ON THE FORMATION AND GROWTH OF CRYSTALS. 
349 
by a brief statement of the modern mode of classifying these 
bodies. 
Crystalline forms are collected into six groups or systems. 
These are comprehended by two classes. To the first belong 
all those whose axes are of equal length, and at right angles to 
each other ; to the second, those, the axes of which are of unequal 
length, and are not always at right angles to each other. 
Class 1. The Equiaxed — Contains one system only, the Cu- 
bical or Regular, and to which a very large proportion of chemi- 
cal and pharmaceutical salts belong. Common salt, alum, and 
iodide of potassium may be mentioned as examples. There are 
those who maintain that all the metals partake of the forms of 
this system. 
Class 2. The Unequiaxed. — Here are included the remaining 
five groups, which are not improperly subdivided into a the dime- 
trie, and & the trimetric. \ 
To the dimetric belong the second system, or the square pris- 
matic, which calomel, percyanide of mercury, and ferrocyanide of 
potassium serve to illustrate ; and the third or rhombohedric, of 
which we have an example is calcareous spar. 
The trimetric contains the fourth or right prismatic, of which 
sulphate of magnesia, sulphate of zinc, and citric acid are modified 
examples. The oblique prismatic well shown in sulphate of soda, 
binoxalate of potash, and tartaric acid ; and, lastly, the doubly 
oblique prismatic, which will be easily remembered from its con- 
taining, among others, sulphate of copper and quadroxalate of 
potash. 
The first point of importance in the production of well-formed 
crystals, is to ascertain the most suitable degree of saturation of 
the solution. 
The proportions usually quoted in books, although frequently 
approximative, are not generally the best. Moreover these pro- 
portions must of necessity vary with the temperature. As a gen- 
eral rule, a completely saturated solution is not the most elegible, 
for, upon the slighest reduction of temrpeature, a considerable 
quantity of small crystals are deposited, while from a solution 
somewhat more dilute, fewer, larger, isolated, and more perfectly- 
formed crystals are generally obtained. A little practice will 
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