542 
RESEARCH FOR ARSENIC AND ANTIMONY. 
found that it is dissolved by ammonia, and not dissolved by 
hydrochloric acid in the cold.* 3d. Arsenic acid is a product 
of the action of strong nitric acid on the hydride. It may be 
procured by evaporating to dryness on a sand-bath, in a 
small porcelain capsule, the nitric acid after it has been 
saturated by the current of arseniuretted hydrogen gas 
passed through it. When quite dry , it appears as a white 
residue, if the nitric acid was pure. It is fixed at the ordi¬ 
nary heat of a sand-bath, and, after exposure to air for a short 
time, it will become moist and deliquescent. It should be 
completely dried before any test is added to it, because the 
presence of traces of nitric acid may interfere with the action 
of the test. If not in a deliquescent state as the result of 
exposure, a small quantity of distilled water should be added, 
to detach the acid from the capsule. One or two drops of 
a strong solution of nitrate of silver should now be added, 
and, if the residue is arsenic acid, a brick-red precipitate of 
arseniate of silver will make itself visible. In place of the 
nitrate, a strong solution of ammonio-nitrate of silver may be 
employed, provided the ammonia is not in excess. This has 
the advantage of neutralizing the solvent effect of any nitric 
acid which may be adhering to the solid arsenic acid, and it 
reveals the presence of small quantities better than the 
nitrate. 
With these combined results, it is conclusively proved that 
arsenic must have been present in the substance analysed. 
We have here, in the production of metallic arsenic, and of 
its two acid compounds with oxygen, as well as in the con¬ 
version of these to sulphide of arsenic and arseniate of 
silver, the maximum of evidence which an analyst could 
desire. Beyond this it appears superfluous to carry the 
chemical proof. The combustion of the gas, as suggested 
by Marsh, with the collection of deposits from the flame, 
while it necessarily entails much loss, is not required in any 
stage. The decomposition of the hydride by a red heat in a 
close tube answers all the purposes of combustion, without 
the risk of loss from the frequent opening of a stop-cock. A 
larger quantity of metallic arsenic can be thus obtained in 
one operation; and any gas which escapes decomposition is 
arrested and decomposed by the nitrate of silver or nitric 
acid. 
The quantity of arsenic present in the distillate may be so 
* The sulphide of cadmium is affected in a contrary manner by these re¬ 
agents. It is insoluble in ammonia, but soluble in hydrochloric acid. 
Cadmium could not be present: it cannot be distilled as chloride, and it 
does not combine with hydrogen to form a hydride. 
