6. Carbon

This is a familiar substance in common life. Anthracite coal and charcoal are examples of impure carbon. Graphite in lead pencils is also carbon, and so are diamonds. When wood chars or food is burned in an overheated oven, the partially decomposed materials become black, revealing the presence of carbon, the other elements with which it was associated being driven out.

An immense quantity of carbon exists in the air, combined with oxygen as carbon dioxid or carbonic acid gas. The average proportion by weight of this compound in the atmosphere is stated to be.06 per cent, and as the weight of a column of air one inch square is fifteen pounds, it follows that over every acre of land there is 26.2 tons of carbon dioxid, or 7.7 tons of carbon. As we know that plants draw their supply of this element from the atmosphere, and as vegetable tissue is its only source to animal life, we are able to assert, with confidence, that the carbon in the tissues of the huinan body was once floating in space. A long time ago, Boussingault determined the average yearly amount of carbon which* was withdrawn from the air by the crops grown on a particular field during a period of five years, and found it to be 4615 pounds. This is no more than is acquired by a large crop of maize. As a matter of fact, plants, as well as animals, contain a larger proportion of this element than of any other, and the amount of this substance which enters into the processes of growth and decay in the vegetable and animal kingdoms is almost beyond comprehension. It is natural to wonder whether the atmospheric supply is equal to the demand. Any anxieties we may have concerning this should be removed by learning that during many years the percentage of atmospheric carbon has not changed appreciably. The processes of decay on the earth's surface, the combustion of wood and coal as fuel and of carbon compounds by animal life, are returning carbon to the air as rapidly as it is being withdrawn. This is the round traveled, - from the air to the plant, from the plant to the animal, and from the animal back to the air, - a cycle in which this element has been moving since life began, and in which it will continue to move so long as life exists.

7. Oxygen

This element is, next to carbon, the most abundant component of vegetable and animal tissues, and it stands second to none in its relation to the vital processes of nearly all forms of life. It is not a substance with which we are familiar by sight, because we ordinarily come in contact with it as a transparent, colorless gas. We live and move in it, for it is an important and uniformly abundant constituent of the atmosphere. The air is over one-fifth oxygen by volume, the proportion by weight being slightly larger. More than twenty-one million pounds of this element are contained in the air above a single acre of land, a quantity which remains remarkably constant, and which is surprisingly uniform over the entire surface of the globe. While it is being continuously withdrawn from the air for the uses of life and to maintain fuel combustion and processes of decay, it is, like carbon, continuously returned.

Vast quantities of oxygen are also contained in water, as this compound, which fills the ocean and lakes, and is abundant in the crust of the earth, is nearly 89 per cent oxygen. It is estimated also that the solids in the crust of the earth are one-half oxygen. That which enters directly into the uses of animal life is, however, chiefly that which is derived from the atmosphere and water.

Not a plant grows or animal lives excepting through the circulation of oxygen, during which it passes into fixed combinations and back again to the free form. Man uses the free oxygen in breathing and returns it to the air in part combined with carbon as carbon dioxid. This compound the plant appropriates, retaining the oarbon for its tissues and giving back the uncombined oxygen to the atmosphere to be again used by animal life. All decay and many other chemical changes require the presence of this element. What we speak of as fire is due to its union with the elements of the fuel.

Oxygen bears an indispensable relation to the mechanical forces that man now employs, for it is the agent which maintains combustion in the furnaces of our industries.

All the activities of life are intimately related to it. When a plant grows, oxygen is torn from its union with other elements by the dominating power of the sun's rays, and energy is stored in vegetable tissue. When this tissue is used as food, the oxygen returns to its former combinations through the opportunities offered by the vital processes of the animal, and the hidden forces of the plant compounds are thus manifested in a variety of ways. Human toil is sustained by the energy thus stored and liberated.