MINEUAI.S. 337 



Observation has shown that every substance in crystallizing lias a 

 tfudency to assume a jicculiar figure. Coinwon salt crystallizes in cubes, 

 Kpsiim suits in six-sided prisms, Alum in octahedrons, Suiar-cunclj/ in 

 obliijue four-sided pri'-nis with wedge-shaped summits. IJut the crj-- 

 slalliuc form in any crystallizable material is liable to be altered by 

 circumstanecs affecting the crystallizing process ; and hence the geome- 

 trical forms which the same identical substances present, often bear no 

 such resemblance to each other as would seem to indicate their rela- 

 tion. There are, nevertheless, a eertixin number of figures pecidiar to 

 every crystallizable body, and the crystals of that substance assume 

 one or otlier ol' these forms, and no other. Common suit, for example, 

 when it has assumed its true crystalline shape, presents itself in the 

 form of cubes ; it is also met with in octahedrons, dodecahedrons, or 

 some ligiu'e appertaining to these solids. ii'w^'ar-f<77H(>/ usually crystal- 

 lizes in oblique tour-sided prisms, and it likewise occurs in cubes and in 

 six-sided prisms with wedge-shajied svuinn its variously moditit^d. Alum 

 crystallizes in octahedrons, but it also occurs in cubes. 



Method ofobtuiuaig Cr^stuls. — 'i'hc method of effecting the crj-sta!- 

 Hzation of such bodies as require a previous state of solution, and 

 among which the class of Salts holds a distinguished rank, consists of 

 heating the solution so as to dissipate gradually part of the water by 

 evaporation. It is thus that chemists proceed tor obtaining crystals of 

 sulphate of potash, muriate of potash, &c. 



The figure of crystals has very little regularity if the water be eva- 

 porated too hastily, as by boiling; but by keeping the saline solution 

 in a gentle heat, very beautifiil and very regular crystals are obtained 

 in a longer or shorter space of time; and there is scarcely any salt 

 which may not be made to assiune avcry distinct form by this process 

 if it be skilfully conducted. — Acciim. 



Cri/stak of Cuiiiplior. — Camphor dissolves readily in spirits of wine. 

 To obtain the crystals it is only necessary to place one drop on a piece 

 of glass ; the glass should be held over a candle a few seconds to ac- 

 celerate the evaporation of the spirit, and then placed in the ■micvu- 

 scope, when the configuration may be seen. 



Crifstak of Siltvr. — This forms a very beautiful and interesting 

 object. In one drop of nitrate of silver put a small piece of very 

 tine brass wire; this must be immediately placed in the microscope, 

 and the crystals will extend gradually till the whole quantity of fluid is 

 evaporated. 



Minerals of all kinds frequently exhibit very curious objects. Sand 

 also should be collected and examined, as it is subject to great variety : 

 r— in fact, a very good Isnowledge might be gained of INIincralogy from 

 small specimens, which maybe obtained at very rcasoaablc prices, and 

 •which occupy but little room. 



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