In addition to these mineral and saline constituents, water also dis solves certain organic substances, so that wo may classify the impuri ties contained in water as merit a nieal, or those that are merely suspended in the water mechanically, and not dissolved, saline impurities, and organic impurities. We will consider these separately.
First, as regards mechanical impurities. They consist, in the first place, of mineral substances which are usually innocuous, and have no effect upon the drinker. Next we have vegetable or animal matters, which are generally of an exceedingly noxious character ; and then wo have living organisms, which perhaps attract the attention of the water-drinker more than the other substance. contained in the water. These organisms consist of animalcule and animals of a larger size. Wo have in the New River water twenty-six species of these animals, in the Thames water twenty-nine species; twenty-four species have been detected in the West Middlesex water, and so on, varying in number in different waters. It has been stated that some of these little animals are very deleterious to health, but this has not been at all clearly made out. Looked at from a chemical point of view, they are exceedingly useful in the water, especially if it be at all impure. Each little animal is a small furnace, which occupies itself in consuming the organic matter, and converting it into its ultimate inorganic com pounds—carbonic acid, water, and ammonia, or nitric acid. They feed upon the dead organic matter in the water, and remove it far more readily than it would be removed by the oxidising property of the air. It is, of course, desirable to remove them before the water is used as a beverage, and this can be done by filtration. The animals themselves cannot pass through the filter, but the ova can. This fact has an important bearing upon the storing up of water, and will be alluded to again presently.
We come now to the saline impurities. The nature of these will be readily seen by an inspection of the following diagram.
The Farnham water Is from the green sand : it has been suggested as a supply for London. That from Loch Katrina I. perhaps purer than the water supplied to any town in the United Kingdom. The Manchester water can always be drawn from the pipes, the supply being continuous, instead of intermittent, and this is now the case in nearly all large towns except London.
Some of these constituents are termed earthy salts, and some of them are alkaiiie sat's. Carbonate of lime, sulphate of lime, and nitrate of lime, carbonate of magnesia, and, occasionally, chloride of calcium and chloride of magnesium, aro salts of the alkaline earths; others are alkellne salts.
The property of ',1hardness" iu water is due to the first of these classes of constituents—to the alkaline earths—the lime and magnesia salts present in the water. Hard water may be defined as a water capable of decomposing a considerable quantity of soap, and forming with the soap an insoluble precipitate. These salts of lime combine with the stearic acid, and form an insoluble stearato of lime, the curdy matter which is produced by washing in such waters. This stearate of lime is of a greasy nature ; it is precipitated in, and fills up, the pores of the akin ; no amount of washing in hard water can thoroughly remove it ; and hence the skin can only be perfectly cleansed in rain water, in softened hard water, or, in fact, in water that does not contain soap-decomposing matters.
Of the above constituents, the carbonates of lime and magnesia are the only ones not soluble in pure water : they are kept in solution by free carbonic acid [Caseiem, Carbonate of]. The amount of hardness possessed by waters varies considerably : it is readily ascertained by the SOAP-TEST. The mode of softening waters is referred to under CALCIUM, Carbonate and Sulphate of. The only perfect method of removing fixed matter from water is, of course, distillation; indeed, this process is carried out on board some of our ships, the water being subsequently aerated for drinking purposes. On land, however, other less expensive processes are available.
The organic impurities contained in solution in water are of animal and vegetable origin, the former being most objectionable. The animal matter contains nitrogen, and is constantly undergoing a fermentive change. Water in this state is highly dangerous to health, and should be carefully avoided. Unfortunately, no test, short of rigid analysis, can be relied upon for detecting this animal matter. Solution of per mauganate of potash is decomposed and decolourised by it, and there fore water that discharges the colour from much of that reagent should be viewed with suspicion.