Liquid air is already used in medical and surgical science. In surgery it is substituted for anæsthetics, deadening any part of the body on which an operation has to be performed. In fever hospitals, too, its cooling influence will be welcomed; and liquid oxygen takes the places of compressed oxygen for reviving the flickering flame of life. It will also prove invaluable for divers and submarine boats.
In combination with oil and charcoal liquid air, under the name of "oxyliquit," becomes a powerful blasting agent. Cartridges of paper filled with the oil and charcoal are provided with a firing primer. When everything is ready for the blasting the cartridges are dropped into a vessel full of liquid air, saturated, placed in position, and exploded. Mr. Knudsen assured the writer that oxyliquit is twice as powerful as nitro-glycerine, and its cost but one-third of that of the other explosive. It is also safer to handle, for in case of a misfire the cartridge becomes harmless in a few minutes, after the liquid air has evaporated.
But the greatest use will be found for liquid air when it exerts its force less violently. It is the result of power; its condition is abnormal; and its return to its ordinary state is accompanied by a great development of energy. If it be placed in a closed vessel it is capable of exerting a pressure of 12,000 lbs. to the square inch. Its return to atmospheric condition may be regulated by exposing it more or less to the heat of the atmosphere. So long as it remains liquid it represents so much _stored force_, like the electricity stored in accumulators. The Liquid Air Company have at their Gillingham Street depôt a neat little motor car worked by liquid air. A copper reservoir, carefully protected, is filled with the liquid, which is by mechanical means squirted into coils, in which it rapidly expands, and from them passes to the cylinders. A charge of eighteen gallons will move the car forty miles at an average pace of twelve miles an hour, without any of the noise, dirt, smell, or vapour inseparable from the employment of steam or petroleum. The speed of the car is regulated by the amount of liquid injected into the expansion coils.
We now come to the question of cost--the unromantic balance in which new discoveries are weighed and many found wanting. The storage of liquid air is feasible for long periods. (A large vacuum bulb filled and exposed to the atmosphere had some of the liquid still unevaporated at the end of twenty-two days.) But will it be too costly for ordinary practical purposes now served by steam and electricity? The managers of the Liquid Air Company, while deprecating extravagant prophecies about the future of their commodity, are nevertheless confident that it has "come to stay." With the small 50 horse-power plant its production costs upwards of one shilling a gallon, but with much larger plant of 1000 horse-power they calculate that the expenses will be covered and a profit left if they retail it at but one penny the gallon. This great reduction in cost arises from the economising of "waste energy." In the first place the power of expansion previous to the liquefaction of the compressed air will be utilised to work motors. Secondly, the heat of the cooling tanks will be turned to account, and even the "exhaust" of a motor would be cold enough for ordinary refrigerating. It is, of course, impossible to get more out of a thing than has been put into it; and liquid air will therefore not develop even as much power as was required to form it. But its handiness and cleanliness strongly recommend it for many purposes, as we have seen; and as soon as it is turned out in large quantities new uses will be found for it. Perhaps the day will come when liquid-air motors will replace the petrol car, and in every village we shall see hung out the sign, "Liquid air sold here." As the French say, "_Qui vivra verra_."