78' Remarks vpon Arsetiic. 



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 moisture 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 possesses 

 no metallic lustre, and if the quantity of arsenic be infinitely 

 small, I see no reason why it all should not undergo this change ; 

 at any rate, the tube D, let it contain any matter that may serve 

 to dry the gas, is of essential importance. 



Another remark to be made about this apparatus is, that some- 

 times all the arsenuretted hydrogen is not decomposed, and it not 

 unfrequently happens that a portion of that decomposed, is thrown 

 out at 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 

 «6c, under the figure before described. The arsenic being col- 

 lected in the tube E, as just mentioned, is described as follows : 



1st. By its volatility. 



2d. By its becoming changed into a white volatile powder 

 when the tube, open at both ends, is heated in an inclined 

 position. 



3d. If we introduce a little nitric acid into the tube it dissolves 

 the arsenic, converting it into arsenic acid, and if this nitric acid 

 solution be evaporated to dryness in a capsule, taking care to add 

 a few 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 crystal of 

 nitrate of silver in the capsule before the solution is added, for it 

 tends to render the test more delicate. 



