One of the most important illustrations of the necessity of control has been found in the celebrated contest between St. Louis and Chicago respecting the diversion of the drainage waters of Chicago from the lake through the drainage canal to the Mississippi River. The sending of such a huge body of sewage into the Illinois River and thence into the Mississippi River is evidently a menace to the inhabitants who live along the banks of these two streams. While it is true that the pollution of a stream is cared for in course of time from natural causes, it is also true that in flowing water very great injury may be done before these natural causes have had an opportunity to completely purify the water. The term "completely purify" may be too strong, for in natural purification the end results of the oxidation processes still remain in the water. While the nitric acid and the nitrates formed therefrom and the ammonia and the common nitrates which remain in the water are in themselves no very great threat to health, the esthetic point of view is offended, by offering for human consumption the purified sewage of large cities in which all the end results of purification remain.

The principle of the natural purification of the water rests upon the oxidation of the organic matter, which is the principal carrier of dangerous infection, by means of organisms which produce a solution of the solid bodies and a change in the chemical nature practically of all the bodies contained in the sewage. These organisms act more rapidly in warm than in cold weather. Other things being equal, the sewage would be more rapidly purified naturally during the summer than during the winter. During the summer, also, the volume of the carrying waters is less, and hence the necessity for purification is greater. These organisms not only attack the carbohydrate bodies and destroy them, but they also convert the nitrogenous bodies into nitric acid and ammonia. It is not safe to depend upon these natural causes for* purification, as they act irregularly and at varying speeds of activity and in varying completeness of final results. The sewage of cities and of towns, should be impounded and purified before being allowed to enter the running streams.