length of the year and the time of the revolution of five planets about the sun. He knew that the solar system was moving toward the east, as compared to the stars, and was able to determine latitude and longitude. It is obvious that he could not have made all these discoveries himself.

Three centuries later, Ptolemy of Alexandria collected all the known facts in a treatise. He rejected the idea of Hipparchus that the earth turns on its axis, though he admitted that this would simplify the explanation, and he held that the earth was the center of the universe and that the heavens were in revolution. This concept of an earthcentered universe dominated for a thousand years. The ideas of the revolution of the earth on its axis and of the rotation of the planets about the sun were revived by the Polish priest and physician, Copernicus (1473-1543). About 1600, the telescope was invented in north Europe. There is much dispute as to the actual inventor. Galileo (1564-1642) heard of this and constructed one for himself with lenses about two inches in diameter. With this he made discoveries so marvelous that he got in trouble with the authorities, but, nevertheless, he overthrew the Ptolemaic interpretation and began a new epoch in human thought. Yet, Hipparchus, like men of to-day, would have said, "I told you so."

The earth, then, is but one of several planets, and by no means the largest, revolving about the sun. The stars which we see are other suns with other planets perchance revolving about them. Our minds are bewildered. We cannot comprehend space without limit nor can we postulate a limit as the ancients did with their idea of a solid firmament, the sky. With improved methods of measurement the distances are seen to be so great that we must figure them in light years rather than miles. It takes the light about four hours to go from the sun to Neptune but it must spend some four years to reach us from the nearest star (Alpha Centauri). If one prefers, let him try to visualize a light

year as about six million million miles. It takes light about 10,000 years to reach us from the outer edge of our stellar system, but the end is not yet, for in 1926 Hubble announced the discovery of another system of suns some 700,000 light years away. The naked eye can see upwards of 4,000 stars while the great telescope at the Mt. Wilson Observatory will reveal nearly 500,000,000. If we use photographic methods another billion reveal themselves and astronomers believe that many billion more exist. Confronted by such numbers the Psalmist's query, "What is man that Thou art mindful of him?" takes on added meaning.

Patient study and repeated observations have given us some idea of the size of these stars. The diameter of the sun is about a hundred times that of the earth, some 800,000 miles, with a mass some 300,000 times that of the earth. The sun is a pygmy after all. Betelgeuse is

gaseous star less dense than our air, but with a diameter of 300,000,000 miles. The sun is bright but Rigel gives about 10,000 times as much light. But the sunlight reaches us in eight minutes while that from Rigel is 400 years on the way.

The discovery that the earth was but one out of countless worlds naturally suggested that its own history must have been the same as that of the others. When the comets and nebulæ were studied and found to be gaseous rather than solid, and when it was realized that many of the stars were in the same condition, it was easy to imagine that such was the origin of our solar system. To Laplace, in 1792, goes the honor of the suggestion of the "nebular hypothesis.' In substance this is that a vast nebula once occupied the entire area of our solar universe. In the process of condensation the various planets and their satellites broke away and gradually cooled while the central sun still retains a large part of its heat. This theory was generally accepted throughout the nineteenth century. It does not fit all the facts known to-day and is being replaced by some of the

"planetary" theories such as that of Chamberlin. This agrees with the older explanation in holding that the earth has split off from the sun but claims that at the time of the separation the earth was gaseous and not hot. To itself the earth attracted much new matter from surrounding space just as the meteorites fall on it to-day. The later process of condensation produced an enormous amount of heat and caused the molten condition. We know that the process of cooling has not been uniform and that there have been a number of glacial epochs when North America as far south as the Missouri and Ohio rivers was covered with ice, and there is evidence of older glaciations elsewhere. The discovery of the radioactive substances raises other doubts. Barrell claims that "if they extend in their surface amount to a depth of 40 miles, they must supply heat to the surface as fast as it is lost by radiation into space. The earth therefore appears not to be growing colder, though losing heat." 1 One of the best of living students, Joly, thinks it quite possible that the heat developed by condensation may again produce a molten globe, or even cause an explosion great enough to tear the earth into pieces through the accumulation of radioactive energy.

It is no longer believed that the earth has a thin skin of crust over a molten center, nor that volcanoes furnish ventholes to the lower inferno. Volcanic phenomena are thought to be due to chemical changes relatively near the surface.

The invention in 1860 of that wonderful instrument, the spectroscope, has given us much added information about the stars. It shows us that the surface temperature of the sun is about 6,000° C. or 10,800° F. "We see carbon dazzling in the crater of the electric light, but the sun is far hotter than that. The sun is so hot that if the carbon stick of the arc lamp was thrust into its depths, the carbon would burst into gas with an explosion." 2 If, however, we compare the sun with other stars it is relatively cool. There

are two stars in the constellation of Argo whose spectra show lines belonging to no known element but bearing such a relation to hydrogen that they are called protohydrogen stars. Helium is present with hints of magnesium and calcium. Taurus and Algol (known as helium-gas stars) show helium along with hydrogen, carbon, oxygen, and nitrogen. The protometallic stars such as Rigel and Sirius reveal such metals as iron, copper, and calcium. The metallic stars, including the sun, Arcturus, and Aldebaran, display the common metals. There are stars so cool that they have fluted spectra which do not show the characteristic lines of their elements. While man cannot rival in his laboratories the conditions of temperature and pressure found in the stars, he can reproduce some of them and by comparing his results with the revelations of the spectroscope, he gets some glimpses of the process of the formation of matter. The significance of this will appear later. It is most interesting to note that all the available evidence shows that the stars contain the same elements found on earth and no others, unless an exception be made of a substance called coronium known only by lines in the solar spectrum and which has not been found on earth. Under the peculiar conditions existing then, in bygone ages, the various substances found on earth as well as in the stars have been created. Further evidence is supplied by the meteorites, composed for the most part of iron, that fall on earth.

CHEMICAL ELEMENTS OF MATTER

There is reason to believe that matter on earth exists in ninety-two different forms to which we give the name of elements. Of these about ninety have been identified and named although the last three to be discovered are known only through the spectroscope and have not been isolated as yet. These bear the names of masurium, rhenium and illinium. In pre-Christian times only nine were known, of

which seven were metals, gold, silver, etc.; the others were carbon and sulphur. Only eighteen more were recognized before the year 1800 while fifty-two were found during the last century. Some sixty-seven of these are metals, four are active gases, seven, inert gases, one, a liquid and seven nonmetallic substances, leaving two as semimetallic and the balance to be determined.

A few of these elements are very common, the majority are very rare. Some are known by name, at least, to every one but it is doubtful if one person in a hundred in the United States can name the three commonest elements on earth. As a matter of fact, two elements form over 75 per cent of the atmosphere, the water and upper ten miles of the crust of the earth. Eight elements form over 97 per cent, thus leaving a very small balance to be supplied by all the rest.

CHEMICAL COMPOSITION OF THE EARTH'S CRUST AND
ATMOSPHERE 8

The proportion of these elements varies greatly in the different layers. Oxygen and silicon are in all silicates