Small, then, as are the atoms when they are packed together in some substance, there are still spaces between them as between the peas in a jar. "Even the diamond is full of holes like the sponge. If the holes were filled up by other carbon atoms the density of the diamond would be doubled, for each hole is just large enough to take one more carbon atom, and there are as many holes as atoms." 8

Furthermore, it is believed that the electrons are in constant revolution so that from the greatest star to the hydrogen atom we find eternal motion.

It is evident that only under conditions of which we have little comprehension could the enormous forces have been exercised which have produced the integration of the more complex elements.

But just such conditions must have existed, as we have noted, in the process of world formation, and may still exist, for all that we know, in the center of the earth where the pressure is enormous.

Matter is much more wonderful to us to-day than it seemed to our ancestors. We catch a glimpse of particles, minute beyond our power of comprehension, charged with electricity, drawn to each other at times, yet separated by the effects of constant motion.

The atoms seem to cling to one another in some such way as two magnets do, when opposite poles are presented to each other; or two charges of electricity of opposite nature. In fact, there is no doubt that both magnetic and electric attractions are at work.

We may now ask why, if there are such forces, the atoms do not all join together into one solid mass? Why are there any gases or even liquids? How is it that there are any atoms at all which do not link up with their neighbors? What prevents the earth from falling into the sun and the final solidification of the entire universe?

The earth does not fall into the sun because it is in motion around the sun, or, to be more correct, because the two bodies are moving around each other. It is motion that keeps them apart; and when we look closely into the matter we find that motion

plays a part of first importance in all that we see, because it sets itself against the binding forces that would join atoms together in one lump. In a gas, motion has the upper hand; the atoms are moving so fast that they have no time to enter into any sort of a combination with each other: occasionally atom must meet atom and, so to speak, each holds out vain hands to the other, but the pace is too great and, in a moment, they are far away from each other again. Even in a liquid where there is more combination and atoms are in contact with each other all the time, the motion is so great that no junction is permanent.

In a solid the relative importance of the attractive forces and the motion undergoes another change: the former now holds sway, so that the atoms and the molecules are locked in their places. Even in the solid, however, the atoms are never perfectly still; at the least they vibrate and quiver about average positions, just as the parts of an iron bridge quiver when a train goes over it. It is difficult to realize that the atoms and molecules of substances which appear to be perfectly at rest→ the table, a piece of paper, the water in a glass-are all in motion. Yet many of the older philosophers grasped the fact.9

So writes Sir William Bragg in his remarkable book which cannot be too highly commended. An atom of hydrogen, oxygen, or nitrogen may unite with another of the same sort to form a gas almost as inert as helium or argon. Yet at extremely low temperatures hydrogen and oxygen as well as helium become liquids. In similar fashion mercury freezes to a solid. Every substance, then, has its freezing or melting points at which it changes form. At ordinary temperatures most elements are solids, but our life depends on the fact that some are gases and others liquids. Hitherto we have spoken as if the elements were found separated in nature. Every one knows this condition to be the exception, not the rule. Our air is a mixture composed of 79 parts nitrogen, 21 of oxygen, about 0.03 per cent of carbon dioxide, and minute amounts of the inert gases, each 1,500 pounds of air containing about 18 pounds of argon for example. In addition it contains large amounts

of water vapor as well as quantities of dust of various sorts. Pure air is a relative term as is pure water, for the latter scarcely exists outside of the laboratory. Chemically pure water contains two parts of hydrogen to one of oxygen but many salts as well as other impurities are found in ordinary In nature, then, we ordinarily find either mixtures, such as the air, or compounds in which the elements have united to form substances whose appearance and properties are widely different from those of the elements, such as water. Indeed, as a rule it is only with the greatest difficulty that we can secure any substance in strictly pure form. The reason for this has been suggested and does not further concern us but we shall encounter the fact on every hand. Thus, some 60 per cent of known inorganic compounds contain oxygen and 30 per cent hydrogen, while other elements, like gold, are rarely found in combinations. These compounds are quite as useful as the pure elements. In as much as the chemical combination of the elements depends in part on temperature and pressure, it is extremely fortunate that past conditions in the history of the earth have been such as to produce them.

