A variety of hydrate of alumina, heavy, and therefore easily washed, is obtained by boiling a solution of alum with a plate of zinc lying on a copper plate until all the alumina has separated. By collecting this on a filter and pouring hot water over it a number of times it is obtained quite pure. Alumina plays a particularly important part in the manufacture of cobalt colours. When treated with a cobalt salt and heated it takes a fine blue shade. In the foregoing those metals and their compounds have been treated which are extensively used in preparing colours without themselves forming coloured compounds. The chromates and prussiates are an exception to this. The ammonia compounds, the alkalis and alkaline earths, also the acids, are used in making many colours, although they do not contain colouring principles. The alumina compounds are in similar case. Themselves colourless, they form at the same time a carrier for the coloured compound and bring it into a suitable form for use as a pigment. An example will explain what we mean by “carrier” of the colouring matter. Logwood contains a very handsome colouring matter which can be extracted by water. In order to be able to employ this colouring matter as a pigment it is combined with alumina, a compound insoluble in water being formed, which is called a lake. In this compound the alumina is to be regarded as the carrier of the colouring matter, which it has fixed in an insoluble form. In dyeing, which in many respects is closely allied to colour making, the property of certain metallic compounds, themselves colourless, of fixing dyes is commonly utilised, the metallic compound being called the mordant. The fabric is first prepared with the metallic compound or mordant, and the colour then formed by bringing the mordanted material in contact with the colouring matter. The “heavy metals” form, among their numerous compounds, a great number of coloured substances, and several of them are distinguished by a great wealth of coloured derivatives; for example, copper, chromium and cobalt form coloured compounds only. Although the use of pigments derived from the heavy metals has been considerably restricted in recent years by the discovery of a series of colouring matters which replace them, yet they are now, and always will be, of very great importance in the manufacture of colours. It is, therefore, necessary briefly to describe the various metals which are used in colour making, so that the manufacturer may know what metals produce harmless colours, permanent and unaltered by the atmosphere, and what do not. The metals are divided into two great groups, designated, according to their specific gravities, the group of the light and of the heavy metals. The light metals comprise the alkali, alkaline earth and earth metals, whose important compounds we have just described. The specific gravity of each of these metals is less than five times that of water.