not a dye, but it forms with each of several metals a differently coloured compound; and thus the metallic salt in the fabric is actually converted into a coloured compound, and the fabric is dyed or printed.  The case is just the same with logwood black dyeing:  without the presence of iron ("copperas,” etc.), sulphate of copper ("bluestone"), or bichrome, you would get no black at all.  We will now try similar experiments with woollen fabrics, taking three simple pieces of flannel, and also two pieces, the one having been first treated with a hot solution of alum and cream of tartar, and the other with copperas or sulphate of iron solution, and then washed.  Turmeric dyes the first yellow, like it did the cotton.  Magenta, however, permanently dyes the woollen as it did not the cotton.  Alizarin only stains the untreated woollen, whilst the piece treated with alumina is dyed red, and that with iron, purple.  If, however, the pieces treated with iron and alumina had been dyed in the Magenta solution, only one colour would have been the result, and that a Magenta-red in each case.  Here we have, as proved by our experiments, two distinct classes of colouring matters.  The one class comprises those which are of themselves the actual colour.  The colour is fully developed in them, and to dye a fabric they only require fixing in their unchanged state upon that fabric.  Such dyes are termed monogenetic, because they can only generate or yield different shades of but one colour.  Indigo is such a dye, and so are Magenta, Aniline Black, Aniline Violet, picric acid, Ultramarine Blue, and so on.  Ultramarine is not, it is true, confined to blue; you can get Ultramarine Green, and even rose-coloured Ultramarine; but still, in the hands of the dyer, each shade remains as it came from the colour-maker, and so Ultramarine is a monogenetic colour.  Monogenetic means capable of generating one.  Turning to the other class, which comprises, as we have shown, Alizarin, and, besides, the colouring principle of logwood (Haematein), Gallein, and Cochineal, etc., we have bodies usually possessed of some colour, it is true, but such colour is of no consequence, and, indeed, is of no use to dyers.  These bodies require a special treatment to bring out or develop the colours, for there may be several that each is capable of yielding.  We may consider them as colour-giving principles, and so we term them polygenetic colours.  Polygenetic means capable of generating several or many.  In the various colours and dyes we have all phases, and the monogenetic shades almost imperceptibly into the polygenetic.  The mode of application of the two classes of colours is, of course, in each case quite essentially different, for in the case of the monogenetic class the idea is mainly either to dye at once and directly upon, the unprepared fibre, or having subjected the fabric to a previous preparation with a metallic or other solution, to fix directly the one colour on that fabric, on which, without such