30 



Supplement to "Nature" /u/y 7, 1923 



not necessary for me to state here how it has been 

 shown in recent years that isotopes are found not 

 only among the radioactive elements, but also among 

 ordinary stable elements ; in fact, a large number 

 of the latter that were previously supposed simple 

 have been shown by Aston's well-known investigations 

 to consist of a mixture of isotopes with dififerent 

 atomic weights. 



The question of the inner structure of the nucleus 

 is still but little understood, although a method of 

 attack is afforded by Rutherford's experiments on the 

 disintegration of atomic nuclei by bombard- 

 ment with a-particles. Indeed, these experi- 

 ments may be said to open up a new epoch 

 in natural philosophy in that for the first 

 time the artificial transformation of one 

 element into another 

 has been accomplished. 

 In what follows, how- 

 ever, we shall confine 



ourselves to a consider- 

 ation of the ordinary 

 physical and chemical 

 properties of the ele- 

 ments and the attempts which have been 

 made to explain them on the basis of the 

 concepts just outlined. ^ 



It is well known that the elements can be 

 arranged as regards their ordinary physical 

 and chemical properties in a natural system 

 which displays most suggestively the peculiar re- 

 lationships between the different elements. It was 

 .recognised for the first time by Mendeleeff and 

 Lothar Meyer that when the elements are arranged 

 in an order which is practically that of their atomic 

 weights, their chemical and physical properties show 

 a pronounced periodicity. A diagrammatic representa- 

 tion of this so-called periodic table is given in Fig. i, 

 where, however, the elements are not arranged in the 

 ordinary way but in a somewhat modified form of a 

 table first given by Julius Thomsen, who has also 

 made important contributions to science in this 

 domain. In the figure the elements are denoted by 

 their usual chemical symbols, and the different vertical 

 columns indicate the so-called periods. The elements 

 in successive columns which possess homologous 

 chemical and physical properties are connected with 



lines. The meaning of the square brackets around 

 certain series of elements in the later periods, tlx 

 properties of which exhibit typical deviations from 

 the simple periodicity in the first periods, will be dis- 

 cussed later. 



In the development of the theory of atomic struc- 

 ture the characteristic features of the natural system 

 have found a surprisingly simple interpretation. Thus 

 we are led to assume that the ordinal number of 

 an element in the periodic table, the so-called atomic 

 number, is just equal to the number of electron 

 which move about the nucleus in th( 

 neutral atom. In an imperfect form, 

 this law was first stated by Van 

 den Broek ; it was, however, fore- 

 shadowed by J. J. Thomson's in- 

 vestigations of the number of elec- 

 trons in the atom, as well as b)- 

 Rutherford's measurements of tin 

 charge on the atomic nucleus. As 

 we shall see, convincing support for 

 this law has since been obtained in 

 various ways, especially by Moseley's 

 famous investigations of the X-ray 

 spectra of the elements. We ma}- 

 perhaps also point out, how the 

 simple connexion between atomic 

 number and nuclear charge offers an 

 explanation of the laws governing 

 the changes in chemical properties 

 of the elements after expulsion of 

 or ^-particles, which found a 

 simple formulation in the so-called 

 radioactive displacement law. 



Atomic Stability and Electro- 

 dynamic Theory. 

 As soon as we try to trace a 

 more intimate connexion between the properties 

 of the elements and atomic structure, we encounter 

 profound difficulties, in that essential differences 

 between an atom and a planetar}- system show 

 themselves here in spite of the analogy we have 

 mentioned. 



The motions of the bodies in a planetary system, 

 even though they obey the general law of gravitation, 

 will not be completely determined by this law alone, 

 but will depend largely on the previous histor}- of 

 the system. Thus the length of the year is not 

 determined by the masses of the sun and the earth 

 alone, but depends also on the conditions that existed 

 during the formation of the solar system, of which 

 we have very little knowledge. Should a sufficiently 

 large foreign body some day traverse our solar system, 

 we might among other effects expect that from that 



