In order to make liquid air an article of commerce the most important condition was a wholesale decrease in cost of production. In 1857 C. W. Siemens took out a patent for making the liquid on what is known as the regenerative principle, whereby the compressed air is chilled by expanding a part of it. Professor Dewar--a scientist well known for his researches in the field of liquid gases--had in 1892 produced liquid air by a modification of the principle at comparatively small cost; and other inventors have since then still further reduced the expense, until at the present day there appears to be a prospect of liquid air becoming cheap enough to prove a dangerous rival to steam and electricity. A company, known as the Liquid Air, Power and Automobile Company, has established large plants in America and England for the manufacture of the liquid on a commercial scale. The writer paid a visit to their depot in Gillingham Street, London, where he was shown the process by Mr. Hans Knudsen, the inventor of much of the machinery there used. The reader will doubtless like to learn the “plain, unvarnished truth” about the creation of this peculiar liquid, and to hear of the freaks in which it indulges--if indeed those may be called freaks which are but obedience to the unchanging laws of Nature. On entering the factory the first thing that strikes the eye and ear is the monstrous fifty horse-power gas-engine, pounding away with an energy that shakes the whole building. From its ponderous flywheels great leather belts pass to the compressors, three in number, by which the air, drawn from outside the building through special purifiers, is subjected to an increasing pressure. Three dials on the wall show exactly what is going on inside the compressors. The first stands at 90 lbs. to the square inch, the second at 500, and the third at 2200, or rather less than a ton pressure on the area of a penny! The pistons of the low-pressure compressor is ten inches in diameter, but that of the high pressure only two inches, or 1/25 of the area, so great is the resistance to be overcome in the last stage of compression. Now, if the cycle-pump heats our hands, it will be easily understood that the temperature of the compressors is very high. They are water-jacketed like the cylinders of a gas-engine, so that a circulating stream of cold water may absorb some of the heat. The compressed air is passed through spiral tubes winding through large tanks of water which fairly boils from the fierceness of the heat of compression.