As a chemist will tell you the air which we breathe is a mixture of two gases--one called nitrogen and the other oxygen; just as syrup, for instance, is a mixture of sugar and water.

Then too, as in syrup, there are different amounts of the two substances in the mixture: as syrup is made up of about one-quarter sugar and three-quarters water, so air is made up of one-fifth oxygen and four-fifths nitrogen.

Now the interesting thing about this mixture, which we call air, is that the only really "live" and vital part of it for breathing purposes is the one-fifth of oxygen, the four-fifths of nitrogen being of no use to our lungs.

In fact, if you split up the air with an electric current, or by some other means, and thus divide it into a small portion of pure oxygen (one-fifth), and a very much larger portion (four-fifths) of nitrogen, the latter would as promptly suffocate the animal that tried to breathe it as if he were plunged under water.[18] It may perhaps be difficult to think of anything burning inside of your bodies where everything is moist, especially as you do not see any flame; but you do find there one thing which always goes with burning, and that is warmth, or heat.

This slow but steady and never-ceasing burning, or oxidation, of the waste and dirt inside your bodies is what keeps them warm.