best. LECTURE IX DYESTUFFS AND COLOURS _Classification._--In classifying the different dyestuffs and colouring matters it is, of course, necessary to consider first the properties of those colouring matters generally, and secondly the particular reason for making such classification. The scientific chemist, for example, would classify them according to theoretical considerations, as members of certain typical groups; the representative of medical science or hygiene would naturally classify them as poisonous and non-poisonous bodies; whilst the dyer will as naturally seek to arrange them according to their behaviour when applied to textile fabrics. But this behaviour on applying to textile fibres, if varied in character according to the chemical nature of the colouring matter, as well as the chemical and physical nature of the fabric--and it is so varied--will make such classification, if it is to be thorough-going, not a very simple matter. I may tell you that it is not a simple matter, and, moreover, the best classification and arrangement is that one which depends both on the action of the dyes on the fibres, and also on the intrinsic chemical character of the dyestuffs themselves. Since the higher branches of organic chemistry are involved in the consideration of the structure and dispositions, and consequently more or less of the properties of these dyes, you will readily comprehend that the thorough appreciation and use of that highest and best method of classification, particularly in the case of the coal-tar dyes, will be, more or less, a sealed book except to the student of organic chemistry. But it may be asked, "How does that highest and best method of classifying the dyestuffs affect the users, the dyers, in their processes?" In reply, I would say, "I believe that the dyer who so understands the chemical principles involved in the processes he carries out, and in the best methods of classifying the dyes as chemical substances, so as to be able to act independently of the prescriptions and recipes given him by the dye manufacturers, and so be master of his own position, will, _ceteris paribus_, be the most economical and successful dyer." Many manufacturers of dyestuffs have said the very same thing to me, but, independently of this, I know it, and can prove it with the greatest ease. Let me now, by means of an experiment or two, prove to you that at least some classification is necessary to begin with. So different and varied are the substances used as colouring matters by the dyer, both as regards their chemical and physical properties, that they even act differently towards the same fibre. I will take four pieces of cotton fabric; three of them are simple white cotton, whilst the fourth cotton piece has had certain metallic salts mixed with thickening substances like gum, printed on it in the form of a pattern, which at present cannot readily be discerned. We will now observe and note the different action on these pieces of