Hitherto we have only considered the energy-supply of the body from one side ; we have regarded it as dependent on the constant supply of things which can be oxidized. But this is only half the question : if substances are to be oxidized there must be a provision of oxygen to oxidize them.

In order that a steam-engine may work and keep warm it is not merely necessary that it have plenty of coal, but it must also have a draught of air through its furnace. Chemistry teaches us that the burning in this case consists in the combination of a gas called oxygen, taken from the air, with other things in the coals: when this combination takes place a great deal of heat is given off. The same thing is true of our bodies ; in order that food matters may be burnt in them and enable us to work and keep warm, they must be supplied with oxygen; this they get from the air by breathing. We all know that if his supply of air be cut off a man will die in a few minutes. His food is no use to him unless he gets oxygen from the air to combine with it; while he usually has stored up in his body an excess of food matters which will keep him alive for some time if he gets a supply of oxygen, he has not stored up in him any reserve, or, if any, but a very small one, of oxygen, and so he dies very rapidly if his breathing be prevented. In ordinary language we do not call oxygen a food,but restrict that name to the solids and liquids which we swallow : but inasmuch as it is a material which we must take from the external universe into our bodies in order to keep us alive, oxygen is really a food as much as any of the other substances which we take into our bodies from outside, in order to keep them alive and at work. Suffocation, as death from deficient air supply is named, is really death from oxygen-starvation.

Why do we need oxygen ? What does a working steam-engine need in addition to coal ? What happens in the furnace of an engine? Why do we need to breathe ? What happens if a man's air supply be stopped ? Why does a man die sooner of want of air than of want of food ? Why is oxygen entitled to be called a food ?

What is suffocation?