An American newspaper reporter for the New York Tribune describes the use of gas at Ypres:
April 24th 1915
The nature of the gasses carried by the German asphyxiating shells remain a mystery. Whatever gas it is, it spreads rapidly and remains close to the ground. It is believed not to be specially deadly – one that rather overpowers its victims and puts them out of combat without killing many. Its effect at Bixschoote may have been due to panic caused by the novelty of the device. Its composition and manner of discharge are probably no mystery to the scientific artillerymen of the Allies. That such devices might be used in war has been known for a long time, but the positive prohibitions of The Hague Conference have prevented the more civilized nations of Europe from going far with experiments in this line.
April 25th 1915
The gaseous vapor which the Germans used against the French divisions near Ypres last Thursday, contrary to the rules of The Hague Convention, introduces a new element into warfare. The attack of last Thursday evening was preceded by the rising of a cloud of vapour, greenish gray and iridescent. That vapour settled to the ground like a swamp mist and drifted toward the French trenches on a brisk wind. Its effect on the French was a violent nausea and faintness, followed by an utter collapse. It is believed that the Germans, who charged in behind the vapour, met no resistance at all, the French at their front being virtually paralysed.
Everything indicates long and thorough preparation for this attack. The work of sending out the vapour was done from the advanced German trenches. Men garbed in a dress resembling the harness of a diver and armed with generators about three feet high and connected with ordinary hose pipe turned the vapour loose towards the French lines. Some witnesses maintain that the Germans sprayed the earth before the trenches with a fluid which, being ignited, sent up the fumes. The German troops, who followed up this advantage with a direct attack, held respirators in their mouths, thus preventing them from being overcome by the fumes.
In addition to this, the Germans appear to have fired ordinary explosive shells loaded with some chemical which had a paralysing effect on all the men in the region of the explosion. Some chemical in the composition of those shells produced violent watering of the eyes, so that the men overcome by them were practically blinded for some hours.
The effect of the noxious trench gas seems to be slow in wearing away. The men come out of their nausea in a state of utter collapse. Some of the rescued have already died from the after effects. How many of the men left unconscious in the trenches when the French broke died from the fumes it is impossible to say, since those trenches were at once occupied by the Germans.
This new form of attack needs for success a favourable wind. Twice in the day that followed the Germans tried trench vapour on the Canadians, who made on the right of the French position a stand which will probably be remembered as one of the heroic episodes of this war. In both cases the wind was not favourable, and the Canadians managed to stick through it. The noxious, explosive bombs were, however, used continually against the Canadian forces and caused some losses.