The position which sulphuric acid occupies in Chemistry is due mainly to three outstanding features. In the first place, it is a strong mineral acid and displaces all other acids from their salts. Secondly, it has a high boiling point (338° C.), and consequently, the displaced acid with the lower boiling point can be distilled from the mixture. Lastly, sulphuric acid can be made very cheaply from materials which are very abundant in Nature, and, therefore, it meets all the requirements of an acid which is to be used for general purposes. SULPHATES All the common metals, except gold and platinum, dissolve either in concentrated or in dilute sulphuric acid, forming sulphates. These salts are highly important and interesting substances. They are all soluble in water, with the exception of the sulphates of calcium, strontium, barium, and lead. Ferrous Sulphate, also called green vitriol and copperas, is obtained by dissolving iron in dilute sulphuric acid. The solution is green, and when it is evaporated, the crystals which separate out look like bits of green glass. It was because of this that the substance was first called green vitriol (_vitrum_ = glass). It is used very largely in dyeing as a mordant. Writing ink and Prussian blue are also made from it. The Alums are double sulphates. They are made by crystallizing solutions of potassium, sodium, or ammonium sulphate together with solutions of iron (ferric), chromium, or aluminium sulphates. In this way, we may have potassium aluminium alum, or iron ammonium alum, and so on, but whichever combination of elements is present, the salt which is formed always crystallizes in octahedra. The chief use of the alums, as also of aluminium sulphate, is as mordants in dyeing. Since a great many metallic salts, particularly acetates and sulphates, are used in the dye industry as mordants, it may be well to explain here very briefly what a mordant is. It must be remembered that almost all the dyes are solids which dissolve in water, yielding intensely coloured solutions. Hence, in most cases, if a fabric is merely dipped in the dye and then dried, the colouring is not permanent, but can be washed out with water. In order to fix the colouring matter, the material is first dipped in the mordant, usually a bath of some metallic salt, and then, generally after exposure to air or after steaming, into the dye bath, with the result that the colour becomes fixed. The first part of the process is called “mordanting” the material. The mordant either adheres to or combines with the fibres, and the dye forms with the mordant a coloured compound called a “lake,” which resists the action of water. The colour is then said to be “fast,” that is, firmly fixed.