252 
Mr F. Koch’s Account of Crystallisations 
the diagonals a : b : c in the fundamental form is rr A 17 : V 48 : 
y/ 16. Supposing p 24., P, Fig. 1. PI. VIII. to be equal to 
the fundamental form, it will follow that the sign of 
Fig. 2. is P — - qo. 
P. Pr + 1. P + go . Pr + go 
a 
Poe h 
Fig. 3. P — go . 
P -f- co . Pr -f go . Mohs . 
a 
e b 
Fig. 4 § P + 1. 
% Pr + 2. Mohs. 
4 
m 
s 
Fig. 5. P — oo . 
|p + l. fpr + 2. Mohs. 
zb zb 
a ms 
The crystals are seldom single, generally many grouped to- 
gether ; occurs massive, in crusts, and in radiated concretions. 
Its colours are greenish and yellowish-white. It is semitrans- 
parent and transparent. The massive specimens, M. Koch re- 
marks, bear a striking resemblance to the precious radiated 
calamine of Hausmann. These varieties were found at the iron- 
forges of Konigshutte. It is also found crystallized, and exhibit- 
ing the following colours at the iron smelting furnace of Zorg ; 
emerald, leek and oil green ; honey, ochre, and straw yellow. 
B. Earths. 
Silica. 
Silica occurs in a remarkably pure state amongst metallurgic 
products, and exhibits the most beautiful and delicate forms. 
Its occurrence, as an artificial production, was first noticed by 
Grignon, who describes it under the name Iron-amianth, and 
Vauquelin first ascertained its true nature. Meyer, Quensell, 
Hausmann, and Stromeyer, were the first who enumerated and 
described it as a product of the forges in the Hartz. It is pure 
silica. Two principal kinds are described by Koch. 
1. Fibrous Silica , which has snow-white and ash-grey colours. 
Its structure is fibrous, concentric and scopiform, with a silky 
lustre. Its forms are globular, botryoidal, and reniform. Is 
friable. 
2. Earthy Silica, — Snow-white. Pulverulent, but becomes 
more and more compact, and passes into fibrous. Glimmering 
