ixxxvi_ Historical Sketch of the Physical Sciences, 1818. 
centrated, the crystals are always of a simpler form and more 
regular than when they are crystallized in a pure liquid. When 
the solution is very concentrated, isolated crystals are formed 
in it, whose faces are crossed hke the hopper of a mill. 
9. The crystallization of a salt may take place in the midst 
of a gelatinous mass without the necessity of any supernatant 
liquid. In that case the crystals contain none of the foreign 
matter, and undergo no change of form; but they are almost 
always isolated and remarkably regular and complete in all their 
arts. 
10. When several salts are in solution in the same liquid, it 
would appear that they are capable of mutually affecting one 
another’s crystallization, even when they are not susceptible of 
uniting or of acting chemically upon each other. Thus common 
salt takes the form of a cubo-octahedron when it crystallizes 
in the midst of a solution of borax, or still better of boracice 
acid. 
11. The forms which the same salt is capable of assuming, 
vary according to the nature of the liquid from which it is pre-_ 
cipitated. Thus alum assumes the cubo-octahedral form when 
it crystallizes in nitric acid, and the cubo-icosahedral form when 
it crystallizes in muriatic acid. 
12. Whenever several salts are capable of mixing chemically, 
that is to say, of uniting without entering into a definite com- 
bination, that salt, whose system of crystallization predominates, 
always assumes particular forms which differ from those which 
it adopts when it is pure. The different salts present likewise, 
in general, different forms in the same system of crystallization, 
according as they contain more or less of acid ; and the double 
salts according as one or other of the component salts exist in 
more or less quantity. 
13. The chemical action which tends to determine a particu. . 
lar form, by altering the composition of a salt, produces different 
effects according to its energy, and often gives occasion at once 
to several varieties of crystals. Thus the action of an insoluble 
carbonate upon alum determines in the same solution octahe- 
dral crystals, cubo-octahedral crystals, cubic crystals, and an 
incrystallizable matter which contains still less acid than the 
preceding. 
14, When simple crystals of different forms belonging to the . 
same salt are dissolved together in the same liquid, two different 
things may happen. If the crystallization takes place slowly, 
the crystals are deposited in succession and separately ; but if 
the crystallization be rapid, a single mixed compound is formed, 
exhibiting crystals partaking at once of all the different simple 
forms. Thus octahedral and cubic crystals of alum may unite . 
and constitute cubo-octahedral crystals. 
15. Crystals of complex form may be sometimes decomposed 
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