formed during various Metallurgical Operations . 253 
and dull. Generally the fine earthy particles are irregularly 
aggregated, seldomer it is globular, reniform, and botryoidal. 
Both kinds are found in smelting-furnaces where siliceous iron- 
ores are smelted. It is sometimes associated with artificial ta- 
bular graphite. 
C. Vitreous Slags. 
Siliceous-enamel ? ( Kieselschmelz.) 
This interesting production of the smelting-furnace is de- 
scribed by Hausmann, in Vol. V. of this Journal, but the de- 
scription by Koch, of which we shall now give an abridged ac- 
count, is much more complete. It appears as a perfect glass, 
and passes gradually into a kind of enamel. Some of the va- 
rieties bear a striking resemblance to glass, others approach 
more to enamel, and there is an uninterrupted transition from 
the one into the other. T wo principal kinds are described : 
1. Glassy Siliceous Enamel. — Is perfectly glassy, friable, 
transparent, or only semitransparent : fracture perfect eonchoi- 
dal, and lustre vitreous ; also small conchoidal, splintery, and 
uneven, and lustre resinous. Colours, pearl, greenish, and red- 
dish grey. Crystallised. Massive. 
2. Opaque Siliceous Enamel. — Very much like enamel. Al- 
ternate from semitransparent to translucent. Fractures uneven, 
even and splintery. Structure fibrous and radiated. Lustre 
resinous, glimmering, and dull. Colours white, grey, yellow, 
and blue. Crystallised, massive, and disseminated. 
The following are some of its regular forms, as given by Koch, 
our space not allowing the insertion of the whole. 
The forms of this artificial mineral belong to the Prismatic 
System of Mohs, and to the Trimetric System of Hausmann. 
The fundamental form is an acute scalene four-sided pyramid. 
The ratio of the axis a , and the diagonals h and c is as 
V 6 : V 3 : 1. 
Fig. (>. P = 123° 54' 58" : 70° 31' 44" ; 141° 3' 28". 
Fig. 7. P. Pr + co . 
P. b 
Fig. 8. The same elongated in the direction of one of the ob- 
tuse terminal edges of P. 
