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disturbance cast-iron shells were indicated, as they required practically no bursting charge; but cast-iron shells have the drawback that they hold much less liquid than steel shell, because the shell wall has to be very much thicker to resist the shock of discharge. It was eventually found that not only every gas but every nature and every calibre of shell required a different bursting charge, and sometimes a different explosive. These all had to be determined experimentally. Later it appeared that certain liquids required a more powerful burster in order that they might be distributed in a fine spray. When a solid was in- troduced, in the shape of diphenylchlorarsine, a still more power- ful burster became necessary in order that the solid might be atomized and dispersed as a cloud. Thus the German 77-mm. shell contained only 125 gr. of this solid enclosed in a glass container, the space between the container and the shell being filled with 600 gi. of explosive.

While the output of chemical shells remained very small the question of cloud formation by lethal shells was of high impor- tance. Not enough shells being available to charge the whole atmosphere over a certain area with a fatal concentration, it was necessary to rely on the effect of each individual shell cloud, which ought to pass over a man or group of men while still at full strength. With the very large quantities of shell that were available later this question was of less importance, as it became possible to produce and maintain very high concentrations over a given area or length of trench. This was facilitated by bringing the larger natures of shell into service, and also by the use of Stokes' bombs and trench-mortar shells, but still more by the Livens projector.

Since gas shells were intended to be used without considering the direction of the wind, the possible effect of a bombardment on one's own troops had to be considered, and a further range of experiments became necessary. The kind of precautions re- quired are indicated in an extract from German Army Orders of June 30 1918:

The following regulations for gas bombardment are made known.

Minimum distance of the objective from our first line:

(1) Wind normal or oblique towards the enemy: for all natures of gas shell the least distance must be 300 metres; below that dis- tance projectiles fired short may fall in our lines. When the wind is steady and the ground favourable, this distance may be reduced if only a small number of projectiles are being fired.

(2) Still weather, or wind normal or oblique towards our lines: (a) Heavy bombardment (several thousand projectiles), ground

favourable for the return of the gas towards our lines:

Blue Cross shells (Diphenyldichlorarsine sternutators). . . 1,000 metres (offensive).

Blue Cross shells. . . 500 metres (defensive).

Green Cross shells (Trichlormethylchloroformate lethal). . . 1,000 metres.

Yellow Cross shells (Sym. dichlordiethylsulphide vesicatory). . 1,000 to 2,000 metres according to the number.

(6) Light bombardments (some hundreds of projectiles), when our troops have been warned and the ground is favourable :

Gas shells of all natures: 300 to 500 metres. These distances are given for general guidance; they may be reduced or increased accord- ing to local conditions.

The influence of the state of the atmosphere and ground condi- tions on the use of gas is naturally of great importance. The first consideration is the wind. Lethal shells will produce the best effect with a wind of three miles an hour or less; with a wind of over seven miles they cannot be used effectively. Lachrymators can be used in higher winds up to twelve miles, but with diminish- ing effect. Heavy rain destroys gas effect. Dry we.ather and a bright sun tend to dissipate the gas quickly. The most favourable atmospheric conditions are little or no wind, moist atmosphere, and no sun. The night usually offers the best conditions for gas. As regards the effect of ground, it will be obvious that anything which protects the gas from the effect of wind assists concentra- tion. Hollow ground, valleys, woods, areas covered with under- growth, and villages make therefore good targets.

Field Organization. The earliest British experiments on a field scale were made with extemporized appliances on the nearest open ground to the source of production of gases under trial. Some experiments not involving danger were made on a ground that had been acquired for flame-projector and explosive trials

at Wembley, and at the Clapham School of Trench Warfare, but it was soon evident that a properly organized experimental ground was essential, and after much search a site was found at Porton near Salisbury. Here trenches and dugouts were made, artillery ranges prepared, and gradually a complete installation provided of laboratories, mechanical workshops, magazines, filling-rooms, gas-chambers, etc. It was now possible to experi- ment on a really scientific basis, while the ground available gave space for trial of many other trench-warfare requirements, among which smoke and incendiary shells and light signals were of great importance. Porton thus became the headquarters of the practical study of gas warfare. The laboratory experiments there were confined, however, to examination of the results of trials. Other laboratory work was done at the Imperial College, at Cambridge and other universities, and in private laboratories.

The first British experiments with gas-projectors showed the difficulties that were likely to arise with defective apparatus, or from changes of wind in the trenches; and it was realized at once that, for the handling of the new weapon, it was necessary to have some chemists in the front line who should be trained in the handling of the material, and who could also advise the troops on the effects of it. The suggestion of raising a Chemical Corps was put forward and approved; and as a result all the universities were invited to nominate students of chemistry, while at the front chemists were withdrawn from the ranks for the new corps. This was the beginning of the Special Brigade R.E. in which a certain number of selected students and officers already serving were given commissions, while others were appointed as non-commissioned officers to give them the necessary author- ity and position.

A certain number of officers and men were also appointed to it who were not chemists but had experience at the front. Thus the officer commanding the Special Brigade suggested working the men at cylinder emplacements in the trenches in pairs; one a chemist and the other an old hand from the infantry. The importance of having trained scientific men in the brigade was shown by the number of valuable suggestions that emanated from the officers, as well as by their extraordinary keenness and effectiveness in the field. After a short time selected officers from the brigade were appointed as chemical advisers at head- quarters of armies and corps, and a central laboratory at General Headquarters was started for examining enemy gas and anti-gas appliances and dealing with urgent problems.

Organization in England. In June 1913, upon the formation of the British Ministry of Munitions, the personnel hitherto engaged on chemical warfare was transferred to the ministry. The time had come for rapid expansion, and a Trench Warfare Department was created by the minister which was responsible for both design and supply, not only of chemical war material but grenades, trench mortars and projectiles, smoke shells, signals and the countless other requirements that modern trench warfare had made necessary. The staff, rudimentary hitherto, was increased in proportion to the requirements of experiment and manufacture, and a scientific advisory committee was formed of eminent specialists in chemistry, physics and physiology. This new department was unique in combining the functions of re- search, design and supply. The other departments of the ministry were concerned with supply only, in response to the demand of the War Office. It was decided after much discussion that this exception should be made for the Trench Warfare Department, because it was recognized that in dealing with so many entirely new products, the manufacture of most of which was attended by considerable danger, it was essential that the designers should be in the closest possible touch with the manufacturers, should be able to explain what was being aimed at, and should advise on difficulties as they arose. The resultant close contact enabled them also to modify their designs during manufacture when necessary; to take account of facilities for supply and manu- facture; and to order supplies in advance as soon as a new production was foreseen.

There is no doubt that this was the right procedure, as was proved by the rate production up to the end of 1915. The weak