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remain in its neighbourhood. By that time catapults were available which could throw a 2-lb. projectile 200 yards, a distance which brought the enemy's trenches within range in many parts of the line. A tinned iron cylindrical grenade was therefore designed to hold 2 Ib. of liquid, which by means of a 5-second time-fuze and detonator was made to burst and dis- tribute the lachrymator in a fine spray. The British War Office and G.H.Q. approved of this grenade, and the manufacture was put in hand. At the same time many other substances were considered, mostly lachrymators and sternutators. Some of these compounds appeared to be very effective under experimen- tal conditions, but were not so in the field.

While these modest tentatives were proceeding, always within the limits laid down by the Convention and the Declara- tion, the first German gas attack took place on the Ypres front on April 22 1915. This immediately altered the whole situation, as it was obvious that in using chlorine an asphyxiant the Germans had transgressed, if not the letter of the Declaration and the Convention, certainly their spirit. Accounts of the sufferings of those who had been exposed without protection to this new form of attack roused great indignation, but its effect- iveness could not be ignored, and after a few days it was decided by the British authorities that preparations at least must be made to reply to the German gas offensive in the same manner. The section of the War Office that had been dealing with lachry- mating grenades was instructed to take up the question, and with the aid of two or three chemists of the highest standing a small council was formed which sat continuously to discuss ways and means and consulted all the most prominent chemists and manufacturers. It soon became evident that the Germans had employed chlorine gas discharged under pressure from cylinders placed in the front line of trenches. A rapid review of all possible means of reply showed that chlorine was the easiest gas to begin with, but the position of Great Britain in this matter was very different from that of Germany. For the ordinary processes of the dye industry, the Gerrhans produced in peace-time very large quantities of liquid chlorine. In England only one or two firms produced it, and that in very small quantities. Moreover, the available containers for transport of the chlorine were not only very few but were much too bulky and heavy for use in the field. The problem therefore was twofold: first, to install appara- tus for an enormously increased supply of liquid chlorine, and secondly to design and manufacture suitable cylinders or dis- chargers for its use in the field. In both cases many initial difficulties were encountered, which were overcome in due course, and on Sept. 25 1915, at Loos, the first British gas attack with chlorine took place.

Meanwhile a very large range of possible gases had been passed under review with the object of discovering substitutes for chlorine. Obviously the thing to aim at was something which was more directly lethal than chlorine, and at the same time would cause less suffering by its effects. It was also realized from the first that the discharge of gas from the front trenches necessi- tated waiting for suitable weather conditions, which was very inconvenient for the arrangement of tactical operations; and it appeared to be necessary to release the gas in the enemy's lines, so as to be independent of wind, which could best be done from gun shells or trench-mortar bombs. The principle was approved and a new class of problems had now to be faced.

Effects of Gases. At this point it will be convenient to consider the effects produced by different varieties of gases, and the methods of employing them.

When considered from the point of view of their physiological effect, war gases may be classed in two main divisions, (a) Lethal, and (b) Irritant. The lethals fall under the two heads: those whose action is instantaneous or practically so (specific), and those whose action is more or less delayed, arid is generally of an asphyxiating character.

A gas is classed as immediately fatal when death follows expo- sure for a period of two minutes to a certain concentration (i.e. a certain proportion in the air breathed). A higher concen- tration may cause instantaneous death. The only known com-

pounds which, in concentrations practically obtainable, produce immediate death, are those containing cyanogen. The chief disadvantage of these is that when the concentration is not sufficient to cause death they have no effect at all, or only tempo- rary faintness, headache or heart trouble. The other lethal compounds may have immediate injurious effects, such as headache, nausea, etc., and in high concentrations may cause death in a short time. In concentrations which are not strong enough to kill, they may cause casualties, which have the dis- advantage that as the action is delayed a man may be able to continue fighting for some time after exposure. Thus in the case of phosgene, a man who does not notice that he has been gassed may die suddenly as much as 48 hours later.

The irritant gases are divided into (a) Lachrymatory (affecting the eyes), (b) Sternutatory (causing sneezing), (c) Vesicatory (blistering).

Lachrymators, on account of the extreme sensitiveness of the eye, can produce an effect in extraordinarily weak concentrations, such as i in 1,000,000 parts, or even less. Protection can be given by well-fitting goggles, but goggles cannot be used when there is a chance of exposure to lethal gases, because they would interfere with the gas-mask. The presence of lachrymators therefore entails the wearing of the complete mask, with all its disadvantages. The principal bases for lachrymators are iodine and bromine.

The sternutators were originally considered from the point of view of putting a man temporarily out of action by a violent fit of sneezing. A more important use suggested itself later, namely, that a man could be prevented by sneezing from adjusting his gas-mask, and would thus be exposed to the action of lethal shell. Similarly, if a sternutator could be found to penetrate the gas- mask it would be impossible for the wearer to keep it on. Early in the war there were many reports of the intended use by the Germans of red pepper and capsicine. Many experiments were tried by the British with capsicine and similar agents, but they did not give good results. In the summer of 1917 the Germans introduced shells filled with diphenyl-arsenious chloride (Blue Cross shell). They used a great many of these, especially on the French front, but not with any great success. In view of their effect on the wearing of the gas-mask, these irritants require further investigation.

Vesicants are only practicable when they act in vapour form. In that case they are by far the most effective of all the irritants, as they attack the skin and all the mucous surfaces. Those first considered were effective only when they reached the skin as liquids, a condition difficult to secure, and they were there- fore dismissed. But the German " mustard gas," also known as Yellow Cross and Yperite (Sym. dichlordiethylsulphide), act- ing as a vapour, was immediately successful. It first appeared in July 1917. It attacked the skin through the clothing, causing burns and irritation which might last from a fortnight to two months. Acting on the eyes it caused blindness, usually tempo- rary. Acting on the bronchial tubes it might cause bronchial pneumonia. It might affect the heart, and also the stomach, causing vomiting and diarrhoea. These effects were sufficiently serious, in view of the large numbers of casualties which were produced, although the proportion of fatal cases was small, being no more than about 3 %. In this connexion it should be noted that substances of the irritant class may also be lethal in high concentrations, i.e. in concentrations higher than those necessary to produce the characteristic temporary disablement.

A very important consideration in the use of gas is what is known as " persistence." Cylinder gas, travelling with the wind, is effective over a particular area only while it passes; but liquid gas splashed on the ground in minute particles from a shell, or left in the ground by the smothered burst of a shell, may be effective for some time, and gases are classed as of " high persist- ence " or " low persistence " according to the time during which their effect remains. This depends on the rate of evaporation. A strong lachrymator may be effective on the surface for perhaps twenty hours; other irritants, such as mustard gas, may remain effective for days in suitable meteorological conditions. Left