=What Is Meant by Gas.= It must be understood that by gases we refer to materials that injure by being carried to the victim in the air. The word “gas” has nothing whatever to do with the condition of the material when in the shell, or the bombs, or the cylinders before released. In every case, the gases are liquids or solids. When the containers are broken open the liquids are volatilized either by the gas pressure or by the force of the explosion of the bomb. =Groups of Gases.= Chemical Warfare gases are divided into three great groups. So far as their actual tactical use on the field of battle is concerned, there are only two groups—persistent and non-persistent. The third is the irritant group. This group affects the eyes and the lungs so as to make the victim very uncomfortable if not completely incapable of action in quantities so small as to cause no injury that lasts more than a few hours. The quantities of such gases needed to force the wearing of the mask is ¹/₁₀₀₀ that needed to cause the same discomfort by the really poisonous gases, such as phosgene. They, therefore, have a very great economic value in harassing the enemy by forcing him to wear masks and to take other precautions against gas. And no matter how perfect gas masks and gas-proof clothing become, their long-continued use will cut down physical vigor in an ever increasing ratio until in two or three days an army may be totally incapacitated. =Smoke.= In Chemical Warfare materials we have another great group which will probably be equal in the future to the three groups just mentioned. That is common smoke. Smoke has a variety of uses. By the simple term “smoke” is meant smokes that are not poisonous or irritating. Such smokes offer a perfect screen against enemy vision, whether it is the man who sights the machine gun, the observer in the lookout station, the cannoneer or even the aeroplane observer. Every shot through impenetrable smoke is a shot in the dark and has a tenth or even less chance of hitting its mark. Smoke affords a means of decreasing the accuracy of firing, much the same as night decreases it, without the inherent difficulties of night action.