does not always take place at the temperature at 

 which it ought to happen. Water, for example, whose freezing 

 point is 32°, may, in suitable circumstances, remain fluid 10° or 

 20° below that point ; and its boiling, which in a metallic vessel 

 takes place at 212°, is very sensibly retarded in vessels of glass. 

 The same liquid may likewise retain in solution a greater quan- 

 tity of carbonic acid than corresponds with the pressure. The 

 effects of this kind are very numerous ; their intensity in deter- 

 minate circumstances ought to be constant ; but as they appear 

 to depend upon the inertia of the molecules, which is general in a 

 very weak force, and which yields to the slightest eftbrt, we are 

 never sure of coming to the point at which this intensity is a 

 maximum ; for example, we have observed that in some experi- 

 ments water remained liquid 18° below its freezing point, but 

 nothing indicates that it may not preserve it in a much more 

 considerable cold. By inertia of molecules, which is necessarily 

 a vague expression, we must understand a resistance to a change 

 of state or equilibrium which may be produced by diflerent 

 causes ; such as the difficulty of a change of place in the mole- 

 cules in a medium perfectly homogeneous ; the viscosity of the 

 solvent 5 the conduction of heat, which, by opposing a resistance 

 to the disengagement or the absorption of heat, may maintain 

 the equilibrium of the molecules ; and, perhaps, also an electric 

 influence. 



It is certain at least that all the effects of which we have just 

 spoken may be prevented or destroyed by causes which appear 

 strangers to affinity. Thus water congeals always at 32", and 

 boils at 212° nearly in glass vessels, it does not take an excess 

 of carbonic acid, or it lets that excess go, when it is agitated. 

 It is true that a piece of ice introduced into water cooled down 

 below 32° will infallibly occasion its crystallization, in conse- 

 quence of the reciprocal affinity of the molecules of water, which 

 is greater in the solid than the liquid state. But this is only an 

 additional method of destroying the inertia; and frequently 

 inert bodies, particularly when they have asperities, produce the 

 same effect. 



Supersaturated saline solutions have a very great analogy with 

 7/ater cooled down below the freezing point, and every thing 



