compounds; that is, chemical elements which are united by certain chemical laws to form substances called compounds. Compounds are quite different in their

Properties of Oxygen. -Oxygen, when carefully prepared, is found to be colorless, odorless, and tasteless. It makes up about one fifth of the air. Combined with other substances, it forms a very large part by weight of water, rocks, minerals, and the bodies of plants and animals. Oxygen has the very important property of uniting with many other sub

stances.

Preparation of Oxygen.

The chemical union of oxygen with any other substance is called oxidation. Oxidation of some sort may take place wherever oxygen is present. This fact has a far-reaching significance in the understanding of the most important problems of biology.

Oxidation in a Match. The simple process of striking a sulphur match gives us another illustration of this process of oxidation. The head of the match is formed of a combination of phosphorus, sulphur, and some other materials. Phosphorus is a chemical element distinguished by its extreme inflammability; that is, it unites with oxygen at a comparatively low temperature, producing a flame. Sulphur is another chemical element that combines somewhat easily with oxygen but at a much higher temperature. The rest of the match head is made up of red lead, niter, or some other substance that will release oxygen, and some glue or gum to bind the materials together. The heat caused by the friction of the match head against the striking surface is enough to cause the phosphorus to ignite; this in turn

ignites the sulphur, and finally the wood of the match, composed largely of the element carbon, is lighted and oxidized. If we could take out the different chemical elements of which the match is formed and oxidize them separately, we should find that the amount of heat needed to start the oxidation of the substances would vary greatly. The element phosphorus, for

oxygen.

example, is kept under water in a glass jar because of the extreme readiness with which it ignites in the presence of oxygen.1

Slow Oxidation. - Oxidation may take place slowly, as may be seen in the rusting of an iron nail. Rust is iron oxide, and is formed by the union of iron and oxygen. This kind of oxidation is said to be a slow oxidation. Slow oxidations are constantly taking place in nature and are a part of the process of decay and of breaking down of complex materials into simpler materials.

Heat given off as Result of Oxidation. One of the most important effects of oxidation lies in the fact that, when anything is oxidized, heat is produced. This heat may be of the greatest use. Coal, when oxidized, gives off heat; this heat boils the water in the tubes of a boiler; steam is generated, wheels of an engine turn, and work is performed. The energy released by the burning of coal may be transformed into any kind of work power. Energy is the ability to perform work. We shall later find that the oxidation of certain materials in the bodies of plants or animals releases The heat of the human body is energy. maintained by constant oxidation of food materials within the body.

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The Composition of Water. If an electric current is passed through water by means of the apparatus shown in the Figure, it

1 The teacher may later introduce experiments in chemistry to demonstrate the physical appearance of such other elements as are used by plants in food making. 2 A little sulphuric acid must be added to make the liquid a better conductor.

is found that the water separates into two gases, one of which occupies twice as much space as the other in the tubes. If we test the gas present in smaller quantity, we find it to be oxygen. The other gas, colorless, tasteless, and odorless like the oxygen, differs from it by igniting with a slight explosion if a burning match or splinter is introduced in it. As it burns, drops of water are formed, showing that it is passing back to its original condition, that is, it is uniting with oxygen to form water. This gas is hydrogen. Hydrogen has a great chemical affinity or liking for other elements, hence it is usually found in nature combined with other elements, as with oxygen to form water.

The Composition of the Soil. The covering of the earth was probably very different in former ages from what it now is. Its molten plastic mass after cooling formed rock. This rock, by the work of the wind, frost, heat, and water, and plants, has in part been broken into small bits. This is inorganic soil, such as sea sand and gravel. Such soil is formed usually of several elements found in rocks, such as calcium, sodium, magnesium, silicon, potassium, and iron combined with oxygen.

A visit to the woods or to a well-kept garden shows us that there is another kind of soil than the inorganic soil just mentioned. This is the rich, dark soil containing humus. Humus is made up in part of dead organic matter, the decayed remains of plants and animals. In such soil we should find relatively more water than in inorganic soil. If we could test the chemical elements to be found in humus, we should find nitrogen, hydrogen, oxygen, and also carbon, an important element found in all organic matter.

