The gas used Using by the German troops at Hulluch was a mixture of chlorine and phosgene , which had first been used Using on 19 December 1915 at Wieltje , near Ypres .

The gas used Using at Wulverghem was a mixture of chlorine and phosgene , which had been used Using against British troops on 19 December 1915 in the phosgene attack Attack at Wieltje , north-east of Ypres .