SEPARATION OF ARSENIC. 
311 
senious acid at a short distance from it, — and the garlic smell 
can be readily detected at either end of the tube in which the 
experiment has been made. 
As the gas produced during the operation is consumed, the 
acid mixture falls into the short limb of the tube, and is thus 
again brought into contact with the zinc, in consequence of 
which a fresh supply is soon obtained. This gas, if submitted 
to either of the processes before described, will give fresh in- 
dications of the presence of the arsenic which the mixture 
may have originally contained; and it will be easily perceived 
that the process may be repeated as often as may be required, 
at the will of the operator, till no further proofs can be ob- 
tained. 
When certain mixed or compound liquors are operated on 
in this apparatus, a great quantity of froth is thrown up into 
the tube, which may cause a little embarrassment by choaking 
the jet. I have found this effect to take place most with 
the contents of the stomach, with wine, porter, tea, coffee, or 
soup, and indeed, with all mucilaginous and albuminous mix- 
tures. The means I adopt to prevent this effect from taking 
place, or, at least, for checking it in a great measure, is to 
grease or oil the interior of the short limb of the apparatus 
before introducing the substance to be examined, or to put a 
few drops of alcohol or sweet-oil on its surface previously to 
introducing the stopcock and its appendages. I have, how- 
ever, found, if the tube be ever so full of froth in the first in- 
stance, that, in an hour or two, if left to itself, the bubbles 
burst, and the interior of the tube becomes clear without at all 
affecting the results. 
In cases where only a small quantity of the matter to be 
examined, can be obtained, I have found a great convenience 
in using the small glass bucket. Under such circumstances, 
the bent glass tube may be filled up to within an inch of the 
short end with common water, so as to allow room for the 
glass bucket, which must be attached to the cork, &c. by 
means of a little platina wire; a bit or two of zinc is to be 
dropped into the bucket, with a small portion of the matter 
