46 THE BEGINNINGS OF LIFE. 



as with salts, they may be broken up into simpler units, 

 which enter separately into combination with the 

 molecules of water. The state of solution is, therefore, 

 to be regarded as a new chemical combination — one 

 which carries with it, like many other such combi- 

 nations, marked differences in physical quality. 



In such respects, therefore, the state of solution differs 

 notably from the mere state of fluidity to which the 

 molecules of a simple body may be reduced by the 

 agency of heat. 



But solution is a state of combination whose dura- 

 bility, like that of all other chemical combinations, is 

 absolutely dependent upon the strength of the affinity 

 existing between the molecules of the solvent and those 

 of the substance dissolved. Solubility, accordingly, is 

 amenable to the influence of all those causes which 

 generally tend to affect the stability of compounds. 

 A little diminution or a little increase of heat may 

 render a pre-existing union no longer possible. Thus, 

 when a hot saturated solution of alum or nitre is 

 allowed to cool, some of the salts crystallize out of the 



(loc. cit. pp. 546, 542) : — ' We cannot, I think, deny that the force 

 represents some modification of chemical affinity, or is, at all events, 

 most closely allied to it. . . . The solubility of salt in water appears to 

 me to result from a kind of affinity which decreases in force as the 

 amount of salt in solution increases. This affinity is opposed by the 

 crystalline polarity of the salt; and when the two forces are equal, the 

 solution is exactly saturated. As is well known, a change in temperature 

 alters this equilibrium ; and, according to my experiments, mechanical 

 pressure relatively increases one or other of these opposing forces, 

 according to the mechanical relations of the salt in dissolving.' 



