132 Chemistry. [Lecture 29, 



salt), in a cubical form : nitrat of soda (cubic 

 nitre), in a rhomboidal : sulphat of soda (Glau- 

 ber's salt), in large four sided prisms : so that, 

 from the appearance, a person accustomed to see 

 crystals can distinguish to what salt they be- 

 long. The common rule of crystallizing salts 

 is to evaporate the fluid very slowly till a pellicle 

 appears at top, and then set it by to crystallize. 

 If we proceed till the salt begins to congeal, when 

 warm the crystallization will be incomplete and 

 irregular. It will be proper to filtrate the 

 liquor, to free it from impurities, before we set 

 it aside to crystallize. If the whole of the salt 

 is not thrown down in the first operation, it 

 must be repeated; The crystals are always 

 better when large, than when small quantities 

 are made. A circumstance, however, often at- 

 tends this which is very troublesome, viz. what is 

 called the vegetation of salts. If we set a glass 

 vessel containing a saline solution in a cool, still 

 place, to crystallize by slow evaporation, as soon 

 as it begins to shoot it will be attracted by the 

 sides of the vessels, and be protruded upwards in 

 the form of a hollow cylinder. As soon as it 

 reaches the top, it will bend down and creep 

 along the outside of the glass, till it becomes a 

 sort of siphon, by which the solution, as the salt 

 is spongy, is drawn over the vessel ; so that, un- 

 less we carefully watch it, we shall lose all except 

 what was crystallized. 



This phenomenon depends on capillary at- 



