In the solution of salt we saw a solid become fluid under the influence of water; in the slaking of lime the fluid water enters into the structure of a solid. If more water is added, this solid dissolves or becomes liquid, as the salt did, and the solution is called =limewater=. By carefully managed evaporation of the water the lime may be recovered in the form of crystals, just as the salt was recovered. But there is this difference, that the salt crystals contain no water, while the lime crystals not only contain water, but contain exactly the same proportion as exists in slaked lime, that is to say, 18 parts water to 56 parts lime. 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.