The degree of chemical complexity capable of existing in the materials found on earth is definitely and sharply fixed by temperature. At a white heat such as exists in the sun's atmosphere, we have seen that only elements can exist, and many of these are decomposed into proto-elements. At a somewhat lower temperature binary compounds, such as the oxides, can remain in equilibrium, in incomplete combination, becoming more and more complete as the temperature falls, and as soon as their existence becomes possible these oxides do exist. Lower still in the scale of temperature, saline compounds, such as the chlorides of the alkalies, and mutually neutralized acidic and basic oxides combined together, can stand the heat. Such bodies as the carbonates of calcium and magnesium can now be present in an incomplete state of combination, partially as oxide and partially as carbonate, in labile balance as the temperature fluctuates up or down, and the pressure of carbon dioxide in the atmosphere

changes. Whenever the environmental conditions make their presence possible, these more complex forms must promptly make their appearance by chemical law.

But it is only at a very much lower temperature that compounds at all complicated in chemical structure can exist in equilibrium, and for those compounds of many hundreds of atoms, which are characteristic of life, the range is narrowly limited.

This note cannot be too strongly sounded that as matter is allowed capacity for assuming complex forms those complex forms appear. As soon as oxides can be, there oxides appear; when temperature admits of carbonates, then carbonates are forthwith formed. These are experiments which any chemist can to-day repeat in a crucible.

It may then be summed up as a general law universal in its application to all matter, although varying in its intensity in different types of matter, and holding throughout all space as generally as the law of gravitation—a law which might be called the Law of Complexity-that matter so far as its energy environment will permit-tends to assume more and more complex forms in labile equilibrium.10

The application of this suggestion to life itself will be considered in the following chapter. That which needs to be noted here is that the development of the world itself has prepared the basis for the continued existence of all organisms and has put in the air or on the surface of the earth the necessary elements, as well as compounds.

More and more it becomes clear that the old idea of creation as a past event gives an inadequate picture of the facts. In so far as we can tell, creation is a process going on in the world about us as much to-day as ever. Water and air are not only modifying the surface of the earth but are being modified themselves.

GEOLOGICAL HISTORY OF THE EARTH

If, as we have abundant evidence for believing, the surface of the earth was once molten, the atmosphere must have contained an enormous amount of water vapor as

compared with to-day, for water could not rest on the melted earth, if, indeed, the heat allowed the hydrogen and oxygen to remain in combination. The atmosphere in this early period must have been rich in carbon dioxide and poor in oxygen. When living organisms appeared, the carbon dioxide was gradually removed from the air and the oxygen taken from the rocks was given to the air until the present mixture was secured.

On the moon gravity is so much less, because the moon has only 1/80.5 the mass of the earth, that all of our common atmospheric gases, as well as carbon dioxide and water vapor, long ago escaped. This is why the moon is dead. So much for the influence of size and mass. But if the moon's atmosphere had always been exceedingly cold, as cold as it would have been if lying as far from the sun as Neptune, probably carbon dioxide, water, oxygen and nitrogen would still be retained, because the molecules would not go fast enough to defy gravity to hold them. So much for temperature influence on the composition of the atmosphere.11

Mars presents an almost unclouded appearance to our eyes, despite the faith of Pickering in the existence of "canals," whereas the earth viewed from Mars would be cloudy a good part of the time. In earlier ages the clouds would have been much heavier. Our water and air are due, then, to the size of the earth as well as to earlier conditions of temperature. It is interesting to ask ourselves whether the amount of water is increasing or decreasing on earth and whether under existing conditions hydrogen and oxygen are being combined to form new supplies. Perchance, instead, the lightning is disrupting the water molecules and setting the gases free.

The winds are changing the face of the earth more than is generally realized. The creeping sand dunes may destroy vegetation over wide areas or block the streams and thus injure man or, in other cases, help him by building up a