82 SCIENCE PRIMERS. [MATERIAL 



55. Mineral bodies may take on definite 

 shapes and grow, or increase in size, by the 

 addition of like parts. 



Water and all the other natural bodies which have 

 hitherto been mentioned, are what are called mineral 

 bodies, although, in common use, the term mineral 

 is usually restricted to ores and metals. Now we have 

 repeatedly had occasion to remark that, under certain 

 circumstances, not only water, but many other mineral 

 bodies, assume regular shapes. The most familiar 

 example is that of the beautiful imitation of leaves 

 and foliage which is presented by the ice which forms 

 on a window in winter. But we have also seen that 

 common salt, lime, gypsum, Glauber's salts and Epsom 

 salts, also assume the crystalline form as they or their 

 compounds with water, are deposited from their 

 solutions. And if a drop of solution of Glauber's salts 

 or of Saltpetre, is allowed to evaporate under the 

 microscope, a wonderful spectacle will be presented. 

 As the salt assumes the solid state, the crystals 

 suddenly appear in the field of view as needles and 

 plates disposed in beautiful patterns, which rival those 

 of hoar frost, though they are quite different from 

 them. In fact, as you will learn if you study 

 crystallography, every crystallizable substance lias 

 its proper crystalline forms and never departs from 

 certain strictly related geometrical figures. 



A crystal of any of these substances will grow if 

 placed under proper conditions. Thus, if a crystal of 

 common salt is hung by a thread in a saturated 

 solution of salt, which is exposed to the air, so as to 

 allow the water to evaporate slowly, the molecules of 

 the salt which is left behind and can no longer be held 



