242 MINERALOGY 



above both curves, is the liquid field, in which all possible mix- 

 tures of water and salt may exist as homogeneous solutions and 

 be in equilibrium at the corresponding temperatures. This field 

 is bounded by the two curves AB and BC. The curve BC is the 

 boundary between homogeneous solution and field III, in which 

 ice crystals and salt solution are in equilibrium. If any point in 

 field I is selected, as p corresponding to a temperature of 20 and 10 

 per cent. NaCl, at 20 this solution is homogeneous, but if cooled, 

 the 10 per cent, solution will meet the curve AB at a point correspond- 

 ing to 6.7, and at that temperature ice crystals will begin to sepa- 

 rate and the conditions of field III will be reached, in which ice crys- 

 tals are in equilibrium with salt solution. If a point on the opposite 

 side of the diagram, as P, be taken, on cooling, the perpendicular 

 from P cuts the curve CB at 5, at which temperature the solution 

 will be saturated in regard to the salt, and salt crystals will form, 

 when the solution or conditions will be those of field IV, in which 

 salt crystals and salt solution are in equilibrium. As the tempera- 

 ture falls, the solution becomes more concentrated in respect to the 

 water content and the temperature of separation follows the curve 

 BC until B is reached, when both salt and ice separate in eutectic 

 proportions of 77 parts water and 23 parts salt, at a temperature 

 of 22.4. Below the line DE or below the point B or a tem- 

 perature of 22.4, liquid solution cannot exist, and field II is 

 the crystalline or eutectic field, separated from the other three, 

 fields in which liquid solutions are possible by the straight line DE, 

 known as the eutectic horizontal. It often happens, and it is 

 indeed the rule, that the temperature will fall below that indicated 

 by the curve of separation without crystals being formed, unless care 

 is taken to prevent it ; such a solution is said to be supercooled. If 

 the 10 per cent, salt solution should fall below 6.4 without the 

 separation of ice, it would be supercooled and in a metastable 

 condition, the solution being supersaturated as regards water; 

 such a condition could not exist if the least particle of ice were pres- 

 ent. On dropping a particle of ice in the supercooled solution 

 there is an immediate separation of ice, a rise of temperature 

 from the heat of crystallization, as well as an increase of the con- 

 centration of salt in solution, and the whole system comes to a 

 state of equilibrium on the curve of separation. A good example 

 to illustrate supersaturation is a solution of sodium sulphate, 

 NaaSO^ 10 H 2 O, dissolved in water and heated in contact with the 

 salt, or saturated at a temperature a little below 32 C. ; if it is 



