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formed does not precipitate, but remains dissolved as a double sulphide of arsenic and ammonium (tersulphide of arsenic and sulphide of ammonium). From this solution it precipitates immediately upon the addition of a free acid.

6. Nitrate of silver leaves aqueous solutions of arsenious acid perfectly bright, or at least produces only a trifling yellowish-white turbidity in them; but if a little ammonia is added, a yellow precipitate of ARSENITE OF SILVER (3 Ag O, As O1) separates. The same precipitate forms, of course, immediately upon the addition of nitrate of silver to the solution of a neutral arsenite. The precipitate dissolves readily in nitric acid as well as in ammonia, and is not insoluble in nitrate of ammonia; if therefore a small quantity of the precipitate is dissolved in a large amount of uitric acid, and the latter is afterwards neutralized with ammonia, the precipitate does not make its appearance again, as it remains dissolved in the nitrate of ammonia formed.

7. Sulphate of copper produces under the same circumstances as the nitrate of silver a yellowish-green precipitate of ARSENITE OF COPPER.

8. If to a solution of arsenious acid in an excess of concentrated solution of soda or potassa, or to the solution of an alkaline arsenite mixed with caustic potassa or soda a few drops of a dilute solution of sulphate of copper are added, a clear, blue fluid is obtained, which upon boiling deposits a red precipitate of SUBOXIDE OF COPPER, leaving arsenate of potassa in solution. This reaction is exceedingly delicate, provided not too much of the solution of sulphate of copper be used. Even should the red precipitate of suboxide of copper be so exceedingly minute as to escape detection by transmitted light, yet it will always be discernible with great distinctness upon looking in at the top of the test tube. Of course this reaction, although really of great importance in certain instances, as a confirmatory proof of the presence of arsenious acid, and more particularly also as a means of distinguishing that acid from arsenic acid, is yet entirely inapplicable for the direct detection of arsenic, since grape sugar and other organic substances also produce suboxide of copper from salts of oxide of copper in the same manner.

9. If a solution of arsenious acid mixed with hydrochloric acid is heated with a clean slip of copper, an IRON-GRAY film of metallic arsenic is deposited on the copper, even in highly dilute solutions; when this film increases in thickness, it peels off in black scales. If the coated copper slip is heated with solution of ammonia, the film peels off from the copper, and separates in form of minute spangles (Reinsch). Let it be observed that these are not pure arsenic, but always contain copper, and will accordingly never completely volatilize. Upon heating them in a tube containing air, arsenious acid sublimes.

10. If an acid or neutral solution of arsenious acid or any of its compounds is mixed with zinc, water, and sulphuric acid, ARSENETTED HYDROGEN (AS H1) is formed, in the same manner as compounds of antimony give under analogous circumstances antimonetted hydrogen. (Compare § 130, 10.) This reaction affords us a most delicate test for the detection of even the most minute quantities of arsenic.

The operation is conducted in the apparatus illustrated by Fig. 23, or in one of similar construction.* a is the evolution flask; b a bulb intended to receive the water carried along with the gaseous current; c a

* I willingly adopt the very convenient form of Marsh's apparatus recommended by Otto in his excellent Manual of Chemistry.

tube filled with cotton and small lumps of chloride of calcium to remove water. This tube is connected with b and d by vulcanized india-rubber; d should have an inner diameter of 7 millimètres (see Fig. 24), and must

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O

Fig. 24.

water.

In experiments

be made of difficultly fusible glass free from lead. requiring great accuracy the tube should be drawn out as shown in Fig. 23. The operation is now commenced by evolving in a a moderate and uniform current of hydrogen gas, from pure granulated zinc and concentrated sulphuric acid diluted with 3 parts of When the evolution of hydrogen has proceeded for some time, so that it may safely be concluded the air has been completely expelled from the apparatus, the gas is kindled at the open end of the tube d. It is advisable to wrap a piece of cloth round the flask before kindling the gas, to guard against accidents in case of an explosion. It is now absolutely necessary first to ascertain whether the zinc and the sulphuric acid are quite free from any admixture of arsenic. This is done by depressing a porcelain dish horizontally upon the flame to make it spread over the surface: if the hydrogen contains arsenetted hydrogen, brownish or brownishblack stains of arsenic will appear on the porcelain; the non-appearance of such stains may be considered as a proof of the freedom of the zinc and sulphuric acid from arsenic. In very accurate experiments, however, additional evidence is required to ensure the positive certainty of the purity of the reagents employed; for this purpose the part of the tube d, shown in Fig. 23, is heated to redness with a Berzelius or gas-lamp, and kept some time in a state of ignition: if no arsenical coating makes its appearance in the narrowed part of the tube, the reagents employed may be pronounced free from arsenic, and the operation proceeded with, by pouring the fluid to be tested for arsenic through the funnel tube into the

flask, and afterwards some water to rinse the tube. Only a very little of the fluid ought to be poured in at first, as in cases where the quantity of arsenic present is considerable, and a somewhat large supply of the fluid is poured into the flask, the evolution of gas often proceeds with such violence as to stop the further progress of the experiment.

Now if the fluid contains arsenic, there is immediately evolved, with the hydrogen, arsenetted hydrogen, which at once imparts a bluish tint to the flame of the kindled gas, owing to the combustion of the particles of arsenic separating from the arsenetted hydrogen in passing through the flame. At the same time white fumes of arsenious acid arise, which condense upon cold objects. If a porcelain plate is now depressed upon the flame, the separated and not yet reoxidized arsenic condenses upon the plate in black stains, the same way as antimony. (See § 130, 10.) The stains formed by arsenic incline, however, more to a blackish-brown tint, and show a bright metallic lustre whilst the antimonial stains are of a deep black color and but feebly lustrous. arsenical stains may be distinguished moreover from the antimonial stains by solution of chloride of soda (compare § 130, 10), which will at once dissolve arsenical spots, leaving antimonial spots unaffected, or removing them only after a considerable time.

