[Pg 204] and substitute for the observed wave-length of the highest frequency line emitted by tungsten—a wave-length which has been accurately measured and found to be ; and, further, if we substitute for , 74, the atomic number of tungsten, and for , 1, if the Moseley law were exact we should obtain, by solving for the wave-length of the highest frequency line which can be emitted by the element whose nucleus contains but one single positive electron. The result of this substitution is (millionths millimeters). Now the wave-length corresponding to the highest observed frequency in the ultra-violet series of hydrogen lines recently discovered by Lyman is and there is every reason to believe from the form of this series that its convergence wave-length—this corresponds to the highest frequency of which the hydrogen atom is theoretically capable—is . The agreement is only approximate, but it is as close as could be expected in view of the lack of exact equality in the Moseley steps. It is well-nigh certain, then, that this Lyman ultra-violet series of hydrogen lines is nothing but the X-ray series of hydrogen. Similarly, it is equally certain that the X-rays series of hydrogen is the ordinary Balmer series in the visible region, the head of which is at In other words, hydrogen’s ordinary radiations are its X-rays and nothing more. There is also an series for hydrogen discovered by Paschen in the ultra-red, which in itself would make it probable that there are series for all the elements of longer wave-length than the series, and that the complicated optical series observed with metallic arcs are parts of these longer wave-length series. As a matter of fact, an [Pg 205] series has been found for a considerable group of the elements of high atomic number. Thus the Moseley experiments have gone a long way toward solving the mystery of spectral lines. They reveal to us clearly and certainly the whole series of elements from hydrogen to uranium, all producing spectra of remarkable similarity, at least so far as the and radiations are concerned, but scattered regularly through the whole frequency region, from the ultra-violet, where the lines for hydrogen are found, all the way up to frequencies or 8,464 times as high. There is scarcely a portion of this whole field which is not already open to exploration. How brilliantly, then, have these recent studies justified the predictions of the spectroscopists that the key to atomic structure lay in the study of spectral lines!