hydrogen.

_The Liquid Air Process._--While several of the foregoing methods of
securing oxygen are successful as far as this result is concerned, they are
not profitable from a financial standpoint. A process for separating oxygen
from the nitrogen in the air has been brought to a high state of perfection
and is now supplying a major part of this gas for oxy-acetylene welding. It
is known as the Linde process and the gas is distributed by the Linde Air
Products Company from its plants and warehouses located in the large cities
of the country.

The air is first liquefied by compression, after which the gases are
separated and the oxygen collected. The air is purified and then compressed
by successive stages in powerful machines designed for this purpose until
it reaches a pressure of about 3,000 pounds to the square inch. The large
amount of heat produced is absorbed by special coolers during the process
of compression. The highly compressed air is then dried and the
temperature further reduced by other coolers.

The next point in the separation is that at which the air is introduced
into an apparatus called an interchanger and is allowed to escape through a
valve, causing it to turn to a liquid. This liquid air is sprayed onto
plates and as it falls, the nitrogen return to its gaseous state and leaves
the oxygen to run to the bottom of the container. This liquid oxygen is
then allowed to return to a gas and is stored in large gasometers or tanks.

The oxygen gas is taken from the storage tanks and compressed to
approximately 1,800 pounds to the square inch, under which pressure it is
passed into steel cylinders and made ready for delivery to the customer.
This oxygen is guaranteed to be ninety-seven per cent pure.