The deposit obtained from the double chloride has a peculiarly matted surface, and is difficult to polish and extremely liable to tarnish on exposure to the air. The deposit obtained from the double sulphate is much easier to polish, but is also very hard and inflexible, and, if thick, is especially liable to separate in lamina3 from the surface on which it is deposited. The solution of the double chloride of nickel and ammonium is better adapted for coating iron with nickel than the double sulphate solution, the double sulphate solution answering better for brass.
Both these solutions become gradually changed in use ; in the second, the ammonium salt is decomposed by the action of the current, and free ammonia is evolved, the solution becoming rich in nickel and losing ammonia, these changes affecting the quality of the deposited metal. These difficulties may be overcome by adding to the bath boracic acid. E. Weston, to whom this improve ment is due, states that an excellent solution may be made by using 5 parts chloride of nickel and 2 parts boracic acid, or with 3 parts sulphate of nickel and 1 part borscic acid. Either of these solutions may have added to them with advantage caustic soda, potash, or lime, until the precipitates formed cease to be dissolved.
For the recovery of nickel from old solutions, a saturated solution of ammonia sulphate should be added to the old solution with constant stirring ; no effect will be observed at first, but in a few moments a deposit of the double sulphate will begin to form. The precipitation should be con tinued until the liquid is colourless ; the resulting salt is quite pure, and may be used direct in making the new solution.
Brass deposits, which are commonly made on zinc articles which have to be afterwards bronzed, may be made as follows :—To 2 gal. of water are added 4 oz. each of carbonate of copper and car bonate of zinc) recently prepared, 8 oz. each of pure cyanide of potassium, pearlash, and common soda, and oz. of white arsenic ; the sides of the tank are lined with one or more brass sheets, joined together, connected to the last carbon or copper of the battery, the intensity of the latter is regulated by the surface of the articles to be electroplated. These are suspended by copper hooks to stout rods of the same metal, all connected to the last zinc of the battery. There are numerous other brass baths in use, and an excellent one, introduced by Watt, is prepared and worked as follows : For each gallon of solution take, say, 2 oz. of sheet brass of the best quality, cut it into strips, and place these in a porcelain capsule or stone jar ; pour on about 4 oz. of commercial nitric acid, to which a little water may be added. Allow the chemical action to proceed in a cool place at
first, and when the red fumes of nitrous gas cease to be visible, apply gentle heat. When the acid has become saturated, the supernatant solution must be transferred to another vessel, and a smaller quantity of acid poured on to the undissolved alloy, and this treated as before. It is advisable not to have the acid in excess ; and if a small fragment only of the metal be undissolved, it is better to put it aside then to run the risk of an excess of acid. The acid solution of the brass alloy is next to be diluted with about 2 qt. of water, and a solution of carbonate of potash, salts of tartar, added gradually, to precipitate the respective metals. For this purpose 1 lb. of carbonate of potash may be dissolved in a quart of cold water, ready for use. Upon adding the carbonated alkali, care must be taken, or the effervescent mixture will overflow, carrying the precipitate with it. This precaution is specially necessary when the precipitation is near completion.
A light bluish-green precipitate is formed in the abovo operation, which must bo allowed to subside. The clear liquor should, however, be tested with a drop or two of the alkaline solution, in order to ascertain if all tho metal has been thrown down. If this is the case, the precipitato may be either washed by filtration, or by the addition of hot water, and frequent subsequent wash ings. Although the washing of the precipitate is a somewhat slow process, it is important to push it as far as possible, sine the salts remaining in the solution do not in any way improve the working or general character of the solution itself.
After the final washing, the precipitate is to be treated with strong liquid ammonia (• 880), which must be added gradually, with constant stirring. The ammonia will redissolve the preci pitate, forming a dark-blue solution. To this now must be added a strong solution of cyanide of potassium, until the blue colour entirely disappears ; and beyond this about 4 or 5 oz. of freo cyanide must be added to assist the solution of the anode. It is even more important in this than in some other eleotro-metallurgical operations that the cyanide be of good quality. The brass solution should now be filtered, and water added to make one gallon. It may be worked either hot or cold, but if the former, it may be advisable to reduce it by adding a little water. The anode should be of the same quality as the alloy from which the solution was made ; indeed the latter would be best prepared from a portion of the sheet of brass which is destined to act as an anode.