On this as well as other occasions the Germans fired some diphosgene and Blue Cross (Sneezing gas), as well as mustard gas. This added to the difficulty of determining areas free from the latter. In the future such mixing of poisonous gases may always be expected and, in addition, gases which have no value other than that of masking the poisonous ones will be fired. While with practically all gases except mustard gas a man is comparatively safe while breathing a concentration very noticeable to the sense of smell, the only safe rule with mustard gas is to consider as dangerous any concentration that can be smelled. For the reason that this gas persists longer in calm areas, woods are always to be avoided, where practicable, and also, since all gases, being heavier than air, tend to roll into depressions and valleys, they should be avoided. There have been a number of authentic cases where batteries in hollows or valleys suffered severely from mustard gas, while troops on nearby knolls or ridges were comparatively free, though the difference in the amount of shelling of the two places was not noticeable. Of great importance with all gases is the posting of a sufficient number of sentries around men sleeping within the range of gas shell. The worst projector gas attack against the Americans was one where the projectors were landed among a group of dugouts containing men asleep without sentries. The result was a very heavy casualty list, coupled with a high death rate, the men being gassed in their sleep before they were awakened. DESTRUCTION OF MUSTARD GAS Prior to the introduction of mustard gas all that was necessary to get rid of gas was to thoroughly ventilate the spot. Thus in trenches and dugouts, fires were found to be very efficient, simply because they produced a circulation of air. In the early days, among the British, the Ayrton fan, a sort of canvas scoop, was used to throw the gas out of the trenches. While this was taken up in the American Service, it did not become very important, since it was found that, under ordinary atmospheric conditions, natural ventilation soon carried the gas out of the trench proper, while fires in dugouts were far more efficient than the fans. Likewise the Ayrton fan smacked too much of trench warfare which had reached a condition of “stalemate”—a condition that never appealed to the Americans and a condition that it is hoped never will.