Fig. 14.
Flames that Laugh.—What makes the candle flame burn steadily is the next problem before us, and we shall see that it is very simple and at the same time most philosophical. It tells us the reason why candles are made round, and not square. The section of a candle being circular, with the wick in the centre, it can, as it burns, get its supply of oxygen from all directions at an equal distance; thus it burns regularly and steadily. If the candle were square, the four corners being at a greater distance from the wick than the sides, we should have four columns of fat standing up at the corners, and as the air rushed in to feed the flame it would come into contact with these, and so the current would be broken and the flame would become unsteady. We can show this by placing some cotton wool on tin dishes, and saturating it with methylated spirits and igniting it. This will give us what are known as laughing flames, because they burn so unsteadily. The air rushing in to feed the flame comes into contact with the wool, which impedes it, and so the flame has a dancing or laughing appearance. This experiment may be made very pretty by rendering the flames coloured. To do this add to the cotton wool, before pouring on the methylated spirit, chloride of copper; this will give a green flame; to another, chloride of strontium; this will colour the flame red; to another, common salt; this will give a yellow coloration. All these should be shown in a dark room.
Fig. 15.
The Importance of Oxygen.—By previous experiment we have seen that oxygen is necessary to a flame, and our ingenious readers may now make a piece of apparatus to prove this. (See Fig. 15.) It consists of two pieces of glass tube standing upright near the two ends of a board, in which there is a covered channel communicating with the two. A small candle is lighted and placed in one of the tubes. The air heated by the flame rises in the tube and causes a corresponding descent of cold air down the other tube. This gives us a good illustration of ventilation produced by artificial heat. So great is the down draught, that if we hold a lighted taper over the mouth of the cold tube the smoke and the flame will be carried down, with the result that the candle is soon extinguished. The reason for this is that the smoke and burnt air from the taper contain insufficient oxygen to feed the candle flame, and it dies. To make this apparatus, obtain a piece of deal board about ten inches long and four inches wide, cut along the middle a groove about three quarters of an inch deep, and about the same width, leaving about half an inch at each end uncut. Cover this groove with a tightly-fitting slip. Over the two ends of the groove are fastened two small blocks of cork pierced with apertures, into which fit the vertical glass tubes; these should be about ten inches high and about three-quarters of an inch bore. Fig. 16, which is a section of one end of the apparatus, shows how a small candle like those used on Christmas trees is held erect by a wooden socket at the end of the groove so as not to impede the current of fresh air.
Fig. 16.
Rates of Combustion.—We must remember that all things do not burn at the same rate. Iron rust is a product of very slow combustion. In using up food to maintain the heat of the body, combustion goes on more quickly than in rusting iron, the candle burns more quickly still, gas still faster, the Bunsen burner faster still. We may get an idea of the different rates of combustion by the two following experiments. On a tin dish place half a thimbleful of gunpowder and lay on it a tiny piece of gun-cotton. Ignite the gun-cotton; it burns so fast that it has no time to set fire to the gunpowder, which may now be ignited in its turn by the taper. Another example is the laying of two long trains of gunpowder, one fine grain and the other coarse. It will be found that the two flames travel at very different rates along the same path.