When an inclosed body of air or other gas is subjected to pressure, its volume is diminished and its density is increased. It is natural to inquire what will happen if the external pressure be increased indefinitely. Will the inclosed substance eventually cease to be gaseous and become a solid or a liquid? The answer to this question, furnished about half a century ago through the researches of Thomas Andrews, is that no amount of pressure will liquefy a gas unless its temperature is below a certain point. This point, known as the _critical temperature_, is widely different for different substances. For most of the atmospheric gases it is exceedingly low. Thus oxygen must be cooled to 118° below zero Centigrade (180° below zero Fahrenheit) before it will liquefy under any pressure, and the critical temperature of nitrogen is still lower. Efforts to liquefy the gases of the atmosphere were unsuccessful for a long time on account of the difficulty of attaining such low temperatures. Nowadays the problem is so completely solved that the manufacture of liquid air is a commonplace commercial enterprise, and millions of gallons are produced every year. Liquid air is the principal commercial source of pure oxygen, nitrogen, and other gases found in the atmosphere. It is also used as a refrigerating substance in various industrial and scientific processes, and new uses are being found for it from year to year. Like many other latter-day miracles, compared with which the alleged feats of necromancy seem tame and puerile, the liquefaction of air is founded on quite simple principles. The earliest commercial process was invented, in its main features, by Linde in 1895, and the newer processes are merely modifications of this one. Experiments of the English physicists Joule and Thomson showed that when a gas under pressure is forced through a small orifice, beyond which it expands, it undergoes a certain amount of cooling. This fall in temperature, known as the “Joule-Thomson effect,” is generally quite small, but Linde devised a means of multiplying it in his “regenerative cooling process.” The air to be liquefied is first compressed to, say, 100 atmospheres, cooled as much as possible by water, and passed through a long spiral tube. At the end of the spiral it escapes through a small nozzle, and is thus somewhat further cooled by the effect above mentioned. This cooled air then passes back around the spiral tube, and causes still more cooling of the air in the latter. The escaping air is again compressed and goes through the same process as before. Thus its temperature grows constantly lower, until finally the stream issuing from the nozzle is a liquid instead of a gas. The liquid collects in a reservoir, from which it can be drawn off when desired.