power for a number of special purposes. Thus for use in coal mines it is safer than electricity because it is free from the danger of sparks. There are a great many cases in which the air itself is used in the process to which the power is applied, as in different kinds of air blast, from the simple bellows to the blowers of blast furnaces; also in aerating apparatus, oil and fuel burners, spraying, cleansing, etc.

A familiar form of air compressor is the hand pump used for inflating bicycle tires. This simple device illustrates two important facts; first, that a considerable amount of energy must be used to overcome the expansive force of the air, and, second, that part of the energy applied to the pump produces heat. That the heat thus produced and dissipated in the surrounding air represents a loss of energy is apparent; but energy is wasted in another way that is, perhaps, not so evident. When a gas is heated its expansive force is increased. Hence, on account of the heating of the air in the tire, the pump has to do more work to accomplish a given amount of compression than it would need to do if the air remained cool.

In order to avoid this loss, the air compressors used for industrial purposes are provided with some sort of device for keeping the air cool during compression. This is accomplished by a spray of water inside the compressor cylinder, or, more commonly, by inclosing the cylinder in a water jacket. In producing high pressures, the air is compressed by degrees in two or more cylinders, and cooled between the successive stages. Lastly, before compressed air is applied to driving tools or machinery, it is often reheated to increase its pressure. For most industrial purposes the pressure of compressed air does not exceed 75 pounds to the square inch (5 “atmospheres”). For charging the tanks of compressed-air locomotives, for liquefying gases, and a few other purposes, much higher pressures are used. In laboratory experiments air has been compressed to the enormous pressure of 60,000 pounds to the square inch, or 4,000 atmospheres. At a pressure of 14,000 pounds to the square inch compressed air has been successfully used for blasting in mines in place of ordinary explosives.

The use of pneumatic tools began in the sixties of the last century, when pneumatic drills were employed with conspicuous success in the construction of the Mont Cenis and Hoosac tunnels. Such tools are now indispensable adjuncts not only of tunneling and mining, but also of nearly every department of metal-working and wood-working, and have contributed incalculably to the welfare of mankind.

Imagine a workman with an ordinary hammer driving such a tool as a chisel, punch, or calking iron, and estimate the amount of work accomplished in the course of a day spent in this wearisome labor. Then consider how such operations are performed with the help of that versatile substance, air. The pneumatic hammer consists of a piston working in a cylinder, to which compressed air is conveyed from a compressor by means of a flexible hose. The hammer is so designed that the air causes the piston to work back and forth with great rapidity. A chisel, rammer, or other percussion tool is loosely fitted in the nose of the hammer, so that the piston will strike it a blow at each forward motion. The workman has nothing to do but hold the tools in place. With a common hammer or mallet a workman will strike from twenty to a hundred blows a minute, according to the nature of the work. The speed of the pneumatic hammer ranges from 1,000 to 20,000 blows per minute, so that its sound is a continuous buzz. Such hammers are used for calking, chipping, riveting, and a great number of other purposes.

In another large class of pneumatic tools work is done by rotation instead of percussion. The piston is replaced by a motor, which turns an auger, drill, or other tool for such operations as boring, screwing, reaming, etc.

The use of pneumatic tubes for transporting letters, parcels, and the like, although suggested as early as 1667, has been in practical operation only since 1854, when a tube 220 yards long was built in London to convey telegraphic dispatches. The articles to be transported are placed in a carrier fitting closely inside the tube and propelled either by introducing air under pressure behind it or by exhausting the air in front of it. Scores of miles of such tubes laid underground are now in operation in London, Paris, Berlin, New York, and other large cities for carrying mail matter. In the United States the pneumatic cash carrier, used in stores, is the commonest application of “pneumatic dispatch,” as this system of transportation is called.

The use of compressed air instead of a brush for applying paint, varnish, and whitewash is a further illustration of the versatile possibilities of air as a means of transmitting power.

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.

The liquid air thus obtained has a temperature of about 315° below zero Fahrenheit. It is generally drawn into a vessel called,