208 — T. S. Hunt on the Constitution . 
From these examples it appears that identity of volume in allied 
and isomorphous species, requires higher equivalents and more 
complex formulas than are generally assigned, and that these new 
formulas are homologous. We deduce the equivalent weight of 
a monometric species like that of a gaseous hydrocarbon from its 
density. 
The rhombohedral species of the calcite group are commonly 
cited as example of identity in equivalent volume, but even this 
group presents exceptions such as we have seen in the mono- 
metric oxyds and chlorids. Calcite is remarkable for the vari- 
ations in the density of different specimens. According to Beu- 
dant* the purest varieties give from 2:5239 to 2:7234. With the 
formula C Ca Os =50 and a density of 2°72, the volume is 18-4, 
and with 2°52 it equals 19:84. The magnesite from Arendal 
which contains only ‘73 p. c. of oxyd of iron, gave to Scheerer a 
density of 3-068; if we take 3 for the pure carbonate, the equiva- 
lent volume will be 14,and calamine with a density of 4:45, gives 
the same number. Dolomite of density 2-9 gives a volume of 
15-9, and diallogite with a density of 3-592, equals 15-6, while 
spathic iron with a specific gravity of 3-8 has a volume of 15°2. 
We give here the volume and the angles of the rhombohedron 
for these species. 
pecies. Volume. Angle R: R, 
Calamine, 14-0 | 107° 40/ 
Magnesite, 14:0 107° 22! 
Spathic iron, | 15-2 107-107 F 
Diallogite, | 15:6 106° 51’. 107° 20’, Levy and Breithaupt. 
Dolomite, 15°9 106° 10'-106° 20/ 
Calcite, 18-4-19'8' 105° 5’-105° 25/ 
Kopp has endeavored to connect these variations in volume 
with the different dimensions of the rhombohedron in the several 
Species, but it seems probable on comparing the corrected num- 
bers, that this apparent relation is accidental. Besides the vari- 
ations in density presented by calcite, it will be seen that the 
difference between the angles of calamine and magnesite, whose 
volumes are identical, is the same as that between the latter and ° 
spathic iron; the angle assigned by Levy and Breithaupt to dial- 
logite, differs only 2’ from that of magnesite, while the volume 
the species is near that of dolomite. The importance to be at- 
tached to the small variations i gular t, is lessened 
by the differences in the angles of the same species, amounting 
to from 5’ to 20’, and by the variations observed by Nickles in 
many artificial crystals, often amounting to 1° and 1° 30’, and 
seemingly dependent upon portions of foreign matters, sometimes 
too small to be detected by chemical analysis.t 
*Annales des Mines, (2,) v. p. 275, cited i ’s Mineralogy, * 
+ Comptes Rend. ie tase ieee sc one ie 
