_Protoxide._ Gold is scarcely affected by pure sulphuric, nitric or muriatic acid; but it is easily oxidized and dissolved by nitro-muriatic acid (that is, a mixture of nitric and muriatic acids) when assisted by a temperature of 150 or 200°. Caustic potash being put into the solution and heated, a brownish black precipitate is obtained; but a part of the oxide remains in solution combined with the muriate of potash, according to Vauquelin; and Proust has observed that the oxide cannot be washed and dried in a moderate heat without a portion of the gold being revived; hence the difficulty of ascertaining in this way the weight of oxygen combining with gold. I have succeeded, as I apprehend, in determining the relative weights of gold and oxygen, by two methods, which mutually corroborate each other. The first is by means of the nitrous gas generated by the solution of gold; and the second is, by finding what quantity of green oxide of iron is converted into red by precipitating a given weight of gold in solution. Ten grains of guinea gold of the sp. gr. 17.3, were repeatedly dissolved in a small excess of nitro-muriatic acid; the quantity and purity of the nitrous gas generated were duly observed and allowance made for the loss occasioned by a small portion of common air originally in the gas bottle. The volume of nitrous gas corrected as above was always found between 1100 and 1200 grain measures, the weight of which may be estimated at 1.6 grains, corresponding to 1.4 grains of oxygen. The small portion of alloy (¹/₁₂) known to be in standard gold is chiefly copper with a small part silver; now it will be seen in the sequel that copper takes ¼ of its weight of oxygen; hence if we allow .8 of a grain for copper and .2 for the oxygen combining with it, we shall have 9.2 gold united to 1.2 oxygen, or 100 gold with 13 oxygen, which is nearly the same as Berzelius has determined by precipitating the gold by mercury.--Again, 10 grains of gold were dissolved as above (= 9.2 pure) and precipitated by a solution of pure green sulphate of iron of the sp. gr. 1.181 and which I had previously proved to contain 9 grains of green oxide in 100 measures. They converted 120 measures of this green sulphate into yellow, which was carefully precipitated afterwards by lime water, dried and weighed. The gold precipitated was found very nearly 9 grains; and the yellow oxide of iron mixed with oxide of copper was nearly 13 grains. Now 120 measures sulphate iron contain 10.8 grains green oxide, and these require ¹/₉ of their weight of oxygen (see the oxides of iron) to be changed into yellow oxide, or 1.2 oxygen. Hence it appears that the oxygen combined with the gold was transferred to the iron unchanged in quantity. It is to be observed however that green oxide of iron not only deoxidates the gold but it semideoxidates the copper also; so that .1 of the transferred oxygen above might be said to be derived from the copper, and the rest, or 1.1