80 SCIENCE PRIMERS. [MATERIAL 



If more water is added, this solid dissolves or becomes 

 liquid, as the salt did, and the solution is called lime- 

 water. By carefully-managed evaporation of the 

 water the lime may be recovered in the form of crys- 

 tals, just as the salt was recovered. But there is 

 this difference, that the salt crystals contain no wa- 

 ter, 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 t\vo. 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 com- 

 bined in definite proportions ; and the slaked lime 

 receives the special name of hydrate of lime. 



Gypsum or Plas'.er 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 mois- 

 ture 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 for 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 in- 

 equalities of its surface ; and, when it sets, it retains the 

 shape which it has thus acquired. S^t plaster of Paris 

 may be perfectly dry; but it nevertheless contains 



