Chemical composition of the atmosphere

We have, up to the present, only considered the physical properties of the atmosphere, and, in fact, we are hardly concerned with its chemical or other properties. Air, however, is a mixture of gases, chiefly nitrogen and oxygen, in the proportion of approximately four-fifths nitrogen to one-fifth oxygen. Of the two main gases, nitrogen is an inert gas, but oxygen is necessary for human life and also for the proper combustion of the fuel used in the engine, there­fore when at great heights the air becomes thin it is necessary to provide more oxygen. In the case of the pilot, this was formerly done by supplying him with pure oxygen from a cylinder, but in modern high-flying aircraft, the whole cabin is pressurised so that pilot, crew and passengers can still breathe air of similar density, pressure, temperature and composition as that to which they are accustomed at ground level, or at some reasonable height. As for the engines, it has always been preferable to provide extra air rather than oxygen, because although the oxygen is needed for combustion, the nitrogen provides, as it were, the larger part of the working substance which actually drives the engine. In piston engines the extra air is provided by a process known as super­charging, which means blowing in extra air by means of a fan or fans; in jet aircraft the principle is fundamentally the same, though simpler because the engine is in itself a supercharger and this, combined with the ‘ram effect’ at the higher true speeds achieved at altitude, keeps up the supply of air as necessary.