These methods of purification have not been neglected of late years. The value of storage is being increas ingly recognized. The three factors making for safety are equaliza tion, sedimentation and devitalization. By equalization is meant the dilution and averaging of any sudden access of pollutions to the water "feeding" a storage reservoir. Sedimentation means the settling out of solid impurities. Devitalization implies the gradual extinction of undesirable bacteria under conditions of storage which are unfavourable to the continued vitality of pathogenic microbes. Nature's method of purification has certain disadvan tages. Just as some things die, so do others multiply, sometimes with embarrassing results. There are the diatoms, the protozoa, green and blue growths, etc. Some of these growths give rise to taste troubles (e.g., tabellaria, synura, uroglena, etc.) ; others exer cise a serious blocking or choking effect on sand filtration processes (e.g., asterionella, synedra, cyclotella, fragilaria, etc.). Copper
sulphate (in doses of o.i to i.o per ,000,000; I to 10 lb. per 1,000,000 gal.) has been proved to be a valuable algicidal agent. The smaller doses have no injurious action on fish, but with the maximum doses care is needed, especially with trout.
A new and interesting development is the suggestion that rapid (mechanical) filters should be used to remove nearly all the suspended matters (including algal and other growths) from water, and worked at the very rapid rate of 100-200 gal. per sq.ft. per hour. In order to cover the additional cost involved it is hoped that it may be feasible to work slow sand filters at six instead of two gal. per sq.f t. per hour as a final filtration process. The underlying idea is that slow sand filters might be worked con siderably faster than is usual if rapid filters were used anteced ently to remove the bulk of the suspended matters. Those who favour these departures usually advocate chlorination as an additional safeguard, or, at all events, as a stand-by measure.