The water thus bound up with the lime into a new solid holds on so firmly to the lime that it requires a red heat to separate the two. The lime and the water are said to be chemically combined; and as the proportion of lime and water in slaked lime, or lime crystals, is always the same, they are said to be combined in definite proportions; and the slaked lime receives the special name of hydrate of lime.
Gypsum or Plaster of Paris is a dry white powder. If mixed with a little water it does not slake after the fashion of quick lime, but the mixture soon sets or becomes hard; and, at the same time, the greater part of the water disappears. In fact, it has combined with the plaster of Paris and forms part of another hydrate, in which, when the superfluous moisture dries, not a trace of water is to be seen. It is this property which is taken advantage of when plaster of Paris is used in making casts and moulds. The fluid plaster is poured over and round the body to be cast; as a fluid, it applies itself conveniently to all the inequalities of its surface; and, when it sets, it retains the shape which it has thus acquired. Set plaster of Paris may be perfectly dry; but it nevertheless contains between ⅐ and ⅛ of its weight of water, fixed and forming an integral part of the solid hydrate. And if the set plaster is strongly heated, the combined water is driven off and it returns to its original state.
Gypsum is found abundantly in nature, in the shape of beautiful transparent crystals which are called selenite. These crystals have the same composition as set plaster, that is to say, they are hydrates. A thin flake of such a crystal viewed with the highest powers of the microscope appears perfectly homogeneous. Nevertheless, there is good reason for the conclusion that it consists of molecules of water and molecules of gypsum which hold together so strongly that they form a hard brittle glassy solid. Moreover, the molecules of the hydrate itself hold together more strongly in some directions than in others. It is very easy to split the crystals lengthwise; while much more force is needed to cut them crosswise and then they do not split, but break.
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.