Tin; omuiN OK MIM.I; \i .- 239 



is orthorhotnbic at ordinary temperatures, but at 500 C. it passes 

 over to an isometric phase. In like manner quartz, when heated 

 to a temperature of 575, /3-quartz is formed, and crystals in passing 

 t his inversion temperature are shattered ; if the temperature is still 

 raised, at 800 C. another inversion point or temperature is 

 reached, when a third phase, or tridymite, is the stable form. 



Different phases of the same substance can exist in equilibrium 

 with each other only when the temperature and pressure are fixed. 



Ice and water are in equilibrium and can exist in contact with 

 each other at C. and under one atmosphere of pressure. When 

 two phases of the same substance are in contact, for each pressure 

 there is a fixed definite temperature of equilibrium, and for every 

 fixed temperature there is also a fixed pressure of equilibrium. 



If ice is heated and the heat is applied at a constant rate, the tem- 

 perature of the ice will rise regularly ; as for every gram of ice a con- 

 st ant amount of heat is required to raise the temperature one de- 

 gree, this quantity of heat is designated the specific heat of the 

 substance. If the temperature is taken every ten seconds and the 

 curve is plotted with temperature and time as ordinates and ab- 

 scissa, the curve will be found to be continuous and regular until 

 C. is reached, when there is an abrupt change in the behavior 

 of the system, as water, or. the liquid phase, begins to form. The 

 rise of temperature is stopped and remains the same, just as long 

 as there is any ice or solid phase present. At this point, Fig. 368, 

 the heating curve runs along horizontally and there is no rise in 

 temperature, since the melting ice absorbs heat in passing in to 

 the liquid phase. This absorbed heat is the heat of fusion; and 

 when water is transformed to the solid phase, ice, the same amount 

 of heat is liberated, and the cooling curve will show the same hori- 

 zontal portion at C. ; such an interruption in the regular trend of 

 the cooling or heating curve is a halting point. Such halting points 

 mark the transition temperatures between two phases of a sub- 

 st ance and are caused by the absorption or liberation of heat. The 

 heat liberated by a solid crystallizing is often termed the heat of 

 crystallization; this is a constant source of heat, available, in 

 magmas that are crystallizing, to counteract radiation and serves 

 to prolong the liquid state and to modify the rate of cooling. In 

 such fluid and crystallizing magmas each substance which crys- 

 tallizes and separates from the solution will have a direct influence 

 upon the temperature at which other substances in turn crystallize ; 

 and each compound dissolved in the magma will also influence 



