22. The Battle of Verdun ended, but the shelling around Verdun went on
Beginning in early 1917 the German batteries which remained in the area, in the positions they had occupied a year earlier, shelled the French positions, and the French responded in kind. The Germans maintained nearly four hundred batteries, and the bombardments were once again supported with observation aircraft, both airplanes and balloons. Aerial bombing expanded over the lines of both sides, adding to the joys of life in the trenches. In the summer of 1917 the Germans added mustard gas shells to their arsenal. Mustard gas is heavier than air, poisoned the soil when it settled into the ground as a liquid, and remained active for weeks.
Mustard gas was not usually lethal, unless the victim was exposed to a large dose, and it caused both internal bleeding and external through lesions and blisters. It also caused blindness and painfully dry eyes. Those exposed to fatal levels of the gas, or its oils in the ground, often took several agonizing weeks to die, and there was little the medical professionals of the day could do to help them. Allied forces began using mustard gas in autumn 1917, and for the rest of the war the Allies launched more gas attacks than the Germans, aided by the prevailing winds of western Europe, which typically blow from west to east.