to the solar system in that it has a central body like the sun called the nucleus and a system of revolving electrons like the planets. The nucleus is positively charged, the whole system being electrically neutral and in dynamic equilibrium. The simplest atom is of hydrogen with one electron revolving around a positive nucleus of equal amount electrically. Such an atom is also similar to the solar system in the relative sizes of its parts and the distances between them. Again the nucleus like the sun has nearly the whole mass of the system since it has been stated that at least in the hydrogen atom it is known to have above 1700 times the mass of the electrons. The process of radiation of such an atom may be quite consistent with the theory of Max Planck, advanced in 1910, the essential point of which is that the energy radiation from an atomic system does not take place in the continuous electrodynamic way but on the contrary that it takes place in distinctly separate emissions, the amount of energy coming from the system in a single emission being to rhy, where is a whole number the frequency of the vibration and h a universal constant. This is what is known as the quantum theory of radiation. In applying it we suppose that an atom can assume various so-called stationary states of equilibrium with its planetary electrons revolving about the nucleus. These states constantly shift from one to another by the jumping of an electron from one orbit to another with an accompanying radiation of definite frequency v, the energy always being equal to a whole number of quantum units.

Various investigations have indicated that the number of electrons in an atom is approximately equal to one half the atomic mass referred to hydrogen. As hydrogen itself must be an exception van den Broeck suggested that the number of electrons in any atom may be equal to the number identifying the element when all are arranged in the order of their increasing atomic masses. Thus hydrogen would have one, helium the second element with an atomic mass of four would have two electrons, carbon six and so on. This "atomic number" arrangement has proved very important in both physics and

chemistry. The most notable early experiments involving the subject of atomic numbers were those of Moseley on the "High Frequency Spectra of the Elements." Many unsuccessful efforts had been made to reflect, to refract, and to diffract Xrays this work beginning at their discovery in 1895. We now know that the difficulty resulted from the extreme shortness of the waves. Reflection of light occurs because the flaws in the surface are small compared with the wave length, but these become important when X-rays are concerned and the surface is much as sandpaper is to ordinary light. Professor M. Laue of Munich however in 1912 suggested that, if X-ray wave-lengths were of the same order of magnitude as atoms, crystals might serve to produce diffraction effects with X-rays by reason of the regular grouping of their atoms as affirmed by crystallographers who have gradually developed the theory suggested by Bravais in 1850 that the atoms of a crystal reside at the angular points of a so-called "space-lattice," the details of which depend on the crystal system. This theory was tested by the experiments of Friedrich and Knipping and at once proved sound. Later W. H. and W. L. Bragg of Cambridge showed that X-rays could be regularly reflected from the cleavage planes of crystals, and their work with that of Moseley resulted in finding the wave-lengths and frequencies of these rays. Suppose for example we have a crystal of the cubic form. If now we know the mass of an atom of which the crystal is made up, then the density of the crystal divided by the atomic mass gives the number of atoms per unit volume. Also N = n3 and (dn)3 = 1 in which N is the number of atoms per unit of volume, n the number on an edge of that volume and d the distance between the atoms. From all this clearly

d

=

I

VN planes.

which is also the distance between the reflecting

=

Furthermore the wave-length of the first order spectrum is found from the usual diffraction formula 1 2d sin in which is the angle of reflection. Thus by means of some simple crystals we have measured X-ray wave-lengths and

then by using these values on unknown and more complicated crystals, the form and atomic arrangements have been investigated. It was shown that there are two kinds of X-rays a more penetrating group or so-called hard rays denoting the K series and a soft kind called the L series. Moseley used a large number of the elements as anodes or targets in X-ray tubes and measured the frequencies of the X-rays produced. He found that the frequency increases with the atomic mass of the element generating the rays. Each element produced two characteristic lines and the pair moved progressively along the photographic plates in passing from one element to the next higher of atomic mass. Thus a remarkable relationship was established expressed by the formula, frequency = A (N − b)2, in which A is a constant and N is a whole number increasing by unity in passing from any element to the next higher in atomic mass, and b = 1 for the K series. If now for any particular element we substitute its atomic number for N and measure the frequency of is X-radiation by experiment, the value of A to apply to all elements can be found. If A thus found be used in the above formula and the latter be solved for N in terms of frequency, it was found by experiment on measuring various frequencies that N is the atomic number in each case. Thus Moseley verified the atomic numbers of aluminum 13, selicon 14, calcium 20, iron 26, cobalt 27, nickel 28, up to silver 47. With elements of higher atomic mass the L series was used and the frequencies measured up to gold 79. In this series five lines were found instead of two. The same formula applied with different values for the constants A and B. In this work it was found that every number from 13 to 79 applied to a known element except three. These three may have once existed and by the gradual process of inorganic evolution become extinct or they may at some remote future time appear in sufficient quantity to be detected, or yet they may now be present having as yet escaped observation. Up to the present time X-ray spectra have been obtained emitted by all the known stable elements except ten. This work of Moseley's was indeed remarkable and it is to be deeply

regretted that he was chosen by his government to present his body as a target for German bullets with a fatal result rather than to use his fertile brain at home for the solution of some of the many great war problems. Before the discovery of radioactivity and of the various radio-elements many atomic number gaps would have appeared between bismuth 208 and uranium 238. At present however the places are practically all filled with the radio elements referred to.

Thus we see a very intimate relation between the structure of the atom and its behavior and radiation as revealed by spectrum analysis. Any new truth discovered in the one is sure to lead to something of corresponding interest in the other. With the addition of X-rays and gamma rays to the list of electromagnetic waves previously known, the table of wave lengths is greatly extended in one direction. It is said that the longest Hertzian waves yet generated are nearly ten miles long while the shortest are a few millimeters. Those used in wireless communication are from three or four miles long down to a few hundred yards. Between the shortest Hertzian waves yet generated and the longest infra-red there is a short gap which will surely be closed in time. The longest infra-red rays are about o.1 of a millimeter and from these to those nearly as short as 10-6, we have the whole visible spectrum and the ultra violet. Then there is another gap before we get to the X-rays of the order of 10-8 cm. and the gamma rays, 10-9 cm.

The following table summarizes these results:

Thus we have briefly surveyed what Kaye calls "the riddle of modern Physics" and while it is not likely that exact agreement will result in the near future surely scientific

investigators are gradually approaching a unity of mind in many details. The speaker in this series six years ago closed his lecture on "What is Electricity" with these words-"we find ourselves now face to face with a single problem instead of many, the solution of which would open wide the door to a comprehensive knowledge of many things now unknown and formerly unknowable. The structure of the atom was here referred to. We must now and we might better then have referred to the duplex aspect of this same problem viz. the structure of the atom and the mechanism of radiation.