2. _Deutoxide._ This oxide is _black_; it contains 25 oxygen on 100 copper: the _black_ oxide is obtained by dissolving copper in nitric or sulphuric acid, then precipitating by lime water or an alkali, and heating the dried precipitate red hot. It may also be obtained by exposing copper to a red heat for some time in common air or oxygen gas, removing the scales and exposing them in like manner, till at length the black oxide is formed. By dissolving 112 grains of copper turnings in 1000 grain measures of 1.16 nitric acid, I obtained 48 oz. measures of nitrous gas, = 30 grains; by oxymuriate of lime I found 2 grains of nitrous gas in the solution, making in all 32 grains = 28 grains of oxygen. If 28 ∶ 112 ∷ 14 ∶ 56, for the weight of an atom of copper; hence the protoxide = 63 and the deutoxide = 70. These weights I adopted in 1806, and have not seen any reason to modify them since. 10. _Oxides of Iron._ Two well known and well distinguished oxides of iron are now universally admitted; the one contains 28 oxygen on 100 iron, the other 42 on 100. 1. _Protoxide._ This is always formed when iron is dissolved in dilute sulphuric or muriatic acid; it may be precipitated from these solutions by the pure alkalies or earths; it appears at first of a dark green, being then a hydrate or combined with water; on a filtre it soon becomes yellow at the surface by attracting oxygen; when dried in a heat of 200° or upwards it becomes black. The quantity of oxygen in it is best ascertained from the hydrogen generated during the solution of the iron. All the authorities I have found nearly concur in their results as under. 100 grains of iron dissolved in dilute sulphuric or muriatic acids yield hydrogen, according to Cavendish (1766) 155 cubic inches. Priestley, from 147 to 162 Lavoisier 163 Vandermonde, Berthollet, } max. 176 and Monge } Vauquelin 160 to 179 Dr. Thomson 163 My own Experiments give 160 ---- Mean 164 = 82 oxygen = 27.9 grains. By precipitating the oxide, and drying it, nearly the same result may be obtained, as 100 iron will yield 128 oxide. This oxide is magnetic.