For a quarter of a century this sun gas remained a mystery; then one day, in 1895, Sir William Ramsay discovered the same band of light when studying the spectrum of the mineral cleveite. The fact that astronomers had been able to single out an element on the sun ninety million miles away before our chemists could find it right here on earth, produced a mild sensation, but the general public attached no special importance to the gas itself. It proved to be a very light substance, next to hydrogen the lightest of gases, and for years it resisted all attempts at liquefaction. Only when Onnes, the Dutch scientist, succeeded in getting it down to a temperature of 450 degrees below zero, Fahrenheit, did the gas yield to the chill and condense into a liquid. The gas would not burn; it would not combine with any other elements, and apparently it had no use on earth, and it might have remained indefinitely a lazy member of the chemical fraternity had not the great world conflict stirred us into frenzied activity in all branches of science in our effort to beat the Hun. Because the gas had no commercial value, there was only a small amount of helium to be found in the whole world. Not a single laboratory in the United States had more than five cubic feet of it and its price ranged from $1,500 to $6,000 per cubic foot. At the lowest price it would cost $3,000,000,000 to provide gas enough for one airship of Zeppelin dimensions and it seemed absurd even to think of a helium airship. AMERICAN CHEMISTS TO THE RESCUE Just before the war it was discovered that there is a considerable amount of helium in the natural gas of Oklahoma, Texas, and Kansas, and Sir William Ramsey suggested that our chemists might study some method of getting helium from this source. The only way of separating it out was to liquefy the gases by subjecting them to extreme cold. All gases turn to liquid if they are cooled sufficiently, and then further cold will freeze them solid. But helium can stand more cold than any other and this fact gave the clue to its recovery from natural gas. The latter was frozen and one after another the different elements condensed into liquid, until finally only helium was left. This sounds simple, but it is a difficult matter to get such low temperature as that on a large scale and do it economically. To be of any real service in aëronautics helium would have to be reduced in cost from fifteen hundred dollars to less than ten cents per cubic foot. Several different kinds of refrigerating-machinery were tried and finally just before the war was brought to a close by the armistice we had succeeded in producing helium at the rate of eight cents per cubic foot, with the prospect of reducing its cost still further. A large plant for recovering helium was being built. The plant will have been completed before this book is published, and it will be turning out helium for peaceful instead of military airships.