174) Mohs’ System of Crystallography 
changing the mutual ratios which they bear to each other. The 
groupe that arises from a fundamental form of determinate dimen- 
sions is named a series (f crystallisations^ or a special series, be- 
cause, as we shall soon see, crystallisations of one and the same 
species are and must be members in some such series. 
60. Examples. — If, in the rhomboidal system, r is put = 
105° 5', the special series of calcareous spar results ; if r is put 
= 133° 26', the special series of tourmaline results; r = 87° 
gives the special series of iron-glance, &c. In the pyramidal 
system, ^=124° 12' determines the special series zircon \ 
p = 129° 30', the special series of vesuvian, &c. The series of 
crystallisations is, hence, that which determines the system of 
crystallisations for a particular case ; the system of crystallisa- 
tions, on the other hand, is the general expression for all pos- 
sible series of crystallisations, which depend upon that system. 
As the fundamental form in the tessular system undergoes no 
alteration in any of its dimensions, only one series of crystallisa- 
tions can have place in that system. 
61. Application. — According to what has already been ad- 
vanced, when a newly discovered species of the mineral king- 
dom is presented to us, we are enabled to fix its system of cry- 
stallisations ; and we have already obtained a general represen- 
tation of all the forms, with their mutual proportions, which can 
possibly belong to that species ; but we are not in a condition to 
determine any particular form, if it is not a limiting form, any 
other way than by its dimensions ; though, when one of these 
forms is determined, the dimensions of all the rest may be com- 
puted from it with the utmost accuracy. The determination of 
any form, from a special series in the first three systems of cry- 
stallisations, must be managed by an immediate and accurate 
measurement. Hence few of them are yet sufficiently deter- 
mined. 
62. Cleamge shews us the System and the Series of Crystalli- 
sations. — The different series of crystallisations are of the high- 
est importance for properly determining the species of minerals, 
the systems of crystallisations for determining their characteristics. 
And as all minerals do not present themselves in a crystallised 
state, the cleavage comes forward to compensate for the absence 
of crystallisation, because any form of cleavage, not a limiting 
