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. If you now transmit through the tube containing the sulphide of arsenic or the 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, § 131, 10, 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 may have separated. My personal experience has convinced me of the infallibility of these combined tests for the detection of arsenic.

The reaction of hydrogen containing arsenetted hydrogen with solution of nitrate of silver will be found in § 134, 6.

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. 39), a fragment of quite

recently burnt charcoal (b) pushed down the tube to within a short distance of the arsenious acid, and first the charcoal then the arsenious acid heated to redness, 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 e 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.

12. If arsenites, or arsenious acid, or tersulphide of arsenic are fused 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 converting 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. 40), and cover it with six times the quantity of a perfectly dry mixture of equal parts of carbonate of soda and of cyanide of potassium. The whole quan

a

Fig. 40.

tity 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; should some water still escape, wipe the inside of the tube perfectly dry with a twisted slip of blotting paper. It is of the highest importance for the success of the experiment to bestow great care upon expelling 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 some time, as the arsenic often requires 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 unaffected, 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 particular y 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 L. v. BABO and myself has shown that the most accurate and satisfactory results are obtained in the following manner. Figs. 41 and 42 show the apparatus in which the process is conducted. a is a bottle for the evolution of carbonic acid. At the bottom is a layer of plaster of Paris. While the plaster is setting the bottle should be placed in a slanting position, so that the surface of the plaster may slope towards d; and before the plaster is quite hard a conical hole should be scooped out of it at d. On the plaster bottom lumps of marble are placed. b is firmly fixed on a by means of two perforated india-rubber stoppers, or preferably by means of one long stopper tapering from the middle towards both ends. b is connected with a by the glass tube e, which is provided with the arrangement represented at f constructed of glass tubes of as large bore as possible. To the glass tube g is attached a short piece of india-rubber tube which reaches into the cavity in the bottom of the bottle. Water and hydrochloric acid are poured in through h, which is afterwards closed by a stopper containing a glass tube. On opening i the acid passes into the evolution bottle and the carbonic acid begins to be formed. On closing i the pressure of carbonic acid drives the hydrochloric acid back into b, and the evolution ceases. The carbonic acid is dried in k, which contains concentrated sulphuric acid. It then passes into the reducing tube

m, which is represented in fig. 42 half its real size; it should have a bore of 8 mm.

When the apparatus is full of carbonic acid triturate the perfectly dry sulphide of arsenic or arsenite in a slightly heated mortar with about twelve parts of a well-dried mixture consisting of three parts of carbonate of soda and one part of cyanide of potassium. The mixture must of course be quite free from arsenic (§ 46). Put the powder upon a narrow slip of card paper, bent into the shape of a gutter, and push this into the reduction-tube down to e; turn the tube now half-way round its axis, which will cause the mixture to drop into the tube between e and d, every other part remaining perfectly clean. Connect the tube now with the gas-evolution apparatus, and pass through it a moderate stream of carbonic acid. Heat the tube in its whole length very gently until the mixture in it is quite dry. When every trace of water is expelled, reduce the gas stream so that the single bubbles pass through the sulphuric acid at intervals of one second, and heat the reduction tube to redness at e (fig. 42). When c is red-hot, apply the flame of a second lamp to the mixture, proceeding from d to e, until the

whole of the arsenic is expelled. The far greater portion of the volatilized arsenic recondenses at h, whilst a small portion only escapes through i, imparting to the air a garlic odor. Advance the flame of the second lamp slowly and gradually up to e, by which means the whole of the arsenic which may have condensed in the wide part of the tube is driven to h. When you have effected this, close the tube at the point i by fusion, and apply heat, proceeding from i towards h, by which means the extent of the mirror is narrowed, whilst its beauty and lustre are correspondingly increased. In this manner perfectly distinct mirrors of arsenic may be produced from 0002 grm. of tersulphide of arsenic. No mirrors are obtained by this process from tersulphide of antimony, or from any other compound of antimony.

13. If arsenious acid or one of its compounds is exposed on charcoal to the reducing flame of the blowpipe a highly characteristic garlic odor is emitted, more especially if some carbonate of soda is added. This odor has its origin in the reduction and re-oxidation of the arsenic, and enables us to detect very minute quantities. This test, however, like all others that are based upon the mere indications of the sense of smell, cannot be implicitly relied on.

§ 133.

e. ARSENIC ACID (AS 0.).

1. Hydrated arsenic acid crystallizes in clear prisms of the formula 2 (3 H 0, As 0,) + aq., which deliquesce in the air. The water of crystallization escapes at 100°; at a higher temperature the water of hydration escapes and the acid fuses. On strong ignition it splits into oxygen and arsenious acid. The anhydrous acid dissolves but slowly

in water. Arsenic acid is poisonous.

