Adjective Colours.—­As regards the artificial coal-tar adjective dyestuffs, the principal are Alizarin and Purpurin.  These are now almost entirely prepared from coal-tar anthracene, and madder and garancine are almost things of the past.  Vegetable adjective colours are Brazil wood, containing the dye-generating principle Brasilin, logwood, containing Haematein, and santal-wood, camwood, and barwood, containing Santalin.  Animal adjective colours are cochineal and lac dye.  Then of wood colours we have further:  quercitron, Persian berries, fustic and the tannins or tannic acids, comprising extracts, barks, fruits, and gallnuts, with also leaves and twigs, as with sumac.  All these colours dye only with mordants, mostly forming with certain metallic oxides or basic salts, brightly-coloured compounds on the tissues to which they are applied.

LECTURE XI

DYEING OF WOOL AND FUR; AND OPTICAL PROPERTIES OF COLOURS

You have no doubt a tolerably vivid recollection of the illustrations given in Lecture I., showing the structure of the fibre of wool and fur.  We saw that the wool fibre, of which fur might be considered a coarser quality, possesses a peculiar, complex, scaly structure, the joints reminding one of the appearance of plants of the Equisetum family, whilst the scaled structure resembles that of the skin of the serpent.  Now you may easily understand that a structure like this, if it is to be completely and uniformly permeated by a dye liquor or any other aqueous solution, must have those scales not only well opened, but well cleansed, because if choked with greasy or other foreign matter impervious to or resisting water, there can be no chance of the mordanting or dye liquids penetrating uniformly; the resulting dye must be of a patchy nature.  All wool, in its natural state, contains a certain amount of a peculiar compound almost like a potash soap, a kind of soft soap, but it also contains besides, a kind of fatty substance united with lime, and of a more insoluble nature than the first.  This natural greasy matter is termed “yolk” or “suint”; and it ought never to be thrown away, as it sometimes is by the wool-scourers in this country, for it contains a substance resembling a fat named cholesterin or cholesterol, which is of great therapeutical value.  Water alone will wash out a considerable amount of this greasy matter, forming a kind of lather with it, but not all.  As is almost invariably the case, after death, the matters and secretions which in life favour the growth and development of the parts, then commence to do the opposite.  It is as if the timepiece not merely comes to a standstill, but commences to run backwards.  This natural grease, if it be allowed to stand in contact with the wool for some time after shearing, instead of nourishing and preserving the fibres as it does on the living animal, commences to ferment, and injures them by making them hard and