and Mineralogy, 175 
form, represents some member in the series of crystallizations he^ 
longing to that species, A limiting form, however, determines 
the system of crystallisations, provided it be not a straight four- 
sided rectangular prism, in which case the choice, lying between 
the pyramidal, prismatic and tessular systems, must be deter- 
mined by the circumstances. 
6S, Determination of the ambiguous case, — ^l^^hen any cleavage 
form is a straight rectangular four-sided prism, though from any 
other quarter the smallest hint were not given us, by which the 
particular system of crystallisations it belongs to could be deter- 
mined, the following simple considerations will be sufficient to fix 
that point. A straight four-sided prism presented to us, may 
be 1. The hexahedron (55.), 2. The limits of the pyramidal sys- 
tem (44), 3. A combination of the limiting forms of the prisma- 
tic system (52.) If the faces of the cleavage, in regard to their 
completeness and lustre, are each of a different nature, the form 
comes under the first case. If two faces of the cleavage are of the 
same, the third of a different nature, then the form must come un- 
der the third case. Examine the cleavage in the direction of the dia- 
gonal. If this, once more, affords a rectangular four-sided prism, 
it indicates the pyramidal system ; if it affords an oblique an- 
gular four-sided prism, it indicates the prismatic system. When 
all the cleavage faces are of the same nature, the form may come 
under any of the whole three cases. Between the second and 
the third a decision must be made as above. If the additional 
cleavage produces a rectangular four-sided prism,''seek to obtain 
the same figure in the direction of another axis. If we obtain 
it, the form is a hexahedron ; if we cannot obtain it, the form is 
the limit of the pyramidal system. 
64. More extensive application by Practice, — It is well known 
how far our , judgments of cleavage and cleavage-forms, are ren- 
dered easy and certain by a little practice. To such a degree is 
this the case, that an observer can often, with great facility, de- 
termine the form in minerals, which, but for this simple artifice, 
would scarcely have exhibited to us any traces of regular confor- 
mation. All these and similar advantages assist us in applying 
the system of crystallisations to the purpose of determining mi- 
nerals ; hence they extend its empire to such a degree, that, as 
experience shews, little can escape it. But, notwithstanding 
