Mineralogy and Geology. 419 
of no importance. The dried precipitate is now to be treated after the 
process recommended by Fresenius, which consists in fusing it with five 
times its weight of a mixture of equa parts of nitre and carbonate of 
so he operation should be performed in a platinum or preferably 
a silver crucible over a spirit lamp, and the mixture kept in fusion ten 
or fifteen minutes to ensure the Priect solution of the chromium. 
3 The chromate of potash is then dissolved out from the mixture of oxyd 
; of iron, alumina and magnesia which may be separated in the ordinary 
manner ; if the precautions above mentioned have been o served, no 
trace of undecomposed ore will be left after treating the mixture with 
hydrochloric acid. A small portion of rs — dissolved in 
the filtrate from the precipitate by carbonate o ay be ob- 
tained by evaporating to dryness. Any silica which on mineral con- 
tained is also dissolved, and may be separated in the 
The presence of a small portion of sulphates prevents the determina- 
tion of the chromic acid by a salt of lead; we, accordingly, supersat- 
urate the solution with hydrochloric acid and boil with alcohol to con- 
nto chlorid of chromium, from which the oxyd is to be precipi- 
adding ammonia in excess and boiling for a few minutes. 
oyed this method several times with perfect success : it 
execution, and being free from any sources of error, yields 
I, March 25, 1848. 
Il. MIngERALOGY AND GEOLOGY. 
that the vertical axis is normally an axis symmetry is aun 
rated by the crystallization, and by optical, thermometric, and acou 
investigations, still extrinsic circumstances cause some variations ae 
f lows = symmetry in the cleavage.* The cleavages observed are as 
o 
, 2. Cleav vage parallel to shies longer diagonal of the primary oan 
In a single direction. Quite commo ; 
n two directions, unequal. More an 
In three directions. Very rare. 
Bre rrr 
| * It would add greatly to the a of these ace ar if the mode of attach. 
a ment of the Top bacon to 2 “ supporting rock were mentioned, as the axis of attach- 
Pte vertical ab exactly) would ¥ under a different condi- 
es of erystal.—J. D. D. 
axes 
Srconp oalieneag Vol. V, No. 15.—May, 1848. vis 
