quantity required for its saturation was two drachms, eighteen grains. The decoction of logwood became of a sappliire blue colour by the addition of sulphate of iron; beyond the point of saturation the blue becomes greenish and dirty. The exact quantity required for sa turation was one drachm, forty eight grains, and the weight of the precipitate was two drachms, twelve grains. All the precipitations in which the oak was used took place readily; that by logwood a little more diffi cultly, hut still more easily than that of galls. These eminent chemists afterwards ascertained, that the quan tity of of substances required to give a black co lour ol equal intensity, to an equal weight of the same cloth, was proportional to the quantities of astringent principle which had been estimated in each kind by the foregoing experiments; but the black obtained by means of the different parts of the oak, did not resist proofs or tests so well as that produced by galls. It appeared also, that logwood alone was incapable of producing so intense a black as galls or oak, nor was the colour so permanent.
233. llaving thus examined the properties of the as tringent principle, with respect to its affinities with so lutions of hon, it ought also to be considered with re specs to its affinities with stuffs, since its action as a inordmt must de pcnd upon its attraction to the latter as well as to colouring matters. Silk acquires by galling. w filch consists in macerating a stuff in a decoction of galls, an addition to its weight which cannot be taken from it again, beyond a certain degree, by repeated washings ; aim which operation, the stuff, when put into a solution of iron, is dyed black, because the astringent principle, decoinposing the sulphate of iron, forms a triple com pound with the oxide of iron and the stuff.
234. A galled stuff is also capable of combining with other colouring particles, the colours of which thereby acquire fixity, if they do not naturally possess it, so that the astringent communicates its durability to the triple compound, or perhaps the more complex one which is formed, but the colour is commonly rendered deeper by this union.
235. As the mordants and colouring particles have a mutual action upon each other, the nature of the latter is often so much altered by their union, that by varying the mordants, we may multiply prodigiously the shades of colour obtained from the same colouring matter. This may be done by simply varying the mode of their application: thus we shall obtain different effects by im pregnating the stuff with the mordant, or by mixing the mordant with the bath, by applying heat, or using re peated exsiccations. The greater or less disposition of the stuff to unite with the colouring matter, gives rise to considerable differences in the mode of applying the mordant. If the attraction of the colouring matter to the stuff be strong, the mordant may be mixed with the colouring matter, and in this state both may be present ed at once to the stuff; but if it be weak, the compound formed by the mordant and colouring matter might, in this way-, be precipitated in the bath without attaching itself to the stuff; and to prevent this inconvenience, the stuff must first be impregnated with the mordant, and afterwards exposed to the colouring matter. In the dyeing of some colours, it is necessary to employ one mordant for the stuff, and another for the colouring principle.