pot, the valuable tin in the form of tinners' waste and wornout tin utensils is all practically wasted. It is scarcely probable that the utilitarian spirit that is so pronounced in our industrial communities will long suffer this discreditable condition of things to exist, and a practical solution of the problem of utilizing tin waste will doubtless sooner or later be found.

Of the many processes that have been attempted for this purpose, only two seem to have given any promise of success. We refer to them briefly as a matter of record.

One plan consists in dissolving off the tin in hydrochloric acid, and throwing down the metal with the aid of granulated zinc. The tin is precipitated as a metallic powder, which, after the solution of any surplus of zinc, may be collected, washed, melted, and run into ingots. This process involves the formation of zinc chloride, for which there is but a limited demand, and much of which would therefore be wasted.

By another plan the tin scrap is roasted in a furnace with free access of air, which oxidizes the layer of tin. The scrap is then removed from the furnace and the film of oxide of tin is shaken off by a simple mechanical procedure, and smelted with other tin ore; or, treated with sulphuric acid, which takes up the oxide of iron mixed with it, and leaves the tin behind. The latter may then be regained by the usual reduction process. The iron scrap left behind by the roasting process may be utilized, and the sulphate of iron (green vitriol) resulting from the purification of the mixed oxides may be crystallized and sold as such, or disposed of to the chemical manufacturers, who may gain the sulphuric acid by distillation, while the residue in the retorts may be utilized in the manufacture of Venetian red. This procedure is the most complete that has yet been devised.

Other plans involve the treatment of the scrap with caustic alkali, and the production of stannate of sodium, which is largely used in dyeing and calico printing.

CHAPTER LXVI.

GALVANIZING BY IMMERSION IN MOLTEN ZINC.

THIS is the name given to the process of coating metals, usually wrought and cast iron, with a coating of zinc, by immersion after suitable preparation, in a bath of the latter metal maintained in a state of fusion; it is therefore strictly analogous to the process of tinning previously described.

The object to be attained is likewise the same, namely, to improve the appearance of the coated metal, and to protect it from the action of corrosive agents.

The name "galvanized iron," applied to iron thus prepared, is somewhat misleading, since it would seem to imply that the coating was effected by galvanic agency. It is intended to apply, however, to the nature of the protective influence, and not to the process.

As we have noticed in the case of tinned iron, two metals in contact, in the presence of moisture, form a galvanic couple, and the result of the galvanic action set up under these conditions will be analogous to that which takes place in a galvanic battery, i. e., the electro-positive element of the couple will be dissolved or corroded, while the electro-negative element remains unaffected or protected, so long as the action continues, the intensity of the chemical action being governed by the relative positions of the two metals in the electro-chemical series.

Tin and iron standing some distance apart, give rise, it was stated, to a decided galvanic action, in which the iron as the electro-positive metal is corroded more rapidly than it would be if untinned. With zinc and iron, the conditions are reversed, zinc is positive to iron, and consequently, whatever galvanic action is set up by their contact in the presence of moisture (or other excitant), must have for effect the solution or corrosion of the zinc and the protection of the iron.

Zinc and iron, however, stand so near to each other in their electrical relations, that they can only originate the feeblest galvanic action. We must look, therefore, for the explanation of the undoubtedly excellent protective influence of zinc upon iron principally to the comparative indifference of zinc to the action of air and moisture.

Unlike tinned iron, "galvanized" iron resists the action of the weather to a remarkable degree. The zinc surface, after a brief exposure, becomes coated with a thin film of oxide, but this seems to adhere tenaciously, and forming a continuous and impenetrable skin, effectually protects the underlying zinc from further oxidation.

So long, therefore, as the zinc surface remains intact, the underlying iron is effectively protected from corrosive action, but, as in the case of tinned iron, wherever the zinc coating is defective or has been removed, there the iron is oxidized. When immersed in water the zinc coating is soon completely oxidized and removed. Salt water acts more energetically; and even the sea-air, charged as it is with salty moisture, is rapidly destructive in its action upon galvanized iron wares. Under ordinary circumstances, however, where it is simply exposed to the weather, a good coating of zinc affords an excellent protection for iron, the latter remaining unaffected for several years, being in this respect, therefore, greatly su

perior to tinned iron, and at the same time much less costly. As compared with tinning, galvanizing is a comparatively new industry, having been introduced in Europe during the early part of the present century, and brought from England to the United States less than fifty years ago. At the present time it has assumed the proportions of a great industry on both sides of the Atlantic.

Sheet iron, for architectural, domestic, and miscellaneous purposes, pipe for gas and water distribution, railings, awning-posts, chains, hoops, and bands, wire and wire cloth, park, cemetery, and garden furniture, kitchen sinks and range boilers, buckets, scuttles, hods, and numberless miscellaneous articles of cast and wrought iron, described as small hardware, comprise principally the articles that are galvanized.

As the galvanizing of sheet iron is one of the principal branches of the industry, and the one that requires the greatest care, a description of this operation as conducted in American works will answer for the rest.

Galvanizing Sheet Iron.-As in the case of the galvano-plastic deposits, so in "galvanizing," the surface of the metal to be coated requires to be freed from oxide and impurities of every description before it will take a proper coating of zinc. The preliminary cleansing, however, does not require to be done with such extreme care as in the case of galvanoplastic deposits.

The first step of the process is therefore the operation of "pickling" (i. e., the removal of the black scale). For this operation there is usually provided a stout, rectangular wooden tank, from five to six feet high, and nine to ten feet long, and wide enough to accommodate from 100 to 200 sheets at a time. This contains the pickling liquid, commonly commercial sulphuric acid, diluted with

from ten to twelve parts of water; and into this liquid, heated by steam, as many sheets are immersed (on edge) as the tank will accommodate. In from one to two hours the scaling of the iron is effected, a workman or two all this time industriously wedging the individual sheets apart with suitably contrived tongs to insure the penetra tion of the pickle to all the sheets. This done, the sheets are withdrawn from the pickle tank and transferred to large, shallow washing-vats of wood, in which they are washed in a stream of fresh water. They are next subjected to an inspection, in order that any obstinately ad hering particles of scale or cinder, that the acid may have failed to take off, may be removed. For this purpose they are passed, one by one, to a bench or table, where a workman, armed with a stiff brush and a triangular-shaped steel scraper, carefully examines each sheet, and removes such patches of scale as may yet adhere in spots upon the pickled sheet. From his hands they pass into a second shallow tank filled with clean water (which should properly be renewed from time to time), and in which they are allowed to remain for 12 or 24 hours. The purpose of this washing is to remove all traces of sulphuric acid and basic sulphate of iron from the pickled sheets, the retention of which would otherwise act inju riously upon the character and the durability of the zinc coating. In order that the work may go on continuously, several of such water tanks must be provided, so that a lot of iron shall always be ready for the next operation. This is termed "clearing," and consists in immersing the sheets from the water tanks, for a few minutes, in commercial muriatic acid contained in a flat wooden tank like the water tanks above described. This treatment removes every trace of oxide from the sensitive iron skin, forms a thin layer of chloride upon it, and renders