Mineral Syiil/iesis. 165 



tion of a substance already formed, but existing in an amor- 

 phous state. 2. The formation of a mineral substance and 

 crystallization at the same time. 



Under the first group, crystals are formed ; by passage of 

 gaseous condition to solid by sublimation ; by a change from 

 fluid condition to solid, and this either from solution ox fusion ; 

 by a change from amorphous body to crystalline by the pro- 

 cess known as devitrification. 



Under the division of sublimation would be included those 

 processes by which a body, in a gaseous state, is changed by 

 rapid cooling to a solid. This is a process easily used in the 

 laboratory, as is seen in heating amorphous arsenic in a glass 

 tube when crystals are formed on the colder portions. 



It is found in many cases that the intervention of certain 

 gases, as hydrogen, nitrogen, fluorine, water, greatly facili- 

 tates the crystallization. These gases are called by the 

 French, '' miner alizersT When amorphous sulphate of cad- 

 mium is heated in a current of hydrogen, greenockite crystals 

 were formed. The action is often obscure and not easily 

 understood, as when crystalline zinc is formed by passing a 

 current of nitrogen over the amorphous sulphate. 



Deville converted amorphous tin oxide into beautiful 

 cassiterite crystals by aid of hydrochloric acid. Freniy, by 

 aid of fluorine, formed sapphires and rubies weighing .075 

 grams, from the amorphous alumina. 



Boric acid was so successfully used by Ebelmen, in the 

 formation of the spinels, that he concluded it must be uni- 

 versally present through the earth's crust. » 



Another method mentioned above was the formation out of 

 solution. This may take place by evaporation, as in all the 

 easily soluble bodies, at ordinary temperatures, seen best in 

 rock salt, sylvine, alum. Or it may take place through con- 

 centration, by lowering the temperature, best illustrated by 

 the precipitation of gypsum from sea water. Finally, it may 

 take place by a variation or change in the character of the 

 solution, as the removal of gases Thus, if an aqueous solu- 

 tion of carbonate of lime loses its carbonic acid, calcite is 

 precipitated. 



A third category named above was the formation of 

 minerals from the fusion of amorphous substances. Many 



