surfaces of many substances which are not naturally conductors, and also for preventing the adherence of two superposed metals. Plumbago is also used for bronzing; in which case, the preliminary purification with hydrochloric acid will be unnecessary. Plumbago of extreme purity and fineness, especially prepared for the use of galvanoplastic operators, may be obtained in the trade.

When plumbago is moistened with a solution of chloride of gold in ether, and then allowed to dry in a shallow vessel exposed to solar light, we obtain a gilded plumbago, which is a far better conductor of electricity than plumbago alone. We may also prepare a silvered plumbago, by calcining at a red heat, and in a closed crucible, plumbago which has been moistened with an aqueous or ammoniacal solution of 5 to 10 parts of nitrate of silver per 100 parts of graphite employed. By incorporating various bronze (metallic) powders with plumbago, a metallized product of more or less conductivity is obtained.

Roseleur's Amalgamating Salt (poisonous).

This is a triple salt of mercury, or rather a mixture of three salts of mercury, namely, the sulphate, nitrate, and chloride. It is liquid, more or less colored, very dense, and gives in water a yellow precipitate, which dissolves in an excess of acid. It produces a violet stain on the skin, and amalgamates copper and its alloys thoroughly and rapidly. It is used for amalgamating the zincs of batteries, and does away with the necessity of using metallic mercury. Though not more economical than mercury alone, it is more easily applied and prevents much trouble in electro-plating establishments. It may be substituted for the nitrate of mercury "quicking" solution, but the converse is not true, as this last when employed as an amalgamating salt, rapidly perforates the zinc.

Amalgamating salt is prepared by boiling the nitrate of mercury with an excess of a powder composed of equal parts of sulphate and chloride of mercury, i. e., mercuric sulphate and mercuric chloride; the liquid remaining after cooling is used.

Sulphate of Copper (poisonous).

(Blue-vitriol. Blue-stone.)

This salt is manufactured in large quantities for dyeing. preserving wood, galvanoplastic operations, etc. It occurs generally in the form of large rhomboidal crystals, translucent and of a fine blue color. Its taste is acid, metallic, and astringent, like ink, but more persisting. On being heated, it first loses its water of crystallization and becomes white; and, at a higher temperature, the sulphuric acid is expelled in the form of dense white fumes, and there remains a blackish-brown oxide of copper. It is freely soluble in water, especially when the latter contains some free acid, and the solution is deep blue. Hot water dissolves much more of this salt than cold, and the excess of the salt crys tallizes out on cooling.

Sulphate of copper solution constitutes the galvanoplas tic baths proper, which are rendered more conducting by the addition of one-tenth (by volume) of sulphuric acid.

Sulphate of copper solution is decomposed, without the aid of the battery, by certain metals, such as iron, zine, and steel, which become coated with copper. Tin and

bismuth are blackened.

Commercial sulphate of copper is frequently impure. containing variable proportions of the sulphates of iron and zinc, which are injurious when their amount is too great. Sulphate of zinc is detected by passing through the acid solution a current of sulphuretted hydrogen gas the sulphide of copper produced is separated by filtration,

and the clear filtrate is treated with ammonia, which produces a white precipitate of oxide of zinc, soluble in an excess of alkali. The iron remains also in the acid liquor filtered from the copper, and its presence is ascertained by the addition of a small quantity of ferrocyanide of potassium, which gives a deep blue coloration. Another process for the detection of iron consists in adding to a small quantity of the solution of sulphate of copper enough ammonia to dissolve all of the oxide of copper at first precipitated, when the brownish oxide of iron will be seen floating in the intensely blue liquid.

The best sulphate of copper is made by dissolving in sulphuric acid the copper scales produced in rolling sheets of this metal. That obtained by the spontaneous oxidization of copper pyrites is always contaminated with iron

or zinc.

We should avoid, for our art, those cheap copper sulphates extracted from old acid dipping liquors, since they contain zinc and other metals, and also nitrate of copper with free nitric acid. Galvanoplastic baths prepared with them are a nuisance. Such sulphates are generally very wet and in small crystals.

Sulphate (of Protoxide) of Iron.

(Ferrous sulphate. Proto-sulphate of iron. Copperas. Green vitriol.)

This salt crystallizes in forms similar to the preceding one, and has a fine green color. Its taste is that of ink, and it is very soluble in water, which it colors a light green. It is rapidly oxidized by contact with the air, and becomes first yellow, then reddish, and is transformed finally into a sulphate of peroxide of iron, which possesses quite different properties.

On heating sulphate of iron it first loses its water of

crystallization and becomes white. At red heat it is de composed into sulphurous and sulphuric acids, which escape, and a red and pulverulent residue which, according to its fineness, its intensity of color, etc., is variously called colcothar, anhydrous sesquioxide of iron, rouge, etc.

The proto-sulphate of iron is employed for precipitating gold from its acid solutions. The sesqui-sulphate does not possess this property.

It is prepared, either by evaporating and crystallizing the liquors used for cleansing iron and steel, or by the oxidization, in the air, of iron pyrites. The salt obtained by this latter process generally contains some copper, which may be precipitated by placing iron scraps in the solution.

Sulphate of Zinc.

(White vitriol. White copperas.)

The trade furnishes this salt in three forms: either in white or opaque plates, or in large transparent crystals. or in a mass formed of a quantity of needle-like crystals. resembling those of sulphate of magnesium or of tin salt.

Its taste is sour, styptic, and metallic, and it is very soluble in water, which remains colorless. It is decom posed by a strong heat, leaving a residuum of oxide of zinc. Its neutral solutions yield a white precipitate with sulphuretted hydrogen, but no precipitate is produced from acid solutions. The sulphide of zinc is the only white metallic sulphide which is known; and this is the reason why zinc paints remain white, notwithstanding the presence of sulphurous gases, which so rapidly blacken lead paints.

Sulphate of zinc is employed for zinc and brass electrobaths, in the preparation of acids for dead lustre dipping. and for a frosted lustre upon clocks and jewelry.

Sulphite (and Bisulphite) of Sodium.

The former salt forms white prismatic crystals, which are rapidly transformed into an amorphous powder by efflorescence.

It possesses no smell, but its taste is flat, saline, and sulphurous. It is very soluble in water, and is gradually transformed into sulphate by the absorption of oxygen from the air. Powerful acids decompose it, with abundant evolution of sulphurous acid, which is easily recognized by its smell of burning sulphur. During this reaction, the solution remains clear, and is thus distinguished from the hyposulphite, which also disengages sulphurous acid, but deposits sulphur in the solution.

Sulphite of sodium, and generally all the soluble sulphites, dissolve the salts of gold, silver, copper, etc., under certain circumstances, and transform them into colorless double salts, which possess more or less stability, and are employed in the preparation of electro-baths, for gilding, coppering, and brassing.

The sulphite of sodium may absorb an excess of sulphurous acid, and thus become a bisulphite, which is always to be preferred to the neutral salt.

The neutral sulphite of sodium is prepared by passing a stream of sulphurous acid gas through a solution of carbonate of sodium until the liquid is neutral to testpaper. If the solution be very concentrated, a quantity of small crystals of bicarbonate of sodium become precipitated in the course of the operation, and it should be stirred to prevent them from obstructing the gas delivery tube. An excess of sulphurous acid decomposes them with an abundant production of carbonic acid. The saturated liquid, if concentrated, crystallizes on cooling; if it is not, it should be evaporated to the proper point.