It

Carbon. Carbon is found in many conditions in nature. makes up a large part of the bodies of plants and animals, and of coal, and it exists in a nearly pure state in the diamond. The presence of carbon can usually be detected by partially burning the substance, carbon showing as a black substance without taste or odor. Carbon may be collected by allowing a candle flame to burn in contact with the underside of a sheet of glass. The black deposit is almost pure carbon.

Oxidation of Carbon and its Result. If we burn a candle in a closed jar containing air, the flame soon begins to flicker, and then goes out. If the cover of the jar is carefully removed, and a

burning match lowered into the jar, the match will at once go out, showing the presence of a gas heavier than air which will not support a flame. We might suspect the presence of nitrogen, but nitrogen would not respond to the test which follows. If we pour into the jar a few spoonfuls of limewater,' a colorless liquid, and shake it up with the gas in the jar, the limewater turns milky in color. This is a test for a compound known as carbon dioxide. This compound was evidently formed by the union of the carbon with the oxygen of the air in the jar.

All organic or living substances, when oxidized, form carbon dioxide. That oxidation of carbon takes place within our own bodies may easily

Hydrogen &
Carbon
in wood

Oxygen in the air

Carbon dioxide

& Water

in the Smoke

Diagram of combustion or rapid oxidation in a stove.

be proved by exhaling through a clean glass tube into some limewater. The heat of our body (98.5° F.) is the result of oxidation taking place within the body. The heat given off from oxidation of wood or coal in a stove is determined by the supply of oxygen we allow to pass to the burning material. If we open the draft, allowing more oxygen to get to the fire, we increase the heat by more rapid oxidation; if we shut off the oxygen supply, we decrease the amount of oxidation. Does this help to explain our deep breathing after doing hard physical exercise?

Problem II. Are mineral matter and water present in living things? (Laboratory Manual, Prob. II.)

(a) Mineral matter.

(b) Water.

Mineral Matter in Living Things. If a piece of wood is burned in a very hot fire, the carbon in it will all be consumed, and eventually nothing will be left except a grayish ash. This ash is well seen after a wood fire in the fireplace, or after a bonfire of dry leaves. It consists entirely of mineral matter which the plant has taken up from the soil dissolved in water, and which has been stored in the wood or leaves.

1 Limewater can be made by shaking up a piece of quicklime the size of your fist in about two quarts of water. Filter or strain the limewater into bottles, and it

is ready for use.

If we were able by careful analysis to reduce a plant and an animal to the chemical compounds of which they were formed, we should discover that both contained mineral material. We have just seen examples of this in plants. Mineral matter is found in bone, in the shells covering mollusks (clams, snails, etc.), and in other parts of the bodies of animals. Water in Living Things. Water forms an important part of the substance of plants and animals. This can easily be proved by weighing a number of green leaves, placing them in a hot oven for a few moments, and then reweighing. The same experiment made with a soft-bodied animal, as the oyster, would show even more water than in leaves. Some jellyfish are composed of over 90 per cent water. The human body contains

about 65 per cent water.

Gases Present.

Some gases are found in a free state in the bodies of plants or animals. Oxygen is of course present wherever oxidation is taking place, as is carbon dioxide. Other gases may be present in minute quantities.

Problem III. The foods that living organisms need. (Laboratory Manual, Prob. III.)

Composition of Living Matter. The living part of a plant or animal is made up of the elements carbon, hydrogen, oxygen, and nitrogen, with a very minute amount of several other elements, which collectively we may call mineral matter. The living part of a plant corresponds closely in chemical composition to the living part of an animal. The sugar found in grains or roots of plants has nearly the same chemical formula as the animal sugar found in the liver of man; the oils of a nut or fruit are of composition closely allied to the fat in the body of an animal. These building materials of a plant or animal may be placed in one of the three following groups of substances: carbohydrates, materials containing a certain proportion of carbon, hydrogen, and oxygen; fats and oils, which contain chiefly hydrogen and carbon with less oxygen; and nitrogenous or proteid substances, which contain nitrogen in addition to the above-mentioned elements. The above three kinds of organic materials also form a large part of the foods of all animals and plants.

Foods. What is a food? We know that if we eat a certain amount of proper foods at regular times, we shall be able to go on doing a certain amount of work, both manual and mental. We know, too, that day by day, if our general health is good, we may