384 
ELEMENTARY CHEMICAL MICROSCOPY 
arsenic acid, thus seriously interfering with the test. Solution 
of the precipitated arsenate in ammonium hydroxide and draw¬ 
ing off will usually effect a partial separation at least, and yield 
a more satisfactory test, but on the other hand the rendering of 
the drop alkaline may lead to the separation of arsenates which 
are soluble in acids but insoluble in alkaline solution. 
Arsenic acid applied as indicated may yield with calcium salts, 
a separation of the compound NH4CaAs04 • 6 H2O, ortho¬ 
rhombic, isomorphous with the corresponding phosphate; the 
crystals appear as large envelope-like crystallites with more or 
less ragged edges. If the solution be dilute hemimorphic forms 
identical with those of ammonium magnesium phosphate are 
seen, but generally of a larger size. Strontium yields minute 
stars and crystalline grains; barium a dense amorphous pre¬ 
cipitate. 
Members of the magnesium group yield colorless crystalline 
double ammonium arsenates isomorphous with their double 
ammonium phosphates. Good crystalline compounds will be 
obtained with the alkaline earths and with the magnesium group 
only when considerable ammonium hydroxide has been added 
to the reagent or when the test drop is distinctly ammoniacal; 
under these circumstances the detection of silver as arsenate may 
be masked. 
Although silver arsenate is of little value as an identity test 
for silver it is of considerable use in detecting arsenates. 
Precautions. 
The arsenic acid may be added directly to the test drop to 
either neutral or to weak nitric acid solutions, but the best and 
most uniform results seem to follow the procedure suggested 
above. 
The amount of ammonium hydroxide added to the reagent 
drop must never he sufficient to neutralize all the arsenic acid and 
give rise to an alkaline solution. 
Note. 
It is of theoretical interest to consider in connection with the 
arsenic acid test for silver, the behavior of compounds of the 
