238 MINERALOGY 



as simple as possible and with one which is familiar to all, as the 

 solution of salt in water a two-component system easily tried and 

 from the consideration of which the application of the terms and 

 diagrams may be appreciated. 



If any crystalline substance is heated and the application of heat 

 is constant and continuous, the temperature of the crystalline solid 

 increases constantly ; in time a temperature will be reached at which 

 the solid passes to the liquid state or the solid is said to fuse or melt. 

 The molecular network or point-system of the crystalline state is 

 broken down. If the temperature of the system when the two 

 phases, solid and liquid, are in contact is measured, it will be found 

 that this temperature is a constant one, providing that the com- 

 pound is chemically pure and the pressure is the same. Thus ice 

 when heated to C. melts and forms water ; this temperature of 

 fusion is the melting point. Water is an exception to the general 

 rule, and the melting point of water is lowered with an increase of 

 pressure, the reverse of the conditions in most other substances. The 

 transition of a solid crystalline substance to a liquid is an abrupt one, 

 and there is no constant decrease of viscosity, with the rise of 

 temperature, to be noted in case of the crystalline substance as is 

 the case with the amorphous solid. In the amorphous solid there 

 is no constant sharply defined temperature of fusion, no abrupt 

 change from solid to liquid. The viscosity simply decreases with 

 the rise of temperature. The line of separation between liquids and 

 solids, when considered from this standpoint, should be drawn 

 between the cyrstalline substances and amorphous substances 

 rather than between solid amorphous substances and the ordinary 

 liquids. 



The difference between amorphous solids, which are excessively 

 viscous liquids, and the ordinary liquid is very little ; and the amor- 

 phous solid is more closely related to liquids than to crystalline 

 solids. 



Water and ice are two modifications of the same chemical 

 substance, between which there is an abrupt change or transition 

 from one to the other. Each of these modifications is known as a 

 phase. Water can exist in three phases: solid, ice; liquid, water; 

 and vapor or gas, steam, depending upon the temperature and pres- 

 sure. Sulphur may exist in two solid crystalline phases, mono- 

 clinic and orthorhombic sulphur; the transition temperature 

 between these two phases at ordinary pressure is 95.6. Many 

 solid compounds as minerals have different solid phases. Leucite 



