Salts. 135 



converted into vapour, the crystal will burst 

 with a kind of explosion, as we see on throwing 

 common salt into the fire ; and this is called the 

 decrepitation of salts. 



The first theory of the crystallization of salts 

 was, that the form of crystals was the form of 

 the ultimate atoms of salts, and that by a regu- 

 lar combination of these atoms the large masses 

 were produced. There are many difficulties in 

 supporting this supposition ; the form of the ul- 

 timate atoms being the same as that of the crystals 

 may be understood of some salts, but of many 

 crystals it cannot. Thus the crystals of salt- 

 petre are prisms of six sides, and then its ulti- 

 mate atoms must have only three sides. The 

 particles of many of the salts are not in contact 

 with one another, as the interposition of the 

 water they contain must effectually prevent the 

 union of these particles ; the shape of the cry- 

 stals cannot then depend on the particles of salt 

 touching one another, as in sulphat of soda, 

 alum, sulphat of iron, &c. But, in fact, cry- 

 stallization is not peculiar to salt, for there are 

 no bodies which do not show some tendency to 

 it in their passage from a fluid to a solid state, 

 as when they have been dissolved or melted. 

 Thus, if melted metallic substances cool slowly, 

 they always show a tendency to crystallization. 

 Some have regular figures, <md appear like knots, 

 trees, <kc. Many metals have oblong masses that 

 resemble those of salt, but are more compact. 