2. Most of the ARSENATES are insoluble in water. Of the so-called neutral arsenates those with alkaline bases alone are soluble in water. Most of the neutral and basic arsenates can bear a strong red heat without suffering decomposition. The acid arsenates lose their excess of acid upon ignition, which passes off in the form of arsenious acid and oxygen. A solution of arsenic acid or of an arsenate in hydrochloric acid may be boiled for a long time without losing chloride of arsenic, provided too much hydrochloric acid is not present. But when the residual fluid contains about half its volume of hydrochloric acid of specific gravity 1·12, traces of terchloride of arsenic begin to escape with the hydrochloric acid.

3. Hydrosulphuric acid fails to precipitate alkaline and neutral solutions; but in acidified solutions it causes first reduction of the arsenic acid to arsenious acid, with separation of sulphur, then precipitation of tersulphide of arsenic. This process continues until the whole of the arsenic is thrown down as As S,, mixed with 2 S (WACKEN RODer, LUDWIG, H. ROSE). The action never takes place immediately, and in dilute solutions frequently only after the lapse of a considerable time (twelve to twenty-four hours, for instance). Heating (to about 70°) greatly accelerates the action. If a solution of arsenic acid, or of an arsenate, is mixed with sulphurous acid, or with sulphite of soda and some hydrochloric acid, the sulphurous acid is converted into sulphuric acid, and the arsenic acid reduced to arsenious acid; application of heat

promotes the change. If hydrosulphuric acid is now added, the whole of the arsenic is immediately thrown down as tersulphide.

4. Sulphide of ammonium converts the arsenic acid in neutral and alkaline solutions of arsenates into pentasulphide of arsenic, which remains in solution as a salt of pentasulphide of arsenic and sulphide of ammonium. Upon the addition of an acid to the solution this salt is decomposed, and pentasulphide of arsenic precipitates. The separation of this precipitate proceeds more rapidly than is the case when acid solutions of arsenates are precipitated with hydrosulphuric acid. It is promoted by heat. The precipitate formed is As S,, and not a mixture of As S, with S..

5. Nitrate of silver produces under the circumstances stated § 132, 6, a highly characteristic reddish-brown precipitate of ARSENATE OF SILVER (3 Ag O, As O,), which is readily soluble in dilute nitric acid and in ammonia, and dissolves also slightly in nitrate of ammonia. Accordingly, if a little of the precipitate is dissolved in a large proportion of nitric acid, neutralization with ammonia often fails to reproduce the precipitate. The ammoniacal solution of arsenate of silver does not deposit silver upon boiling (difference between arsenic and arsenious acids).

6. Sulphate of copper produces under the circumstances stated § 132, 7, a greenish-blue precipitate of ARSENATE OF COPPER (2 Cu O, H O, As 0,).

7. If a dilute solution of arsenic acid mixed with some hydrochloric acid is heated with a clean slip of copper the metal remains perfectly clean (WERTHER, REINSCH); but if to one volume of the solution two volumes of concentrated hydrochloric acid are added, a gray film is deposited on the copper, as in the case of arsenious acid. The reaction is under these circumstances equally delicate as with arsenious acid (REINSCH).

8. With zinc in presence of sulphuric acid, with cyanide of potassium, and before the blowpipe, the compounds of arsenic acid comport themselves in the same way as those of arsenious acid. If the reduction of arsenic acid by zinc is effected in a platinum capsule, the platinum does not turn black (difference from antimony).

9. If a solution of arsenic acid, or of an arsenate soluble in water, is added to a clear mixture of sulphate of magnesia, chloride of ammonium, and a sufficient quantity of ammonia, a crystalline precipitate of ARSENATE OF AMMONIA AND MAGNESIA (2 Mg Ŏ, N HO, As 0 + 12 aq.) separates; from concentrated solutions immediately, from dilute solutions after some time. If a small portion of the precipitate is dissolved on a watch-glass in a drop of nitric acid, a little nitrate of silver added, and the solution touched with a glass rod dipped in ammonia, brownishred arsenate of silver is formed. Or if a small portion of the precipitate is dissolved in hydrochloric acid and hydrosulphuric acid is passed into the solution with warming, a yellow precipitate is formed. (Differences between arsenate and phosphate of magnesia and ammonia.)

§ 134.

Recapitulation and remarks.—I will here describe first the different ways adapted to effect the detection or separation of tin, antimony, and arsenic, when present together, and afterwards the means of distinguishing between the several oxides of the three metals.