OBJECTS.] INTRODUCTORY. 81 



between 4- and \ of its weight of water, fixed and form- 

 ing 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 homo- 

 geneous. 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 pro- 

 portion 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 dif- 

 ferent 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 con- 

 stituents. 



