THE BEGINNINGS OF LIFE. 47 



solution; when heat is applied to a solution of lime, 

 some of it becomes precipitated ; whilst, when either 

 heat or cold is applied to a solution of sodic sulphate 

 already at a temperature of 33' C, some of this salt 

 separates from the state of solution l . Here, as in 

 other cases of decomposition, the molecules of the dis- 

 solved substance and of the solvent, being themselves 

 different, are differently affected by the influence of the 

 same change or disturbing influence 2 . And we must 

 suppose the amount of difference induced to be so great 

 as to weaken or wholly destroy the affinity which had 

 previously held them together, so that the molecules of 

 the water under the new conditions are no longer able 

 to hold asunder the molecules of the substance with 

 which they were previously in combination. Or a 

 similar effect may be brought about by the addition of 

 a considerable quantity of a substance more soluble 

 than that which is already in solution 3 . Thus, sodic 

 chloride crystallizes from its aqueous solution on the 



1 Mr. Sullivan considers ('Rep. of Brit. Assoc.' 1859, p. 292) that the 

 solubility of very many salts (like that of sodic sulphate) attains a maxi- 

 mum at some particular temperature, above or below which it diminishes. 

 This temperature may frequently be above iooC; hence the common 

 belief that solubility always increases with rise of temperature, because 

 temperatures higher than 100 C are rarely resorted to. Calcic sulphate 

 (gypsum) is less soluble in boiling than in cold water, and is quite 

 insoluble in water at I4OC. 



2 See vol. I. pp. 98-104. 



3 Even saturated solutions of certain substances, however, will permit 

 a solution of some other salts without occasioning a precipitation of those 

 originally dissolved. 



