Rain Water

Next to clear mountain-spring water which has run through gravel and been well aerated, rain water at the close of a shower is the purest form of natural water, excepting where it has fallen through a very dusty or smoky atmosphere or has run over a dirty roof. The first drops of a shower carry down with them the impurities of the air, including traces of ammonia and nitric acid. No natural water possesses absolute chemical purity, but water collected at the end of a hard or long shower is very pure, having been distilled by previous evaporation.

River water which has run over a rocky or gravel bed, and has been well aerated, is quite pure and assimilable if properly protected from sources of pollution. Free oxidation makes much organic matter harmless. Flowing water by oxidation and dilution becomes purified to a great extent of injurious organic matter, but is not necessarily deprived of living germs of contagion, such as those of typhoid fever or cholera, and the question whether their number and virulence is at all diminished in such water is still a matter of controversy.

It is now so well recognised that the impurities of water which are chiefly dangerous to man are of organic germ origin that in examining water which is suspected of being the cause of a typhoid-fever epidemic a chemical analysis of the nitrates present, unaccompanied by a proper bacteriological examination, is entirely useless.

Distilled Water

Distilled water is absolutely pure, but it has a flat or metallic taste from absence of air and salts. It is now extensively furnished for drinking, and is largely in use on vessels at sea. Our modern naval vessels carry apparatus for its preparation. Distilled water has been recommended as a solvent for calculi, but it is of doubtful efficacy. It erodes lead pipes and cisterns.

Filtered Water

Filtration as employed to render drinking water more pure is a very delusive process, unless great care be taken as to the construction of the filter and the rate of filtration. Extensive experiments made at Providence, R. I., and Lawrence, Mass., have proved that the benefit of sand infiltration of city water results only when the filters are frequently cleaned. The sand layer should not be less than 30 centimetres thick, nor the rate of water flow fall below 100 millimetres per second. Frequent bacteriological examination of the water should be made. Koch has shown that it is not the sand which "filters," but the layer of mud or slime which is deposited upon it and which retains bacteria. Hence the reason for not allowing filters to become clogged with mud in which germs multiply. During the Hamburg cholera epidemic of 1892, the neighbouring town of Altona suffered much less severely from the disease than did Hamburg, for, although the Altona water supply was contaminated by Hamburg sewage, the water was thoroughly filtered.

A great variety of patent filters have been devised, and they have the power of making water appear pure by decolourising it and removing the larger organic particles and grosser impurities which it may contain, but it has been conclusively shown that the bed of a filter which has been in use for a short time furnishes a favourable soil or culture medium for the development of germs, which increase very rapidly until they reach such numbers that they are washed out with the filtered water, rendering it much more injurious, in some instances, than it was before. Many of these germs are doubtless harmless, but if the microbes of infectious diseases once gain access to such a soil they multiply and become highly dangerous.