Glauber’s salts and Epsom salts are other examples of solids which dissolve in water and separate in the crystalline form as the water evaporates; and which, like lime and gypsum, combine with a definite proportion of water to form crystalline compounds. In fact, each of these glassy brittle solids contains more than half its weight of water. Thus we see that two bodies, of which water is one, may combine together to give rise to something different from either. And we are thus led to the science of =chemistry=, which tells us exactly how bodies combine, what comes of their combination, and how compounds may be separated into their constituents. 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 has 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 in solution, deposit themselves on the crystal in regular order and increase its size without changing its form. And, in this way, the small crystal may =grow= to a great size. The large crystals of sugar candy, which consist of sugar and water deposited from a strong syrup or saturated solution of sugar, grow in the same fashion, upon threads suspended in the evaporating syrup. In this mode of growth you will observe that the enlargement is effected by addition to the outside of the growing body; and moreover the matter which is added, namely, the salt or the sugar, already exists as salt in the brine or as sugar in the syrup.