In the dyeing of wool and felt with coal-tar colours, it is in many cases sufficient to add the solution of the colouring matters to the cold or tepid water of the dye-bath, and, after introducing the woollen material, to raise the temperature of the bath. The bath is generally heated to the boiling-point, and kept there for some time. A large number of these coal-tar colours show a tendency of going so rapidly and greedily on to the fibre that it is necessary to find means to restrain them. This is done by adding a certain amount of Glauber's salts (sulphate of soda), in the solution of which coal-tar colours are not so soluble as in water alone, and so go more slowly, deliberately, and thus evenly upon the fibre. It is usually also best to dye in a bath slightly acid with sulphuric acid, or to add some bisulphate of soda. There is another point that needs good heed taking to, namely, in using different coal-tar colours to produce some mixed effect, or give some special shade, the colours to be so mixed must possess compatibility under like circumstances. For example, if you want a violet of a very blue shade, and you take Methyl Violet and dissolve it in water and then add Aniline Blue also in solution, you find that precipitation of the colour takes place in flocks. A colouring matter which requires, as some do, to be applied in an acid bath, ought not to be applied simultaneously with one that dyes best in a neutral bath. Numerous descriptions of methods of using coal-tar dyestuffs in hat-dyeing are available in different volumes of the _Journal of the Society of Chemical Industry_, and also tables for the detection of such dyestuffs on the fibre. Now I will mention a process for dyeing felt a deep dead black with a coal-tar black dye which alone would not give a deep pure black, but one with a bluish-purple shade. To neutralise this purple effect, a small quantity of a yellow dyestuff and a trifle of indigotin are added. A deep black is thus produced, faster to light than logwood black it is stated, and one that goes on the fibre with the greatest ease. But I have referred to the use of small quantities of differently coloured dyes for the purpose of neutralising or destroying certain shades in the predominating colour. Now I am conscious that this matter is one that is wrapped in complete mystery, and far from the true ken of many of our dyers; but the rational treatment of such questions possesses such vast advantages, and pre-supposes a certain knowledge of the theory of colour, of application and advantage so equally important, that I am persuaded I should not close this course wisely without saying a few words on that subject, namely, the optical properties of colours.