(4) Fill two bottles with air from the lungs, as in (3) having previously introduced a cutting from a plant into one of the bottles. Allow them to stand in the sun for a day or two. Then test both bottles with a burning match. If properly done, the result will be very striking. The end of the cutting should be in the water of the dish. This experiment will not succeed excepting with bottles such as are used for chemicals, which have their mouths carefully ground. Common bottles allow the air to enter between the bottle and the glass.[1] [Footnote 1: See note on page 13.] [Illustration: FIG. 3.] 4. _Fuel_.--Light a match and allow it to burn until half charred. Blow it out gently, so as to leave a glowing spark. When this spark goes out it will leave behind a light, gray ash. We have to consider the flame, the charred substance, and the ash. Flame is burning gas. In all ordinary fuels, carbon and hydrogen, in various combinations and free, make the principal part. The first effect of the heat is to set free the volatile compounds of carbon and hydrogen. The hydrogen then begins to unite with the oxygen of the air, forming water, setting free the carbon, which also unites with oxygen, forming carbonic acid gas. The burning gases cause the flame. The following experiment will illustrate this. [Illustration: Fig. 4.] (5) Fit a test-tube with a tight cork, through which a bit of glass tubing, drawn out into a jet, is passed, the tubing within being even with the cork. Place some bits of shaving in the tube, cork it, and make the cork perfectly air-tight by coating it with bees wax or paraffine. Heat the test-tube gently over an alcohol lamp. The wood turns black, and vapor issues from the jet, which may be lighted (Fig. 4). Care should be taken to expel all the air before lighting. (6) That the burning hydrogen forms water by uniting with the oxygen of the air, may be shown by holding a cold glass tumbler over the jet, or over any flame. The glass will be dimmed by drops of moisture. The charred part of the wood is charcoal, which is one form of carbon. Our ordinary charcoal is made by driving off all the gases from wood, by burning it under cover where only a little air can reach it. The volatile gases burn more readily than the carbon, and are the first substances to be driven off, so that the carbon is left behind nearly pure. In the same way we have driven off all the gases from the half-burned match and left the carbon. The teacher should have a piece of charcoal to show the pupils. It still retains all the markings of the wood. If the combustion is continued, the carbon also unites with the oxygen of the air, till it is all converted into carbonic acid gas. This was the case with the match where we left the glowing spark. The gray ash that was left behind is the mineral matter contained in the wood.