Metallic copper, perfectly pure, does not possess any deleterious properties. We have already cited instances[306] sufficiently conclusive to establish this fact. It becomes, therefore, a subject of no little interest to enquire, under what circumstances it may become poisonous by combination. _M. Orfila_ observes that it has been long maintained, that milk heated, or allowed to remain in vessels of copper not oxidized, dissolved a portion of this metal, and acted as a poison. _Eller_, a philosopher of Berlin, has, however, very clearly proved such an opinion to be incorrect. He boiled in succession, in a kettle well freed from verdegris, milk, tea, coffee, beer, and rain water; after two hours boiling, he found it impossible to discover, in any of these fluids, the least vestige of copper. _M. Drouard_ has also shewn that distilled water, left for a month together on the filings of this metal in a glass bottle, did not dissolve an atom of it. The celebrated toxicologist above cited, after relating these important facts, concludes by observing, that the phenomena are very different, if, instead of pure water, we substitute that which contains a certain quantity of muriate of soda. _Eller_ has demonstrated the presence of a very small quantity of copper in water, which contained 1/20th of its weight of muriate of soda, and which had been boiled in a brass kettle. This fact is of the highest importance, for it will explain the reason why highly seasoned aliments have proved deleterious, when cooked in vessels of copper. But we are indebted to _Mr. Eller_ for a still more important discovery; he found that if, instead of heating a simple solution of common salt in copper vessels, the salt be previously mixed with beef, bacon, and fish, the fluid resulting from it does not contain an atom of copper.[307] In relating this fact, _M. Orfila_ observes, “however astonishing it may appear, it is quite correct, _M. Eller_ was the first to announce it, and I have several times ascertained the truth of it; it is probable,” continues _Orfila_, “that the combination of several kinds of aliments destroys the effect of the solution of the muriate of soda; which consequently ought to render the cases of poisoning by aliments cooked in copper vessels, _which are not oxidized_, extremely rare.” Copper combines with sulphur, and affords a black sulphuret. OXIDE OF COPPER. By oxidation, copper becomes poisonous. The substance may be easily recognised by the change of colour which it produces in ammonia; this alkali will dissolve it instantly, and assume a beautiful blue colour. It is wholly insoluble[308] in water. In oils and fatty matter it is easily and copiously dissolved at the ordinary temperature of the atmosphere. Such bodies also, when boiled in vessels of perfectly clean copper, facilitate their oxidation, especially if left to cool a few minutes before they are poured out.