78 Remarks upon Arsenic. 
made, it would appear that it was; for the gas passing over the 
chloride of calcium is deprived entirely of its moisture, which 
does not happen in the other case; and the dryness of the gas 
must evidently augment the delicacy of the instrument, for if the 
gas contain méisture when it arrives at the point where it is de- 
composed, the arsenic, as it is liberated, will combine with the 
oxygen of the moisture, forming arsenious acid, which 
no metallic lustre, and if the quantity of arsenic be infinitely 
small, I see no reason sinc epi eet not undergo this change ; ; 
at any rate, the tatis-D; any matter that may” serve 
to dry the gas, Se Sa at 
Another ren remark to be made about this anintiant is, ahaa 
} hydrogen is not decomposed, and it not 
povnanra happens that a portion of that decomposed, is thrown 
outat the extremity of the tube along with the gas. To obviate 
these little inconveniences, I have used the following means, 
which appear to be of some service. In the interior of the tube 
E, at the point where it is heated, are placed very small fragments 
of charcoal, that have been heated to redness in a close vessel 
before being introduced, and bending the same tube, as is seen in in 
abc, under the figure before described. 'The arsenic b 
lected in the tube E, as just meutioned, is described as follows 
Ist. By its volatility. 
2d. By its becoming changed into a white relate powder 
when the tube, open at both ends, is heated in an inclined 
position. 4 Sie 
“Bd. If-we introduce a little nitric acid into the tube it dissolves 
perenne; converting it into arsenic acid, and if this nitrie acid 
solution be evaporated to dryness in a capsule, taking care to add 
afew drops of hydrochloric acid to the nitric acid before com- 
mencing the evaporation—(the reason of this precaution is, that 
most nitric acid of commerce contains an organic substance, 
which gives to the residue a more or less black appearance, and 
by the addition of a few drops of hydrochloric acid chlorine is 
generated, which serves more or less to destroy this substance, 
and therefore afford us a whiter residue )—the residue will give a 
red precipitate if we add to it a drop or two of a concentrated 
solution of silver, and it is often well to place a small erystal of 
nitrate of silver in the-capsule age occatREn! ee 
tends to render the test more delicate. ~ fy ee 
