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. CHEMICAL ACTION OF OXYGEN (a) _Upon Substances_ [Sidenote: Effect of air upon iron and wood] Upon some substances oxygen acts at ordinary temperature. Iron becomes covered with rust when exposed to air and moisture. Wood and other vegetable and animal substances undergo slow decomposition when exposed to the air. This is partly due to the action of oxygen at ordinary temperature. [Sidenote: Pure oxygen aids combustion] A splinter of wood will burn brilliantly in a jar of pure oxygen, and much more rapidly than in common air. Pure oxygen gas will cause many substances to burn which will not burn in air. Iron can be burned in pure oxygen, leaving only a reddish powder. [Sidenote: Formation of iron-rust] When iron rusts the carbon dioxid and water vapor combine chemically with the iron, and form what is known as a basic hydroxid or carbonate of iron. The process is somewhat complex. When iron burns in oxygen a red powder is formed--ferric oxid, Fe₂O₃. Iron dissolves in water, or moisture from the air containing carbonic acid, forming acid ferrous carbonate-- Fe + 2H₂CO₃ = FeH₂(CO₃)₂ + H₂ Iron + Carbonic acid = Acid ferrous carbonate + Hydrogen This acid ferrous carbonate, on drying or further oxidation, is converted into _iron-rust_. If we represent _iron-rust_ by the formula Fe₂O₃. 2Fe(OH)₃, the equation is as follows: 4FeH₂(CO₃)₂ + O₂ = Fe₂O₃. 2Fe(OH)₃ + H₂O + 8CO₂ Acid ferrous carbonate + Oxygen = Iron-rust + Water + Carbon dioxid (b) _In Living Bodies_ The most interesting action of oxygen at ordinary temperature, however, is that which takes place in our bodies and the bodies of all other animals. [Sidenote: Rate of blood circulation] [Sidenote: Oxidation of waste matter]