The methods employed for purifying water for potable purposes are classified as follows: First, boiling; second, treatment with chemicals; third, filtration; fourth, treatment with chemicals and subsequent filtration. By means of one or a combination of the above methods, it is possible to convert the water supply which has become contaminated into a safe and palatable beverage.

In describing these methods of purification, it is not the intention to approve of them as a substitute for the securing of pure, uncontaminated water. That should be the object of every householder and municipality. Methods of purification should not be permitted to interfere with, or delay in any way, attempts to secure pure water at the source. In order that the sources be uncontaminated, the general practice of pouring waste and sewage into running streams must be regarded as entirely reprehensible. Many years will elapse before the running waters of the country can be protected from such contamination. Meanwhile we must be content with doing the best possible to purify the contaminated waters.

The day will doubtless come when all pollution of the streams of the country will be forbidden by national, state or municipal laws. Already the state of Ohio has made a start in this direction, and the state of Indiana, as well as some other states, has instituted a careful sanitary survey of running streams for the purpose of determining the extent and character of the pollution. Among the great cities Baltimore has already established a sewage disposal plant, which will be described further on. International questions also arise in connection with this subject. The Great Lakes are becoming contaminated, thus presenting a problem which can only be dealt with by international means. Attention will be called in coming pages to some of the systems of sewage disposal, notably that of Chicago, which places upon individuals, towns, corporations and states, below the city of Chicago, a serious problem in connection with the diluted Chicago sewage. These problems will be discussed only in so far as is necessary to convey to the public the knowledge of how pollutions occur and the necessity of remedying or preventing them.