Exp. with a Candle. Take a tallow candle, and by means of a lighted match raise its temperature sufficiently high to start an action between the carbon in the candle and the oxygen of the air; in other words, light the candle. A match is composed of wood, sulphur, and phosphorus. The latter is a substance which unites with oxygen very easily; that is, at a low temperature. By friction against any hard object, sufficient heat is aroused to effect a union between the phosphorus of a match and the oxygen of the surrounding air; the flame is then conveyed to the sulphur, or the heat thus generated causes a union between it (the sulphur) and the oxygen, sulphur burning somewhat less freely than phosphorus; this gives enough heat to ignite the wood, and with its combustion we get sufficient heat to light the candle, or to start a chemical union between the combustible portion, carbon chiefly, of the candle and the oxygen of the air. Allow the candle to burn for a time, then put over it a tall lamp-chimney; notice that the flame grows long and dim. Next place on the top of the chimney a tin cover, leaving a small opening, and make an opening into the chimney from below, with a pin or the blade of a knife placed between it and the table; note that the candle burns dimly. Then exclude the flow of air by completely covering the top; in a moment, as soon as the oxygen inside the chimney is consumed, the candle will go out.
This shows (1) that air—in other words, oxygen—is necessary to cause the candle to burn; (2) that by regulating the draft or flow of air the intensity of the combustion may be increased or diminished; (3) that by completely excluding air the candle is extinguished. This experiment with the candle illustrates the way in which coal is consumed in a stove. By opening the drafts and allowing the inflow of plenty of oxygen, combustion is increased; by partially closing them it is diminished, and by the complete exclusion of air burning is stopped.
The products of the burning of coal are carbonic acid and a small amount of ash. Twelve weights of coal, not counting the ash, will unite with thirty-two weights of oxygen, giving as a result forty-four weights of carbonic acid. Accompanying the union there is an evolution of light and heat. The enormous amount of carbonic acid given out daily from fires is taken up by plants and used by them for food. In the course of ages these plants may become coal, be consumed in combustion, and, passing into the air, thus complete the cycle of change.
Fuel and Kindlings. The common fuels are coal, coke, wood, gas, coal-oil, and peat. For kindling, newspaper is good because, being made of straw and wood-pulp, it burns easily, and also because printers' ink contains turpentine, which is highly inflammable.
COMPOSITION OF THE BODY
Before entering upon the study of foods it is well to consider the composition of the human body, that some idea of its chemical nature may be gained. In the United States National Museum at Washington may be found some interesting information on this subject. From there much that is contained in the following pages is taken.
A complete analysis of the human body has never been made, but different organs have been examined, and chemists have weighed and analyzed portions of them, and from such data of this nature as could be obtained, estimates of the probable composition of the body have been calculated. Thirteen elements united into their compounds, of which there are more than one hundred, form it.
The following table gives the average composition of a man weighing 148 pounds.
| Oxygen | 92.4 | Sulphur | .24 |
| Carbon | 31.3 | Chlorin | .12 |
| Hydrogen | 14.6 | Sodium | .12 |
| Nitrogen | 4.6 | Magnesium | .04 |
| Calcium | 2.8 | Iron | .02 |
| Phosphorus | 1.4 | Fluorin | .02 |
| Potassium | .34 |
Prof. Atwater.