Duds that were collected showed that the mustard-gas liquid was a chemical called dichlorethyl sulphide, a liquid that gives off its vapour only slowly. The shell themselves were similar to the previous gas shell except that the small one have a new type of fuse--a very simple and quick-acting fuse which bursts the shell before it can get into the ground, and consequently produces a very little crater. This of course helps to spread the gas round more than if a big hole were formed. The respirators keep out the Yellow Cross gas completely, and the blanket protection of dugouts will also keep out the gas splendidly. Of course if a dugout gets a direct hit with a mustard shell there is nothing for it but to leave it empty for some days, as the liquid cannot be removed by ventilation with either fans or fires. A case that will illustrate what I mean was one in which a three-inch mustard-gas shell got a direct hit on a doctor’s dugout and gassed him and his orderlies. Some time afterward the remaining orderlies thought they ought to send the doctor’s things down the line and went in and got them out of the dugout. They noticed a faint smell but did not worry about it, and soon afterward found themselves gassed in consequence. A fire was then placed in the dugout to clear it. In the meantime the medical sergeant secured another dugout by clearing out some infantrymen. In the evening the infantry felt soul-sick and wanted somewhere to sleep, so they went into the original gassed dugout and slept there. In the morning they all went down, gassed. Where there has been no direct hit and the mustard-gas vapour gets into the dugout, it can be cleared out just like ordinary gas, by ventilation either with fans or by means of fires. For clearing dugouts a great deal of reliance is placed nowadays on building small fires inside. A dugout with two entrances can be very quickly cleared by means of fires, as a through draft is produced, which carries the gas away with it; but difficulty is frequently found in getting the necessary fuel for the fire and in keeping the stuff handy. Bundles of firewood and kindling material are supposed to be kept in the dugouts ready for use; but, as has already been explained, the Tommies are always on the lookout for combustible materials for their own fires, and continual inspection has to be made to see that the special supplies for ventilation are kept available. One officer told me that he always had the supplies of wood, paper and kerosene kept in an army-biscuit tin which was closed and sealed; because, as he said, no Tommy would ever investigate the contents of a biscuit tin unless absolutely forced to do so for lack of other food.