air, by volume, is oxygen and the remaining seventy-nine per cent is the
inactive gas, nitrogen. But for the presence of the nitrogen, which deadens
the action of the other gas, combustion would take place at a destructive
rate and be beyond human control in almost all cases. These two gases exist
simply as a mixture to form the air and are not chemically combined. It is
therefore a comparatively simple matter to separate them with the processes
now available.

_Water._--Water is a combination of oxygen and hydrogen, being
composed of exactly two volumes of hydrogen to one volume of oxygen. If
these two gases be separated from each other and then allowed to mix in
these proportions they unite with explosive violence and form water. Water
itself may be separated into the gases by any one of several means, one
making use of a temperature of 2,200 to bring about this separation.

[Illustration: Figure 7.--Obtaining Oxygen by Electrolysis]

The easiest way to separate water into its two parts is by the process
called electrolysis (Figure 7). Water, with which has been mixed a small
quantity of acid, is placed in a vat through the walls of which enter the
platinum tipped ends of two electrical conductors, one positive and the
other negative.

Tubes are placed directly above these wire terminals in the vat, one tube
being over each electrode and separated from each other by some distance.
With the passage of an electric current from one wire terminal to the
other, bubbles of gas rise from each and pass into the tubes. The gas that
comes from the negative terminal is hydrogen and that from the positive
pole is oxygen, both gases being almost pure if the work is properly
conducted. This method produces electrolytic oxygen and electrolytic