and it is dyed a fine red, composed of a tannate of antimony and Magenta. You now see, no doubt, the necessity of sharply discriminating between two classes of colouring matters, which we may term _colour acids_ and _colour bases_ respectively. There are but few acids that act like tannic acid in fixing basic aniline dyestuffs, but oleic acid and other fatty acids are of the number. A curious question might now be asked, namely: "Could the acid colour Alizarin, if fixed on cotton cloth, combine with a basic aniline colour, _e.g._ Aniline Violet, and act as a mordant for it, thus fixing it?" The answer is, "Certainly"; and thus an Alizarin Purple would be produced, whilst if Magenta were used in place of Aniline Violet, an Alizarin Red of a crimson tone would result. _Chrome Mordanting of Wool and Fur._--In studying this subject I would recommend a careful perusal of the chapter on "Mordants" in J.J. Hummel's book, entitled _The Dyeing of Textile Fabrics_, and pages 337 to 340 of Bowman's work on _The Wool-Fibre_. In the treatment of wool or fur with bichrome (potassium bichromate) we start with an acid salt, a bichromate (K_{2}Cr_{2}O_{7}) and a strong oxidising agent, and we finish with a basic substance, namely, oxide of chromium, in the fibres of the wool or fur. If we desire to utilise the whole of the chromic acid in our mordanting liquor, we must add to it some sulphuric acid to set free the chromic acid from the potassium with which it is combined. Bichromate of potash with sulphuric acid gives sulphate of potash and chromic acid. The question of the proper exhaustion of bichromate baths is an important economic one. Now we must remember that this chromic acid (CrO_{3}) oxidises our wool or fur, and must oxidise it before it can of itself act as a mordant by being reduced in the process to hydrated chromic oxide, Cr_{2}O_{3} + 3 H_{2}O. [2 CrO_{3} (chromic acid) = Cr_{2}O_{3} (chromic oxide) + O_{3} (oxygen).] It is this hydrated chromic oxide in the fibre that yields with the Haematein of the logwood your logwood black dye. Mr. Jarmain finds that it is not safe to use more than 3 per cent. (of the weight of the wool) of bichromate; if 4 per cent. be used, the colour becomes impaired, whilst if 12 per cent. be employed, the wool cannot be dyed at all with logwood, the phenomenon known as "over-chroming" being the result of such excessive treatment. I think there is no doubt, as Professor Hummel says, that the colouring matter is oxidised and destroyed in such over-chroming, but I also think that there can be no doubt that the wool itself is also greatly injured and incapacitated for taking up colour. Now the use of certain coal-tar black dyes in place of logwood obviates this use of bichrome, and thus the heavy stress on the fibre in mordanting with it. It also effects economy in avoiding the use of bichrome, as well as of copper salts; but even thus, of course, other problems have to be solved before it can be finally decided which is