Naturally in practical work, the reduction cannot be made to proceed rigidly in the definite steps described by Eitner on p. 206, but all go on in different proportions together, though by supplying the acid in proper quantities, and at proper intervals, they may be made in the main to follow in the given order. Both on this account, and because neither the exact amount of chromic acid in the skins, nor the sulphurous acid lost by escape into the air can be exactly determined, the reduction cannot be conducted on theoretical principles, but the best conditions must be empirically determined. Eitner states that 12 parts of thiosulphate dissolved in 400 parts of water, and 6 parts of (40 per cent.) hydrochloric acid are sufficient for 4 parts of bichromate per 100 of wet pelt employed in the chrome bath, of which not more than one-half to two-thirds is absorbed; and that if equal parts of bichromate and acid are employed in chroming, the acid used in reducing may be lessened to 5 parts. In this case it must not be forgotten, that if the partially exhausted chrome-bath is used for a second pack of skins, which are afterwards finished in a bath of full strength, nearly the whole quantity of bichromate used in making up one bath will be absorbed by the skins. The amount of acid consumed in reduction will be greater, the more rapidly it is added, owing to increased escape of sulphurous acid. It is better to add the acid, previously diluted with water, in 8 or 10 successive portions, more rapidly at first, and more slowly during the latter half of the operation, each portion of acid being added as soon as no further change of colour appears to be caused by that already given. These changes are the more rapid the lighter the goods. The colour darkens at first to olive-brown, then gradually becomes green, and finally blue, and when this colour is uniform throughout the thickness of the goods, no further acid need be added. For goods which have been chromed in an acid bath, Eitner states that no acid will be needed for the first twenty to thirty minutes. It is important to have a sufficient excess of thiosulphate in the bath when reduction is complete, in which case the goods may be left for some hours or overnight in the bath, to complete “neutralisation,” but Eitner prefers to use a fresh bath of 1¹⁄₂ parts of thiosulphate in 400 parts of water for this purpose, the bath being used, after settling, for making up the reduction bath for the next lot of goods, for which 1¹⁄₂ parts less thiosulphate is used. The goods must be kept in motion during reduction, either in a drum or a covered paddle.