A reliable solution may be made by dissolving nitric acid in about five times the quantity of water. Another solution is made by mixing a solution of nitric acid and water (l to 10 parts respeetivelyi with about an equal quantity of potassium chloride dissolved in 16 of water. A mixture of nitric acid 20 parts with 1 of muriatic acid, may be used, or a solution containing equal quantities of nitric acid and water and a few small pieces of copper may be tried.