Question: When did the Germans begin using chlorine gas on the Western Front?
About the article "World War I", section "Trench warfare begins".

Candidate passages:
Stoltzenberg was the main protagonist at the Second Battle of Ypres (22 April to 25 May 1915) in Belgium where the Germans used poison gas for the first time on the Western Front.
The first gas attack occurred against Canadian soldiers and also against a force of mostly colonial soldiers from French Africa known as French colonial troops.
The gas used was Chlorine gas.
Mustard gas, also called Yperite from the name of this city, was also used for the first time near Ypres in the autumn of 1917.
Prior to that, Stoltzenberg had injured his eye in an incident involving a chlorine gas cylinder which exploded and blinded him in the left eye. 
The most widely reported and, perhaps, the most effective gas of the First World War was mustard gas.
It was a vesicant that was introduced by Germany in July 1917 prior to the Third Battle of Ypres.
The Germans marked their shells yellow for mustard gas and green for chlorine and phosgene; hence they called the new gas "Yellow Cross".
It was known to the British as "HS" ("Hun Stuff"), while the French called it "Yperite" (named after Ypres).
Mustard gas is not a particularly effective killing agent (though in high enough doses it is fatal) but can be used to harass and disable the enemy and pollute the battlefield.
Delivered in artillery shells, mustard gas was heavier than air, and it settled to the ground as an oily liquid resembling sherry.
Once in the soil, mustard gas remained active for several days, weeks, or even months, depending on the weather conditions.
The skin of victims of mustard gas blistered, their eyes became very sore and they began to vomit.
Mustard gas caused internal and external bleeding and attacked the bronchial tubes, stripping off the mucous membrane.
This was extremely painful.
Fatally injured victims sometimes took four or five weeks to die of mustard gas exposure.
One nurse, Vera Brittain, wrote: "I wish those people who talk about going on with this war whatever it costs could see the soldiers suffering from mustard gas poisoning.
Great mustard-coloured blisters, blind eyes, all sticky and stuck together, always fighting for breath, with voices a mere whisper, saying that their throats are closing and they know they will choke.
"The polluting nature of mustard gas meant that it was not always suitable for supporting an attack as the assaulting infantry would be exposed to the gas when they advanced.
When Germany launched Operation "Michael" on 21 March 1918, they saturated the Flesquières salient with mustard gas instead of attacking it directly, believing that the harassing effect of the gas, coupled with threats to the salient's flanks, would make the British position untenable.
Gas never reproduced the dramatic success of 22 April 1915; however, it became a standard weapon which, combined with conventional artillery, was used to support most attacks in the later stages of the war.
Gas was employed primarily on the Western Front—the static, confined trench system was ideal for achieving an effective concentration.
Germany also made use of gas against Russia on the Eastern Front, where the lack of effective countermeasures resulted in deaths of over 56,000 Russians, while Britain experimented with gas in Palestine during the Second Battle of Gaza.
Russia began manufacturing chlorine gas in 1916, with phosgene being produced later in the year.
However, most of the manufactured gas was never used.
The British Army believed that the use of gas was needed, and mounted more gas attacks than the Germans in 1917 and 1918 because of a marked increase in production of gas from the Allied nations.
Germany was unable to keep up with this pace despite creating various new gases for use in battle, mostly as a result of very costly methods of production.
Entry into the war by the United States allowed the Allies to increase mustard gas production far more than Germany.
Also the prevailing wind on the Western Front was from the west, which meant the British more frequently had favorable conditions for a gas release than did the Germans.
Though the United States had never used chemical weapons in World War I, it had began a large scale production of an improved vesicant gas known as Lewisite, for use in an offensive planned for early 1919.
By the time of the armistice on 11 November, a plant near Willoughby, Ohio was producing 10 tons per day of the substance, for a total of about 150 tons.
It is uncertain what effect this new chemical would have had on the battlefield, however, as it degrades in moist conditions. 
The Second Battle of Ypres was the first time Germany used chemical weapons on a large scale on the Western Front in World War I.
168 tons of chlorine gas was released on 22 April over a four mile (6 km) front.
Around 6,000 troops died within ten minutes from asphyxiation.
The gas affects the lungs and the eyes causing respiration problems and blindness.
Being denser than air it flowed downwards forcing the troops to climb out of trenches.
Eventually reserve Canadian troops held the front, being away from the attack, using urine-soaked cloths as primitive respirators.
A Canadian soldier realized that the ammonia in urine would react with the chlorine, neutralizing it, and that the water would dissolve the chlorine, allowing the soldiers to breathe through the gas.
This is the first recorded response and defense against chemical attacks using respirators. 
The village of St. Julien had been comfortably in the rear of the 1st Canadian Division until the poison gas attack of 22 April, when it became the front line.
