According to German Wikipedia, the very first chemical warfare attack was made
by French troops in August 1914, using Xylolbromid (a kind of tear gas).http://de.wikipedia.org/wiki/Gasangriff#Erster_Weltkrieg
But yes, the first really killing stuff used was chlorine gas, which was simply
poured out from big flasks by the Germans in the 2. Battle of Flanders near Ypres
on 22 April 1915. The gas was heavier than air and sank into the British trenches.
It is nowadays estimated, that it killed 1200, and wounded 3000 men.
On 31. Mai 1915, Germany used Chlorine gas with 5% Phosgen added.
On 22 February 1916, France was the first nation to use pure Phosgen gas.
Phosgen gas is held responsible for the majority of the gas victims in the Great War.
The gas was later fired in gas granades.
Mankind can be perfidious at war, and so the later artillery tactics were to fire
a mix of gas granades. One gas, which would only just irritate the upper airways and
which could penetrate the gas mask filters, caused the soldiers to take off the masks.
Then they were exposed to the other, lethal gasses.
Even without a parachute, I would have prefered to be a pilot.