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.