~Impregnation.~--Since the filaments from which artificial silk is obtained are solid and rod-like in form, as opposed to the tubular structure of cotton and ramie filaments, it is rather surprising that the fabric should take up the lighting fluid in the necessary quantity (_vide_ p. 295). It is found that a 50 per cent. solution of nitrates gives the best results, the impregnation requiring half an hour; a warm bath is usually employed. It is usual to add to the bath a quantity of thorium hydroxide, since the thorium nitrate of commerce generally contains nitric acid, which has a bad effect on the fabric.[537] The excess of solution is removed by means of a glass or porcelain centrifuge, not, as with cotton or ramie mantles, by use of a wringer; drying must be carried out very slowly. The fabric is not cut into lengths before impregnation, as in the case of cotton or ramie, but is immersed in the lighting fluid in long strips. [537] _Vide_ Buhlmann, _D. R. P._ 188427, 1907; also _E._ 6828, 1907. ~‘Fixing.’~--If the dried fabric, impregnated with the necessary salts, be finished and burnt off in the usual way, the oxide skeleton is extremely fragile, and soon falls to powder. The reason for this lies probably in the explosive decomposition of the nitrates, the weight of organic matter relative to the salts being very much less than in the case of ramie or cotton fabrics (_vide_ p. 295). The additional ammonia bath advocated by Knöfler (_vide supra_) was therefore adopted by Plaisetty, and the nitrates in the dried impregnated fabric are converted into hydroxides by this treatment. For this process, to which the name ‘Fixing’ has been given by Böhm, numberless alternative proposals have been made. Plaisetty’s ammonia fixing gives a mantle which, after burning off, is exceedingly elastic and strong, but it is nevertheless open to serious objections. Thus the nitrates may be to some extent dissolved out by the fixing bath before precipitation of the hydroxides has occurred; to remedy this, the impregnated fabric must be very thoroughly dried before fixing, and in this case, apart from the trouble involved, the acid of the commercial nitrates will attack the fabric unless addition of thorium hydroxide has been made to the impregnating fluid. Again, the conversion into oxides is not complete, the outer layer first formed preventing free diffusion of the alkaline fluid. Finally, since ammonium nitrate is formed in the reaction, a very thorough washing is necessary to remove this salt.