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SELECTED ARTICLES. 
to be examined, and three or four drops of diluted sulphuric 
acid (acid 2, water 14;) and the whole is then to be intro- 
duced into the mouth of the short limb of the tube. The 
production of gas under this arrangement is much slower, and 
of course, requires more time to fill the tube, than in the for- 
mer case; but the mode of operating is precisely the same. 
Indeed, it is of great advantage, when the quantity of arsenic 
present is very minute, not to allow the hydrogen to be 
evolved too quickly, in order to give it time to take up the 
arsenic. 
A slender glass funnel will be found of service when as 
much as a table spoonful, or even a tea spoonful of matter, can 
be obtained for examination. In this case, the tube is to be 
partly filled with common water, leaving a sufficient space for 
the substance to be examined; a piece of zinc is to be sus- 
pended from the cork by a thread or wire, so as to hang in 
the axis of the tube; and the fluid to be operated on, having 
previously been mixed with dilute sulphuric acid, is then to 
be poured through the funnel carefully, so as to surround the 
zinc, avoiding, as far as possible, to mix it with the water be- 
low, and the stopcock and its appendages are to be replaced in 
the mouth of the tube; the production of the gas then goes on 
as before stated, and the mode of manipulating with it is ex- 
actly the same as described in the foregoing part of this paper. 
It will be necessary for me, in this place, to explain the 
methods I employ after each operation, to determine the 
integrity of the instrument, so as to satisfy myself that no 
arsenic remains adhering to the inside of the tube, or to the 
cork and its appendages, before I employ it for another opera- 
tion. 
After washing the apparatus with clean water, a piece of 
zinc may be dropped in, and the tube filled to within half an 
inch of the top of the short limb; two drachms of diluted sul- 
phuric acid are then poured in, and the stopcock and cork 
secured in its place; hydrogen gas will in this case, as before, 
be liberated and fill the tube. If the gas as it issues from the 
jet be then inflamed, and a piece of window glass held over it 
