By this method the oxygen and the hydrogen are separated by
electricity.
3 By the _liquefaction of air_, which is a very recent and a
very scientific method.
By this method the air is cooled down until it liquefies.
At normal atmospheric pressure it liquefies at a temperature of
--312.6°F., but under pressure of about 585 pounds it liquefies
at a temperature of --220°F. After the air has been liquefied, it
is allowed to go back to vapor by exposing it to the surrounding
heat of the atmosphere, and this vaporization separates the
nitrogen from the oxygen, as the nitrogen boils at a temperature of
--318°F., while the oxygen boils at a temperature of --294°F. There
is a difference of about 24° in the boiling points of these two
gases, which at this low point amounts to more than the difference
between the boiling points of alcohol and water, and this
difference is sufficient to separate the oxygen from the nitrogen.
Production of oxygen by the liquefaction of air is the latest,
cheapest, and most approved method, and is now becoming extensively
used in obtaining both oxygen and nitrogen for commercial use.
[Sidenote: Properties of oxygen]
Oxygen is tasteless and odorless. It is slightly heavier than air. When
subjected to an extremely high pressure and low temperature it becomes
liquid.