cially in cities where the large consumption of coal and of coal-gas charges the atmosphere with sulphur compounds.

Of all the suggestions that have been made to accomplish this object none appear to have given such satisfactory results as a varnish of collodion (soluble gun-cotton, photographer's cotton), a solution of gun-cotton in a mixture of ether and alcohol. All other varnishes appear to impart a yellowish tint to the silver or silvered wares; but collodion varnish is quite colorless. The articles should be carefully brushed over with the varnish, with an elastic brush, making sure that the entire surface is covered. The film of collodion will protect the underlying metal for a long time.

Lustrous Gold or Green on Brass.- Puscher recommends the following procedures for producing a golden coloration upon brass (Kunst u. Gewerbe, 1883, 59). Dissolve 4 parts each of caustic soda and milk sugar in 100 parts of water. Heat the solution to boiling for a quarter of an hour; remove the vessel containing the dark yellow solution from the fire, and add with stirring 4 parts of a cold saturated solution of sulphate of copper. When the fluid has cooled off to about 165° Fah., and after the separated suboxide of copper has settled, the clean, polished articles of brass are introduced into it, in a vessel of wood with perforated bottom. In about two minutes the golden coloration will probably be dark enough, and the articles are removed, washed and dried in sawdust. If the articles are permitted to remain for a longer time in the liquid, the yellowish color at first developed will change to a lustrous bluish-green, and finally to iridescent colors. In order to obtain uniform colorations, the temperature must be maintained between 140° and 165° Fah.

The copper bath may be used repeatedly, and for this purpose should be kept in a well-stoppered flask. When it has become spent, it may be revivified by the addition of one part of caustic soda, of the quantity of water evaporated, heating to boiling, and the addition of 21 parts of sulphate of copper. If, instead of milksugar, glycerine or a bitartrate is used, the colors obtained are not so uniform.

Colorations on Surfaces.-Puscher (Neueste Erfind., etc.) proposes the following procedure for obtaining a permanent dark or

light green coating, resembling enamel on articles of zinc, espe cially those of sheet zinc. Fifty parts of hyposulphite of sodium are dissolved in 500 parts of boiling water, and the solution poured at once, in a fine stream, into 25 parts of strong sulphuric acid. [We should prefer to reverse the operation, namely, to pour the acid into the salt solution.-W.] The milk of sulphur that separates will soon ball together in lumps, and settle. The hot liquid containing sulphate of sodium and sulphurous acid is decanted, and the cleansed zinc placed in it. In a short time it will acquire a very brilliant light green coating, which only needs to be washed and dried.

By exposing it, and for a longer time, to this hot bath, the coating grows thicker, and the color darker and more brilliant. The temperature should not be allowed to fall below 145° Fah., to insure a fine, brilliant deposit.

By dipping the articles thus treated in diluted hydrochloric acid (1 acid to 3 water) sulphuretted hydrogen is evolved, and this enamel-like coating loses its lustre and becomes lighter in color. Aqueous solutions of aniline colors have little effect upon this dull surface, and none whatever on the brilliant coating.

The effect of “marbling" can be obtained by moistening the gray zinc and applying hydrochloric acid in spots with a sponge, then rinsing off, and, while still wet, flowing over it an acidified solution of sulphate of copper, which produces the appearance of black marble. As this has a dull surface it should be varnished.

By adding 15 parts of chrome alum and 15 more of the hyposulphite to the above solution, the article treated will take on a brownish color. The above treatment may be applied, according to Puscher, to articles of sheet and cast zinc [and also to galvanized wares.-W.].

Colored Coatings for Metals.-The following method of proce dure for obtaining decorative effects on metal surfaces, although it has never yielded entirely satisfactory results in our hands, is so highly spoken of by some other writers (notably by Urquhart) that we are induced to attribute our want of success to our having failed in giving it proper care. It is as follows:

In a quart of pure water dissolve one ounce of hyposulphite of

sodium. Stir into this another solution, made by dissolving an ounce of acetate of lead in a pint of water.

For use, heat the solution in a glass or earthenware pan to about 195° Fah., and immerse the metal required to be colored. The coating is one of lead sulphide, and its depth of color will depend upon the time the metal is immersed. In a few minutes brass articles of small size may be coated with any color, varying from golden yellow to the tint of clean copper or red gold, to carmine, down to dark red, from light aniline blue to bluish-white, then to reddishwhite or brown.

