254 Mr F. Koch's Account of Crystallisations 
Fig. 9. Pr. P + oo • 
d' e. 
Fig. 10. P — oo . P + go . 
a. et 
Fig. 11. The same as Fig. 15., only elongated in a direction 
perpendicular to the former. 
Fig. 12. Similar figure. 
Fig. 13. Supposed to be gP — 4. ^Pr — 3. 
4 * 
Fig. 14. The sign of the faces p is — Pr — 3. 
These crystals occur either single or variously grouped. In 
general sharply bounded crystals seldom occur, the edges being 
more or less rounded, thus appearing as having been melted on 
the edges. 
The siliceous enamel, Koch remarks, he has hitherto found 
only in the slag formed during the formation of common grey 
crude iron by means of charcoal. It is formed at the same time 
with the slag, and occurs in it in crystals, crystalline masses, 
and in larger and smaller particles, with a crystalline or curved 
surface, and sometimes the greater part of the slag is composed 
of siliceous enamel. Koch further remarks, that, for the forma* 
tion of crystals, repose and slow cooling are necessary ; that, in 
those cases where the slag is principally composed of the ena- 
mel, the most frequent varieties are the common and fibrous, 
and the fibrous is generally disseminated in the other : He fur- 
ther observes, that crystals, having an opaque and glassy charac* 
ter, sometimes occur in it : On the contrary, in those cases in 
Which the slag predominates, the enamel occurs in it, in imbed- 
ded masses and crystals. Where the crystals are perfect, and 
with sharp edges, the slag is easily fusible : On the contrary* 
where the crystals are imperfect, rounded, without sharp edges 
and angles, the slag appears to have been more viscid and is diffi- 
cultly fusible. These facts shew that the substance of the ena- 
mel is less fusible than that of the slag, and must have cooled 
Sooner ; consequently, that these crystals were consolidated be- 
fore the slag in which they are contained. Koch, in a note, re- 
marks, cc Does not the occurrence of this enamel in a mass, 
having different properties from itself, throw some light on the 
