CRYSTALLOGRAPHY. 7 



state of fusion, or of gas or vapor, or of solution. In the case 

 of iron the crystallization takes place from, a state of fusion, and 

 while the result is ordinarily only a mass of crystalline grains, 

 distinct crystals are sometimes formed in any cavities. If in 

 the cooling of a crucible of melted lead, bismuth, or sulphur, the 

 crust be broken soon after it forms, and the liquid part within be 

 turned out, crystals will be found covering the interior. Here, 

 also, is crystallization from a state of fusion. When frost or 

 snow-flakes form it exemplifies crystallization from a state of 

 vapor. If a saturated solution of alum, made with hot water, 

 be left to cool, crystals of alum after awhile will appear, and 

 will become of large size if there is enough of the solution. A 

 solution of common salt, or of sugar, affords crystals in the 

 same way. Again, whenever a mineral is produced through 

 the change or decomposition of another, and at the same time 

 assumes the solid state, it takes at once a crystalline structure, 

 if it does not also develop crystals. 



Further, the crystalline texture of a solid mass may often be 

 changed without fusion : e. g., in tempering steel the bar is 

 changed from coarse-grained steel to fine-grained by heating 

 and then cooling it suddenly in cold water, and vice versa, and 

 this is a change in every grain throughout the bar. 



Thus the various processes of solidification are processes of 

 crystallization, and the most universal of all facts about miner- 

 als is that they are crystalline in texture. A few exceptions 

 have been alluded to, and one example of these is the mineral 

 opal, in which even the microscope detects no evidence of a 

 crystalline condition, except sometimes in minute portions sup- 

 posed not to be opal. But if we exclude coals and resins this 

 mineral stands almost alone. Such facts, therefore, do not 

 affect the conclusion that a knowledge of crystallography is of 

 the highest importance to the mineralogist. It is important 

 because — 



1. A study of the crystalline forms and structure of minerals 

 is a convenient means of distinguishing species — the crystals 

 of a species being essentially constant in structure and in 

 angles. 



2. The most important optical characters depend on the 

 crystallization, and have to be learned from crystals. 



3. The profoundest chemical relations of minerals are often 

 exhibited in the relations of their crystalline forms. 



4. Crystallization opens to us nature at her foundation work 

 md illustrates its mathematical character. 



