310 
SELECTED ARTICLES. 
the gas will be arsenuretted hydrogen. The first portions are 
to be allowed to escape, in order that they may carry with them 
the small quantity of common air left in the apparatus; after 
which the cock is to be closed, and the gas will be found to 
accumulate in the shorter leg, driving the fluid up the longer 
one, till the liquor has descended in the short leg below the 
piece of zinc, when all further production of gas will cease. 
There is thus obtained a portion of gas subject to the pressure 
of a column of fluid, of from seven to eight inches high: when, 
therefore, the stopcock is opened, the gas will be propelled 
with some force through the jet, and, on igniting it as it issues 
(which must be done quickly by an assistant,) and then hold- 
ing horizontally a piece of crown or window-glass over it, in 
such a manner as to retard slightly the combustion, the arsenic 
(if any be present) will be found deposited in the metallic state 
on the glass; the oxygen of the atmosphere being employed 
in oxydizing the hydrogen only during the process. If no ar- 
senic be present, then the jet of the flame as it issues has a very 
different appearance; and although the glass becomes dulled 
in the first instance by the deposition of the newly formed 
water, yet such is the heat produced, that in a few seconds it 
becomes perfectly clear, and frequently flies to pieces. 
If the object be to obtain the arsenic in the form of 
arsenious acid, or white arsenic, then a glass tube, from a 
quarter to half an inch in diameter (or according to the size 
of the jet of flame,) and eight or ten inches in length, is to 
be held vertically over the burning jet of gas, in such a manner 
that the gas may undergo perfect combustion, and that the 
arsenic combined with it may become sufficiently oxydized; 
the tube will thus, with proper care, become lined with arse- 
nious acid in proportion to the quantity originally contained 
in the mixture. 
When the glass tube is held at an angle of about forty-five 
degrees over the jet of flame, three very good indications of 
the presence of arsenic may be obtained at one operation; 
viz. metallic arsenic will be found deposited in the tube at the 
part nearest where the flame impinges, — white arsenic, or ar- 
