Gas & Fire
The first gas to be used on the battlefield was tear gas in 1914. In 1915 the Germans used siphons to spread a chlorine cloud over the French trenches. Few months later the British did the same, starting a competition for the improvement of the weapon (in its effects and in the delivery mechanism) and of the defenses against it.
German chemistry, led by Nobel prize Fritz Haber, was the main engine of this innovation. In 1916 the German army used phosgene, a toxic gas that was more difficult to detect than chlorine. In 1917 mustard gas was first used at Ypres, spread though artillery shells. It was a powerful vesicant gas, able to create internal and external injuries and to linger on the bottom of the trenches.
The losses caused by chemical warfare, estimated in about half a million, are not comparable to those caused by artillery fire or fire arms. In many cases its success was dependent on wind, that could send the toxic cloud back to the attackers. The horrible death the gas could deliver, the perceived cowardice of its use and the iconography that accompanied it, made it nonetheless one of the symbols of World War One. Its psychological effect went well beyond the actual casualty figures, so much so that popularization magazines often avoided the subject and, atypically, refrained to imagine future evolution (with some exceptions).
Flamethrowers were another chemical warfare innovation that found its first application during the war and that were used for the first time by the Germans. Its portable version was composed by cylinders, strapped to the soldier's back and filled with oil and pressurized gas. A muzzle in the operator hands was used to light up the oil spray and to direct the flame.