Notwithstanding the great variety of iron ores, they may be all, as far as analysis is concerned, arranged under three heads; namely, sulphurets, oxides, and salts. The first are distinguished by their general bronze colour, but more particularly by the suffocating smell of sulphureous acid gas, which they afford by being heated to redness in the open air. The second consist of iron united with oxygen, and are by far the most common of all. Nearly the whole of the iron ores in use are of this kind, containing also different proportions of earthy matter in their composition. The third division comprehends such as consist cf the oxide of iron combined with some acid, and hence are called salts; the principal varieties of these are the phosphates, sulphates, arseniates, and carbonates. The various processes employed at our great iron works for the reduction of the different species of iron ore, are given under the article Iron.