✓ 
Gryftals of Alum. 103 
E reprefents an eight-fided Figure, con¬ 
fining of two triangular, two hexagonal, 
and four quadrilateral Planes, two whereof 
flope downwards from the upper hexangular 
Plane, and the other two upwards from the 
under one. 
The Figure at F is compounded of eight 
Planes, the undermoft whereof is a large 
equilateral Triangle, from each Side of which 
a doping quadrilateral Plane proceeds. Thefe 
Planes are Trapezoids, each of them having 
a Side in common with that of the Triangle, 
another fhorter one parallel thereto, and two 
others doping one towards the other, and 
towards the Side of the Triangle, in a Di¬ 
rection of about twenty Degrees. 
The Plane which lies parallel to the 
Triangle (which Plane in the prefent Fi¬ 
gure is next the Eye) is hexagonal, three 
of its Sides being made up of the fhorter 
parallel Sides of the Trapezia; the other 
three (which are lefs than thefe, being cut 
down perpendicularly, fo as to meet the 
Angles of the large Triangle) form three 
other fmaller Triangles, each whereof has 
one Side in common with the Hexagon, 
and the other two with the neighbouring 
Trapezoids. 
Our Alum is obtained from a blueifh mi¬ 
neral Subftance refembling Slate, which 
after being calcined and fleeped in Water 
• H 4 for 
