Calcium carbonate is the most common cause of hardness, and the hardness produced by it is remediable by boiling or chemical means. Calcium carbonate (chalk) is rendered soluble in water, by the carbonic acid contained in the latter, a double bicarbonate being thus formed. The air contained in the interstices of the soil through which water passes, often contains 250 times as much carbonic acid as ordinary air. The water, in percolating through the soil, dissolves this carbonic acid, and thus is able to take up a considerable amount of chalk. The amount of hardness in any given water is expressed in degrees, one degree being equivalent to a grain of calcium carbonate in a gallon of water. =Clarke’s soap test= is employed to detect the amount of hardness. It consists of a solution of soap of a known strength. Soft water will form a lather at once with this; hard water will only form a lather after all the calcium salt is neutralised. The amount of Clarke’s solution required before a lather is produced, will give an estimate of the amount of hardness. =To Determine the Total Hardness= take 70 c.c. of the water and place in a stoppered bottle. From a burette run in a sufficient quantity of the standard soap solution (of which 1 c.c. equals 1° of hardness), to produce a lather on shaking the water, which remains unbroken after standing five minutes. Thus, if 7·5 c.c. of the soap solution were required, the hardness is 6°·5, as 1 c.c. of the solution is required to produce a lather in soft water. The 6°·5 means 6·5 milligrammes of calcium carbonate in 70 c.c. or 6·5 grains in a gallon of the water. =To Determine the Permanent Hardness= boil 70 c.c. of the water in a flask for half-an-hour; allow the precipitated carbonates of calcium and magnesium to settle. Some of the latter will be re-dissolved. Carefully decant, and make up the liquid to the original 70 c.c. with distilled water. Filter through fine filter paper and estimate hardness as above. The amount of soap wasted in consequence of the hardness of water is very great. Thus, in the case of water of one degree of hardness, as every gallon contains one grain of chalk, 7,000 gallons would contain 7,000 grains—that is, a pound. But every grain of chalk wastes 8 or 9 grains of soap; therefore, a pound of chalk, contained in 7,000 gallons, would waste about 8½ pounds of soap. But nearly all waters are harder than this, and they not uncommonly possess a hardness of 20° or more. If the hardness be 20°, the waste would be 170 pounds of soap. This quantity would be easily used annually in a family of seven or eight persons, if we include the washing of clothes. The amount of money thus wasted can be easily estimated.