Part 7

Air is made up principally of two elements, nitrogen and oxygen. It also always contains vapor of water and carbonic acid. Its average composition is as follows:

Nitrogen 78.49% Oxygen 20.63% Aqueous Vapor .84% Carbonic Acid .04%

These are mixed together, not chemically united. Oxygen and nitrogen do unite chemically, but not in the proportions in which they exist in the air. Nitrous Oxid (N_{2}O), sometimes called "Laughing Gas," is one of the compounds of nitrogen and oxygen.

FIRE

=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 Carbon 31.3 Hydrogen 14.6 Nitrogen 4.6 Calcium 2.8 Phosphorus 1.4 Potassium .34 Sulphur .24 Chlorin .12 Sodium .12 Magnesium .04 Iron .02 Fluorin .02 PROF. ATWATER.

It will be seen from this that oxygen, carbon, hydrogen, and nitrogen constitute nearly the whole, the other elements being in very small proportions.

PRINCIPAL CHEMICAL COMPOUNDS IN THE BODY

The following interesting table, obtained at the National Museum, gives the principal compounds of the body. Some of the more rare organic compounds are omitted.

WATER:--A compound of oxygen and hydrogen.

PROTEIN { ALBUMINOIDS { MYOSIN AND SYNTONIN OF MUSCLE COMPOUNDS, { or { (sometimes called "muscle { Proteids. { fibrin"). composed { { Albumen of blood and milk. Casein mainly of { { of milk. { { { Collagen of bone and } Carbon, { { tendons. } which { Gelatinoids. { Chondrigen of cartilage, } yield Oxygen, { { gristle, } gelatin. { Hydrogen, { { { Hemoglobin. { The red coloring matter of blood. Nitrogen. { {

FATS, { { } These make up the { { Stearin, } bulk of the fat of composed { Neutral { } the body. mainly of { Fats. { Palmitin, } They are likewise { { } the chief { { Olein, etc. } constituents of Carbon, { { } tallow, lard, etc. { Oxygen, { Complex { Protagon, } Found chiefly in { Fats, { } the brain, spinal Hydrogen, { containing { Lecithin, } cord, nerves, etc. { phosphorus { } { and nitrogen. { Cerebrin. }

CARBOHYDRATES, { Glycogen, "animal starch." Occurs in the composed { liver and other organs. of { Inosite, "muscle sugar." Occurs in various Carbon, { organs. Oxygen, { Lactose, "milk sugar." Occurs in milk. Hydrogen. { Cholesterin. Occurs in brain, nerves, and other { organs.

{ Phosphate of lime, or calcium } { phosphate. } Occurs chiefly { Carbonate of lime, or calcium } in bones and { carbonate. } teeth, though { Fluorid of calcium, or calcium } found in { fluorid. } other organs. { Phosphate of magnesia, or } { magnesium phosphate. } {

MINERAL { PHOSPHATE OF POTASH, OR POTASSIUM } SALTS. { phosphate. } { Sulphate of potash, or potassium } Distributed { sulphate. } through the { Chlorid of potassium, or } body in the { potassium chlorid. } blood, muscle, { Phosphate of soda, or sodium } brain, { phosphate. } and other { Sulphate of soda, or sodium } organs. { sulphate. } { Chlorid of sodium, or sodium } { chlorid. }

Now, since the body is composed of these substances, our food, including air and water, should contain them all in due proportion, that the growth, energy, and repair of the body may be healthfully maintained.

THE FIVE FOOD PRINCIPLES

For convenience of comparison foods may be divided into five classes: Water, Protein, Fats, Carbohydrates, Mineral Matters.

Some scientists include air in the list, but it has been thought best in this work to speak of it separately as the greatest necessity of life, but not in the sense of a direct nutrient.

An average composition of three of the principles is as follows:

{ Carbon 53 PROTEIN { Hydrogen 7 { Oxygen 24 { Nitrogen 16