Risks from Leaf Mould. Practically the whole surface of the earth is covered with some form of vegetation--grass, trees, or other green plants. These dying down and decaying year after year, form a layer of vegetable mould such as you can readily scratch up on the surface of the ground in a forest or old meadow; this is known as leaf mould, or _humus_. As the water soaks through this mould, it becomes loaded with decaying vegetable matter, which it carries with it down into the soil. Most of this, fortunately, is comparatively harmless to the human digestion. But some of this vegetable matter, such as we find in the water from bogs or swamps, or even heavy forests, will sometimes upset the digestion; hence, the natural dislike that we have for water with a marshy, or "weedy," taste. [Illustration: NATURE'S FILTER-BED The spring water is pure; the brook may gather infection as it goes.] Nature's Filter-Bed. When, however, this peaty water soaks on down through the grass, roots, and leaf mold, into the soil, it comes in contact with Nature's great filter-bed--the second place in the circuit where the water is again made perfectly pure. This filter-bed consists of a layer of more or less spongy, porous soil, or earth, swarming with millions of tiny vegetable germs known as bacteria. 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