Several other forms of filter have also been employed with success, and also methods in which the treated water traverses tanks with sloping partitions on which the carbonate of lime is deposited. The latter plan was originally patented in France by Gaillet-Huet, and has been introduced into England by Stanhope. So far as is yet known, from the tanner’s point of view, it is hardly necessary to make any distinction between lime and magnesia, either or both of which may be considered simply as “hardness.” A hard water probably softens dried hides more slowly than a purer water, though it is possible that the observed difference in the time required may be due in many cases to the lower temperature of wells from which hard water is generally derived. In the actual “limes” the hardness of the water can have no appreciable influence, though if sodium sulphide be used alone for unhairing, a certain waste occurs from temporary hardness which may render it advisable to add a little lime. It is in washing the hides free from lime that the influence of hard water is first distinctly felt. If limy goods, after unhairing, are placed in a water with much temporary hardness, the same action occurs as in Clark’s water-softening process, and chalk is deposited in the surface of the hides, making them harsh and apt to “frize” or roughen the grain in “scudding.” The common, but not wholly satisfactory expedient is to add a little lime, or better, a few pailfuls of lime liquor to the water before putting in the hides. The best plan is to use a properly softened water. Permanent hardness is not injurious in this way. Unfortunately it is not the grain alone which is injured by the use of hard water for washing the hides, but on coming into the liquors the precipitated bases combine with the acids and tannins, forming compounds which oxidise and darken when exposed to the air, and which are the commonest causes of stains and markings on all descriptions of leather. Even when goods are drenched or bated before tanning the injury is not prevented, since the weak organic acids which are capable of removing the lime (as such) from the hide have little effect on the precipitated carbonate, which can only be dissolved by the use of stronger acids. It must be noted that the same injurious effect on limed goods is produced by free carbonic acid, which may be present even in soft waters.