CRYSTALLOGRAPHY 



li-rovery, crystals were always considered to he necessarily 



solid-. 



It it is wished to crystalli/e any substance, and thus (litain 

 crystals of any conipound for study, it will lie n<T<--:trv to tiring 

 the substance into one of those conditions of mutter in which the 

 molecules are free to move, and then to reverse the process under 

 ;uch conditions that the transition to the solid state will take place 



;ery slowly. Each molecule will be fixed upon the network with 

 all equivalent lines of force parallel, providing always that the 

 instance is capable of forming crystals. Several cases may ari.-e : 



L. The substance may be a gas; all gases, with the exception of 



icliuin, have been solidified by decreasing the temperature and 

 increasing the pressure. Carbon dioxide, a gas at ordinary tem- 



jeratures, becomes a liquid at*^8.2 C.. Liquid carbon dioxide is 

 >ld in the market in iron tubes. At ordinary temperatures these 

 t ulies are subjected to a pressure of 60 atmospheres. If a small 

 jet of the carbon dioxide be allowed to escape in a beaker, by the 

 sudden expansion and vaporization a large amount of heat is ab- 

 sorbed and the temperature of the remainder caught in the beaker 

 falls below the freezing point, and snowlike crystals of solid carbon 

 dioxide are formed. 2. The substance to be crystallized may be a 

 liquid ; leaving out of consideration supercooiing, if the tempera- 

 ture of a liquid is decreased, at a definite temperature, the freez- 

 ing point, crystalline nuclei will appear. From these as centers 

 crystallization will take place until all the liquid has become 

 solid. These centers of crystallization may be seen on the surface 

 of any pool of water just as ice begins to form. 3. The substance 

 to be crystallized is a solid ; substances in this class will fall 

 under three divisions: A. Solids which when heated volatilize 

 without fusion. If the metal arsenic is heated, it volatilizes 

 without fusion; on resolidification, out of contact with oxygen of 

 the air, the metal will be crystalline. Solids formed in this way 

 are known as sublimates. B. When heated, the substance fuses 

 without chemical change. Such metals as silver, copper, lead, 

 in fact most of the elements, may be cryst.dli/ed in this way. 

 The crystals of the igneous rocks, as the feldspars, olivine, 

 a-ugite, etc., have been formed from a fusion ; only here there 

 has been a segregation, or a separation of the various kinds 

 of molecules at the same time. C. The substance is either 

 infusible, or is decomposed when heated. When a solid is 

 dissolved in a liquid, its molecules pass off from the surface and 