Urquhart remarks of this process, that it is not a good plan to plunge the articles into the solution without previous preparation; it is by far best to chemically clean them as for plating, and, before dipping in the coloring liquid, to keep them for a few minutes in hot water, unless the coating is to be very slight indeed. Steel and iron articles may also be treated, and given a fine blue color, without the aid of such great heat as is necessary in "bluing" or oxidizing. Copper articles do not, of course, show the lighter tints. If the cleaning is well done, the adhesion will be perfect, so perfect indeed, that the burnisher may be used with impunity; but it is not prudent to use the scratch-brush. Instead of burnishing, however, the surface may be finished by a soft and smooth buff, which will impart a lasting polish.

The solution will not keep long in the heated state, as it deposits its sulphide upon the bottom of the vessel if no metals are present.

It is stated that an exceedingly beautiful red and green coloring can be given to brass articles by omitting the lead, and using in its stead an equal weight of sulphuric acid (see Colorations on Zinc Surfaces, page 618). If the immersion continues, the red changes to a fine, brilliant green, and then to green and brown, with a splendid iris glitter. The coating is very durable, unlike the others, which are but lacquered over with pale lacquer (Urquhart).

The same authority affirms that clean brass and copper may be covered with a firmly adherent pure black coating by placing them very near to the flames of burning straw. It will not rub off, and may be polished with a soft cloth.

REFINING AND EXTRACTION OF METALS BY ELECTROLYSIS (on the Commercial Scale).

Although this subject does not fall properly within the province of this work, it may be of use to some of our readers to notice that great progress has been made during the past few years in the application of the electrolytic process to the extraction of metals from their ores-and the refining of impure metals (copper, argentiferous lead) on the commercial scale. In these operations, it need scarcely be explained, steam-driven dynamo-machines are used. Such electro-metallurgical processes are carried on, on a scale of greater or less magnitude, in Germany, England (Wales), and the United States.

For special information on this subject, the reader is referred to the files of the technical Journals, particularly to Dingler's polytech. Journal, and the Engineering and Mining Journal, in which the progress of this branch of the metallurgical industry is very fully recorded.

Depositing Copper on Steel Wire.-An interesting operation showing the perfection to which the electrolytic deposition of copper has been brought, is the manufacture of what is known as "Compound Wire," which is carried out on a large scale at Ansonia, Connecticut, by the Postal Telegraph Company.

This compound wire consists of a steel wire core, weighing 200 pounds to the mile, and having a tensile strength of 1650 pounds, on which copper is deposited continuously by the galvanic process, and of any desired thickness. On the present scale of these works, the electric currents for depositing the copper are generated by twenty-five large steam-driven dynamo-machines, with the aid of which 10,000 pounds of copper are deposited per day, representing 20 miles of compound wire" carrying 500 pounds of copper to the mile. At the time of this writing, the company has announced its intention to shortly double the capacity of its works. The "compound wire" is used for telegraphic purposes, for which on account of its high conductivity and strength it is claimed to possess many advantages over the galvanized iron wire at present generally used for that purpose.

LISTS OF AMERICAN AND BRITISH PATENTS BEARING ON THE SUBJECT OF THE ELECTRO-DEPOSITION OF METALS.

AMERICAN PATENTS.

1850. December 10. No. 7,821. G. Mathiot. Electrotyping. 1855. September 4. No. 13,516. J. A. Adams. Electrotyping machine. 1856. December 2. No. 16,141. W. Filmer and E. Bookhout. Mode of backing electrotype plates.

1857. July 7. No. 17,741. A. H. Jocelyn. Method of backing electrotype plates.

1858. May 25. No. 20,353. S. P. Knight. Production of electrotype

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plates.

September 14. No. 21,509. H. Lovejoy and R. Wheeler. Machine for coating electrotype moulds.

1859. May 3. No. 23,836. W. H. Elliot. Mode of forming curved elec

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trotype plates.

November 1. No. 25,953. T. Crossley. Electrotype printing

block.

1860. November 20. No. 30,663. A. Berthoud. Electro-plating with

alloys of gold.

1861. November 12. No. 33,721. J. von Liebig. Electro-plating with copper, etc., the surfaces of mirrors, etc.

1862. February 18. No. 34,470. J. E. Walcott. Process of electro-plating iron and other metals with copper.

66 March 11. No. 34,640. M. Miller, Jr.

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steel wires for piano-strings, etc.

Mode of electro-plating

October 21. No. 36,750. C. Beslay. Process of electro-plating iron,

steel, etc.

1865. December 26. No. 51,714. J. D. Grüneberg. Manufacture of plated

metal.

1867. February 19. No. 62,251. S. A. Chapman. Plated-ware burnish

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66

1868.

ing machine.

April 2. No. 63,512. S. Hallock. Surface conductor for electro

typing.

May 28. No. 65,084. E. Hunter. Compound for silver plating. March 10. No. 75,258. M. L. Forbes. Frame for plating spoons, etc., heavier at certain parts than at others.