CRYSTALLOGRAPIY. r 
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, 1s crystallization from a state of fusion. When frost or 
snow-flakes form it exemplifies: crystallization from a state of 
vapor. Ifa saturated solution of alum, made with hot water, 
be left to cool, crystals of alum after awhile will appear, and 
will become of dee ge size if there is enough of the solution. <A 
solution of common salt, or of sugar, affords crystals in the 
same way. Again, whenever & aninvese. Fe produced through 
the change or decomposition of another, and at the same time 
assumes the solid state, it takes at once a erystalline 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 i in every grain throughout the bar. 
Thus the various processes of solidification are processes of 
erystallization, 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 
erystalline 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 
erystallization, 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 
and illustrates its mathematical character. 
