The fourth group, of blistering gases, contains only one substance used during the war, dichlorethyl sulphide, or “mustard gas.” This is really a liquid, whose vapour is not only poisonous when breathed, but blisters any part of the skin with which it comes into contact even. To take an example, a drop of the liquid was put on a piece of paper and left for five minutes on a man’s sleeve. The vapour penetrated his coat and woollen shirt, causing a blister the effects of which lasted six weeks. And yet evaporation is so slow that ground contaminated by the liquid may remain dangerous for a week. Mustard gas caused more casualties to the British than all other chemical weapons put together. Such are the weapons which chemistry has given us. It is often asked why chemists cannot produce something which will put our foes comfortably to sleep and allow us to take them prisoners. The answer is that such substances exist, but that in small amounts they are harmless, in large amounts fatal. It is only over a moderate range of concentrations that their effect is merely stupefying. One has only to think of the familiar case of chloroform vapour, and the skill required to give neither too much nor too little. It would be logical to speak of explosives under the heading of chemical warfare, but there is curiously little chance of explosives becoming any more effective. We know fairly well the maximum amount of energy which can possibly be got out of a chemical action, and, though explosives might perhaps be made which were about twice as destructive as our best (or worst) to-day, they would probably be far less stable, and therefore less safe to their users.