But despite everything that could be done, the Zeppelin eventually proved a failure as a war-vessel because it was so very costly to construct and operate and could so easily be destroyed, and the Germans began to build huge airplanes with which bombing-raids could be continued.
Strange to say, however, although the Germans were ready to admit the failure of their big airship, when the war stopped the Allies were actually building machines patterned after the Zeppelin, but even larger, and expected to use them for bombing-excursions over Germany. This astonishing turn of the tables was due to the fact that America had made a contribution to aëronautics that solved the one chief drawback of the Zeppelin.
A BALLOON GAS THAT WILL NOT BURN
When we entered the war against Germany, our allies placed before us all their problems and among them was this one of the highly inflammable airship. Could we not furnish a substitute for hydrogen that would not burn? It was suggested to us that helium would do if we could produce that gas cheaply and in sufficient quantity. Now, helium has a history of its own that is exceedingly interesting.
Every now and then the moon bobs its head into our light and we have a solar eclipse. But our satellite is not big enough to cut off all the light of the big luminary and the fiery atmosphere of the sun shows us a brilliant halo all around the black disk of the moon. Long ago, astronomers analyzed this flaming atmosphere with the spectroscope, and by the different bands of light that appeared they were able to determine what gases were present in the sun's atmosphere. But there was one band of bright yellow which they could not identify. Evidently this was produced by a gas unknown on earth, and they called it "helium" or "sun" gas.
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.
The reduction in the cost of helium is really one of the most important developments of this war. By removing the fire risk from airships we can safely use these craft for aërial cruises or for quick long-distance travel over land and sea. For, even in time of peace, sailing under millions of cubic feet of hydrogen is a serious matter. Although no incendiary bullets are to be feared, there is always the danger of setting fire to the gas within the exhaust of the engines. Engines have had to be hung in cars well below the balloon proper. But with helium in the gas-bags the engines can be placed inside the balloon envelop and the propellers can operate on the center line of the car.