Hard and Soft Water. Water that is used for domestic or manufacturing purposes is described as either hard or soft. Soft water produces a soap lather almost at once; hard water forms at first a scum or curd which has no detergent properties, and only after a time gives the soap lather which is required. The difference is due to the relative amount of dissolved solid contained in the water. Only distilled water or rain water collected in the open country is perfectly soft, for this is the only kind of water which on being evaporated to dryness leaves no solid residue. In districts where the underlying strata are composed of hard insoluble rock, such as granite or millstone grit, the water contains very little dissolved matter and is relatively soft. In a limestone or chalk country, water is very hard and in many cases has to be softened either before delivery or before use. The chief impurities which cause hardness are the chlorides, sulphates, and bicarbonates of magnesium and calcium. The chlorides and sulphates are not affected in any way by boiling, and the hardness which is due to them is said to be “permanent.” The bicarbonates, on the other hand, are decomposed when the water is boiled, and then they cease to cause the water to be hard. This part of the hardness is spoken of as “temporary” hardness. Let us now consider what calcium bicarbonate is and how it is formed. It is a compound of calcium carbonate and carbonic acid, and is formed by the solvent action of carbonic acid on limestone or chalk. The compound is soluble in water; but when the solution is boiled, the carbonic acid is broken up, carbonic acid gas is expelled from the solution, and calcium carbonate is formed. Temporary hardness is the more troublesome. In the first place, the bicarbonates, especially that of calcium, often form the greater part of the dissolved impurity. Moreover, when the water is boiled, although the hardness is removed, the insoluble calcium carbonate is a source of trouble, for it gradually settles down into the hard mass known as “fur” in kettles and “scale” in boilers. It is perhaps necessary at this point to emphasize the fact that matter _suspended_ in water does not make it hard, and it is only matter which is _dissolved_ which makes any difference in this respect. Since the softening of temporary hard water by boiling has the undesirable feature of introducing solid matter into the boiler, it is customary now to treat this water chemically. The following is the process most generally used. Quicklime or slaked lime is stirred into the water until the mixture gives a faint brown coloration when a drop of silver nitrate is added to a small test portion. Unsoftened water is then added until a sample just ceases to give this test. The temporary hardness has then been removed, and it is only necessary to allow the suspended matter to settle.