Liquid air is a mixture of oxygen and nitrogen which have been compressed until they form a liquid. When the pressure on the liquid is released, it turns back into these two gases. The liquid is kept under great pressure in strong steel cylinders. Liquid air has a great many uses, because it enables large amounts of these two gases to be stored in small spaces. The aqualungs used by skin divers consists of small cylinders joined to a mouthpiece. Liquid air is also used by pilots, firemen and astronauts.
Once liquid air has been made, the liquid oxygen can be separated from it for use in hospitals. People with lung trouble are helped by being given an increased amount of oxygen to breathe. Other gases, liquefied in the same way, are used as anaeshetics to put people to sleep during operations.
Liquid oxygen is widely used in industry because a flame fed with additional oxygen is much hotter than an ordinary flame. The oxyacetylene torch consists of two cylinders, one of liquid oxygen and the other of acetylene. The two gases are mixed together and fed through tubes to a special blowpipe. When they are lit, they can produce a flame with a temperature of about 3,0000 Centigrade (5,4320 Fahrenheit) or twice the melting point of iron. This torch is used to cut and to weld metals.