NATURE 



[July 7, 1923 



The same is true in a somewhat less striking way of 

 the electronic conception of electricity. This idea 

 became current in a vague form and was shown to be 

 a suitable foundation for the known phenomena of 

 electricity ; it was not till some years later that the 

 fundamental experiments on the conduction of 

 electricity through gases first led to a practical demon- 

 stration of the existence and main properties of the 

 electron. Just as with atoms, a break-away from 

 statistical deductions was necessary before the electron 

 could be assigned a definite form. The demonstration 

 of its existence and properties, though it belongs 

 historically (1897) to the nineteenth century, is in 

 fact the starting point of what we have called twentieth- 

 century physics. 



This concentration on the statistical side was of 

 course inevitable, for the phenomena to which current 

 theories could be applied were mainly concerned, as 

 we have said, with the properties of matter or electricity 

 in bulk. There were, of course, striking and significant 

 exceptions which were already well known for many 

 years before 1900 — for example, optical spectra. 

 These had long been recognised as essentially character- 

 istic of particular atoms or molecules, obscured little 

 if at all by any process of averaging. But optical 

 spectra are too complicated and their conditions of 

 excitation too obscure to have formed then a possible 

 basis on which to build theories of atomic structure 

 with any real chance of success. It was necessary to 

 wait first for direct experimental evidence of the more 

 fundamental properties of individual atoms which are 

 unaffected by the widest possible range of external 

 circumstances. It is clear that it is such properties 

 that any atomic model must first set out to reproduce. 



The discovery of the nature and properties of X-rays 

 might have provided a new and more hopeful starting 

 point. Here we have evidence of fundamental pro- 

 perties which remain constant and characteristic in 

 all known circumstances. But even this evidence — 

 even, for example, an empirical formulation of Moseley's 

 law — would scarcely have been simple and direct 

 enough for a starting point, and in fact was not 

 available until after the first essential ideas had been 

 otherwise won. The evidence necessary for the start 

 had clearly to refer directly to individual atoms and 

 be such as to lay down with absolute convincingness 

 the main features of atomic structure. It was pro- 

 vided first by the study of radioactivity, and it is 

 difficult to see, as we have tried to show, how any 

 other evidence could have been sufficiently powerful 

 for the purpose. The radioactive evidence soon made 

 it clear that here physicists were concerned with pro- 

 cesses connected with the most intimate structure of 

 the individual atom, which outside conditions (physical 



NO. 2801, VOL. 1 12] 



or chemical) were powerless to affect ; and concerned, 

 too, with energy transformations in a single atom so 

 large that the resulting effects could actually be 

 detected. This made it clear that the atom must have 

 an innermost structure, a place apart, the seat of 

 gigantic forces. Ideas of the atom thus began t - 

 tend generally in the right direction, and crv-stallised 

 into the nuclear atom when the nature of the a-particle 

 had been established and the phenomena of its scatter- 

 ing worked out. 



It was at this point (191 1) that Prof. Bohr's contribu- 

 tions began, and it is convenient to specify the situation 

 in somewhat more detail. It was known that the 

 atom must almost certainly consist of a heavy nucleus, 

 of extremely small size, with a positive electric charge ; 

 this nucleus must probably behave, so far as the rest 

 of the world or even the rest of the atom was concerned, 

 as a massive point charge. The nuclear charge must 

 be neutralised in the natural atom by a system of 

 satellite electrons in number equal to the number of 

 units in the nuclear charge. Their arrangement was, 

 however, quite unknown, except that they must with 

 the nucleus compose a structure on the scale of the 

 atom of gas theory — a scale which is exceedingly large 

 and open compared to the dimensions of the nucleus 

 and the electrons themselves. The exact number of 

 satellite electrons or units of nuclear charge was also 

 uncertain, but, by the results of X-ray and a-particle 

 scattering, must be about half the atomic weight. It 

 was almost certain that it was two for helium and one 

 for hydrogen. If these views were to be accepted the 

 hydrogen atom must be very simple — a single heavy 

 nucleus with a unit positive charge, and somewhere 

 near it a single electron : it must also yield the known 

 series spectrum of hydrogen. This was the problem 

 presented to Prof. Bohr. He maintained from the 

 first, and justly as is now admitted by all, that there 

 was no possibility of a solution within the domain of 

 classical electrodynamics, and that the ideas of the 

 quantum theory must be invoked. How these ideas 

 lead inevitably to the accepted hydrogen atom of 

 to-day is set forth at length in the first of his three 

 essays, " The Theory of Spectra and Atomic Constitil- 

 tion," referred to in Nature of April 21, p. 523, and, 

 more shortly, in the present supplement 



The next essential step was the final assignment of 

 atomic number ,^\i\^ connected up once and for all the 

 ordinal number of any atom in the periodic table of the 

 elements, its nuclear charge, the number of its satellite 

 electrons, and its characteristic X-ray spectrum. This 

 assignment, which was, of course, the result of a system- 

 atic survey of X-ray spectra, was to some extent 

 directly inspired by the successful theory of the 

 hydrogen atom, and without that theory the full 



