22

COMBUSTION AND DECAY.

the flame is bright, we are sure that it contains a gooddeal of carbon, for the flame of pure hydrogen is verypale indeed, and the brightness of a flame, such as thatof a candle, is entirely due to the carbon contained inthe tallow, which is burned at the same timo with thehydrogen, both combining with oxygen of the air.

60. As the products of the combustion of a candleare carbonic acid gas and water, it would be natural toexpect that a cold substance held over the flame of acandle would take from the vapour of water thusformed the heat necessary to its existence in the state ofvapour, and consequently condense it into the fluid form.This is an experiment which may very easily be made,for we have only to hold a cold glass at a little distanceabove the flame of the candle, and we shall soon find itlined with a fine dew of water condensed in thismanner.

61. Carbon is unable to combine with oxygen atcommon temperatures; it requires to be heated beforeit can enter into combination with that substance; butwhen a compound substance containing carbon is ex-posed to the air, it usually happens that if the otherelements which it contains combine with oxygen, thecarbon also is then able to combine with oxygen, andforms carbonic acid gas.

62. Thus when a plant dies and decays, its elementsseparate and form new compounds; but the carbon isnot set free in the form of black charcoal—it isslowly combined with oxygen.