What do you mean by 'tested'

Do you mean they used gas as a weapon, or that they tested gas on prisoners as an experiment?

If the former, then yes. The French used tear gas as early as August 1914 and the Germans used in in October 1914. This was not successful, the gas being barely noticeable.

The Germans were the first to use de facto poison gas, with the first effective use being at Ypres in April 2015, where around 170 tons of chlorine was deployed against French colonial troops. These troops broke and ran, but the hole in the line was plugged by Canadian and French troops.

The Germans again used chlorine on the Eastern Front in August 1915, but again, the initial advantage gained was quickly recovered by surviving defenders.

The British used chlorine at Loos in September 1915, though adverse wind conditions made this less than successful.

The main problem with chlorine was that it was extremely irritating, had a visible colour, and thus could be detected and countered long before the dose became fatal.

Phosgene was the next gas to be developed and was considerably more insidious. It was colourless and less immediately irritating which meant that a fatal dose could be inhaled before countermeasures were taken. Phosgene kills slowly however - a gassed soldier could take 24 hours to die, meaning he could keep fighting and resisting until reinforcements were brought up.

Mustard gas was the next weapon to be developed and was truly horrific. Although not effective at killing, its action could cause permanent blindness and exposed skin would form huge blisters. It would destroy mucus membranes, causing vomiting, internal and external bleeding, all of which was severely painful. Fatal cases could take weeks to die.

Mustard gas could remain present in soil for days or even weeks, which meant it was unsuitable for supporting an attack.

Overall, the impact of gas on the war was slight. While it aroused great fear and did cause barbaric suffering to its victims, it could cause as great a problem for the side using it as it did for the opposing force.

It also failed to cause mass casualties - The British and Empire forces sustained only 6,000 fatalities from gas during the course of the entire war. Not a great number for a force that would number almost two million men by 1918.