At 5 o'clock the following morning the 3rd Guards Brigade passed through the Battalion, which was withdrawn to watch the exposed right flank. The visibility was good, and a few sentries were all that were required, while the remainder of the Battalion obtained some rest. On the 5th the Battalion relieved the Welsh Guards in the front line, and Lord Lascelles decided to move the Battalion Headquarters farther forward, and to hand over what had been the Welsh Guards Headquarters to the Medical Officer for an aid-post. Nos. 3 and 4 Companies were placed in the front line, with Nos. 1 and 2 in support. The right of the Battalion was not in touch with any troops, there being a gap of some 500 yards, and this was accounted for by the fact that the ground was covered with wire of the old Hindenburg line and of the old British line facing it. This wire was almost impenetrable laterally, and was at right angles to the line held by the Battalion. The enemy was in considerable strength in front, and held some 400 yards west of the Canal du Nord as an outpost line in the old maze of trenches, with a strong defensive position behind the Canal. The ground sloped down to the Canal, and the farther the Battalion advanced, the more they were overlooked from the opposite slope; but the necessity of gaining touch with the 1st Battalion King's Royal Rifles made an advance necessary. The line of resistance, about 600 yards behind the front line, which the Battalion received instructions to dig, was nearly finished, when the enemy put a concentrated gas bombardment on the valley, where the Company Headquarters of the two companies in support were placed. For an hour the Germans bombarded the valley with sneezing-gas shells, and all the officers and men kept on their masks, but when the gas bombardment appeared to cease and was succeeded by one of H.E. shells, every one incautiously took off his mask. This new bombardment proved to be one of mustard gas. By the time this was realised every one was being sick, and all the officers and N.C.O.'s were casualties. Lord Lascelles came up from Battalion Headquarters to see what had happened, and met Captain Dury being led away blind. There were 61 men in No. 1 Company and 30 men in No. 2 who had been gassed, in addition to the following officers: Second Lieutenant S. Calvocoressi, Captain G. Dury, Second Lieutenant W. B. L. Manley, Lieutenant H. P. Gordon, and Second Lieutenant R. K. Henderson.