Carbonate of calcium, though practically insoluble in pure water, is soluble in water containing carbonic acid. When this carbonic acid is neutralised by lime or soda, the carbonate is precipitated. The sulphate of calcium is unacted upon by lime, but by the action of caustic soda is converted into free lime and sodium sulphate. The lime then neutralises the free carbonic acid in the water, and forms carbonate of calcium, which is of course precipitated. These reactions may be represented by the following equations:— Calcium Carbonic Lime. Calcium carbonate. acid. carbonate. 1. CaCO_{3} + CO_{2} + CaO = 2 CaCO_{3}. Calcium Caustic Calcium Sodium sulphate. soda. hydrate. sulphate. 2. CaSO_{4} + 2 NaOH = CaH_{2}O_{2} + Na_{2}SO_{4}. Calcium Carbonic Calcium Water. hydrate. acid. carbonate. CaH_{2}O + CO_{2} = CaCO_{3} + H_{2}O. If sodium carbonate be used instead of caustic soda, the decomposition will take place thus:— Calcium Sodium Calcium Sodium sulphate. carbonate. carbonate. sulphate. CaSO_{4} + Na_{2}CO_{3} = CaCO_{3} + Na_{2}SO_{4}. It will be seen from the above equation that in the boiling processes the lime salts are removed from the water at the expense of an equivalent quantity of caustic soda. The amount thus decomposed is not sufficiently large to make it advisable {214} on that account to purify the water. It is indeed so small that processes of purification, based upon the use of lime and caustic soda, are now largely used, the cost for chemicals rarely exceeding 1_d._ per 1000 gallons. But for the reasons we have stated above, and also from the fact that it serves to remove dissolved iron and organic matter from a water, such a purification process is in certain cases advisable. The processes now in use consist, as we have indicated, in the addition to the water of lime and caustic soda or carbonate of soda, the quantities being regulated according to the hardness of the water, and the relative proportions of carbonate and sulphate of calcium. The method of removing carbonate of calcium from water by the addition of lime is due to the late Dr. Clark, of Aberdeen, and the process is still called after him. It is in use in its original form in many places. The plant necessary consists simply of a tank for mixing the water and the lime, and of large settling tanks in which the carbonate of calcium subsides. Various modifications of this process have been proposed, chiefly in the direction of improved plant. In the Porter-Clark process the carbonate of calcium is removed by passing the water after the addition of lime (in the form of lime-water) through a filter-press. [Illustration: FIG. 81.]