Air-tight tanks with adequate arrangements for filtering the incoming air are probably more hopeful, as mustard gas will not poison motors as it does men. (The motors would, of course, have their own air-supply, as it would hardly be practicable to filter air in the quantities needed by them.) To support the tanks and to tackle specially protected machine-gunners use will probably be made of immune infantry. One attack of gas-poisoning, whether by the lungs or skin, produces no immunity to a second attack――in fact, it generally increases the sensitivity of the victim. If a vapour is discovered against which immunity can be conferred, it will be the most effective weapon in history as long as its secret is kept. On the other hand, some people are naturally immune. The American Army authorities made a systematic examination of the susceptibility of large numbers of recruits. They found that there was a very resistant class, comprising 20% of the white men tried, but no less than 80% of the negroes. This as intelligible, as the symptoms of mustard gas, blistering, and sun-burn are very similar, and negroes are pretty well immune to sun-burn. It looks therefore as if, after a slight preliminary test, it should be possible to obtain coloured troops who would all be resistant to mustard gas blistering in concentrations harmful toward most white men. Enough resistant whites are available to officer them. One sees, then, the possibility of warfare on somewhat the following lines:―― Heavy concentrations of artillery would keep an area say thirty miles in length and ten in depth continuously sprayed with mustard gas. After allowing, say, two days for the development of blisters, the gassing of the positions within two or three miles of the front line is discontinued, but a long-range bombardment, especially of roads, goes on. Suddenly, behind the usual barrage of high explosive shells appears a line of tanks supported by negroes in gas-masks. They meet with but little opposition in the area still reeking of gas, and occupy the hostile lines to a depth of two or three miles. A counter-attack, even if successful, involves concentration in an area under gas-bombardment and enormous casualties from blistering. The only satisfactory counter-attack would be from the air. In this way the side possessing a big superiority of mustard gas should be in a position to advance two or three miles a day.