$Carbonic Acid Gas.$--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.)
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.
$Our Use of Oxygen.$--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.)
$Convincing Proof.$--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.)