a porcelain plate, from which it is easily separated after cooling.

Chloride of Zinc (poisonous).

(Hydrochlorate or muriate of zinc. Butter of zinc.)

This substance is more or less gray, or white in color, according as it has been prepared in iron or porcelain vessels, or has been more or less thoroughly desiccated. It is caustic, greasy, and hot to the touch. It absorbs moisture with such energy that a few minutes of exposure to the air are sufficient to cause it to completely liquefy. It may be distilled without decomposition like mercury and other volatile substances, and then possesses the appearance of butter, whence its name of "butter of zinc."

Chloride of zinc is employed for aiding soldering, brazing, or welding operations, for which uses it should be as neutral as possible in order that the metals shall not be corroded by the free acid. It enters into the composition of the brassing or zincing baths.

It is prepared by dissolving zinc in hydrochloric acid, filtering the solution after it has been left standing for a few days in contact with an excess of zinc, and evaporating it cautiously to igneous fusion. At this moment, abundant and heavy white fumes are disengaged. The mass is then cast into thin plates by pouring it out on porcelain dishes; and these plates are broken up at once after cooling, and kept in well-stoppered glasses.

Cyanide of Silver (poisonous).

(Prussiate, or hydrocyanate, of silver.)

This substance is white, becomes slowly black when exposed to light, and is insoluble in water and in cold acids, which, however, will dissolve it with the aid of

heat. It is dissolved and decomposed by the sulphites hyposulphites, and chlorides; the cyanides and prussiates form with it double salts. A cyanide of silver is always formed when a soluble silver salt is treated with a cyanide.

If introduced dry into a small tube, closed at one end. and drawn out at the other extremity, leaving but a fine aperture, it yields, at a red heat, a deposit of metallic sil ver with carbon, and cyanogen gas which escapes at the small aperture, and which when ignited burns with a mingled blue and purple flame.

Cyanide of silver is prepared by passing cyanogen gas through, or adding hydrocyanic acid to, a cold solution of nitrate of silver. The precipitate formed is thoroughly washed, and kept in a moist condition in blue or black bottles.

We have given, in the article on Cold Electro-gilding Baths, all the directions necessary for properly preparing this substance. It should not be forgotten that, whatever be the silver salt brought in contact with cyanide of potas sium, it is first transformed into cyanide of silver, which is redissolved in an excess of cyanide of potassium, to form the double cyanide of potassium and silver, which last constitutes the silvering bath.

Cyanide of Copper (very poisonous).

(Hydrocyanate of copper. Prussiate of copper.)

A few electro-brassers, and many gilders and nickel platers use cyanide of copper.

Two salts are called by this name; one (improperly so called) is the ferro-cyanide, a powder of a maroon Vandyke-brown color, obtained by the precipitation of a soluble copper salt with ferro-cyanide of potassium; the other, the cyanide, a dirty white powder with a

greenish

yellow tinge, resulting from the precipitation of a soluble copper salt by cyanide of potassium.

Whatever be its mode of production, it is freely soluble in all the alkaline cyanides.

By solution in an excess of cyanide it forms the double cyanide of potassium and copper, constituting the bath used for electro-coppering.

Cyanide of Ammonium (poisonous).

(Hydrocyanate of ammonia. Prussiate of ammonia.) We may obtain with this salt the same baths as with the cyanide of potassium. It is, however, rarely used, because it is easily decomposed, and has a disagreeable cadaveric smell. It is obtained by exactly neutralizing ammonia with hydrocyanic (prussic) acid.

Cyanide of Calcium (poisonous).

(Hydrocyanate, or prussiate, of lime.)

This salt is employed for decomposing the carbonates formed in baths of cyanide of potassium. It will be readily understood how, by double decomposition, an insoluble carbonate of calcium is formed, while a proportional quantity of cyanide of potassium is reconstituted. This salt, in the majority of cases, should be preferred to hydrocyanic acid.

A solution of cyanide of calcium is obtained by adding hydrocyanic (prussic) acid to freshly-slaked caustic lime in excess. By filtration, the excess of lime remains behind, and the cyanide of calcium is found in the filtrate. This salt cannot be obtained in the solid state, is decomposed by heat, and it is better to use it when recently prepared.

Cyanide of Gold (poisonous).

(Prussiate, or hydrocyanate, of gold.)

It is of an earthy yellow color, and behaves with reagents very much like the cyanide of silver. Cyanide of gold is prepared by precipitating a solution of chloride of gold with a solution of cyanide of potassium. An excess of alkaline cyanide must be avoided, as it will dissolve the precipitate and form a double cyanide of gold and potas sium. This salt is employed for the preparation of gilding baths, and is preferred to the chloride for this purpose, as it avoids the objection of introducing chloride of potas sium into the gilding solution.

Cyanide of Potassium (extremely poisonous).

(White prussiate of potash.)

No product is more important to the electro-plater than the cyanide of potassium, which is the basis of most of the baths employed, and the purity of which is so necessary for the success of the operation.

At the same time, nothing more closely resembles a good cyanide than a bad one; a fact that has aided many manufacturers in producing, at cheap rates, products which have nothing in common with real cyanide but the

name.

It may be formed by introducing dry ferro-cyanide of potassium in an iron crucible heated to redness, and maintaining the mass in a state of fusion for about a half hour, or as long as gas continues to be given off, keeping the crucible closed as much as possible during the operation. In Liebig's plan, eight parts of dry ferro-cyanide of potassium are mixed with three parts of dry carbonate of potassium and fused as above.

Cyanide of potassium, or rather all of the alkaline

cyanides at first precipitate metallic solutions, and then redissolve the metallic cyanide to form double salts. A certain number of metals, however, should be excepted, such as platinum, bismuth, antimony, tin, etc.

Cyanide of potassium forms with the salts of peroxide of iron a precipitate of Prussian blue. This color is developed especially in the presence of an excess of hydrochloric acid, but disappears on the addition of an alkali.

Ferrocyanide of Potassium (poisonous).

(Yellow prussiate of potassa.)

This product is manufactured on a large scale, and has many uses in the arts. It occurs in the shape of fine yellow, and semi-translucent, crystals with mother-of-pearl lustre, which break gradually and without noise. The fracture is jagged, and filled with a multitude of small bright spots.

The dried salt is yellowish-white; at a higher temperature it decrepitates, and is decomposed into white cyanide of potassium, and cyanide of iron, the cyanogen of which in its turn is expelled as gas, while the iron is reduced to the metallic state, or to that of a carbide.

The solution of ferrocyanide of potassium is strawyellow, and, like the simple cyanide of potassium, precipitates and redissolves afterwards nearly all metallic salts. Its solvent power, however, is much less energetic. The metallic anodes are but slightly dissolved in the baths composed of yellow prussiate, which renders the use of such baths expensive.

The ferrocyanide is somewhat poisonous, and does not emit any smell, or absorb moisture.

It is prepared by carbonizing animal matters, such as blood, horn, hair, etc., with a mixture of carbonate of