which form 59.8 per cent of the lithosphere. Hydrogen makes up II per cent of the hydrosphere and only 0.22 per cent of the lithosphere. Carbon is found chiefly in the carbonates, usually appearing as limestone.

It is evident that the above table ignores the make-up of the great central core of the earth of which we have little direct knowledge, since our deepest boring scarcely penetrates more than two miles below the surface. It is known that the mean specific gravity of the earth is about 5.4 or double the density of the lithosphere. Hence, the core must be much heavier and must weigh between 7 and 8. The probability is that the core is largely composed of iron whose specific gravity is 7.9.

We must conclude that the center of the earth consists of metals, very probably molten and that iron is probably the chief component. This central mass makes up something more than half of the whole earth-mass. Over it there lies a lighter mass whose composition seems to correspond to that of igneous rock. In the deeper layers of this material prevail rock containing the heavier iron, magnesium and aluminium silicates, higher up come lighter silicates in which the iron and magnesium are replaced by the lighter metals, calcium, potassium and sodium, and uppermost lie layers richest in silica (quartz), which is the lightest of such rocks. Very probably all these silicates, save the uppermost, are in molten condition. This molten material corresponds to the magmas thrown out by volcanoes.*

In this process of earth creation, the heavier elements find their way to the center as the phenomenon of rotation would lead us to expect, while the uncombined gases form the atmosphere from which it is conceivable that some of the lightest occasionally make their way into the outer spaces and are lost to earth. The word atmosphere is in common use, but the equally valuable words hydrosphere (the water bodies) and lithosphere (the rock crust) are seldom heard.

The appearance and properties of many of these elements have been well known, but until very recently there has been little beyond surmise as to their nature and mutual relations. Long ago the Greeks, speculating as to the essence of matter, thought that it might be made up of minute particles which could not be further subdivided to which they gave the name of atoms. The word survived but the idea was lost for centuries. During the Middle Ages men sought in vain for some way to transmute the baser metals into gold. The atomic theory was revived by Boyle (1661) in his Skeptical Chymist and by the first of the nineteenth century received considerable attention from such men as the school-teacher Dalton, who devised our system of naming chemical compounds and who recognized that all atoms of any given element were alike. Meantime many new substances were discovered and studied. In 1774, Priestley and Scheele both found oxygen and the hoary idea that fire was a separate entity, phlogiston, was soon destroyed. Before 1800, Cavendish had discovered hydrogen and found that water was a combination of this with oxygen. A stream of discoveries followed and many new elements were revealed. In 1868, Johannsen of Paris, using a spectroscope, found an unknown element in the sun to which the name of helium was given. In 1895, Ramsay proved its presence on earth. Then came the epoch-making research of the Curies and the finding of radium in 1898.

Radium has revolutionized human thought or, more correctly perhaps, has given evidence for a long-dormant suspicion. It had long been known that matter would combine to form compounds but the elements seemed permanent and distinct, occupying space, but inert. Radium did not comply with these conditions. It did things regardless of the conditions man might impose upon it. It gave off rays of various sorts which were soon classified. The alpha rays proved to be positively electrified and to travel some

20,000 miles a second and to be stopped by a sheet of aluminum 1/500 of an inch thick. The beta rays are negatively electrified, travel 10,000 miles a second and are stopped by a sheet of aluminum 1/25 of an inch thick; while the gamma rays, apparently the same as X-rays, will pierce a plate of aluminum up to 20 inches or take a photograph through a foot of iron. We now know that what is happening is the break-up of the radium atom, after an average duration of two thousand years, regardless of external conditions, with the release of tremendous energy. If the ninety-two elements are arranged in tabular form based on atomic weights, we have what is called the Periodic Table and some striking facts are revealed. The beginnings of such a table are given below. We start with the lightest element, hydrogen, as number I and find the

the elements fall into periodic groups and that substances occupying similar places in the different periods have very similar properties. Thus each period starts with an inert gas

and if the entire table were given we should find that krypton, xenon, and niton headed the later periods. Such a coincidence is hardly accidental. It is known that the nucleus of each atom is charged with positive electricity and the number of charges corresponds to the number of electrons about the nucleus. Thus hydrogen has one electron and a nucleus with one charge, lithium has three electrons and three charges.

"Now it is reasonable to suppose that the properties of an atom as manifested by its relation to any other will be determined by some arrangement of its electrons and especially of those which are most on the surface and are first presented to the other atom. Thus, lithium, sodium and potassium, probably behave alike because they all have the same external presentment of electrons; and so with carbon and silicon, with fluorine and chlorine, and so on." It appears that there is some definite arrangement of the electrons on the nucleus and that the most stable condition is when the outer shell consists of eight electrons. If one or two are lacking the atom seems to make effort to get them.

It is, in fact, this power that Chlorine possesses of dragging to itself an electron from other atoms, and upsetting their combination in order to get it, which makes the substance so actively poisonous. In the same way, sulphur has two gaps to fill up, and its behavior is largely governed by that fact.®

Atoms may share electrons with one another, each being capable, apparently, of counting them in its own structure, just as two houses may have the same party wall. . . . In the diamond,

...

each atom is surrounded by four other carbon atoms, with each of which it shares two electrons. So each atom is provided with an external shell of eight electrons none of which it has entirely to itself. This kind of combination is generally very strong and molecules so formed hold together well."

To this class then belong the inert gases.

Granted that the atoms of the different elements consist of a nucleus and its electrons arranged in some definite

fashion, is there any evidence of relationship between the elements? The answer to this was found in the radioactive substances. The element having the greatest atomic weight and standing at the top of the natural scale is uranium 238.2. This is followed by thorium, actinium and radium. It has been learned that the beta ray given off by radium is the atom of helium. Moreover, Rutherford has been able to bombard electrically the nitrogen atom and secure the atoms of hydrogen. According to press reports early in 1926, scientists at Amsterdam by exposing lead to intense heat in high vacuum secured mercury and thallium and Taylor of Princeton by the collision of hydrogen molecules and mercury produced peroxide of hydrogen. There is good reason to believe that lead is a product of the disintegration of uranium. Thus we find both in nature and in the laboratory that a more complex atom can be reduced and give us substances already known to us as elements. Whether elements other than the radioactive ones degenerating we have no idea, nor can we as yet do much to disrupt the atom, but a glimpse has been given us of its structure. On the basis of present knowledge it seems that the hydrogen ion, the nucleus of the hydrogen atom minus the accompanying electron, is the long-sought unit of positive electricity, the proton, which enters into the composition of all atoms. Shall we call it matter or electricity?

are

A few paragraphs ago our minds were being taxed to grasp the immensities of the interstellar spaces. At the other end of the scale of magnitude stand the atoms. So small are they that all ordinary measures fail us. Light cannot show details unless the objects are larger than the length of the rays. The X-rays are 10,000 times finer than light and so can reveal the atom. Let the reader see if he can visualize the diameter of the hydrogen atom, which is put at I.IX 10-8 centimeters, or that of uranium whose diameter is about two and one-half times greater.