potassa and iron scraps. The mass is then lixiviated with water, and the solution allowed to crystallize.

Cyanide of Zinc.

(Hydrocyanate of zinc. Prussiate of zinc.)

A few brass electroplaters still employ this salt, although it is costly, and does not present any real advantage over the other zinc salts. It is white or dirty white, according as the zinc is free from or contaminated with iron.

It is insoluble in water, but soluble in ammonia, and in the earthy or alkaline sulphites and cyanides, with which it forms double salts, suitable for zinc electro-baths. Its solution is the more easy according as it has been more recently prepared.

Cyanide of zinc is obtained by incompletely precipi tating with cyanide of potassium a solution of sulphate, nitrate, chloride, or acetate of zinc. The precipitate is drained upon a filter of paper or muslin, and completely washed to remove the remaining soluble zinc salt.

Gelatine.

(Isinglass. Glue.)

Every one is familiar with gelatine, and with the fact that it is extracted by acids, or by superheated water, from bones, skin, cartilage, and similar substances; and that it is more or less colored, according to its degree of purity. The commoner sorts are called glue, and are employed for making galvanoplastic moulds. The least colored are preferred, because the casts are more delicate.

which

Cold water swells up and softens gelatine, but hot water will dissolve it in all proportions. This property, is very useful for the production of the moulds, is, on the contrary, disadvantageous when the mould is in the gal vanoplastic bath. This inconvenience is partly remedied

by adding to the gelatine, before pouring it upon the pattern, a few hundredths of tannic acid, which combines with it, and forms a kind of leather which resists the action of the liquids better. At all events, gelatine moulds should be rapidly coated with the metallic deposit, otherwise they will give very imperfect, or valueless, copies.

While it is true that hot water dissolves a great proportion of gelatine, which sets on cooling, this property disappears after too prolonged ebullition, when a syrup is left which will not coagulate.

Benzole.

(Benzine. Light oil from coal tar.)

When coal tar is distilled, there remains in the retort a thick mastic called pitch, and the distillate is composed of essential oils, having different specific gravities and points of ebullition. Those which are lighter than water are collected apart, and are deprived of their coloring matter by more or less prolonged treatment with concentrated sulphuric acid, and then with soda. After the proper washings and one or several distillations, a liquid is obtained which is colorless, smelling strongly of coal gas, with a sharp and bitter taste, and which is completely insoluble in water, although it imparts its odor to the latter. This product becomes oxidized and reddens under the action of solar light, when it has not been perfectly rectified.

Benzole is an excellent solvent of all the oils, resins, gums, varnishes, fats, etc., and is therefore very useful in our art. It is much superior to the alcohol and essence of turpentine formerly employed for removing stoppingoff varnishes, and may be used in the cold, which is a great advantage with inflammable substances. A small proportion of naphthaline is sufficient to give a pink, red,

or brown tinge to benzole; but this is of no consequence for our operations. The preparation of this substance involves such great danger from fire that we cannot recom mend our readers to undertake it. Moreover, it is found in the trade very cheap, and pure enough for our pr poses.

Phosphate of Ammonium.

This salt, which is absolutely necessary for the compe sition of baths for thick platinum deposits, is obtained by the exact saturation of phosphoric acid with ammonia.

The liquid obtained is then evaporated at a gentle heat, and a few drops of ammonia are now and then added, in order to compensate for that removed by the decomposi tion of small quantities of the salt. When the liquid becomes syrupy it is set aside to crystallize in a cool place.

This salt may also be prepared by decomposing, with carbonate of ammonium, the acid-phosphate of calcium. resulting from the digestion in sulphuric acid of ground

and calcined bones.

Phosphate of Sodium.

(Tribasic phosphate of soda.)

This salt crystallizes in fine, transparent, and colorles prisms; its taste is slightly bitter and saline, and it effe resces, losing part of its water of crystallization. It is so uble in distilled water without producing any precipitat but causes a deposit of white phosphate of calcium calcareous waters. The composition of this salt will best understood by supposing it to consist of phosphor acid containing three atoms of hydrogen, two of whi have been replaced by sodium. Hence its name tribasic.

At a temperature of about 400° Fah., phosphate of sodium diminishes in volume, and loses all of its water of crystallization, but not that combined. By still raising the temperature, it melts to a glass and loses the combined water, which it will not reacquire except by remaining a very long time in solution. Its nature and properties have been entirely changed; and it gives now a white precipitate with nitrate of silver, instead of a yellow one, as was the case previous to its transformation into pyrophosphate or bibasic phosphate. It is capable of combining with a metallic base, in place of the equivalent of water lost. It is this property which renders the pyrophosphate valuable for the preparation of baths for gilding by simple immersion, and of tinning baths either by the battery, or by the method of double affinity. In these cases it assimilates respectively an equivalent of oxide of gold or of oxide of tin.

Phosphate of sodium is used for hot electro-gilding baths, and is prepared by treating calcined and powdered bones with sulphuric acid, and letting the mixture rest for several days. The acid phosphate of calcium is then removed by washing the residue, and the filtered liquid is saturated with carbonate of sodium until carbonic acid is no longer disengaged. The clear liquid is then concentrated until it marks 33° Baumé, and is allowed to crystallize once or several times.

Pyrophosphate of Sodium.

(Bibasic phosphate of soda.)

The commercial salt is generally in the form of a white powder, odorless, and with a hot, saline, alkaline, and then bitter taste. It is soluble in water, but not so readily as the preceding salt, and requires distilled water,

in preparing baths with it, since it produces a precipitate in ordinary calcareous waters.

The pyrophosphate of sodium gives a white precipitate with nitrate of silver, whereas that with the ordinary tribasic phosphate is yellow.

It is employed for the preparation of simple-immersion gilding baths; and is obtained by fusing the ordinary dried tribasic phosphate, which by this operation loses an equivalent of combined water, and becomes bibasic. The temperature required is quite high, and few crucibles will stand the heat and the action of this substance which acts as a flux.

Plumbago.

(Black-lead. Graphite.)

This is nearly pure carbon, and is found in crystalline or amorphous masses in several countries, as in England, Russia, Germany, Ceylon, the United States, etc. This carbon is black, with a metallic lustre, soft to the touch, without smell or taste, and is difficult to ignite.

Plumbago, in the natural state, is generally mechani cally mixed with a variable proportion of iron and earths, which may be removed more or less perfectly by washing the previously finely pulverized mineral with dilute hydrochloric acid.

Plumbago varies very much in quality. The best plumbago for our purposes is very black, and without much lustre, except after rubbing; it should firmly adhere to articles of wax and plaster of Paris, and should not become detached from them by being immersed into a liquid. The best manner of ascertaining its quality and purity is to apply a galvanoplastic deposit upon it. The more promptly it becomes regularly coated the better it is.

It is employed for imparting electric conductivity to the