A second group are poisonous only in very high concentrations, but irritate the eyes when present in amounts so small that one part in five million may render a man blind with weeping in a few seconds. There is no evidence, so far as I know, that anyone was killed or even permanently blinded by these substances; but they had a great momentary effect. They can be kept out by respirators, or even goggles.
The third group of poisonous smokes, mostly arsenic compounds, were little developed during the war. They are, however, weapons of very great efficiency, and it is well known that they would have been used by the British at any rate on a very extensive scale in 1919.[A] In small amounts, these smokes merely make one sneeze. In somewhat larger amounts they cause pain of the most terrific character in the head and chest. The pain in the head is described as like that caused when fresh water gets into the nose when bathing, but infinitely more severe. These symptoms are accompanied by the most appalling mental distress and misery. Some soldiers poisoned by these substances had to be prevented from committing suicide; others temporarily went raving mad, and tried to burrow into the ground to escape from imaginary pursuers. And yet within forty-eight hours the large majority had recovered, and practically none became permanent invalids. These substances, when in the form of smoke, will penetrate any of the respirators used in the late war, though the British box-respirator would stop all but a little of them in the concentrations then used. In future they will probably be used in much larger concentrations and in finer particles than those formed by the German smoke-shells. It is extraordinarily difficult to produce a respirator which will completely stop very fine smoke, for the following reason. In a gas the molecules (or ultimate particles) are moving very rapidly, with speeds of several hundred yards per second, continually colliding and rebounding. A gas molecule, therefore, will probably hit the sides of a fairly narrow passage through which it is drawn. But a smoke particle is moving at a speed measured in inches per second, and is far less likely to hit the wall of the respirator, and be held by its absorbent surface. If we try to make the passages through which air is drawn very narrow, as by sucking in our air through cotton-wool (which will stop most smokes), we find that we have created an appalling resistance to breathing. There is an electrical method of removing smoke-particles completely, but it would probably more than double the weight of respirators, and does not appear to be either water-proof or fool-proof.
[A] The American “Lewisite,” of which so much was heard in 1918 and 1919, is a substance of this class.
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.
Of course, if we could utilize the forces which we now know to exist inside the atom, we should have such capacities for destruction that I do not know of any agency other than divine intervention which would save humanity from complete and peremptory annihilation. But the remoteness of the day when we shall use these forces may best be judged by an analogy. Some thousands of years ago someone first realized that the sun, moon and stars were not mere bodies as large as a plate or a house, but very large, and moving very fast. It was an obvious idea that their motions might be exploited in some way. Wise men observed them and hoped, for example, to increase the probability of success in their own enterprises by beginning them when Jupiter was in the ascendant. These attempts were unsuccessful, though far more valuable to humanity than most of the methods successfully employed for the same purposes, such as fraud, violence and corruption. They led to astronomy, and so to all modern physics. We now know that the only probable way of harnessing the kinetic energy of the heavenly bodies is to employ tidal power to create electric currents. But five thousand years ago “hitching one’s wagon to a star” was a reasonable project and not a poetic metaphor. The reason we cannot do it is a simple matter of scale. And the reason why we cannot utilize subatomic phenomena is just the same. We cannot make apparatus small enough to disintegrate or fuse atomic nuclei, any more than we can make it large enough to reach to the moon. We can only bombard them with particles of which perhaps one in a million hit, which is like firing keys at a safe-door from a machine-gun a mile away in an attempt to open it. We do occasionally open it, but the process is very uneconomical. It may be asked why we cannot bring our machine-gun nearer, or improve our aim. To do this we should require to construct apparatus on the same infinitesimal scale as the structure of the chemical atom. Now we can arrange atoms into various patterns. For example, we can arrange carbon, hydrogen and oxygen atoms in patterns which constitute the molecules of sugar, glycerine, or alcohol at will. This is called chemical synthesis. We have been doing it by rule-of-thumb methods for thousands of years, and are just beginning to learn a little about it. But even chemical molecules are much too large for our purposes. We can no more ask a chemist to build our apparatus than expect a theatrical scene-painter or a landscape-gardener to do us a miniature. We know very little about the structure of the atom and almost nothing about how to modify it. And the prospect of constructing such an apparatus seems to me to be so remote that, when some successor of mine is lecturing to a party spending a holiday on the moon, it will still be an unsolved (though not, I think, an ultimately insoluble) problem.
To see how chemical weapons are likely to be used in future we must study their employment in the late war. Lachrymatory gas was only once used under ideal conditions—by the Germans in the Argonne in 1915. They captured a fairly extensive French trench system and about 2,400 prisoners, almost all unwounded, but temporarily blind. When they gave the number of prisoners, the French authorities not unnaturally protested that this number was practically equal to the total of their casualties. And this was quite true. The French were unprotected. They were deluged with shells giving off a vapour which temporarily blinded them. They could not even run away. The Germans walked across, removed their rifles, and formed them up in columns which marched back, each led by a German in goggles. In order to make future wars humane it would only be necessary to introduce the two following rules:—
1. No goggles or other eye protection shall be worn;