The instructor explains that if they take four deep breaths it will prove fatal: "One breath and you catch the first spasm, two and you are mad, three and you are unconscious, four and you are dead. If you keep your presence of mind and hold your breath you will have six seconds to get on your gas helmet or respirator." The attack, remember, is a surprise in the dark; brain-splitting gas shells are dropping on all sides, and it is hard to keep cool and hold one's breath in the moment of sudden surprise and panic. We are told that there are fifteen mistakes which are easily possible in getting on this complicated helmet, or if there is one big blunder in the sudden surprise the man is done for. Before going through the death chamber, helmets are inspected, to see that they are sound and unpunctured, and the men are drilled in the open to practice putting them on quickly. Suddenly the warning whistle of an imaginary gas attack sounds. One backward fling of the head and the steel helmet falls off, for there is no time to lift it off. A dive into the bag carried on the chest and the respirator is grasped and with one skilful swoop it is drawn over the face. Your nose is pinched shut by a clamp, your teeth grip the rubber mouthpiece, and, like a diver, you must now get your one safe stream of pure air through the respirator. You draw in the air from a tube which rises from a tin of chemical on your chest. Then you can breathe in the dense, deadly, greenish chlorine vapor, for as it passes through the respirator filled with chemicals, it is absorbed, neutralized, oxidized, and purified into a stream of pure air. All about you may be choking fumes of death which would kill you in four seconds, yet you will be completely immune, breathing a purified atmosphere.