Part IL. § vii.] DETERMINATION OF ROCKS. 193 — 
“or even sometimes for quartz. If, however, a drop of solution of 
molybdate of ammonia be placed upon one of these crystals, a yellow 
precipitate will appear if it be apatite. Nepheline, which is another 
hexagonal mineral likewise abundant in some rocks, gives no yellow 
precipitate with the ammonia solution, while if a drop of hydro- 
chloric acid be put over it crystals of chloride of sodium or common 
salt will be obtained. ‘These reactions can be observed even 
with minute crystals, by placing them under the microscope and 
using an exceedingly attenuated pipette for dropping the liquid 
on the slide. 
Recently two ingenious applications of chemical processes to the 
determination of minute fragments of minerals. have been made. In 
one of these, devised by Boricky,’ hydrofluosilicic acid of extreme 
purity is employed. This acid decomposes most silicates, and forms 
from their bases hydrofluosilicates. A particle.about the size of a 
pin-head of the mineral to be examined is fixed by its base upon a 
thin layer of Canada balsam spread upon a slip of glass, and a drop 
of the acid is placed upon it. The preparation is then set in moist 
air near a saucer of water under a bell-glass for twenty-four hours, 
after which it is enclosed in dry air, with chloride of calcium. 
In a few hours the hydrofluosilicates crystallize out upon the balsam 
and can be examined with the microscope. Those of potassium take 
the form of cubes, of sodium hexagonal prisms, &c. 
The second process consists in utilizing the colorations given to 
the flame ofa Bunsen burner by sodium and potassium. An elongated 
splinter of the mineral to be examined is first placed in the outer or 
oxidizing part of the flame near the base, and then in the reducing 
_ part further up and nearer the centre. The amount of sodium present 
in the mineral is indicated by the extent to which the flame is 
coloured yellow. The potassium is similarly estimated, but the flame 
is then looked at with cobalt glass, so as to eliminate the influence of 
the sodium.” 
Another process has been devised by M. Thoulet for making a 
qualitative and even quantitative analysis of the powder of a rock. 
It consists in the use of a solution of iodide of mercury in iodide of 
potassium, which ata temperature of 11°C. hasa density of 277. The 
_ powder of a rock being introduced into this liquid, those particles 
whose specific gravity exceeds that of the liquid will sink to the 
_ bottom, while those which are lighter will float. This process allows 
of the separation of the felspars from each other, and at once eliminates 
the heavy minerals such as hornblende, augite, and black mica.* 
| 4. Blow-pipe Tests—The chemical tests with the blow-pipe are 
‘simple, easily applied, and require only patience and practice to give 
great assistance in the determination of minerals. If unacquainted 
* Archiv Naturwiss. Landesdurehforschung von Béhmen, iii. fasc. 3, 1876. 
? Szabo, “Ueber eine neue Methode die Felspathe auch in Gesteinen zu bestim- 
men.” Buda- Pest, 1878. 
> Fouqué et Michel-Lévy, op. cit. p. 117. 
O 

