More recently, the use of organic salts of thorium and cerium for impregnation has been proposed by Dr. F. W. Wirth;[550] in fixing the impregnated fabric with hydrogen peroxide, the cerium is not removed in solution, since the weak organic acid formed will not dissolve cerium peroxide. The same author has also suggested[551] the addition to the fixing bath of substances which absorb oxygen from the air--_e.g._ sodium hydrogen sulphite, resorcinol, tannin--to prevent removal of cerium. He has also advocated impregnation with amorphous salts,[552] which will obviate the necessity for any subsequent fixing treatment, the hypophosphites and double compounds with ammonium oxalate being specified. Attempts have been made to achieve the same end by other methods. Thus Silbermann[553] has proposed a preliminary treatment with alkalies (mercerisation); the fabric is treated with concentrated sodium hydroxide solution in absence of air for half an hour, pressed through rollers, and plunged into the impregnating solution. Two years previously a patent was taken out by Drossbach[554] to protect the use of colloidal solutions of the hydroxides. To a boiling suspension of well-washed, freshly-precipitated thorium hydroxide, a solution of a small quantity of the nitrate is gradually added; after half an hour a colloidal solution is obtained, which, after the addition of the required quantity of cerium nitrate, and dilution to a suitable extent, is employed directly for impregnation. The patent states that this solution is more readily absorbed than the ordinary nitrate solution, but the statement has been questioned. [550] _Chem. Zeitg._ 1911, ~35~, 752. [551] _Zeitsch. angew. Chem._ 1912, ~25~, 922. [552] _Chem. Zeitg._ 1911, ~35~, 752. [553] _Chem. Zeitg._ 1911, ~35~, 1037. [554] _D. R. P._ 212842, August, 1909; _vide_ also Kreidl and Heller, _E._ 17862, 1909, and _D. R. P._ 228203, 1910. Artificial silk itself is of a colloidal nature, the solidification of the filaments of cellulose during the manufacture being rather in the nature of a coagulation than of a precipitation; it is to this fact that the fibres owe their solid, rod-like structure, and it is probably this circumstance also which determines the very intimate absorption of the hydroxides or peroxides produced by fixing. It is well known that colloidal substances under some circumstances possess the property of clinging tenaciously to foreign bodies, exhibiting the so-called phenomenon of adsorption. The strength and elasticity of the oxide skeleton, obtained when the fixed and dried fabric is subjected to the operation of burning off, are presumably to be referred to such a relation between the cellulose of the fibres and the insoluble thorium and cerium compounds, precipitated by one of the methods of fixing described.