Our Use of Oxygen.—We are breathing out carbonic acid gas; and on breathing through a piece of glass tubing into some of the clear lime-water we shall see that it will be turned milky in just the same way as when the candle burned. We are using up oxygen to support life, the candle uses up oxygen to support life, and in both cases the product is carbonic acid gas, as we have proved by means of the lime-water test. (See Fig. 7.)
Fig. 7.
Convincing Proof.—All that we have done up to the present supports our statement that the matter of the candle is not destroyed. In fact we have accounted for all its parts excepting that of a little mineral ash which will be left after the candle has burned away. We may, however, show in a very convincing way that our contention is true. An ordinary gas chimney is obtained, and at about three inches from one end a piece of wire gauze is placed, and the open end filled up with quicklime, at the lower end a cork is fixed upon which a short piece of candle is placed. There must also be a hole in the cork for the admission of air; when all is ready, carefully counterpoise the scales. Then remove the cork and light the candle and quickly replace. After burning a short time it will be found that the chimney glass bears down the beam because of increased weight. The products of the burning candle have united with the oxygen of the air, and these products, consisting chiefly of carbonic acid gas and water, have been caught by the quicklime. Because of the added oxygen they are heavier than the original candle. (See Fig. 8.)
Fig. 8.
Capillary Attraction.—There is still one interesting thing to illustrate about the burning candle, and that is the way in which the particles of fat ascend the wick to reach the flame. This is accomplished by what is known as capillary attraction. A very good illustration of this is afforded by a piece of salt standing upon a plate, on which is poured some salt water coloured blue with indigo or ink. The liquid will rise up the pillar of salt, and eventually reach the top. It rises by the force of capillary attraction. Let the pillar of salt represent the wick of the candle, and the coloured water, the fat, and the illustration is complete.
Fig. 9.
Analysis of Candle Flame.—Our candle can still give us some useful and suggestive illustrations of flame and combustion. We have seen that unconsumed gaseous vapours can be obtained from the flame by means of a bent glass tube. In the candle flame (Fig. 9) we see that this is because of the way the flame is built. The part marked o is the gaseous chamber, i is the luminous part, and e is where combustion is complete. On taking a sheet of clean white paper and pressing it down on the candle flame for a moment or two we shall get the fact of this hollow chamber demonstrated by the smoke ring upon the paper, which will appear thus—