Another source of hydrogen originating with the electric furnace is “hydrolith,” which consists of calcium hydride.  Metallic calcium is prepared from lime in the electric furnace.  Then pieces of the calcium are spread out in an oven heated by electricity and a current of dry hydrogen passed through.  The gas is absorbed by the metal, forming the hydride (CaH_{2}).  This is packed up in cans and when hydrogen is desired it is simply dropped into water, when it gives off the gas just as calcium carbide gives off acetylene.

This last reaction was also used in Germany for filling Zeppelins.  For calcium carbide is convenient and portable and acetylene, when it is once started, as by an electric shock, decomposes spontaneously by its own internal heat into hydrogen and carbon.  The latter is left as a fine, pure lampblack, suitable for printer’s ink.

Napoleon, who was always on the lookout for new inventions that could be utilized for military purposes, seized immediately upon the balloon as an observation station.  Within a few years after the first ascent had been made in Paris Napoleon took balloons and apparatus for generating hydrogen with him on his “archeological expedition” to Egypt in which he hoped to conquer Asia.  But the British fleet in the Mediterranean put a stop to this experiment by intercepting the ship, and military aviation waited until the Great War for its full development.  This caused a sudden demand for immense quantities of hydrogen and all manner of means was taken to get it.  Water is easily decomposed into hydrogen and oxygen by passing an electric current through it.  In various electrolytical processes hydrogen has been a wasted by-product since the balloon demand was slight and it was more bother than it was worth to collect and purify the hydrogen.  Another way of getting hydrogen in quantity is by passing steam over red-hot coke.  This produces the blue water-gas, which contains about 50 per cent. hydrogen, 40 per cent. carbon monoxide and the rest nitrogen and carbon dioxide.  The last is removed by running the mixed gases through lime.  Then the nitrogen and carbon monoxide are frozen out in an air-liquefying apparatus and the hydrogen escapes to the storage tank.  The liquefied carbon monoxide, allowed to regain its gaseous form, is used in an internal combustion engine to run the plant.

There are then many ways of producing hydrogen, but it is so light and bulky that it is difficult to get it where it is wanted.  The American Government in the war made use of steel cylinders each holding 161 cubic feet of the gas under a pressure of 2000 pounds per square inch.  Even the hydrogen used by the troops in France was shipped from America in this form.  For field use the ferro-silicon and soda process was adopted.  A portable generator of this type was capable of producing 10,000 cubic feet of the gas per hour.

The discovery by a Kansas chemist of natural sources of helium may make it possible to free ballooning of its great danger, for helium is non-inflammable and almost as light as hydrogen.