Some of the first fighting in the village involved a hasty stop, which included the stand of Lance Corporal Frederick Fisher of the 13th Battalion CEF's machine-gun detachment; who twice went out with a handful of men and a Colt Machine-gun and prevented advancing German troops from passing through St. Julien, into the rear of the Canadian front line; Fisher was killed the next day using the same tactics.
On the morning of 24 April the Germans released another cloud of chlorine, towards the re-formed Canadian line just west of St. Julien.
Word was passed among the Canadian troops to urinate on their handkerchiefs and place these over their noses and mouths.
The countermeasures were insufficient and German troops took the village.
Next day the York and Durham Brigade units of the Northumberland Division counter-attacked, failed to secure their objectives but established a new line closer to the village.
On 26 April the Northumberland Brigade attacked again and gained a foothold in the village but were forced back with the loss of more than The 2nd Royal Dublin Fusiliers had hundreds of casualties and with no respite took part in the battles at Frezenberg and Bellewaarde.
On 24 May the battalion was subject to a German chlorine gas attack near St. Julien and was nearly annihilated.
The German Army first used chlorine gas cylinders in April 1915 against the French Army at Ypres.
Yellow-green clouds drifted towards the Allied trenches.
The gas had a distinctive smell, like pineapple and pepper.
At first the French officers assumed that the German infantry were advancing behind a smoke screen and the troops were alerted.
When the gas arrived at the Allied front-trenches soldiers began to complain about pains in the chest and a burning sensation in their throats.
After the first German chlorine gas attacks, Allied troops were supplied with masks of cotton pads that had been soaked in urine.
It was found that the ammonia in the pad neutralized the chlorine.
These pads were held over the face until the gas dispersed.
Other soldiers preferred to use handkerchiefs, a sock, a flannel body-belt, dampened with a solution of bicarbonate of soda, tied across the mouth and nose until the gas passed over.
Soldiers found it difficult to fight like this and attempts were made to develop a better means of protecting men against gas attacks.
By July 1915 soldiers were given efficient gas masks and anti-asphyxiation respirators.
Private W. Hay of the Royal Scots arrived in Ypres just after the chlorine gas attack on 22 April 1915. 
The first instance of large-scale use of gas as a weapon was on 31 January 1915, when Germany fired 18,000 artillery shells containing liquid xylyl bromide tear gas on Russian positions on the Rawka River, west of Warsaw during the Battle of Bolimov.
However, instead of vaporizing, the chemical froze and failed to have the desired effect.
The first killing agent employed by the German military was chlorine.
Chlorine is a powerful irritant that can inflict damage to the eyes, nose, throat and lungs.
At high concentrations and prolonged exposure it can cause death by asphyxiation.
German chemical companies BASF, Hoechst and Bayer (which formed the IG Farben conglomerate in 1925) had been producing chlorine as a by-product of their dye manufacturing.
In cooperation with Fritz Haber of the Kaiser Wilhelm Institute for Chemistry in Berlin, they began developing methods of discharging chlorine gas against enemy trenches.
According to the fieldpost letter of Major Karl von Zingler, the first chlorine gas attack by German forces took place before 2 January 1915: "In other war theaters it does not go better and it has been said that our Chlorine is very effective.
140 English officers have been killed.
This is a horrible weapon ...".
By 22 April 1915, the German Army had 168 tons of chlorine deployed in 5,730 cylinders opposite Langemark-Poelkapelle, north of Ypres.
At 17:30, in a slight easterly breeze, the gas was released, forming a gray-green cloud that drifted across positions held by French Colonial troops from Martinique who broke ranks, abandoning their trenches and creating an 8,000-yard (7 km) gap in the Allied line.
However, the German infantry were also wary of the gas and, lacking reinforcements, failed to exploit the break before the 1st Canadian Division and assorted French troops reformed the line in scattered, hastily prepared positions 1,000 to 3,000 yards apart.
The Entente governments quickly claimed the attack was a flagrant violation of international law, but Germany argued that the Hague treaty had only banned chemical shells, rather than the use of gas projectors.
In what became the Second Battle of Ypres, the Germans used gas on three more occasions; on 24 April against the 1st Canadian Division, on 2 May near Mouse Trap Farm and on 5 May against the British at Hill 60.
The British Official History stated that at Hill 60,On August 6, German troops used chlorine gas against Russian troops defending the Fortress of Osowiec.
Surviving defenders drove back the attack and successfully retained the fortress.
Germany used chemical weapons on the eastern front in an attack at Rawka, south of Warsaw.
The Russian army took 9,000 casualties, with more than 1,000 fatalities.
In response, the artillery branch of the Russian army organized a commission to study the delivery of poison gas in shells.