3 
Chemistry. 109 
quantity of nitric acid is very small, and that of the iodine ‘predomi- 
nates, it is necessary to make a comparison within a standard liquid ; for 
this purpose 50 grains of distilled water containing half a grain of 
iodide of potassium, to which 50 grains of sulphuric has been added 
gradually, and afterwards 50 grains of a concentrated solution of 
protosulphate of iron is used; this fluid is of a yellowish tinge. If 
nitric acid be present, even to the amount of go'yo part, it is made evi- 
dent by the above test, a brownish color appearing, which becomes more 
evident to the eye by comparison with the standard liquid just descri- 
bed; when the quantity of nitric acid increases, the color becomes 
darker. If a bromide be present, instead of an iodide, the nitric acid 
is ascertained with the same facility, by its imparting a greenish hue to 
the liquid ; 50 grains of pure water, containing half a grain bromide of 
potassium, to which 50 grains sulphuric acid are added, will indicate 
zesa part of nitric acid by the development of a brownish tinge ; this 
being brought about first by the liberation of hydrobromic acid by the 
sulphuric acid, and the subsequent decomposition of it by the presence 
of free nitric acid. The presence of organic matter affects the delica- 
cy of this test. Je bed. 
19. Preparation of pure Phosphoric Acid ; by Dr. Grecory, (Med. 
Gazet. 1845, and Chem. Gazet. May, 1845, p. 216.)—This author has 
recommended in his recent work on chemistry, the preparation of phos- 
phoric acid directly from burnt bones; the only difficulty attendant 
upon it, being the separation of the phosphate of magnesia, which he 
has advised to do by means of alcohol. ‘This method, however, upon 
more recent experiments, has proved not to answer, and Dr. G. there- 
fore recommends the following :—Having removed the lime by means 
of sulphuric acid, and evaporated the filtered liquid, (filtering again if 
any sulphate of lime separates during the evaporation,) to the consist 
ency of syrup, a few drops of sulphuric acid are added to make sure 
that no lime remains. Of course if turbidness ensues, the lime has not 
been completely separated before, and the addition of sulphuric acid, 
filtration after adding some water, and evaporation must be repeated till 
the syrupy liquid continues quite clear, when sulphuric acid is added. 
This liquid now contains only the phosphoric acid, the magnesia, and 
the excess of sulphuric acid. It is concentrated, heated in a covered 
platinum crucible until the whole of the sulphuric acid has been expell- 
ed, and the residue has acquired a low red heat. On cooling it forms 
a glass perfectly colorless and transparent, which contains only phos- 
phoric acid and the magnesia of the bones. This glass when boiled 
with water dissolves rather slowly but completely. When the solution — a 
is again concentrated in a capsule of platinum until most of the water 
is expelled, and the temperature rises to between 595° and 600° Fahr., 
