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Water is Always in Motion. The part of our water supply which we take

directly, in the form of drinking water, is, however, unfortunately

anything but free from danger of impurities. The greatest difficulty

with water is that it will not stay put--it is continually on the

move. The same perpetual circulation, with change of form, but without

loss of substance, which is taking place in the engine and in our

bodies, is taking place in the world around us. The water from the

ocean, the lakes, and the rivers is continually evaporating under the

heat of the sun and rising in the form of vapor, or invisible steam,

into the air. There it becomes cooler, and forms the clouds; and when

these are cooled a little more, the vapor changes into drops of water

and pours down as rain, or, if the droplets freeze, as snow or hail. The

rain falls upon the leaves of the trees and the spears of the grass, or

the thirsty plowed ground, soaks down into the soil and seeps or

drains gradually into the streams and rivers, and down these into the

lakes and oceans, to be again pumped up by the sun. All we can do is to

catch what we need of it, on the run, somewhere in the earthy part of

its circuit.



Why our Drinking Water is Likely to be Impure. Every drop of water

that we drink or use, fell somewhere on the surface of the earth, in the

form of rain or snow; and if we wish to find out whether it is pure and

safe, we must trace its course through the soil, or the streams, from

the point where it fell. Our drinking water has literally washed all

outdoors before it reaches us, and what it may have picked up in that

washing makes the possibilities of its danger.



As it falls from the skies, it is perfectly pure--except in large cities

or manufacturing centres, where rain water contains small amounts of

soot, smoke-acids, and dust, but even these are in such small amounts as

to be practically harmless. But the moment it reaches the ground, it

begins to soak up something out of everything that it touches; and here

our dangers begin.



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.



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.