These gases and salts are eagerly sucked up by the roots of plants, so that the soil bacteria are our best friends, changing poisonous decaying things into harmless plant-foods. They are the chief secret of the fertility of a soil; and the more there are of them the richer a soil is.
This makes fourteen times as many deaths from typhoid in proportion to the population as occur in Germany.
New York City, for instance, goes forty miles up into the hills to the great Croton reservoir for its water supply; and as this is proving insufficient, is preparing to go ninety-five miles up into the Ramapo Hills to secure control of a whole country-side for a permanent source of supply. Portland, Oregon, nearly twenty years ago, with then a population of some 75,000, built an aqueduct sixty miles up into the mountains to a lake on the side of Mt. Hood, and has reaped the advantages of its foresight ever since, in a low death rate and a rapid growth (200,000 in 1910), as well as a financial profit on its investment. Los Angeles, California, is preparing to build an aqueduct a hundred and thirty miles, and tunnel two mountain ranges in order to reach an inexhaustible supply of water.
Of late, currents of electricity are passed through the water (setting free oxygen or ozone) which make the purifying of it much more rapid and complete.
It is, however, often considered safer to pass the water through still another filter bed, consisting of layers of charcoal, which has the power of gathering oxygen in its pores, to attack and oxidize, or burn up, the remaining impurities in the water. A sort of scum forms over the surface of the last and finest bed of sand or charcoal, and if this scum is not too frequently removed, though it makes the filtering slower, the water comes out purer. On examining this scum, we find it to consist of a thick mat of our old friends, the purifying bacteria of the soil. So that the last step of our artificial filtration is simply an imitation of nature's great filter-bed.
Several streams emptying into the Ohio River from a thickly settled region are said to be actually pumped out into waterworks systems, used for drinking, washing, and manufacturing, and run back into the river again through sewers by the different cities along its banks, at such frequent intervals that every drop of water in them passes through waterworks systems and sewers three times before it reaches the mouth of the stream.