CRYSTALLIZATION. 
which must be placed above ABCD (fig. 23.) in such 
a manner, that the points d', b', c', d', (fig. 24.) answer 
to the points, a, b, c, d, (fig. 23.) By this disposition, 
the squares Aa, Bb, Cc, Dd, (fig. 23.) which compose 
the four outermost rows of squares parallel to the dia 
gonals AC, BD, remain uncovered. It is evident also, 
that the borders QV, ON, IL, GF, (fig. 24.) project 
by one range beyond the borders AB, AD, CD, BC, 
(fig. 23.), which is necessary, that the nucleus may be 
enveloped towards these edges : For if this were not 
the case, re-entering angles would be formed towards 
the parts AB, BC, CD, DA of the crystal; which 
angles appear to be excluded by the laws which deter 
mine the formation of simple crystals, or, which comès 
to the same thing, no such angles are ever observed in 
any crystal. The solid must increase, then, in those 
parts to which the decrement does not extend. But as 
this decrement is alone sufficient to determine the form 
of the secondary crystal, we may set aside all the other 
variations which intervene only in a subsidiary manner, 
except when it is wished, as in the present case, to con 
struct artificially a solid representation of a crystal, 
and to exhibit all the details which relate to its struc 
ture. 
The superior face of the second lamina will be A'G 
LK (fig. 25.) It must be placed so that the points 
a", b", c’, d' correspond to the points d', b, c', d'. (fig. 
24.), which will leave uncovered a second row of cubés 
at each angle, parallel to the diagonals AC and BD. 
The solid still increases towards the sides. The large 
faces of the laminæe of superposition, which in fig. 24. 
were octagons, in fig. 25. arrive at that of a square ; 
and when they pass that term they decrease on all sides ; 
M 
VoL. III. 
177 
Chap. II.