If water is thus an absolute requisite for life, pure water is a no less imperative necessity for health, and as the possibilities of contamination are very numerous, owing to its great solvent powers, the sources of our drinking supply should be very carefully watched.
Water constitutes about three-fourths of the surface of the earth, and the greater part of the bodies of man and other animals; some vegetables contain as much as 95 per cent, of this fluid.
A healthy individual requires from 3 to 5 pints of water daily, nearly one-third of this quantity being contained in articles of diet, and the rest supplied to the system in the form of liquids.
Rain-water is the purest of all forms ordinarily met with, if collected in clean vessels as it falls. This only applies to country districts. In towns the rain carries down with it blacks, dust, and organic matter in suspension in the air. Rivers are probably the most usual sources of supply for our drinking-waters, and where due care is exercised to prevent contamination from sewers, factories, etc., this variety of water is one of the least objectionable. A still better source for our large towns is the water of mountain lakes, conveyed direct in covered channels.
A certain amount of saline constituents, especially of the sulphates and chlorides of the alkaline earths, must be present in order to render river and lake waters safe from the contamination of lead house-pipes, if these are used for distributing the fluid, as they are in most of our larger cities and towns.
The way in which these soluble salts act is by forming with the metal an insoluble coating over the inner surface of the pipe, which mechanically precludes the water from acting upon the metallic surface. It is on account of the purity of rain-water from these saline compounds that lead pipes or lead-lined cisterns should never be used for its conveyance or retention.
Rain-Water.—When rain-water falls upon the surface of the ground, a portion of the moisture runs off into brooks, creeks and rivers, but a much larger part soaks downwards through the earth, and after a few hours or days finds its way through the soil into the subterranean streams and reservoirs which feed our wells. In the course of its journey it may meet with materials from the animal kingdom, which will change it into a slow and insidious poison or a swift agent of destruction. One of the most fatal diseases of humanity, typhoid or enteric fever, is particularly apt to be transmitted by the medium of contaminated well-water; and the same may be said of Asiatic cholera, happily now very rare with us due chiefly to modern improvements in sanitation and hygiene.
Recent observations on the propagation and spread of typhoid or enteric fever have shown that in all the instances of excessive malignity, when great numbers were swept away in a few weeks, the cause of the fearful mortality lay in contaminated water-supplies. It is most
