The destruction of pathogenic germs in water by chemical processes has long been practised and with various re-agents. Among those which have been used particularly for this purpose are sulphate of copper and so-called hypochlorite of lime. On account of the great expense of the sulphate of copper and its extremely poisonous effect upon animal life, especially fish, the greater use of the lime compound has of late years come into vogue. It is now used in many parts of the United States and Canada to such an extent that when one speaks of the chemical treatment of water for the destruction of pathogenic germs by chemical means, the chlorin-lime compound is the one indicated unless otherwise specified. It is a cheap and effective method for destroying certain of the threatening organisms which are usually present in polluted water.

The destruction of germs in water must not be confounded with the process of eliminating silt. For this purpose certain electrolytes, like alum or ferrous sulphate and lime, are more generally employed and are much more effective than the hypochlorite of lime. Also when vegetable organisms are to be destroyed, such as algae, there is nothing so effective as copper sulphate. Hence, hypochlorite of lime is not the ideal substance to use as a precipitating agent when the waters are subsequently to be filtered through sand or other filters. The treatment of water with hypochlorite is also not a decolorizing process.

In order that the germicidal action of the hypochlorite may be most effective, it is highly important that suspended and organic matters be previously precipitated. There is one point of excellence in the use of hypochlorite found in the fact that any excess of it is extremely disagreeable both to the taste and to the nose, and hence reveals itself directly in the water. When an excess of the re-agent has been employed, the water becomes to that extent unpotable. For this reason the use of hypochlorite of lime requires a careful chemical supervision, in order that the quantities which are necessary for purification may not be exceeded. In all these cases the only excuse for the use of chemicals of any kind is the choice between two evils. There is always a danger of using too little or too much of the chemical. In the first case the water, which is supposed to be free of danger, is not so; and in the second place, the excess of the chemical may prove injurious. The ideal solution of the problem in all cases would be to seek a source of unpolluted water, but as this is not always obtainable, we must expect that the use of purifying re-agents of the kind mentioned is a necessity or at least a precaution which should not be omitted.