240 MINERALOGY 



the temperature at which crystals of every other compound con- 

 tained in the solution will begin to form. 



If two substances as salt, NaCl, and water are taken, and the 

 salt is dissolved in the water, a two-component system will result. 

 From a saturated solution of salt in water, salt crystals separate 

 without water, and water will separate as ice without salt ; these 

 conditions are the simplest possible. Salt crystals and ice are the 

 two solid phases and dissolve in each other to the liquid phases. If 

 one substance is dissolved in another, as salt in water, the temper- 

 ature of fusion is always lowered or the temperature of solidifica- 

 tion of both components is lowered. The change in temperature 

 at which each will solidify caused by the addition of the other is 

 known as the lowering of the freezing point. If water at 60 C. 

 is surrounded by a temperature of 30 and its cooling curve is 

 plotted, Fig. 368, the curve will be regular until C. is reached, 

 when ice crystals will form and the temperature will halt at this 

 point as long as there is any water to be transformed to ice and 

 absorb heat, when the lowering of the temperature will proceed 

 in a regular manner, as before. If to the water, or liquid phase, 

 salt be added to, say, an amount of 15 per cent., then if the system 

 be cooled as before, when C. is reached, the curve will show no 

 indication of a halting point and no ice crystals' will form until the 

 temperature has fallen far below the freezing point of water, as 

 the freezing point of water has been depressed by the addition of 

 salt, and ice will not form until a temperature is reached corre- 

 sponding to the amount of salt added. In the case of a 15 per cent, 

 solution this temperature is 12.2. As ice crystals separate, 

 the composition of the remaining solution is constantly enriched 

 in salt and impoverished in water by the ice formed, which is 

 unmixed with salt. As the percentage .of salt increases, this 

 lowers still further the freezing point of the remaining solution, 

 (indicated by the curve AB), until the temperature is lowered to 

 or reaches 22.4, where there is a halting point, until the liquid 

 phase disappears. At this point both salt and ice are formed and 

 solidify as a whole ; the temperature, owing to the heat of crystal- 

 lization, remains constant until the liquid phases have entirely 

 disappeared, after which the cooling will proceed in a regular man- 

 ner. If the mixture of salt and water which solidifies as a whole 

 when the temperature of 22.4 is reached be analyzed, its com- 

 position will always be found constant, and it is composed of 77 

 per cent, of water and 23 per cent, of salt, and no matter what 



