Snippet from Wikipedia: Second Battle of Ypres

During the First World War, the Second Battle of Ypres was fought from 22 April – 25 May 1915 for control of the tactically important high ground to the east and south of the Flemish town of Ypres in western Belgium. The First Battle of Ypres had been fought the previous autumn. The Second Battle of Ypres was the first mass use by Germany of poison gas on the Western Front.

Background

The German chemist Walther Nernst, who in 1914 was as a volunteer driver, proposed to Colonel Max Bauer, the German general staff officer responsible for liaison with scientists, that they could empty the opposing trenches by a surprise attack with tear gas. Observing a field test of this idea, the chemist Fritz Haber instead proposed using chlorine gas that was heavier-than-air.

The German commander Erich von Falkenhayn agreed to try the new weapon but intended to use it in a diversionary attack by the 4th Army. Falkenhayn wanted to use the gas to cover the transfer of Imperial German Army units to the Eastern Front to assist its ally Austria-Hungary with the Gorlice–Tarnów offensive against the Russian Empire. The gas would be released by siphoning liquid chlorine out of cylinders; the gas could not be released directly because the valves would freeze; wind would carry the gas to the enemy lines. 5,730 gas cylinders, the largest weighing 40 kg (88 lb), were carried into the front line. Installation was supervised by Haber, Otto Hahn, James Franck and Gustav Hertz.

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