(^Crystallisation of Eyidote and of Glauber-Salt. 309 
responding to the most perfect cleavage, are parallel to the prin- 
cipal axis of the fundamental form. The faces r and T be- 
long to a horizontal prism, the first being those on the side of 
the observer, and the second the other faces contiguous to the 
same apex of this form. According to accurate measurement 
by the reflective goniometer, the difference between the two 
angles formed by the intersection of M with r and T is rather 
less than that indicated by Haiiy, since they are r upon M — 
116® 17', and T upon J/=115° 24'. The incidence of r upon 
T is=128® 56'. In itself the choice of the position of these forms 
is quite arbitrary, but that one will be the most eligible which 
allows the simple forms most easily to be traced to the prismatic 
forms of other species, by certain analogies, which will be point- 
ed out in another paper. The relations of the simple forms to- 
wards each other, developed upon the supposition of the face 
O 
M being Pr -f go , or a plane parallel to the axis of the funda- 
mental pyramid, are so simple, that there can be no doubt that 
the position chosen is the one most agreeable to nature. I 
. shall proceed by developing these simple forms in several ex- 
amples taken from nature. 
One of the most ordinary combinations is the Varike amyhi- 
hexaMre of Haiiy, that represented by Fig. 1 . It is, at the same 
time, one of those which contain the least number of simple forms. 
o o 
Pr P r 
Besides the above mentioned faces of H — ^ (r), 
and Pr -}- CO (il/), it contains only the faces of 
- or n. the in- 
2 
tersection of which, over the face r, produces an angle of TO"* 
33' ; its supplement, or the angle of incidence of n upon is 
= 109° 27'. The angles of this variety are sufficient for calcu- 
lating the fundamental form, and the angles of all secondary 
forms relative to this species. 
The fundamental form of the species is a scalene four-sided py- 
ramid, Fig. 10., in which AX, the axis of the form, includes with 
AP, a line perpendicular to the base, an angle of 0° 33', which, in 
the method of Mohs, is called the Inclination of the Axis. The 
algebraic formulae for calculating the angles of this form, are 
obtained from the values of the four lines AP, MB, MC, and 
MP, called a, b, c, and d. They are, 
