As a matter of fact prussic acid has not been used by the Germans simply because it is not poisonous enough. It is not so poisonous, for example, as phosgene, and a lot of captured German documents showing the relative toxicity of different vapours always put it on a rather low basis. It was this and not a desire to avoid utter barbarity which decided the Germans not to use it. The ordinary German soldiers, just like ourselves, still consider prussic acid as the most dangerous possible material, and whenever they have a story to tell of a new gas being invented or being got ready to use against us they will tell you in awestruck tones that it is prussic acid. The most valuable piece of information which we got was a complete set of notes of some very secret lectures given to specially selected senior officers at a conference in Germany. We gathered that this conference was held behind closed doors and triple lines of sentries, and all that kind of thing, and I cannot of course indicate how the information came into our hands, but there it was. It described a lot of new gases which had been considered, and stated among other things that they intended to make a big gas attack against either the French or ourselves in Flanders in December, 1915, some time before Christmas when the wind was favourable. For this purpose they were going to use a mixture of chlorine with another gas, phosgene--the amount of phosgene to be twenty per cent of the whole. Now phosgene had been realised by our own chemists as a very likely gas to be used. I cannot say that it is more poisonous than chlorine, but it is infinitely more deadly because it is much more difficult to protect against and is more insidious in its nature. For one thing, though it is an asphyxiant like chlorine it is possible for a man to be only slightly gassed and think he is all right, and then, especially if he takes any exercise in between, to fall dead several hours later from heart failure. The information was so complete that our arrangements to provide a helmet which would protect against phosgene were hastened as much as possible; and it was as well that they were, for the attack actually did come off just about the time and place mentioned, in the Ypres salient.