All these explosives are made from nitric acid and this used to be made from nitrates such as potassium nitrate or saltpeter.  But nitrates are rarely found in large quantities.  Napoleon and Lee had a hard time to scrape up enough saltpeter from the compost heaps, cellars and caves for their gunpowder, and they did not use as much nitrogen in a whole campaign as was freed in a few days’ cannonading on the Somme.  Now there is one place in the world—­and so far as we know one only—­where nitrates are to be found abundantly.  This is in a desert on the western slope of the Andes where ancient guano deposits have decomposed and there was not enough rain to wash away their salts.  Here is a bed two miles wide, two hundred miles long and five feet deep yielding some twenty to fifty per cent. of sodium nitrate.  The deposit originally belonged to Peru, but Chile fought her for it and got it in 1881.  Here all countries came to get their nitrates for agriculture and powder making.  Germany was the largest customer and imported 750,000 tons of Chilean nitrate in 1913, besides using 100,000 tons of other nitrogen salts.  By this means her old, wornout fields were made to yield greater harvests than our fresh land.  Germany and England were like two duelists buying powder at the same shop.  The Chilean Government, pocketing an export duty that aggregated half a billion dollars, permitted the saltpeter to be shoveled impartially into British and German ships, and so two nitrogen atoms, torn from their Pacific home and parted, like Evangeline and Gabriel, by transportation oversea, may have found themselves flung into each other’s arms from the mouths of opposing howitzers in the air of Flanders.  Goethe could write a romance on such a theme.

Now the moment war broke out this source of supply was shut off to both parties, for they blockaded each other.  The British fleet closed up the German ports while the German cruisers in the Pacific took up a position off the coast of Chile in order to intercept the ships carrying nitrates to England and France.  The Panama Canal, designed to afford relief in such an emergency, caved in most inopportunely.  The British sent a fleet to the Pacific to clear the nitrate route, but it was outranged and defeated on November 1, 1914.  Then a stronger British fleet was sent out and smashed the Germans off the Falkland Islands on December 8.  But for seven weeks the nitrate route had been closed while the chemical reactions on the Marne and Yser were decomposing nitrogen-compounds at an unheard of rate.

England was now free to get nitrates for her munition factories, but Germany was still bottled up.  She had stored up Chilean nitrates in anticipation of the war and as soon as it was seen to be coming she bought all she could get in Europe.  But this supply was altogether inadequate and the war would have come to an end in the first winter if German chemists had not provided for such a contingency in