THE SELF-PURIFICATION OF WATER.
Dr. Guiseppe Cao in the Giornale della Reale Societa Italiana d’lgiene, Milan, thus writes of the self-purification of water: That the water of lakes and rivers, even though infected with immense quantities of refuse materials, is purified spontaneously, and after a certain time is freed from an infection and returns to its pristine condition, is now no longer disputed. What factors cause this purification ; what part must be attributed to each; and what is the mechanism of the action of each? The selfpurification of water is much more easily accomplished, when the foreign matter is broken up into small particles, this being determined by the velocity of the current, the condition of the bed of the river or lake, etc. When the particles are very small, the molecular cohesion with the water is all the greater, while the separation of the matter multiplies the surface of contact of the particles with the liquid mass; the surface for the nutrition of bacteria is also increased, these bacteria assuming the office of demolition of the foreign matter. A great number of the bacteria, according to the researches of Kruger and Frankland, are dragged to the bottom by the precipitation of the solid matter, and in this way sedimentation plays an important role in the purification of water. While it may be admitted that sedimentation is the principal factor of purifies -tion in sluggish streams, in swift currents the velocity itself is a purifying agent, carrying away all particles of matter which could cause infection. Sunlight and temperature, also, fill an important role. It has been seen that bacteria may be destroyed in the water by sunlight even at a depth of three feet, while temperature has a very important influence, there being much fewer bacteria in the water in the winter than in the summer. Many organisms have an action on the purification of water namely, the bacteria of putrid fermentation, protozoa, molluscs, crustaceans, etc. In the work of destruction plants are also concerned, the cryptogamia, the algae, etc. All of these organisms grow better in impure water which is rich in organic substances, and they must certainly destroy a notable part of the impurities. The part in purification which is assumed by dilution of the foreign matter is in relation to the mass of pure water and the character of the water sources, this action always increasing with distance from the source of contamination. Also the movement of the water, either in itself or by the aeration which is produced, has a powerful action on the purification of water.