SEPARATION OF ARSENIC. 
313 
as before described, and any arsenic remains, it will be ren- 
dered evident by being deposited on the glass; if so, this ope- 
ration must be repeated till the glass remains perfectly clean, 
after having been exposed to the action of the gas. 
When I have had an opportunity of working with so large 
a quantity of mixture as from two to four pints (imperial mea- 
sure), I then have employed the instrument which is, indeed, 
but a slight modification of one of the instantaneous light ap- 
paratuses, now so well known and used for obtaining tire by 
the aid of a stream of hydrogen gas thrown on spongy plati- 
num. It will, therefore, be of importance only for me to de- 
scribe the alteration which I make when I employ it for the 
purpose of detecting arsenic. In the first place, I must ob- 
serve, that the outer vessel which I use, holds full four pints, 
and that the jet of the stopcock is vertical, and its orifice is 
twice or three times larger than in the instrument as generally 
made for sale, and also that there is a thread or wire attached 
to the cork of the stopcock for suspending a piece of zinc 
within the bell-glass. 
With an instrument of this description, I have operated on 
one grain of arsenic in twenty-eight thousand grains of water 
(or four imperial pints), and have obtained, therefrom, up- 
wards of one hundred distinct metallic arsenical crusts. 
Similar results have been obtained with perfect success from 
three pints of very thick soup, the same quantity of port wine, 
porter, gruel, tea, coffee, &c. &c. 
It must, however, be understood, that the process was al- 
lowed to proceed but slowly, and that it required several days 
before the mixture used ceased to give indication of the pre- 
sence of arsenic, and, also, a much larger portion of zinc and 
sulphuric acid was employed from time to time, than when 
working with the small bent tube apparatus, in consequence 
of the large quantity of matter operated on under this arrange- 
ment. 
With the small apparatus, I have obtained distinct metallic 
crusts, when operating on so small a quantity as one drop of 
VOL. II. — NO. iv. 40 
