MINERALS. 337 



Observation has shown that every substance in crytallizing has a 

 tendency to assume a peculiar figtire. Common suit cr\slallizes incubts, 

 Epsom sulta in six-sided prisms, xUum in octahedrons, Siigar-candjf in 

 obhipie fonr-sided prisms with wedge-shaped summits. But the cry- 

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

 circumstances 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 certain number of figures peculiar to 

 every crystallizable body, and the crystals of that substance assimie 

 one or other of these forms, and no other. Common salt, for example, 

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

 form of cu!)es ; it is also met with in octahedrons, dodecahedrons, or 

 some figure ajiperlaining tu these solids. Sugar-eantlii usually crystal- 

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

 si-x-sided prisms with wedge-shaped summits variously modiiied. Alton 

 crystallizes in octahedrons, Init it also occurs in cubes. 



Method ofobtainbtg Crystals. — The method of effecting the crystal- 

 lization of such bodies as re(|uire a previo\is 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 for obtaining crystals of 

 sulphate of potash, muriate of potash, &c. 



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

 porated Too hastily, as by boiling; b\it b.y keeping the saline solution 

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

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

 which niay not be made to assume a very distinct form by this process 

 if it be skilfully conducted. — Accnm. 



Cnist'ils of Camphor. — Camphor dissolves readily in sjiiritsof wine. 

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

 ol" 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 micro- 

 scope, when the configuration may be seen. _ ^ 

 Crystals of Silver. — This forms a very beautiful and interesting 

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

 fiiie iM-ass 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 ol jects. Sand 

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

 — -m fact, a very good knowledge might be gained of Mineralogy trom 

 small specimens which maybe obtained at very reasonable prices, and 

 which occupy but little room. 



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