In the early days it was very difficult to get officers to realize the absolute necessity of night drill in the adjustment of the mask. For various reasons, including surprise, gas attacks were probably eighty to ninety per cent of the time carried out at night. Under such conditions confusion in the adjustment of the mask is inevitable without a great deal of practice before hand, especially for duty in trenches with narrow spaces and sharp projecting corners. There are numerous instances of men waking up and getting excited, who not only gassed themselves, but in their mad efforts to find their masks, or to escape from the gas, knocked others down, disarranging their masks and causing the gassing of from one to three or four additional men. The confusion inherent in any gas attack was heightened in the latter stages of the war by heavy shrapnel and high explosive bombardments that accompanied nearly all projector and cloud gas attacks for that very purpose. The bombardment was continued for three or four hours to cause exhaustion and removal of the mask and to prevent the removal of the gassed patients from the gassed area. DETECTION OF GASES Efforts were made by the enemy and by all the Allies throughout the war to invent a mechanical detector that would show when gas was present in dangerous quantities. While scores, perhaps hundreds, of these were invented none proved simple, quick, or certain enough in action to make their adoption desirable. In every case it was necessary to rely on the sense of smell. Thus it was that as the war wore on, more and more attention was given to training officers and non-commissioned officers to detect various kinds of gases in dangerous quantities by the sense of smell. In the American Gas Defense School for officers this was done wholly by using captured German gases. This was because certain gases have quite different smells, depending upon the impurities in the gas and also upon the solvents sometimes mixed with them. Thus the German mustard gas has a mustard smell, while the Allies mustard gas, due to a slight difference in the method of manufacture, has a very perfect garlic odor. Not only must officers and men who handle gas training know the smell of the various gases, but they must know when the concentration of each is high enough to be dangerous. This is not easy to learn because the strength of the various gases in dangerous concentrations varies through wide limits. Not only does the strength of the gases vary and the sharpness of the odors accordingly, but the mingling of poisonous gases with other gases from high explosive and shrapnel tends to obscure these odors and make them more difficult of detection. DECEPTIVE GASES