which is soluble in water, usually amounts to about 2i per cent. and the colouring matter to about 1-55th, or 1-60th part of the silk ; the wax seldom exceeds the 1-100th part of the silk, and is frequently less than half that quantity. All these substances are removed by scouring with soap ; but this operation must be slightly modified, according to the purposes for which the silk is intended : it ought not to be so complete for silks which are to be dyed, as for those which are to be white, and a difference ought to be observed in the former case, ac cording to the colour to be given. For common colours, it is generally thought sufficient to boil the silk for three or four hours, in a solution of twenty pounds of soap for each hundred of silk, taking care to fill up the kettle from time to time, to replace the water carried off by evaporation. The quantity of soap is increased for those silks which are to be dyed blue ; and still more for those which are to be scarlet, cherry-colour, &c. these colours requiring a brighter ground for their display than such as are of a more delicate hue.
190. Silk which is to remain white commonly under goes three operations. In the first of these, which is in tended to remove the gummy matter, it is kept in a so lution of thirty pounds of soap to a hundred of silk, the solution being very hot, but not boiling. When it has been immersed a sufficient length of time for the remo val of the gum, which is known by the whiteness it ac quires, it is taken out and wrung. In the second opera tion, the silk is put into bags of cloth, and boiled in a weaker solution of soap for about an hour and a half; taking care to keep the bags constantly stirred, to pre vent those which touch the bottom receiving too much heat. The third operation is intended to soften the white, and render it more pleasing, by giving it a slight shade of some delicate colour. For this purpose, a so lution of silk is employed, the proper strength of which is determined by its mode of frothing ; this is slightly tinged with a colour of the shade to be given, and the silk is then moved about in it until it acquires the de sired hue. At Lyons, where silk is scoured of a beau tiful white, no soap is used in the third operation ; but after the second, the silks are washed, fumigated with sulphur, and azured with river water.
191. Roard thinks, that instead of employing the soap at different times, it is more advantageous to employ the whole quantity to be used at once, and by that means shorten the boiling to an hour, or a little more ; the silk is thus left with more of its natural softness and lustre, as well as stronger and more elastic. He recommends, indeed, that when it is to be dyed of a crimson, or any other colour which would not be affected by the yellow ish tinge of the silk in its raw state, the latter ought not to be completely removed ; because, when this is done, the dyed colour possesses less brilliancy than it would otherwise have.
192. As soap seems to impair the lustre of silk, the Academy of Sciences, in 1761, proposed, as the subject of a prize dissertation, to find a method of scouring it without soap ; and the prize was adjudged to 1I. Ri gaud of St Quentin, who proposed substituting for soap a solution of soda, or carbonate of soda, so much diluted as not to affect the texture of the silk. But some incon venience must have attended the practice of this method, as it is not adopted, though generally known, and easy of execution. Roard, indeed, affirms, that the gummy matter of the silk is less effectually removed by soda than by soap.
19". The c Ct llomb 11,5 given an account of a n ell 1 of stout ing silk by the ;alit n of utter alone, ci-1. 1 excites ttC141011. 11.11111g I Oiled silk for about t rt e It ors i11 common w att r, he found that its weight emit i lied a ut toc•clf;litb; and upon repeating annuli. do loss t I weight amounted nearly to ooe-tourt 1. The silk still rt tallied a yellos‘ish or cha in.ls col ,er; and though it did not possess a sufficiently blight •ri and for the more brilliant colours, it answer t d %%ell for those which w ere not affected by the tinge It rt tallied. It took a finer and more glossy black, I r impk, than if it had been scoured in the usual way with soap.
t 9-1 When silk is intended fur the manufacture of lb lids and gauzes, its maul al elasticity and stiffness shout] I be preset.% cd as much as possible ; but as the le thuds usually employed for removing the colouring matter, also &Knel it of the gum, it became a desi clt lawn] to whiten silk without affecting its elasticity.