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been established, we may now proceed with the process of reduction proper. If, for the purpose of the examination of the materials, only a provisional reduction-tube has been used, it is now exchanged for another provided with more narrows, and if the contents of the drying-tube should have become too moist, this is also exchanged for another which lies ready prepared and connected by India-rubber with the new reduction-tube; the whole takes but a few moments. It may, possibly, also be necessary to replace the zinc, or, at least, to pour off the liquid, which may have become too much saturated by sulphate of zinc.* It is hardly necessary to mention that if a replacement of any parts of the apparatus has taken place, or the bottle itself has been opened, care must be taken to drive out all atmospheric air from the apparatus by a sufficiently long-continued evolution of gas before proceeding any further.

While the reduction-tube is in a state of vivid incandescence at the part immediately before the first narrowing and the escaping gas burning, the liquid to be tested for arsenic is poured into the bottle through the funnel-tube, and washed down with a little water. This liquid is either the one mentioned ($13), prepared by treating the precipitate obtained by sulphureted hydrogen with nitric acid, fusion with nitrate and carbonate of soda, etc., or the one men

* If a three-necked Woulff's bottle has been used, and the middle neck been provided with a cork, through which a syphon-tube has been tightly fitted, the liquid may be removed through the syphon by closing the opening of the reduction-tube with the finger, or better by compressing one of the India-rubber connection-tubes.

tioned (914), prepared by treating the same precipitate with nitric and sulphuric acid; in both the arsenic, if present, is contained as arsenious or arsenic acid.

If the liquids under consideration contain arsenic, the escaping hydrogen gas becomes mixed with arseneted hydrogen, and in passing through the ignited part the latter is decomposed, and immediately beyond this place, at the narrow portion of the tube, a mirror or coating of arsenic is deposited.

Having thus obtained a mirror or a distinct coating, the flame of the lamp is made smaller, so that the tube remains no longer incandescent.

In consequence of this the flame of the escaping gas immediately assumes a bluish-white color, and the abovementioned spots are formed on porcelain when this is held into the flame; of these spots a sufficient number is prepared to serve as material for further experiments.

It is desirable to obtain, at least, a second metallic mirror in the reduction-tube; but after that the tube is turned in such a manner that its originally ascending leg hangs downward; the flame of the escaping gas being extinguished it is inserted into a test-tube, or a small beaker-glass, partly filled with a solution of nitrate of silver. On then diminishing the heat, and thereby preventing the decomposition of the arseneted hydrogen, this gas passes through the solution of the nitrate of silver, by which it is completely decomposed. The solution blackens in consequence of the elimination of metallic silver, and the liquid contains arsenic in the state of arse

nious acid, together with the excess of the silversalt. To prevent the latter from becoming too considerable, it is advisable to take at first a dilute solution, acidulated with a drop or two of nitric acid, and adding gradually of a more concentrated one, in case a large deposit of metallic silver should be formed. In this experiment the reduction-tube may be substituted by a common delivery-tube, bent at a right angle.

$ 20. Simple as the above-described experiments are, their execution, if they shall lead to reliable results, requires the careful observance of some precautions. The following remarks contain all that can be said about them :

The liquid to be tested for arsenic should not be poured into the bottle at once, but at first only a part of it, and the effect be well observed. · If the liquid contains arsenic the evolution of gas will increase considerably after its addition, owing to the precipitated arsenic forming a galvanic pair with the zinc; the pure zinc then dissolves as rapidly in the acid as impure zinc. Suppose now a very large quantity to have been added, the evolution of gas may, for this reason, and also because the amount of acid is increased, become very violent, and the operator would be obliged to allay it by pouring cold water into the apparatus, or by surrounding the bottle with cold water. It is, on this account, also advisable not to add the liquid in question until the evolution of gas is more moderate.

If the reduction-tube is sufficiently heated, if the evolution of gas is not too rapid, and if the escaping gas does not contain a large amount of arseneted

hydrogen, the whole of the arsenic is deposited, the whole of the arseneted hydrogen becomes decomposed, and no appreciable quantity of the reduced arsenic is mechanically carried away by the current. But if the reverse is the case, the flame of the burning gas becomes colored, and spots may be obtained on porcelain during the process of reduction.

Spots on porcelain are very readily obtained if the amount of arseneted hydrogen is large, but it requires some nicety of management if only traces of this gas are present.

The opening of the tube must not be too narrow; the current of gas must not be too strong —the gas must not escape with violence, the flame must not be pointed—and the porcelain must be held horizontally and close to the opening of the tube. According to my experience, it is best to cause the spots to be deposited on the inner surface of small porcelain dishes, which must be new, or at least very white and clean ; it is convenient to collect the spots on different dishes, so as not to be obliged to make the tests of the subsequent examination of them in one and the same dish. Instead of dishes, small saucers may be used, provided the porcelain be genuine; delf with a lead-glazing is quite inadmissible. The temperature of the dishes should be that of the surrounding air; if they are held into the flame while too cold, water is deposited, and the arsenic floats as a brown coating on the drop. On the other hand, a spot of the dish must not be held into the flame for any great length of time; otherwise, the place becomes so hot that a deposition of arsenic can no

longer take place, and sometimes the arsenic-spot even disappears.

$ 21. It is, as above indicated, one of the great advantages of this apparatus, that it furnishes different and equally characteristic proofs for the presence of arsenic. The principal object ought to be, to obtain one or several depositions of metal in the reduction-tube. If these not merely appear as transparent coatings, but as real metallic mirrors, then there is sufficient arseneted hydrogen mixed with the escaping gas to produce spots on porcelain, which, in this case, should be collected at the same time. When a sufficient number of spots has been obtained, the gas is passed, for a length of time, through the solution of nitrate of silver, and thus the loss of any recoverable arsenic prevented.

In this process, as originally described by Marsh, attention was only paid to the formation of the spots on porcelain; and it was Berzelius, if I mistake not, who first noticed the deposition of metallic arsenic from the gas by heating: an observation which has, afterwards, become of the highest importance.

$ 22. It is only the acids of arsenic, and the solutions of the arsenites and arsenates of potassa, soda, lime, and similar bases, when acidulated with sulphuric acid, and chloride of arsenic, which cause the formation of arseneted hydrogen; neither sulphide of arsenic nor metallic arsenic produce the slightest trace of this gas. The first-mentioned compounds even do not give rise to the formation of this gaseous compound, unconditionally; the absence of certain substances is required. Free chlorine, nitric acid,

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