8

COMBUSTION.

of the air then spoiled or used up ; and if it is, why doesnot fresh air enter the bottle and supply its place ? Thetruth is, only a small portion, perhaps about one-fifthof the bulk, of the air is able to feed the flame of acandle; the remainder, which cannot feed flame northe life of an animal, is of a different kind from the airwhich can; and we find that the common air whichwe breathe is a mixture of two kinds of air, orGAS, as it is called by chemists;—the one kind,which we might call good air, which supports the lifeof animals, and is essential to the burning of fires andcandles; and the other, or bad air, in which animalsand plants die, and which immediately puts out fireand lights.

20. Chemists call the good air oxygen, and the bad airnitrogen; but we must not suppose that because thenitrogen appears thus useless, it is really so ; for it is,in fact, of very great importance, as we shall hereaftersee. In the experiment just mentioned of burning thecandle in a large bottle, the oxygen is all combined withthe elements of the tallow, and the nitrogen left un-changed, which puts out a fresh-lighted candle plungedinto it.

21. When a candle burns, it gradually disappears ; itgrows shorter and shorter, and at last, when all thetallow is burnt, the candle goes out ; but we must nottherefore suppose that it is utterly destroyed. A changehas taken place; the tallow, or rather its elements,have combined with the oxygen of a portion of air, and