The action of ordinary oxygen gas is a separate matter. The atmosphere which we breathe consists of one part by volume of this gas and of four parts of nitrogen gas. It is the oxygen which is necessary to us and to all animals, and the nitrogen is merely an inert diluting accompaniment of the essential oxygen gas. It is, of course, easy to increase or to diminish the proportion of oxygen in the air breathed accordingly as one introduces additional pure oxygen or, on the other hand, diluting nitrogen into a collapsible bag or sac from which one continuously draws breath. Such a bag can be connected by a tube to a helmet or mask enveloping the head. The expired air is discharged by a specially provided passage. It used to be thought that it was dangerous to breathe pure undiluted oxygen, although the proportion of oxygen to nitrogen in the air taken into the lungs might be increased to as much as a half without injury, and, indeed, with great benefit in some serious conditions of collapse. Dr. Leonard Hill, F.R.S., of the London Hospital Medical College, has, however, recently shown that oxygen gas, of 97 per cent. purity, may be breathed continuously for as much as two hours without any ill-effects or sense of inconvenience. Contrary to what has been stated, it is neither exciting nor unpleasant. He has made the experiment on himself and on some of his assistants, and in doing so has made use of such an apparatus as that above-mentioned—so as to ensure the in-take of undiluted oxygen.