8 MINERALOGY 



move as free as those of a gas. Just as water evaporates in the 

 air, the solid may be said to evaporate in the liquid ; this continues 

 until the liquid is no longer able to hold more of the solid, and 

 equilibrium between the liquid, solid, and the dissolved substance 

 is established, when the solution is said to be saturated. This 

 condition will remain as long as the temperature, pressure, and 

 solubility remain constantV Crystals of copper sulphate may be 

 obtained from a saturated solution by cooling the solution. Salts 

 with few exceptions are more soluble the higher the temperature. 

 Again, crystals may be obtained from a saturated solution by de- 

 creasing the amount of the solvent ; let the solution slowly evapo- 

 rate, both processes will be combined, as the slow evaporation will 

 cool the solution; counteracting this 'decrease of temperature is 

 the heat of crystallization, for wheitf crystals are forming there heat 

 is being liberated. Perfect crystals may be secured by suspending 

 a small crystal on a thread in the slowly evaporating saturated 

 solution, at the same time guarding against any sudden change in 

 temperature. It is also well to mechanically revolve the growing 

 crystal to insure its being surrounded by solution of the same con- 

 centration, when the deposition will be uniform. 



Crystals may also be formed from solution by a decrease of the 

 solubility, produced, as in precipitation, by the addition of some 

 reagent in which the dissolved salt is less soluble, or as in the salting 

 out process by the addition of a common ion. All sulphates are 

 insoluble in alcohol ; if alcohol is poured carefully over the surface 

 of the copper sulphate solution so as to lie as a layer covering the 

 surface, it will mix slowly with the solution and the solubility of 

 the sulphate will be decreased gradually, producing perfect little 

 crystals of copper sulphate. 



A large number of chemical compounds, especially the more 

 insoluble salts, may be prepared in crystalline form by chemical 

 precipitation. If to a neutral solution of calcium chloride a solu- 

 tion of sodium carbonate is added, a white, flocculent, amorphous 

 precipitate of calcium carbonate is produced which on standing 

 becomes crystalline. In the first rapid separation the more un- 

 stable amorphous solid is formed, which becomes crystalline, not by 

 the rearrangement of the molecules in the solid, but by a slow trans- 

 fer of molecules from the unstable amorphous solid to the crystal- 

 line nuclei by resolution, the crystalline form being the more stable. 



The methods mentioned are the more important; there are 

 modifications and combinations of these which are applicable to con- 



