198 SEVENTH REPORT—1837. 
Again, carbonate of lime presents itself in three forms : 
Vrs =; aM. 
Rhomboid in calc | Rt. Rh. Prism in | Ob. Rh. Prism in 
spar arragonite and Rt. | Obliq. Rh. baryto 
Rh. baryto calcite | calcite ; 
and though the third form in this case may result from the 
combination of the rhomboid of calc spar with the Rt. Rh. 
prism of heavy spar, yet it is not impossible that it may arise 
from a true trimorphism. 
44. Even simple substances are not exempt from the suspicion 
of assuming more than two forms. Thus, in many of its combi- 
nations with their metals, sulphur belongs to the regular system 
to which the metals themselves also belong. It is not easy to 
see how regular forms should result from the union of a cube 
with either of the known forms of sulphur; it may be con- 
sidered probable, therefore, that in certain circumstances sul- 
phur may be isomorphous with the metals which belong to 
the regular system. 
Further, it is not unworthy of notice that, among substances 
assuming regular forms, iron pyrites (FeS,) and glance cobalt, 
Fe tS: +e bAs, alone exhibit the so called pyritohedral 
faces. And though we cannot draw any certain conclusions in 
relation to our present subject from the phenomena exhibited 
by bodies belonging to the regular system; yet the circum- 
stance now mentioned seems to indicate a connexion between 
the two minerals not common even among such regular forms. 
This connexion is most likely to be such as that which exists 
among the octohedral minerals RR, of which magnetic iron is the 
type, and among the garnets, namely, that the analogous mem- 
bers of the formule by which their chemical constitution is re- 
presented are respectively isomorphous, that is to say, that in 
the two minerals FeS, and R 3 arsenic and sulphur are 
2 
isomorphous, and may replace each other. In addition, there- 
fore, to the two known forms of sulphur, there are two others 
in which we may still expect to find it, or sulphur may be ¢eéra- 
Aimorphous. 
