It would be impossible to mention all of the numberless proposals which have been put forward for fixing; nor are the great majority worthy of mention.[538] One of the most important was that of Albrecht,[539] by which hydrogen peroxide is substituted for ammonia. This reagent, as is well known, precipitates from solutions of thorium salts ‘peroxide’ compounds (_vide_ p. 255); from the fabric impregnated with the nitrate, free nitric acid is liberated in the reaction. Since the peroxide is soluble in nitric acid, two baths are used, the fabric being allowed to remain for a short time only in the first, which becomes strongly acid, and being then transferred to the second, in which the precipitation is completed. The burning off of the product so treated proceeds quite quietly, and leaves a very hard and elastic skeleton. The method, however, has the grave disadvantage that cerium salts are not precipitated under these conditions, but escape into the solution. To remedy this, arbitrary additions of cerium nitrate are made to the fixing bath, but the percentage of cerium, and therefore the lighting power (_vide_ p. 293) of mantles fixed by this method is liable to fluctuate. A modification has been introduced[540] in which various salts are added to the hydrogen peroxide fixing baths to prevent the withdrawal of the cerium salt; these are chiefly acetates of the alkali metals and allied compounds. The fabric requires washing after this treatment. [538] A large number of patents are mentioned by Böhm, _J. Gasbel._ 1909, ~52~, 855. [539] _D. R. P._ 188427, September, 1907; _E._ 15295, 1907. [540] _Vide_ _E._ 2240, 1908. Cerofirm Gesellschaft, by Brit. Cerofirm Co.