ON THE DETECTION OF ARSENIC. 
287 
quantity as not to be separated from the filter, then take it up 
by means of caustic ammonia, which you afterwards evaporate 
in a watch glass; the sulphuret which remains may be de- 
tached from the glass and collected. You then transform it to 
arsenic acid; for this purpose, throw it by little at a time up- 
on nitre in a state of fusion, and contained in a glass tube 
closed at one end. The sulphuret of arsenic oxydizes with a 
slight effervescence and without deflagaration; then dissolve 
the salt which remains in a few drops or least possible quan- 
tity of water, add to the liquid an excess of lime water, and 
boil it, to collect together the arseniate of lime. Expose this 
salt to a moderate red heat, mix it with recently burned char- 
coal, and introduce the mixture into a glass tube, narrowed 
towards one extremity and terminated by a hollow ball, (see 
fig. 3,) into which the mixture is received. 
Commence by heating the tube to drive out any moisture. 
Then expose the part A to the flame of a blowpipe, until the 
glass begins to melt. The arsenic is then reduced and collects 
in the narrow part of the tube, where it spreads over a surface 
so small that the minutest quantities may be recognised. One- 
tenth of a grain of sulphuret of arsenic is sufficient to give a 
decided reaction. 
In such delicate experiments as are required in the detection 
of arsenic, where even traces should not escape observation, 
you must be certain that the reactives employed are in no 
wise contaminated with arsenic. For instance, it might hap- 
pen that the hydrochloric acid contains some, for the reason 
that sulphuric acid is often made from pyrites or arseniuretted 
sulphur, and in this case the arsenic passes along with the hy- 
drochloric acid during its preparation. For this reason, you 
must first convince yourself that your sulphuric acid, for dis- 
engaging the sulphuretted hydrogen, and the hydrochloric 
acid, are free from arsenic, which may be done by passing over 
them a current of sulphuretted hydrogen. 
Where the poison has been produced by arsenic acid, sul- 
phuretted hydrogen reacts with difficulty, and in a less de- 
cided manner. In this case you must supersaturate your acid 
