Caustic potass in a solution not too diluted causes a fine azure-blue precipitate, the hydrated peroxide of copper.
Oxide of arsenic, with the previous addition of a few drops of ammonia, causes a fine apple-green or grass-green precipitate, the arsenite of copper. This test, which is both delicate and characteristic, has been already fully considered under the head of Arsenic.
The process by fluid reagents, as hitherto laid down, merely proves the presence of copper, but does not indicate the acid with which the oxide is combined. In order to determine whether it is sulphuric acid, the fluid must also be tested with nitrate of baryta followed by nitric acid: a heavy white precipitate is thus produced, which the excess of nitric acid does not redissolve.
4. Artificial Verdigris.
Artificial verdigris is a common pigment, which is met with in the form either of earth-like masses, or of a light powder of a greenish-blue colour and peculiar disagreeable smell, approaching that of vinegar. Like blue vitriol it has a strong metallic, astringent taste. The effect of heat is peculiar. Some acetic acid is in the first place distilled over; a portion of the acid, however, is decomposed and reduces the oxide; and a low red heat is sufficient to make the outer crust of the verdigris distinctly copper-red, when the material is contained in a glass tube.
Artificial verdigris varies somewhat in composition. Foreign verdigris contains chiefly the hydrated diacetate, with a little carbonate, oxide, and even metallic copper, along with particles of the fruit and fruit-stalks of the grape. British verdigris consists of little else than the hydrated diacetate. It is known by the following characters. Ammonia dissolves it almost entirely, forming a deep violet solution. Diluted sulphuric acid dissolves it, evolving an odour of acetic acid, and forming a solution of sulphate of copper, which may be known by the tests for that salt. Boiling water converts it partly into an insoluble brown powder, which is oxide of copper in union with a small proportion of acetic acid, and partly into a greenish-blue neutral acetate, which is dissolved, and may be known by the four tests for sulphate of copper, and the want of action of nitrate of baryta.
It may be right to notice shortly three other salts of copper, the nitrate, the ammoniacal sulphate, and the muriate. The nitrate forms a violet solution, which is acted on by reagents in the same way as the dissolved acetate, but has not any odour of vinegar. The ammoniacal sulphate [ammoniated copper—ammoniuret of copper], has been occasionally used in medicine. It forms, when solid, small scaly crystals, of an intense violet colour and strong ammoniacal odour; and when dissolved it retains its peculiar colour even though very much diluted.—The muriate of copper has a lively grass-green colour, and is acted on by reagents in the same way as the solution of verdigris.
Of the corrosion of copper by articles of food and drink.—To these observations on the chemical history of copper a few remarks must be added relative to the action of various articles of food or drink upon the metal. Unpleasant accidents have often happened from the use of copper vessels in the preparation of food; and it is therefore necessary for the medical jurist to know the circumstances, so far as they have been investigated, under which the poison may be dissolved.
Dr. Falconer found, that distilled water kept several weeks on a polished plate of copper, neither injured its lustre, nor acquired any taste, nor become coloured with ammonia;[[1050]] and Drouard afterwards observed, that distilled water, kept for a month on copper filings, did not contain any of the metal.[[1051]] Eller of Berlin, however, remarked, that water, if it contain a considerable quantity of common salt, as four ounces in five pounds, or a twentieth part, will give slight traces of copper after being boiled in a brass pan; and that if the pan be made of copper, a powder is procured by evaporation, which when treated with acetic acid yields so much as 20 grains of acetate of copper.[[1052]] But it is a singular circumstance, also observed by the same experimentalist, that if beef of fish be boiled with the usual allowance of salt, and with the addition also of various vegetable substances, the liquid does not yield any copper. This observation has been lately denied by Professor Orfila; who says he found copper deposited on a plate of iron in salt water in which beef had been boiled, and that he also obtained copper from the beef itself.[[1053]] The quantity thus dissolved, however, must be exceedingly small, if the copper be kept clean and free of oxide; for copper vessels, although they have often been the source of fatal accidents, if carelessly used in the preparation of food, have appeared under careful management to be quite harmless. An excellent practical confirmation of this will be found in Michaelis’s Commentaries. He states, that in the Orphan Hospital of Hallé, the food was in his time prepared in large copper vessels, which were kept remarkably clean; and that out of a population of eight or nine hundred he never heard of any one having suffered from symptoms of poisoning with copper.[[1054]] Several other saline matters promote the solution of copper in water. Thus Dr. Falconer found that alum has this effect when aided by heat; and probably nitre and Epsom salt possess the same quality.[[1055]] Their mode of action is not very well known.
It is a common though erroneous idea, that milk, heated or allowed to stand in a copper vessel, becomes impregnated with the metal. Eller has shown, that, on the contrary, if the vessel be well cleaned, milk, tea, coffee, beer, and rain-water, kept in a state of ebullition for two hours, do not contract the slightest impurity from copper;[[1056]] and the same remark has been also made by Dr. Falconer with respect to cabbage, potatoes, turnips, carrots, onions, rice, and barley.[[1057]]