Other uses of hydrogen besides ballooning have already been referred to in other chapters.  It is combined with nitrogen to form synthetic ammonia.  It is combined with oxygen in the oxy-hydrogen blowpipe to produce heat.  It is combined with vegetable and animal oils to convert them into solid fats.  There is also the possibility of using it as a fuel in the internal combustion engine in place of gasoline, but for this purpose we must find some way of getting hydrogen portable or producible in a compact form.

Aluminum, like silicon, sodium and calcium, has been rescued by violence from its attachment to oxygen and like these metals it reverts with readiness to its former affinity.  Dr. Goldschmidt made use of this reaction in his thermit process.  Powdered aluminum is mixed with iron oxide (rust).  If the mixture is heated at any point a furious struggle takes place throughout the whole mass between the iron and the aluminum as to which metal shall get the oxygen, and the aluminum always comes out ahead.  The temperature runs up to some 6000 degrees Fahrenheit within thirty seconds and the freed iron, completely liquefied, runs down into the bottom of the crucible, where it may be drawn off by opening a trap door.  The newly formed aluminum oxide (alumina) floats as slag on top.  The applications of the thermit process are innumerable.  If, for instance, it is desired to mend a broken rail or crank shaft without moving it from its place, the two ends are brought together or fixed at the proper distance apart.  A crucible filled with the thermit mixture is set up above the joint and the thermit ignited with a priming of aluminum and barium peroxide to start it off.  The barium peroxide having a superabundance of oxygen gives it up readily and the aluminum thus encouraged attacks the iron oxide and robs it of its oxygen.  As soon as the iron is melted it is run off through the bottom of the crucible and fills the space between the rail ends, being kept from spreading by a mold of refractory material such as magnesite.  The two ends of the rail are therefore joined by a section of the same size, shape, substance and strength as themselves.  The same process can be used for mending a fracture or supplying a missing fragment of a steel casting of any size, such as a ship’s propeller or a cogwheel.

[Illustration:  TYPES OF GAS MASK USED BY AMERICA, THE ALLIES, AND GERMANY DURING THE WAR

In the top row are the American masks, chronologically, from left to right:  U.S.  Navy mask (obsolete), U.S.  Navy mask (final type), U.S.  Army box respirator (used throughout the war), U.S.R.F.K. respirator, U.S.A.T. respirator (an all-rubber mask), U.S.K.T. respirator (a sewed fabric mask), and U.S.  “Model 1919,” ready for production when the armistice was signed.  In the middle row, left to right, are:  British veil (the original emergency mask used in April, 1915), British P.H. helmet (the next emergency mask), British box respirator (standard British army type), French M2 mask (original type), French Tissot artillery mask, and French A.R.S. mask (latest type).  In the front row:  the latest German mask, the Russian mask, Italian mask, British motor corps mask, U.S. rear area emergency respirator, and U.S.  Connell mask]