These eagerly pick out all the decaying vegetable substances of the water and feed upon them, changing them into harmless carbon dioxid water, and small amounts of _ammonia_.

Not only will this filter-bed, or spongy mat of bacteria, burn up and remove all traces of vegetable decay, but if the rain happens to have soaked through the decaying body of a bird or animal or insect, the bacteria will just as eagerly feed upon these animal substances and change them into harmless gases and salts.[13] By the time the rain water has reached the deeper layers of the soil, it is again perfectly pure and has also, in seeping through the soil, picked up certain mineral salts (such as _calcium_, _sodium_, and _magnesium_) which are of use in the body; so that in an open or thinly settled country, the water in streams, rivers, and lakes is usually fairly pure and quite wholesome.

That is why, in ancient times, the great majority of villages and towns and camps were situated on the bank of some stream, where a supply of water could easily be obtained.
CAUSES AND DANGERS OF POLLUTED WATER Wells--the Oldest Method of Supplying Water.

It was long ago discovered that, by digging pits or holes in the ground, the rain water, in its steady flow toward the streams and lakes, could be caught or trapped, and that if the pit were made deep enough, a sufficient amount would accumulate during the winter or spring to last well on into the summer, unless the season were unusually dry.

These pits, or water traps, are our familiar _wells_, from which most of our water supply, except in the large cities, is still taken.