http://avalon.law.yale.edu/19th_century/dec99-02.asp
"The Contracting Powers agree to abstain from the use of projectiles the object of which is the diffusion of asphyxiating or deleterious gases."
I remember reading a history that stated that during WW1 the Germans first released chlorine gas from cylinders when the wind was blowing to the West arguing that as the cylinders were not projectiles they were not breaching the convention.
And when the British retaliated with chemical shells the Germans claimed that it was the British who were first breaching the convention and the Germans deployed chemical shells themselves.