We will first take note of the methods used by Nature in purifying water. Nature’s method of obtaining chemically pure water is by evaporation and condensation. The result of this method is the snow and rain water. About three-fourths of the earth’s surface is covered with water. The sun acts as a great furnace and the atmosphere as a vast still. In this way we see that a great quantity of water is distilled and returned to the earth in a state of purity. It is estimated by Summerville that “186,240 cubic miles of water are annually raised from the surface of the globe in the form of vapor chiefly in the intertropical seas.” It is entirely erroneous to assume that the impurities in water are eliminated by freezing. Ice may contain great quantities of impurities, even so-called pathogenic germs. It has been shown that many germs are not destroyed by freezing. It is true, however, that ice will be purer than the water from which it is taken. There are certain processes which result in a purifying influence as water is crystallized. As ice forms it excludes suspended matter and even under certain conditions will exclude dissolved substances. It will thus be seen that even though ice may not be pure, the danger from its use is greatly reduced. It is not advisable to use water from melted ice for drinking purposes. Manufactured ice is supposed to be free from impurities, but this is not necessarily true. If ice is produced artificially from distilled or properly filtered water it will naturally be pure, provided it does not become polluted in the process of manufacture. At one time it was believed that river water would become pure in a flow of seven miles. This, however, could not be true since the distance of the flow is of less importance than the time required for the flow. The greatest aid in this method of water purification comes from the impurities mixing with the volume of water as it flows. The impurities become diluted and if sufficient time elapses the bacteria and microörganisms will die a natural death. The principal process involved in self-purification of water is the oxidation of nitrogenous organic matter which is a chemical process. In the course of time the microörganisms die as a result of the biological action. Then there is the effect of dilution, sedimentation, especially in lakes and still water, and the effect of sunlight. Food for the bacteria is lessened as a result of the destruction of the organic matter through oxidation. The minute infusoria, amœbæ and water worms feed upon organic matter and bacteria and thus aid in the process of purification. The plants known as algæ play a very important part also in water purification. They take up the organic substances and perform other functions similar to the vegetation on the land. The purifying effects of water vegetation is very important.