_Alumina_, except as clay, is rarely present in waters, and probably harmless in any water likely to be used in tanning. _Soda_ is sometimes present in considerable amount, as sulphate, chloride, or carbonate. The sulphate is probably inoperative. The chloride, if present in material quantities, prevents plumping, and may be the cause of thin and soft leather, and in large amounts will greatly impede the proper exhaustion of many tanning materials. Sodium carbonate is sometimes present in considerable quantities, as in some of the waters of the Leeds district. It may coexist with temporary hardness, and produces similar injurious effects. Waters in which it is present cannot have any real permanent hardness. It may be neutralised by the very cautious addition of an acid; or by admixture of a permanent-hard water. It tends to increased plumping in the limes, but neutralises the free acids of the tan-liquors which are necessary in sole-leather tanning. _Copper_, _lead_, and other metallic bases are not likely to be present in any waters used for tanning in quantities sufficient to be injurious. _Sulphuric acid_ rarely occurs free in water, and then only in such traces as would be harmless for tanning, though possibly injurious to steam boilers. As sulphates it is most common. Alkaline sulphates are not known to have any deleterious action. The sulphates of lime and magnesia are the principal cause of permanent hardness, q.v. Iron sulphate is sometimes found in colliery waters. _Nitrates and nitrites_ in water are usually the result of “previous” sewage contamination, and are only important as an indication of the possible presence of the putrefactive ferments, and are of little moment in waters only used for manufacturing purposes, while they seem to be even useful in promoting the “working” of bran drenches, by supplying the nitrogen required by the ferment. _Chlorine_ is seldom or never present in water in the free state, but only in the form of chlorides, most frequently of sodium chloride (common salt), the effect of which has been referred to above, and also at p. 88. The action of other chlorides is probably similar as regards the swelling of hide. _Magnesium chloride_ is very objectionable as a constituent of boiler-waters, as it liberates hydrochloric acid at high temperatures, and corrodes the plates at the surface of the water. This injury can be prevented by addition of soda. _Carbonic acid_ has been referred to under temporary hardness. Its presence in the free state is a matter of some importance to the tanner (see p. 99). _Silicic acid_ in a soluble form is present in some waters in considerable quantity. Such waters are said to harden leather, but of this the writer has no personal experience.