The use of lime for softening temporary hard waters was originally proposed by Thomas Henry, F.R.S., of Manchester, but was first applied as a practical process by Clark, who, after adding the requisite quantity of lime to the water in a mixing vat, allowed it to stand in a large tank to clear by subsidence, the precipitated carbonate of lime taking from 6 to 12 hours to settle. The process in its original form is a perfectly satisfactory one, except for the capacious settling tanks which are required, which in some cases are inconvenient and expensive. Messrs. Archbutt and Deeley[62] have patented a modification of the Clark process, by which the time of subsidence is much shortened, and according to which the precipitated carbonate of lime of previous operations is allowed to remain in the tank, and the fresh charge of water and lime is mixed up with it by means of steam-injectors, which blow in a current of air through perforated pipes at the bottom of the tank, and at the same time very slightly warm the water. The action goes on much more rapidly at a slightly raised temperature than in the cold; and rather curiously, the stirred up precipitate, instead of increasing the time of clearing, settles rapidly and carries down with it that formed in the new operation. It is particularly suitable for treating waters containing magnesia, from which a compound of lime and magnesia is apt to be precipitated in a colloid form which chokes filter-cloths and will not readily settle. After softening, the water is usually “carbonated” by passing the gases produced by burning coke into the floating exit-pipe through which it falls, in order to retain any remaining traces of carbonates of lime and magnesia in a soluble form, and prevent their subsequent precipitation in the pipes. The apparatus is made by Messrs. Mather and Platt, of Manchester, and its arrangement is shown in Figs. 19 and 20. [62] Journ. Soc. Chem. Ind., 1891, p. 511. [Illustration: FIG. 20.] Several modifications of the Clark process have been introduced, in which the precipitation is carried on continuously instead of intermittently. The most important of these is the Porter-Clark, in which one portion of the water to be softened flows through an agitator containing excess of lime, with which it forms saturated lime-water, which is passed slowly up a cylinder where it deposits the excess of suspended lime. The clear lime-water so produced is mixed with a fresh portion of the water to be softened in a second cylinder also provided with an agitator, the proportion of the two liquids being regulated by cocks. The carbonate of lime is at once precipitated, and is removed by passage through a filter press. This process is in successful operation on a considerable scale at Messrs. Hodgsons’ tannery at Beverley.