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Second, or Finishing Solution

Dissolve 4½ troy ounces cyanide of potassium, been polished; it is therefore a necessary and 14 ounces cyanide of silver, in 1 gallon practice in the electroplating factories to polrain or distilled water. This solution should ish the articles before plating. This is done be used with one large cell of Smee's battery, by means of a circular brush, more or less observing that the silver plate is placed as near the surface of the articles to be plated as possible. hard as required, fixed upon a lathe, and a thin paste made of oil and pumice-stone ground as fine as flour. By this process the surface of any article can be smoothed and polished; but a little experience is required to ensure success, and enable the operator to polish the surface equally without leaving brush marks.

After this the article must be cleaned in potash before it is plated. 3714. Boettger's Test for the Silver on Silver-Plated Metals. The test fluid consists of a saturated solution of bichromate of potassa in nitric acid, specific gravity 1.2.

Any dirt or varnish having been removed by strong alcohol from the metallic surface to be tested, a drop of the test fluid is applied to it 3717. To Recover Silver from Copper. by means of a glass rod, and immediately Instead of stripping off the silver by means afterwards washed off with some cold water. of acid, as in No. 3716, it is a more common If pure silver is present (as regards silver and preferable mode to brush off the silver coins, these are left in contact with the test by the operation just described. In this case fluid for a greater length of time), there will the brushings must be collected, dried, and appear clearly a blood-red colored mark burned; this may be done in an iron pan, (chromate of silver). Upon German silver keeping it at a red heat until all carbonaceous the test liquid appears brown, but after wash- matters are consumed; the remainder is fused ing with water the blood-red colored mark with carbonate of soda or potash, when the does not appear; the so-called britannia-metal silver is obtained, in combination with a little is colored black; on platinum no action is vis- copper. ible; metallic surfaces coated with an amal- 3718. Cyanide of Silver and Potasgam of mercury yield a reddish speck, which, sium, its Decomposition During the however, is entirely washed off by water; on Plating Process. The silver salt in the lead and bismuth the test liquid forms a yel- plating solution is a true double salt, being, low-colored precipitate; zinc and tin are as already described, a compound of 1 equiboth strongly acted upon by this test liquid, which, as regards the fo former metal, is entirely removed by water, while, as regards the latter, the test liquid is colored brownish, and addition of water produces a yellow precipitate which somewhat adheres to the tin. valent of cyanide of silver, and 1 of cyanide of potassium-two distinct salts. In the decomposition of the silver solution by the electric current, the former, cyanide of silver, is alone affected; the silver is deposited, and the cyanogen passes to the positive plate or electrode. The cyanide of potassium is therefore set at liberty upon the surface of the article