When it is remembered that water forms from 60 to 70 per cent. of the body, and that it enters into the composition of every living tissue, it can be readily understood how important an article of diet it is. In all the processes of metabolism or constant change, by which alone life and function are maintained, water plays a part. Thus, in digestion, the food principle must be dissolved before assimilation can take place; in respiration the expired air is charged with moisture; in circulation the blood must be fluid; in the secretions, by means of which the body machine is maintained in working order, and in the excretions by means of which the refuse is cast out, the presence of water is an absolute necessity.

To drink pure water is the custom of the natural man; to drink impure water is the almost invariable habit of the civilised man. In the days of the Greeks, the Romans, the Egyptians, and the powerful and interesting races of India and Asia Minor, the provision of pure water for the people was looked upon as a national obligation; and the ruins of the great aqueducts, by means of which the water of mountain and sky and lake was carried immense distances to the cities, to-day cumber the ground whence the people now draw their water supply from polluted wells. It is only of recent years, and not till the ravages of cholera and typhoid had at last taught the people and municipal authorities the lessons which the sanitarians had never ceased to preach in season and out of season, that the absolute necessity of a perfectly pure water supply for drinking purposes has been recognised.

In London the water supply is derived from the Thames, the Lea, and the New River, rivers which receive sewage and filth of every description along almost the whole length of their course. Elaborate and costly precautions are, however, taken by the water companies to get rid of all the unwholesome and dirty particles with which the water is more or less charged. This is done by means of filtration and by the formation of large filter beds. Nature's way of cleansing polluted water is twofold - either to pass it through immense filter beds of gravel, beneath which it collects in underground reservoirs, which may be naturally tapped by "faults" in the strata, leading to the formation of springs of pure water, or by shafts purposely sunk; or foul water is purified by means of the flow of a river, during which the solid particles sink to the bottom, and the organic pollutions, such as germs, etc., are oxidised and destroyed. Spring or well water often contains inorganic materials, dissolved out of the soil through which it has passed, such as lime, iron, etc. River or stream water is the pleasantest to drink, as it contains much air, and a small proportion of organic salts, which give it an agreeable flavour. So long, however, as it is not considered a crime against humanity and the State to pollute a river or stream, unfiltered river water is dangerous to drink.

Water is purified in three ways: by filtration, by distillation, and by boiling.