free oxygen. By
_free oxygen_ is meant oxygen in the form of gas.

COMPOSITION OF WATER.--Now, water, in which harmful germs live, is
one-third oxygen. Nevertheless, the germs thrive in water, because the
oxygen is in a compound state, and, therefore, not an active agent. But
if oxygen, in the form of gas, can be forced through water, it will
attack the germs, and destroy them.

COMMON AIR NOT A GOOD PURIFIER.--Water may be purified, to a certain
extent, by forcing common air through it, and the foulest water, if run
over rocks, will be purified, in a measure, because air is intermingled
with it. But common air is composed of four-fifths nitrogen, and only
one-fifth oxygen, and, as nitrogen is the staple article of food for
bacteria, the purifying method by air is not effectual.

PURE OXYGEN.--When, however, oxygen is generated from water, by means of
electrolysis, it is pure; hence is more active and is not tainted by a
life-giving substance for germs, such as nitrogen.

The mechanism usually employed for purifying water is shown in Fig. 90.

A WATER PURIFIER.--The case (A, Fig. 90) may be made of metal or of an
insulating material. If made of metal it must be insulated within with
slate, glass, marble or hard rubber, as shown at B. The case is provided
with exterior flanges (C, D), with upper and lower ends, and it is
mounted upon a base plate (E) and affixed thereto by bolts. The upper
end has a conically-formed cap (F) bolted to the flanges (C), and this
has an outlet to which a pipe (G) is attached. The water inlet pipe (H)
passes through the lower end of the case (A). The electrodes (I, J) are
secured, vertically, within the case, separated from each other
equidistant, each alternate electrode being connected up with one wire
(K), and the alternate electrodes with a wire (L).

[Illustration: _Fig. 90._ ELECTRIC WATER PURIFIER]

When the water passes upwardly, the decomposed or gaseous oxygen
percolates through the water and thus attacks the germs and destroys
them.

THE USE OF HYDROGEN IN PURIFICATION.--On the other hand, the hydrogen
also plays an important part in purifying the water. This depends upon
the material of which the electrodes are made. Aluminum is by far the
best material, as it is one of nature's most active purifiers. All clay
contains aluminum, in what is known as the sulphate form, and water
passing through the clay of the earth thereby becomes purified, because
of this element.

ALU