You see this is quite a different process of pigment colouring from that of rubbing or working a colour mechanically on to the fibre. Let us now turn to the substantive colours (Group I.), and see if we can further sub-divide this large group for the sake of convenience. We can divide the group into two--(_a_) such colours as exist ready formed in nature, and chiefly occur in plants, of which the following are the most important: indigo, archil or orchil, safflower, turmeric, and annatto; (_b_) the very large sub-group of the artificial or coal-tar colours. We will briefly consider now the dyestuffs mentioned in Group (_a_). _Natural Substantive Colours._--Indigo, one of the most valuable dyes, is the product of a large number of plants, the most important being different species of _indigofera_, which belong to the pea family. None of the plants (of which _indigofera tinctoria_ is the chief) contain the colouring matter in the free state, ready-made, so to say, but only as a peculiar colourless compound called _indican_, first discovered by Edward Schunck. When this body is treated with dilute mineral acids it splits up into Indigo Blue and a kind of sugar. But so easily is this change brought about that if the leaf of the plant be only bruised, the decomposition ensues, and a blue mark is produced through separation of the Indigo Blue. The possibility of dyeing with Indigo so readily and easily is due to the fact that Indigo Blue absorbs hydrogen from bodies that will yield it, and becomes, as we say, reduced to a body without colour, called Indigo White, a body richer in hydrogen than Indigo Blue, and a body that is soluble. If this white body (Indigo White) be exposed to the air, the oxygen of the air undoes what the hydrogen did, and oxidises that Indigo White to insoluble Indigo Blue. Textile fabrics dipped in such reduced indigo solutions, and afterwards exposed to the air, become blue through deposit in the fibres of the insoluble Indigo Blue, and are so dyed. This is called the indigo-vat method. We can reduce this indigo so as to prepare the indigo-vat by simply mixing Indigo Blue, copperas (ferrous sulphate) solution, and milk of lime in a closely-stoppered bottle with water, and letting the mixture stand. The clear liquor only is used. A piece of cotton dipped in it, and exposed to the air, quickly turns blue by absorbing oxygen, and is thus dyed. The best proportions for the indigo-vat are, for cloth dyeing, 4000 parts of water, 40 of indigo, 60 to 80 of copperas crystals, and 50 to 100 of dry slaked lime. The usual plan is to put in the water first, then add the indigo and copperas, which should be dissolved first, and finally to add the milk of lime, stirring all the time. Artificial indigo has been made from coal-tar products. The raw material is a coal-tar naphtha called toluene or toluol, which is also the raw material for saccharin, a sweetening agent made from coal-tar. This