Gas

one of the three states of matter, as distinguished from a solid and a liquid. Some elements, as oxygen and nitrogen, are gases under ordinary conditions. Most liquids and solids can be converted into gases by the application of sufficient heat. All gases have a tendency to expand or increase in bulk; that is, to fill the entire space in which they are confined. Many gases are combustible. Heat expands a gas; loss of heat contracts it. Ordinarily a gas expands or contracts 1/273 of its volume at zero centigrade for every degree of increase or decrease in temperature. Its volume is changed also by a change of pressure. Twice the pressure gives half the volume. Hydrogen, oxygen, and nitrogen are important gases. A ton of coal yields ordinarily 8,000 cubic feet of gas. It has been estimated that under ordinary circumstances a quart of gas contains nearly 1,000,000,000,000,000,000,000,000 molecules. See Boyle