4 28 
SECOND REPORT -1832. 
Specular iron.2 F + 3 G 
Corundum.2 A1 + 3 O 
Peroxide of manganese ... 2 Mn + 3 O 
all of which belong to the same group, occurring in the same 
form ; and isomorphous with this we have also 
Ilmenite and titanic iron 
from Orendhal .... 
Kibdelophan and titanic! t # # ,F+Ti 
iron from Gastein . . j 
• • • 
in which it will be seen that the titaniate of iron F + Ti = 
(F + O) + (Ti + 20) contains exactly the same number of atoms 
as the peroxide of iron, which in the ilmenite it partially, and 
in the kibdelophan entirely replaces *. 
Differences between the angles of similar crystals of the same 
group .—It lias been already mentioned (p. 422,) that the gene¬ 
ral law of Mitscherlich has undergone some modification since 
it was first announced by that distinguished philosopher. This 
modification has arisen from the necessity of admitting two new 
elements into the investigation; namely, temperature, and the 
relative chemical affinities of the constituent atoms. 
The chief support of the doctrine of isomorphism is drawn 
from the observed identity of the forms and angles of similar 
salts of the same acid with different bases, or of the same base 
with different acids. But while, in general, the similar com¬ 
pounds of the isomorphous groups agree perfectly in crystal¬ 
line form, a few instances have been found in which the angles 
of the similar crystalline compounds of members of the same 
group differ from each other by a quantity sometimes exceed¬ 
ing two degrees. Thus the obtuse angle in the right rhombic 
prisms of the following substances has been found to differ. 
\ . . . . (Fe + Ti) -fadFe 
Sulphate of barytes . 101 42 
— strontian 104 00 
-— — —- lead . . . 103 42 
O / 
Carbonate of barytes 118 30 
______—.—_ strontian 117 32 
— -.— lead ... 117 18 
Arragonite.110 10 
Carbonate of lime 
• — iron . 
manganese 107 20 
zinc 
105 05^ 
107 00 
20 
40 
> 
107 
These form rhomboids, 
not rhombic prisms. 
* A valuable list of isomorphous mineral groups, by Professor Miller of 
Cambridge, will be found in p. 118. 
