per cent.) if time be allowed for the weaker solution to act. The quantities of “hypo” and hydrochloric acid given for the second bath are often somewhat insufficient, and have to be slightly increased to complete the reduction. The reactions which take place are represented by the following formulæ, in which the weights of the materials taking part in the reaction are also given below the symbols. In the first bath-- Potassium Hydrochloric Potassium Chromic Water. dichromate. acid. chloride. acid. K₂Cr₂O₇ + 2HCl = 2KCl + 2CrO₃ + OH₂ 294 + 73 = 149 + 200 + 18 As ordinary concentrated hydrochloric acid does not contain more than about 30 per cent. of actual HCl,[116] about 2·5 parts would be required to completely decompose 2·94 parts of dichromate, while in Schultz’s formula 2·5 parts of hydrochloric acid are used to 5 parts of dichromate. This excess has been found useful in the production of a good leather, both to prevent accidents from an overdose of hydrochloric acid, and because of the modifying effect of an excess of neutral salt on the action of the chromic acid (cp. p. 82). [116] Acid of S.G. 1·16 (32° Tw.) contains 31·5 per cent. of HCl by weight or 36·6 grm. per liter, and therefore is practically 10 × normal strength. Acid of S.G. 1·2 (40° Tw.) contains 39·1 per cent. or 469 grm. per liter. The reactions which take place in the second bath are somewhat complicated. Eitner, in a valuable series of articles on chrome tannage, which have been appearing in the ‘Gerber’ since January 1900, states that even better results are obtained by using the hydrochloric acid in slight excess, as the action of chromic acid (in the presence of the potassium chloride of the chrome-bath) is not swelling but hardening to the skin, and the slight swelling action of the hydrochloric acid tends to counteract this, and also to facilitate the subsequent reduction. The two views are not contradictory, as the excess of bichromate behaves to the hide as an alkaline salt, which also produces a slight swelling effect, and it is quite probable that better results are attained when the solution is either alkaline or acid, than when the potassium chromate is exactly decomposed. Eitner recommends the use of four parts by weight of bichromate, and four parts of the strongest hydrochloric acid, dissolved in 400 parts of water, for each 100 parts of wet pelt, which should yield about 40 parts of dry leather. He states that if such a bath be used, it may be safely and economically exhausted by a second pack of skins, which is impossible in a bath containing excess of unacidified bichromate. He gives[117] the following explanation of the successive changes which take place when acid is gradually added during the reduction, but points out that in practice the reactions always to some extent go on simultaneously. [117] Gerber, p. 297, 1900.