56 THE BEGINNINGS OF LIFE. 



If, instead of pouring a certain amount of a solution of 

 potassic sulphate into one of baric chloride, we allow 

 the mixture to take place gradually by means of 

 dialysis, then crystals of calcic sulphate are formed 

 rather than an amorphous precipitate. It has been 

 ascertained by M. Fremy^ that insoluble compounds 

 generally, which appear in the laboratory as a result 

 of double decomposition in the form of amorphous 

 precipitates, can almost invariably be obtained in a 

 crystalline condition when the chemical reaction is 

 allowed to take place very slowly. This may be 

 brought about by making the saline solutions mix after 

 osmosis — either through membranes, wooden vessels, 

 or porous porcelain. By one or other of these 

 methods, he obtained many very insoluble salts in the 

 crystalline condition — such as the sulphates of baryta, 

 strontia, and lead, the carbonates of baryta and lead, 

 oxalate of lime, chromate of baryta, and several sul- 

 phides 2. These facts are highly interesting because a 



^ ' Compt. Rend.' t. Ixiii. p. 714. 



^ We find in Watts's Dictionary the following statement, which has an 

 explanatory bearing upon what has been above stated : — ' The more slowly 

 the liquified body is brought back to the solid state, and the more the 

 liquid is kept at rest, the smaller the number and the greater the size 

 and regularity of the crystals ; but if the solvent be cooled or separated 

 quickly the crystals are numerous, but small and ill-defined. In the 

 former case, the particles of the solidifying body have time to unite 

 themselves regularly with those which separate first from the fluid and 

 form nuclei of crystallization ; if, on the contrary, the crystalHzation 

 takes place rapidly, a great number of particles solidify at the same time, 

 each forming a nucleus to which other portions attach themselves, and 



