tion. A possible loss of sulphuretted hydrogen on mixing the sulphide of zinc with sesquichloride of iron may be prevented by conducting the decomposition in a flask, connected with a U-tube containing sesquichloride of iron.

3. CARL MOHR'S METHOD.*

This method is based upon the following considerations:

I. If a solution of acetate of zinc, acidified with acetic acid, is mixed with an excess of ferricyanide of potassium, the whole of the zinc is thrown down in the form of a reddish yellow precipitate of ferricyanide of zinc Zn, (CyFe).

II. If solution of iodide of potassium is now added in excess, we have this decomposition:-2[Zn, (Cy ̧Fe,)]+2KI + 2(A, HO)=3[Zn, (Cy,Fe)]+2(KO,A)+H2(CysFe)+21.

III. 1 eq. liberated iodine corresponds, accordingly, to 3 eq. zinc.

IV. If iodide of potassium is made to act upon ferricyanide of zinc in a neutral fluid the liberated iodine acts upon the ferrocyanide of potassium present in that case, which leads to the formation of a little ferricyanide of potassium; the remaining free iodine, therefore, will not indicate, with accuracy, the quantity of zinc present. But whereas the reaction actually takes place in acid solution of acetate of zinc, as above directed, it may be assumed that acetate of potassa and free hydroferrocyanic acid are formed; and as iodine exercises no appreciable action upon the latter substance, the iodine liberated in the process indicates, with tolerable accuracy, the amount of zinc present.

The process is as follows:

Treat the ore with aqua regia, as in 1, a, and drive off the greater part of the free acid; nearly neutralise with carbonate of soda, add acetate of soda in excess, boil, filter, and wash with boiling water mixed with a little acetate of soda. The solution is iron-free; it contains the whole of the zinc, but, in presence of manganese, also the whole of the latter

* Dingler's Polyt. Journ. 148, 115.

metal. Hence the process is not applicable in the presence of manganese.

Mix the solution of zinc, prepared as directed, with ferricyanide of potassium in slight excess, i.e. until a sample of the clear supernatant fluid gives a blue precipitate with a salt of protoxide of iron. Then add a sufficient quantity of iodide of potassium. The fluid acquires a brown colour, in consequence of the liberation of iodine; the white precipitate of ferrocyanide of zinc is suspended in the brown fluid. Determine now the free iodine by means of hyposulphite of soda, and calculate 3 eq. zinc for each eq. iodine. The results obtained by C. Mohr are very satisfactory. The method can be employed only if the acetic acid solution contains no other heavy metal besides zinc, and, more particularly, no manganese.

BLOWPIPE REACTIONS OF ZINC.

ZINC BLENDE, BLACK JACK, SULPHIDE OF ZINC.-Alone, decrepitates violently. Suffers no remarkable change on ignition; does not fuse, and gives off but a very slight odour of sulphurous acid, being very difficult to roast.

On charcoal, an annular deposit of oxide of zinc is formed when heated violently in the outer flame.

Soda attacks it feebly; but the zinc is reduced in a good fire, with the deposition of oxide of zinc on the charcoal. CARBONATE OF ZINC, CALAMINE.-Alone, gives off no water, but becomes a white enamel, which behaves like oxide of

zinc.

OXIDE OF ZINC.-Alone, becomes deep yellow when heated. This assay must be made by daylight. It reassumes its white colour on cooling. It does not fuse, but gives off a vivid light during incandescence. It is gradually evaporated in the reducing flame, during the continuance of which a yellow ring is deposited on the charcoal, which becomes white on cooling.

With borax it fuses readily, and gives a transparent glass, which, with a large proportion of oxide, becomes milky by flaming. It assumes an enamel-white appearance on cool

ing. In the reducing flame the metal sublimes, and covers the charcoal with a white film.

With microcosmic salt it behaves as with borax, except that the metal sublimes less readily with the first than the

second.

Soda does not dissolve it; but acted on by this reagent on charcoal, it is reduced, and covers the charcoal with a coating of oxide.

453

CHAPTER XIV.

ASSAY OF MERCURY.

MERCURY is found in the native or metallic state, and as sulphide or cinnabar :

Native mercury, Hg.

Sulphide of mercury, cinnabar, Hg2S.
Bituminous sulphide of mercury.

There are other minerals of mercury met with, but hitherto not in sufficient quantity to be worked for the metal. They are:

Zinciferous subsulphide of mercury.
Zinciferous sulphide of mercury.

Selenide of mercury.

Subchloride of mercury.

Iodide of mercury.

Silver amalgam (see Silver).

is

Assay of Mercurial Ores.-The determination of mercury is always made by distillation. In case the mercury present in the form of native mercury, or oxide of mercury, it is distilled without any addition. The ore (say from 500 to 1,000 grains) is placed in an iron or earthenware retort, which is set over a suitable fire, and the heat raised gradually, and kept up, until the whole of the mercury has passed over. The mercury which passes over is collected either in the neck of the retort, or a receiver fitted for that purposesuch as a glass flask kept cool by affusion with water. When but a small quantity is operated upon (say 150 to 200 grains), it is most convenient to use a glass retort, or bent tube retort, heating it gradually over a charcoal fire, taking care to keep the upper part so hot, that no metallic mercury

may adhere to it. It must be heated nearly to the melting point of the glass, and until all the mercury has come over.

When the operation is finished, the neck is cut off, weighed, the mercury detached, and weighed again: the loss of weight is the amount of mercury. Or the metal may be detached by means of a feather, and allowed to fall into a basin of water, which, if heated for a few seconds, will cause the mercury to collect into one globule: the water may be decanted, and the mercury dried at the ordinary temperature, and weighed.

The mercury wholly condenses in the neck of the retort, under the form of a metallic dew. Some may by chance pass off; but in order to prevent such an occurrence, the beak of the retort is plunged into water, or a small dossil of linen, moistened with water, introduced into the neck, the end of which is plunged into water, by which means the neck of the retort is kept constantly cool, and the mercury is found deposited on the linen, from which it detached by shaking in water.

may

be

When large quantities of substances containing mercury are operated upon, it is necessary to heat very strongly towards the end, in order that the centre of the mass may receive a sufficient amount of heat to effect its decomposition. Naked glass retorts cannot be used; and either coated glass or porcelain retorts must be employed. In the large way, as in the distillation of amalgams, &c., cast-iron retorts are used.

As before stated, all substances containing mercury, either in its metallic state or as oxide, are distilled without addition, but with the others it is necessary to employ some reagent, which will separate and retain the sulphur, selenium, &c.; which reagent may be a metal, as iron, copper, or tin; or black flux, or a mixture of quick-lime and charcoal: iron filings are most often used. For cinnabar about 50 per cent. of iron filings is required, in order to prevent any it being sublimed; the true quantity required is only about 24 per cent., but an excess is necessary, in order, as before stated, to prevent loss: 50 per cent. of iron filings may be employed for the selenides, &c. When black flux is used, from about 50 to 70 per cent. is employed. Caustic

of