In order to prepare ferric oxide suitable for an artists’ pigment the following process may be used: 17 parts of soda are dissolved in 68 parts of water; the solution is boiled in an iron pan, and 10 parts of crystallised ferrous sulphate are added in small quantities with continual stirring. The boiling and stirring are continued until the green vitriol has completely dissolved, when the greenish white precipitate is allowed to settle, washed several times with water, and then exposed to the air in thin layers. The precipitate, which begins to turn yellow during washing, becomes in a short time ochre yellow in the air, being changed into ferric hydroxide. After drying and calcining, a fine red powder of pure ferric oxide is formed. The shade depends on the temperature at which the substance is calcined: the higher the temperature and the longer the heating is continued, the darker is the product. =Vogel’s Iron Red.=—This preparation, which is particularly brilliant and therefore highly suitable for an artists’ colour, is made by adding a saturated solution of oxalic acid to a boiling solution of green vitriol. The greenish yellow precipitate of ferrous oxalate is collected on a filter and well washed with water. After drying, the precipitate is heated in a shallow iron dish to a temperature of 200° C., at which the ferrous oxalate decomposes and is converted into a soft fiery red powder consisting of pure ferric oxide. By igniting this powder in covered crucibles the different shades of ferric oxide can be obtained. =Macay’s English Red.=—Seven hundred and four parts of ferrous sulphate, 1,000 parts of copper chloride, and 1,678 parts of common salt are dissolved, the solution boiled and the precipitate ignited. In the manufacture of fuming or Nordhausen sulphuric acid, ferric oxide is obtained as a residue. It is then known under the names of English red, _caput mortuum_, colcothar, rouge and Indian red as a very cheap pigment. It is also used as a polishing material. Fuming sulphuric acid is made by heating green vitriol at a white heat in retorts placed in furnaces; sulphur dioxide and trioxide are evolved, whilst in the retorts there remains a residue of almost pure ferric oxide, containing small quantities of basic ferric sulphate, which can only be decomposed by long continued violent heating. The vapours of sulphur trioxide are caught in receivers containing oil of vitriol, in which they dissolve and produce fuming sulphuric acid.