The

If the heat of a Berzelius, or gas-lamp, is now applied to the part of the tube d, shown in Fig. 23, a brilliant arsenical mirror makes its appearance in the narrowed portion of the tube behind the heated part; this mirror is of a darker and less silvery white hue than that produced by antimony under similar circumstances; from which it is, moreover, distinguished by the facility with which it may be dissipated in a current of hydrogen gas without previous fusion, and by the characteristic odor of garlic emitted by the escaping (unkindled) gas. If the gas is kindled whilst the mirror in the tube is being heated, the flame will, even with a very weak current of gas, deposit arsenical stains on a porcelain plate.

The reactions and properties just described are amply sufficient to enable us to distinguish between arsenical and antimonial stains and mirrors; but they will often fail to detect arsenic with positive certainty in presence of antimony. Now, in cases of this kind, the following process will serve to set at rest all possible doubt as to the presence or absence of arsenic :

Heat the long tube through which the arsenetted hydrogen passes to redness in several parts, to produce distinct metallic mirrors; then transmit through the tube a very weak stream of dry hydrosulphuric acid gas, and heat the metallic mirrors with a common spirit-lamp, proceeding from the outer towards the inner border. If arsenic alone is present, yellow tersulphide of arsenic is formed inside the tube; if antimony alone is present, an orange-red or black tersulphide of antimony is produced; and if the mirror consisted of both metals, the two sulphides appear side by side, the sulphide of arsenic as the more volatile lying invariably before the sulphide of antimony. It you now transmit through the tube containing the sulphide of arsenic, sulphide of antimony, or both sulphides together, dry hydrochloric gas, without applying heat, no alteration will take place if sulphide of arsenic alone is present, even though the gas be transmitted through the tube for a considerable time. If sulphide of antimony alone is present, this will

entirely disappear, as already stated, and if both sulphides are present, the sulphide of antimony will immediately volatilize, whilst the yellow sulphide of arsenic will remain. If a small quantity of ammonia is now drawn into the tube, the sulphide of arsenic is dissolved, and may thus be readily distinguished from sulphur which perhaps may have separated. My personal experience has convinced me of the infallibility of these combined tests for the detection of arsenic.

Marsh was the first who suggested the method of detecting arsenic by the production of arsenetted hydrogen.

11. If a small lump of arsenious acid (a) be introduced into the pointed end of a drawn-out glass tube (Fig. 25) and a fagment of very recently burnt charcoal (b) pushed down the tube to within a short distance of the arsenious acid, and the flame of a spirit-lamp applied, first to the piece of charcoal, then to the arsenious acid, a MIRROR OF METALLIC ARSENIC will form at c, owing to the reduction of the arsenious acid vapor by the red-hot charcoal. If the tube be now cut between b

and c, and then heated in an inclined position, with the cut end c turned upwards, the metallic mirror will volatilize, emitting the characteristic odor of garlic. This is both the simplest and safest way of detecting pure arsenious acid.

a

b

Fig. 25.

12. If arsenites, or arsenious acid, or tersulphide of arsenic are fused together with a mixture of equal parts of dry carbonate of soda and cyanide of potassium, the whole of the arsenic is reduced to the metallic state, and so is the base also, if easily reducible; the eliminated oxygen converts part of the cyanide of potassium into cyanate of potassa. In the reduction of tersulphide of arsenic, sulphocyanide of potassium is formed. The operation is conducted as follows:-introduce the perfectly dry arsenical compound into the bulb of a small bulb-tube (Fig. 26), and cover it with six times the quantity of a perfectly dry mixture of equal parts of carbonate of soda and cyanide of potassium. The whole quantity must not much more than half-fill the bulb, otherwise the fusing cyanide of potassium is likely to ascend into the tube. Heat the bulb now gently with a gas or spirit-lamp; should some water still escape upon gently heating the mixture, wipe the inside of the tube perfectly dry with a

twisted slip of paper. It is of the highest importance for the success of the experiment to bestow great care upon the expulsion of the water, drying the mixture, and wiping the tube clean and dry. Apply now a strong heat to the bulb, to effect the reduction of the arsenical compound, and continue this for awhile, as the arsenic often requires

Fig. 26.

some time for its complete sublimation. The mirror, which is deposited at b, is of exceeding purity. It is obtained from all arsenites whose bases remain either altogether untouched, or are reduced to such metallic arsenides as lose their arsenic partly or totally upon the simple application of heat. This method deserves to be particularly recommended on account of its simplicity and neatness, as well as for the accuracy of the results attainable by it, even in cases where only very minute quantities of arsenic are present. It is more especially adapted for the direct production of arsenic from tersulphide of arsenic, and is in this respect superior in simplicity and accuracy to all other methods hitherto suggested. The delicacy of the reaction may be very much heightened by heating the mixture in a stream of dry carbonic acid gas. A series of experiments made by Dr. V. Babo and myself has shown that the most accurate and satisfactory results are obtained in the following

manner :

Figs. 27 and 28 show the apparatus in which the process is con ducted.

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A is a capacious flask intended for the evolution of carbonic acid; it is half-filled with water and lumps of solid limestone or marble (not

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