When the man exhales, as we all know, the air which he then discharges from his lungs is deficient in oxygen and instead contains carbonic acid gas. The latter must be got rid of and replaced by pure oxygen. The exhaled air is therefore led down a pipe to the regenerator, where it comes into contact with several trays of caustic soda, a chemical which has a great affinity for carbonic acid. The result is that the latter gas is extracted from the impure air, finding a more congenial home in the caustic soda. It is then necessary to restore the normal quantity of oxygen, and so, as the air passes on, it meets, in a little apparatus known as an injector, a spray of pure oxygen from the cylinders. Thus, after being purified and re-oxygenated, the air passes on through more pipes to the helmet or mouth-piece, to be breathed once more. The apparatus contains sufficient oxygen and caustic soda for this to go on for a space of two hours. But during times of extra exertion a man needs more air than at others, for which provision has to be made, and so on his chest the rescuer carries a flexible bag divided into two compartments. Through one of these the exhaled air passes on its way to the regenerator, while through the other the oxygenated air flows on its way to the man's mouth. When he is breathing hard, then, during a moment of extra exertion, and when, therefore, he is turning out bad air faster than it can be purified, and drawing in pure air faster than it can be produced, this bag comes to his aid. From the store of oxygenated air in one side of it he draws the extra which he requires, while the other side stores up temporarily the excess of vitiated air, until the regenerator is able to overtake its work. Thus at all times, whether breathing ordinarily or heavily, the apparatus can respond to his demands. The spray of oxygen as it escapes from the cylinders into the injector has the effect of driving the air along, so that the circulation through the tubes and the regenerator is automatic, and the foul air flows away from the man's mouth and the new air comes back to him quite without effort on his part. As time goes on, of course, and the stored oxygen becomes used up, the pressure in the cylinders falls, which fall, shown upon a little pressure-gauge, tells the man how much longer time he has before he must return for fresh supplies of oxygen and soda. Fresh cylinders of oxygen can be connected up very quickly in place of the empty ones, while a fresh regenerator can be put in, or new caustic soda supplied, in a very short time.