7. " " " magnesium chloride, 20° do. Not at all plumped. 8. " " " common salt, 20° do. do. do. (1 German degree of hardness corresponds to 1 of lime in 100,000.) The peculiarities which were shown by the hide pieces on removal from the water were maintained throughout the tanning, which was conducted in imitation of the German method, the hide being swollen and coloured through in weak birch-bark liquors, made with distilled water and acidified in each case with equal quantities of lactic acid, and finally laid away till tanned in a mixture of oak bark and valonia. No. 6, from magnesia sulphate, was the best; then No. 2; No. 3 was less good, but all the pieces from 1 to 6 were firm, close, and of good substance and texture, No. 1 having swelled well in the sour liquor. On the other hand, 7 and 8 scarcely swelled in liquor, but remained flat throughout, and were looser, thinner, and of finer fibre. From this experiment it is clear that while sulphates and carbonates exert a favourable influence on plumping, chlorides do the reverse, not only not plumping themselves, but placing the hides in an unfavourable condition for the plumping action of acids in the liquors. These experiments are quite borne out by the writer's experience in practice. The water at the Lowlights Tannery, which in dry weather is mostly obtained from beds of what was originally sea-sand, and which consequently contains a very abnormal proportion of chlorides (up to 68 pts. NaCl per 100,000), requires special and very careful management to make thick leather, notwithstanding its containing a considerable quantity of calcium and magnesium sulphates. These facts also indicate the importance of the thorough removal of salt from hides intended for sole-leather. Plumping is not a desirable thing in leather intended for dressing purposes, and it is possible that the use of a small percentage of salt in the liquors or wash waters might enable bating to be dispensed with. Like a bate, salt would dissolve a small proportion of hide substance (see p. 19). There is no practicable means of removing chlorides from water, but Eitner suggests the addition of a small quantity of sulphuric acid to water containing much temporary hardness (bicarbonates), by which it is converted into permanent (sulphates), which, as we have seen, plumps better. For this purpose about 2·8 oz. of ordinary English vitriol (sp. gr. 1·490) per 100 cub. ft. of water is required for each part of lime Ca(OH)_{2} per 100,000 (see p. 97 for testing of water). A simpler guide is to add enough to purple, but not to redden litmus paper, even after moving it about in the water for some minutes. The acid must of course be well mixed by plunging. It must be borne in mind that Eitner's experiment was on sweated hides, and that with limed hide, which is kept plump by the dissolved lime retained in the hide, the conditions are different,