the evidence is very strong that they originate in the innermost ring of electrons. Barkla has shown, moreover, that the heavier elements give out a second characteristic type of radiation very much softer than the first, which again is connected in a simple way with the atomic weight of the element.
This radiation from elements of small atomic weights is exceedingly soft, so soft, indeed, that it has not yet been detected from any element with an atomic weight less than 90. This softer type of radiation probably originates in the second shell of electrons, counting from the inside of the atom. By the study of these radiations we thus get, in the case of the heavier elements, evidence of the existence of two groups of electrons. The radiation from the outer of these groups is so much softer than that from the inner, that if the increase in softness were to continue at the same rate, we should not expect, except perhaps for elements heavier than lead, to obtain radiations from a third ring which could be detected by the methods hitherto applied to Röntgen rays. The method thus breaks down as we approach the most interesting part of the atom.
I think, however, that we may hope before long to have at our disposal methods by which we can produce and investigate Röntgen rays of a much softer type than those hitherto used. Röntgen rays are usually generated by shooting rapidly moving electrons against a solid target; the greater the speed of the electrons the harder are the rays they produce. The softest characteristic radiation yet detected is that from aluminium; this type of radiation is produced by electrons moving at a speed corresponding to about 3,000 volts, and is so easily absorbed that it is difficult to work with in the open air. By working inside a very good vacuum, and