RESEARCH POR ARSENIC AND ANTIMONY. 
543 
small as only slightly to darken a solution of nitrate of silver, 
and to yield a hair-brown film of metallic arsenic bv heating 
the tube. In cases in which the quantity of arsenic is very 
small, it will be convenient to extemporise a Marsh’s appa¬ 
ratus, by fitting to a test-tube a cork traversed by a funnel- 
tube and a short exit-piece bent at right angles. This should 
be connected with a piece of tubing containing broken 
chloride of calcium, so that the gas as it passes may be 
dried. A tube, drawn out for about two thirds of its length 
to almost a capillary bore, should be fitted by a cork to the 
other end of the chloride-of-calcium tube. Zinc is placed in 
the apparatus, the funnel-tube is then fitted, and the distil¬ 
late, suspected to contain traces of arsenic, is poured through 
the funnel. The gas is tested by holding filtering-paper, 
wetted with a solution of nitrate of silver at the capillary 
aperture of the tube. If arsenic is present, a minute black 
spot appears. This may, however, be due to antimony. If 
owing to the presence of sulphur, the spot will be brown, 
and the gas will discolour paper moistened with a solution of 
acetate of lead.* The tube may now be heated red-hot within 
half an inch of the contracted portion, taking care that the 
contracted or capillary part of the tube, which is to receive 
the arsenic, is not overheated. A metallic deposit of arsenic 
is soon formed in the narrow portion of the tube. From the 
1-1000th to the 1-2000th of a grain of arsenic may have its 
presence thus indicated. 
When the quantity of arsenic is at a minimum, no other 
evidence than this thin metallic deposit may be obtainable. 
In general, however, besides the metallic deposit as a result 
of heat, it will be found, by passing the gas through a bent 
* In all cases, it will be found convenient to dry the arseniuretted 
hydrogen gas by means of a chloride-of-calcium tube attached to the appa¬ 
ratus. A piece of lead-paper, introduced at the mouth of this tube, will 
effectually stop any sulphuretted hydrogen that may be produced. It may 
here be observed, that neither arseniuretted, antimoniuretted, nor phosphu- 
retted hydrogen darkens a solution of acetate of lead; and it is always proper 
to test the gas by lead-paper applied to the open end of the conducting- 
tube, before passing the current into a solution of nitrate of silver. Sul¬ 
phuretted hydrogen would otherwise cause thick black flakes in the solu¬ 
tion, and might give rise to an error respecting the presence of arsenic. 
In cases in which the quantity of arsenic is sufficiently large, a tube with 
a stop-cock may be resorted to. This gives a command over the rapidity 
of the current. 
The zinc which has been used in one experiment is not fitted for use a 
second time. Some arsenic is always deposited upon it, which is evolved 
when hydrogen is again generated. I have substituted aluminum with 
hydrochloric acid as well as potash, but I have not found this new metal to 
present any advantages over zinc. It contains no arsenic, but much iron 
and carbon. 
