308 
Mr. William Phillips on the Modifications 
The crystals of this substance, however, are not ordinarily m 
the general form of figure 20 : the planes of the 12th modification 
which are situated in the acute angles of the prism, are frequently 
so large as materially to reduce the size of the plane P and its op- 
posed plane, giving to the crystal a prismatic form in the direction 
of its shorter axis ; and with the planes of the 12th are often asso- 
ciated those of the 4th modification, which are situated on the obtuse 
angles, both prevailing to such a degree as to give to the crystal the 
form of an octohedron of which the summits are deeply truncated. 
The planes of the 1st and 9th modifications also are sometimes very 
large, and occasionally reduce the crystal to the general form of the 
cube, exhibiting the planes of several other modifications on the 
edges and angles. 
I have heretofore noticed the fact, that no reliance can be 
placed on . measurements taken by the goniometer on the natural 
planes of crystals, however brilliant and perfect they may appear, 
without comparing the results of many attempts. It is even diffi- 
cult to find two crystals that agree perfectly ; they more often vary 
from one to twenty minutes, so that the following measurements 
amount only to approximations ; but such at least as were made by 
the assistance of the reflecting goniometer may be considered as 
being within a few. minutes of the truth. But the planes of some 
of the modifications are almost always rough and without reflection; 
the measurements of these, therefore, could only be obtained by the 
aid of the common goniometer, and are denoted by an asterisk: 
some few are altogether omitted, because that instrument would not 
assist even in approximating the angle, on account of the minute- 
ness of the planes, as is the case with those of the 7th, 11th, and 
15th modifications, which I have observed only on very small 
crystals. 
