SOLUTIONS 131 



the solvent alone disappears, and the solute remains behind. A 

 few solutions may be cooled below the saturation point, with- 

 out causing them to drop any of the solute. Such solutions are 

 said to be super-saturated. When a bit of the solute is dropped 

 into such a solution, however, the solution at once throws 

 down the extra solute and returns to the saturated condition 

 again. 



111. Crystallization. When a saturated solution is allowed 

 to cool, or the amount of the solvent is reduced by evapora- 

 tion or otherwise, particles of the solute begin to appear, usu- 

 ally in definite and characteristic forms known as crystals. A 

 crystal, once formed, may go on increasing in size by the addi- 

 tion of more material, but it always maintains its character- 

 istic form. Snowflakes are crystals of water, and sugar and 

 salt are other forms of crystals. In general, the slower the 

 process of crystallization goes on, the larger the crystals are 

 likely to be. When more than one solute is found in a solvent, 

 each tends to crystallize out by itself. 



112. Mineral Waters. Water, as it falls in rain, is nearly 

 pure, but when it sinks into the soil, it begins to take up the 

 soluble materials it encounters so that when it appears again 

 in the form of springs, wells, and the like, it usually holds con- 

 siderable matter in solution. In many parts of the earth are 

 extensive beds of salt, iron, gypsum and other minerals that 

 have been carried in watery solutions to the places in which 

 they are found, and left as the water evaporated. When 

 water carries enough mineral matter in solution to give it an 

 appreciable flavor, it is called a mineral water. The common- 

 est substances in mineral waters are salt, soda, iron, sulphur, 

 and lime. Waters containing considerable potash or soda are 

 called alkali waters. In arid regions, the water, after pene- 

 trating the soil and dissolving out some of the soluble alkalis, 

 may rise to the surface by capillarity and evaporate, leaving the 

 alkali behind and thus rendering the soil unfit for cultivation. 



