288 
ON THE DETECTION OF ARSENIC. 
liquid with hydrosulphate of ammonia, then heat it gently for 
an hour, and precipitate with hydrochloric acid. The preci- 
pitate, which in all likelihood may be nothing more than sul- 
phur, is treated as has just been directed. If it contains no 
arsenic, lime water will produce no precipitate. (Berzelius.) 
While one or two of, these experiments, by their results, 
may tend to confirm suspicion as to the presence of arsenic — 
the concurrent testimony of the whole is necessary to form a 
proper conviction in the mind of the operator. 
The ordinary tests of ammoniacal nitrate of silver, and am- 
moniacal sulphate of copper, give uncertain results. Phos- 
phoric acid, for instance, precipitates nitrate of silver yellow, 
like arsenious acid, and decoctions of onion, and unburnt cof- 
fee, when mixed with potash, form with salts of copper green 
precipitates, which resemble Scheele's green. 
The reduction alone may be regarded as positive proof, and 
renders the others superfluous ; when this does not succeed, 
the result is always doubtful. An operator, not much accus- 
tomed to these operations, may often imagine he perceives 
the smell of arsenic in the odor of animal matters and yet be 
mistaken. Upon this subject it is well to repeat, that no phy- 
sician or chemist should give testimony in court regarding 
the detection of arsenic, unless he has himself assisted in the 
investigation, or been a witness to the examination conducted 
under skilful hands, and who have affixed their signatures and 
seals to the vessels containing the mixture. 
The processes thus far detailed for testing arsenic, though 
good in themselves, have in a great measure given way to a 
more recent one, for which we are indebted to the ingenuity 
of Mr. James Marsh. It consists in the application of hydro- 
gen gas, and possesses the advantage of simplicity, in separat- 
ing minute portions of arsenic from organic substances, by a 
prompt and convenient method. 
It is founded upon the property which hydrogen, in a nas- 
cent state, possesses, of reducing arsenious and arsenic acids, 
and forming, with their metallic radical, arseniuretted hydro- 
gen gas, which separates spontaneously, by reason of its elas- 
