222 DETECTION OF ARSENIC IN ORGANIC MIXTURES. 
other a little above it; the first serves to hold the zinc, the 
second to break any bubbles which may arise. The cen- 
tral curve just spoken of, and the convexity of which is 
directed upwards, serves to prevent the gas from backing 
and escaping through the longer limb, and to prevent the 
zinc from falling into the tube below the lowest bulb in the 
shorter limb ; a pointed piece of glass-tubing, loosely fitting 
the lower part of the shorter limb, should be placed in it. 
A cap, into which a stop-cock screws, is cemented to the 
upper part of the shorter limb, and two pieces are to be 
ground into the upper opening of the stop-cock, and so 
adapted that they may be removed at pleasure. One of 
these pieces is a jet for burning the gas ; the other, a right- 
angled tube, to which another of Berlin glass, 6 inches long 
and §th of an inch in bore, can be connected by means of 
caoutchouc or bladder; to the other end of this another 
right-angled tube is adapted, and its lower limb made to dip 
into a solution of nitrate of silver. When used, the arsenic- 
ated zinc is introduced into the bulb, the stop-cock screwed 
on, and the right-angled tube fixed on. Dilute sulphuric 
acid, of spec. grav. 1080 (1-7,) is then added, when the gas 
is evolved, and must be transmitted slowly through the 
solution of nitrate of silver until it begins to blacken it; the 
stop-cock is then turned, the right-angled tube removed, and 
the jet substituted. The ordinary tests may then be ap- 
plied. 
The solution of nitrate of silver may also be tested by 
precipitating with slight excess of muriatic acid, gently 
boiling for a short time and filtering. Evaporate the filtered 
liquid to dryness, then redissolve the residue in a little dis- 
tilled water, neutralize with a drop of ammonia, and make 
it boil so as to expel any excess of the latter. On testing 
with nitrate of silver, the arseniate is deposited if arsenic had 
been present ; while there is no change, or but a white 
cloudiness, if it had been antimony or sulphur. 
The only impediment the author has found to this process 
