Fig. 3.
On carefully observing the flame of the candle we shall see that it really consists of three distinct parts. Round the wick it looks black, this is really a hollow chamber filled with unconsumed hydrogen. Next to this is a bright luminous cone, and outside of that is an almost invisible covering of blue flame. In the black space gas is unconsumed, in the luminous part the combustion is only partial, but outside of all, where there is most oxygen, the combustion is complete, and the flame can hardly be discovered. Now when the flame is quite steady the tube must be gently inserted at an angle into the black cone; after a few minutes, on applying a light at the end of the tube, although the candle is still burning, we shall see that this free hydrogen will burn there too with a small bluish flame.
The Candle’s Carbon.—As the candle burns, another part of its constituents is passing off into the air as soot or carbon, and this can be shown by holding a sheet of white paper or cardboard in the top of the flame, or better still, a cold saucer, on which there will be a copious deposit of black soot. This is another proof that as a candle burns it is not destroying matter, but only changing its form; from the white fat of the candle, black sooty carbon is liberated by the process of incomplete combustion that is going on. (See Fig. 5.)
Fig. 5.
Carbonic Acid Gas.—When substances containing carbon are burnt, one of the products is an invisible gas, commonly called carbonic acid gas. After an explosion in a mine, all the workings are filled with a deadly gas, which often kills more men than the explosion. This is called choke damp, and is the same as carbonic acid gas. Whenever a fire burns—gas, lamp, coal fire, or candle, this gas is one of the products. Let us fasten a piece of wire round our candle, and, after lighting it, lower it down into a glass bottle with a wide mouth. At first the candle burns dimly, and then, when a current of air is established, brightens. Now cover the mouth of the jar with a piece of card or the hand, and we shall see that the candle again burns dimly and quickly goes out. The jar now contains a considerable quantity of this carbonic acid gas. We may prove its presence by pouring into the jar a little clear lime-water and shaking it up. The carbonic acid gas will turn the lime-water milky. (See Fig. 6.)
Fig. 6.
Lime-water can be purchased at any chemist’s very cheaply, or it can be made by pouring water on a piece of quicklime, well shaking it, and then allowing it to settle. The clear lime-water may then be poured off. The lime may be used again and again until it is all dissolved.