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.

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 Heat the bulb now of potassium is likely to ascend into the tube.

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

a

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.

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

chalk, as this would not give a constant stream of gas). B is a smaller flask containing concentrated sulphuric acid. The flask A is closed with a double-perforated cork, into the one aperture of which is inserted a funnel-tube (a), which reaches nearly to the bottom of the flask; into the other perforation is fitted a tube (b), which serves to conduct the evolved gas into the sulphuric acid in B, where it is thoroughly freed from moisture. The tube c conducts the dried gas into the reductiontube C, of which Fig. 28 gives a representation on the scale of one-third of the actual length. The tubes which I employ for the purpose in my own experiments, have an inner diameter of eight millimètres.

When the apparatus is fully prepared for use, 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. 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 halfway 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 evolve a moderate stream of carbonic acid, by pouring some hydrochloric acid into the flask A. Heat the tube C in its whole length very gently with a spirit-lamp, until the mixture in it is quite dry; when every trace of water is expelled, and the gas-stream has become so slow that the single bubbles pass through the sulphuric acid in B at intervals of one second, heat the reduction-tube to redness at c, by means of a spirit or gas-lamp; when c is red-hot, apply the flame of a second gas or larger spirit-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 surrounding air the peculiar odor of garlic. Advance the flame of the second lamp slowly and gradually up to c, 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 pointi 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 as little as the th part of a grain of tersulphide of arsenic. No mirrors are obtained by this process from tersulphide of antimony, nor from any other compound of antimony.

13. If arsenious acid or one of its compounds is exposed on a charcoal support to the reducing flame of the blowpipe, a highly characteristic garlic odor is emitted, more especially if some carbonate of soda is added to the examined sample. 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.

§ 132.

é. ARSENIC ACID (AS O2).

1. Arsenic acid is a transparent or white mass, which gradually deliquesces in the air, and dissolves slowly in water. It fuses at a gentle red heat without suffering decomposition; but at a higher temperature it is resolved into oxygen, and arsenious acid, which volatilizes. It is highly 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, the free acid being resolved into arsenious acid and oxygen.

3. Hydrosulphuric acid fails to precipitate alkaline and neutral solutions of arsenates; but in acidified solutions it produces a yellow precipitate of PENTASULPHIDE OF ARSENIC (AS S1). This precipitate never forms instantaneously, and in dilute solutions frequently only after the lapse of a considerable time (twenty-four hours, for instance). Heat promotes its separation. The pentasulphide of arsenic manifests the same deportment as the tersulphide with the various solvents and decomposing agents mentioned in the preceding paragraph. 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 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 ammonio-pentasulphide of arsenic (pentasulphide of arsenic and sulphide of ammonium). Upon the addition of an acid to the solution, this double sulphide 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.

5. Nitrate of silver produces under the circumstances stated § 131, 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.

6. Sulphate of copper produces under the circumstances stated § 131, 7, a greenish-blue precipitate of ARSENATE OF COPPER (2 Cu O, H O, As 0,).

7. With zinc in presence of sulphuric acid, with copper, 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 dish, the platinum does not turn black, as is the case in the reduction of antimony by zinc (§ 130, 8).

8. 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 O, N H, O, As 0, + 12 aq.) separates; from concentrated solutions immediately, from dilute solutions after some time.

$133.

4

Recapitulation and remarks.-I have again made the separation and positive identification of the oxides belonging to the second division of the sixth group the object of a most careful study, and my endeavors have been crowned with complete success. I will here describe first the different ways best adapted to effect the detection or separation of tin, antimony, and arsenic, when present together in the same compound or mixture, and afterwards the most reliable means of distinguishing between the several oxides of each of the three metals.

1. If you have a mixture of sulphide of tin, sulphide of antimony, and sulphide of arsenic, triturate 1 part of it, together with 1 part of dry carbonate of soda, and 1 part of nitrate of soda, and transfer the mixed powder gradually to a small porcelain crucible containing 2 parts of nitrate of soda kept in a state of fusion at a not over-strong heat; oxidation of the sulphides ensues, attended with slight deflagration. The fused mass contains binoxide of tin, arsenate and antimonate of soda, with sulphate, carbonate, nitrate, and nitrite of soda. You must take care not to raise the heat to such a degree, nor continue the fusion so long, as to lead to a reduction of the nitrite of soda to the caustic state. Treat the fused mass, poured out upon a piece of porcelain, with cold water until it is completely softened; then filter the fluid off from the undissolved residue, which contains the binoxide of tin and antimonate of soda nearly unacted on. Mix the filtrate, which contains the arsenate of soda and the other salts, with nitric acid to distinctly acid reaction, then with a sufficient proportion of solution of nitrate of silver; a precipitate of chloride of silver forms (if the reagents employed or the precipitated sulphides contained a chlorine compound) and some nitrite of silver. Filter, and carefully add to the filtrate dilute solution of ammonia, whereupon the characteristic reddish-brown precipitate of arsenate of oxide of silver will make its appearance, first in the uppermost stratum of the fluid where the solution of ammonia comes first into contact with it, but subsequently, upon complete neutralization of the free acid, in every part of the fluid.

Wash now the filter containing the residuary binoxide of tin and antimonate of soda once with water, then three times with a mixture of equal parts of water and spirit of wine, dry, incinerate,* and put the ash into a tube of difficultly fusible glass, sealed at one end, measuring some eight or ten centimètres in length, and having an inner diameter of from five to seven millimètres; add to the ash in the tube four times the quantity of cyanide of potassium, and heat over a Berzelius or gas-lamp. This effects the reduction of the binoxide of tin and

* With small quantities of substance this may be done most conveniently by twisting the little filter together, inserting it into a spiral coil of platinum wire, and holding this in the outer mantle of a flame.

+ Had not the nitrate of soda been removed by washing, as directed, this part of the process would be attended with an explosion.