80 



SCIENCE 



[N. S. Vol. XLI. No. 1046 



order, a, /?, j3. So the old difficulty of find- 

 ing places in the periodic table for the 34 

 radioactive substances now known has dis- 

 appeared, since they have but ten different 

 atomic numbers and require therefore but 

 ten places in the periodic table. Soddy has 

 introduced the term isotopes to designate 

 two elements occupying the same place in 

 the table. Isotopes are chemically insep- 

 arable and probably have identical spectra, 

 but they have different atomic weights. 



It is evident that much remains to be 

 done before we have very definite ideas of 

 the structure of the nucleus atom. Many 

 questions are entirely unanswered. For ex- 

 ample, in how many rings do the electrons 

 lie 1 For hydrogen and helium as for nebu- 

 ulium and protofluorine (if they exist) the 

 electrons are so few that they doubtless all 

 lie in one ring, but there are reasons for 

 believing that in atoms of higher atomic 

 weight there are two or more rings. With 

 a large number of electrons present — with 

 the 100 electrons of the gold atom for in- 

 stance — there may indeed be several con- 

 figurations which will satisfy the condi- 

 tions of stability. Even for comparatively 

 light atoms Bohr^° supposes that as many 

 as five rings exist. Again from what part 

 of the atom of a radioactive substance do 

 these ejected a- and /?-partieles come? 

 Soddy^^ believes that both originate in the 

 nucleus, but that the chemical and the 

 electro-chemical properties are controlled 

 by the outer ring of the electrons. Mose- 

 ley regards the similarity of the X-ray 

 spectra of different metals as satisfactory 

 evidence that such radiation originates in- 

 side the atom, while light radiation is de- 

 termined by the "structure of the sur- 

 face." Rutherford^^ and Bohr both raise 

 the important question whether atomic 

 nuclei contain electrons, and both conclude 

 that they do. These and many other ques- 

 tions have already been asked but only 



tentative and provisional answers have 

 thus far been given. Doubtless there is a 

 field here for much important experimental 

 and theoretical work in the immediate fu- 

 ture — a field which American physicists 

 will seek to cultivate with their European 

 brethren, who have done about all of the 

 work thus far. 



These hasty considerations perhaps suf- 

 fice to show the varied character of the 

 lines of evidence that have been developed 

 during the past three years to give support 

 to some form of nucleus atom. Radioactive 

 phenomena. X-ray radiation and chemical 

 properties seem to give united testimony 

 for it. Doubtless the final type of atom 

 has not yet been described, for it is easy to 

 criticize the views of Nicholson, of Bohr 

 or any other who has proposed a model, but 

 it is probable that some form of nucleus 

 atom will soon receive general recognition. 



1. Poincarg, Annual Eep. Smithson. Institution, 

 1912, p. 199. 



2. P. Zeemann, Phil. Mag., 43, 226-239; 44, 

 55-60 and 255-259. 



3. J. J. Thomson, Phil. Mag., 7, 237-265, 1904. 



4. H. A. Wilson, Proo. Amer. Phil. Soo., 366 

 (1911); Phil. Mag., 21, 718 (1911). 



5. Nagaoka, Phil. Mag., 7, 445 (1904). 



6. Geiger and Marsden, Proc. Poy. Soo. A, 82, 

 495 (1909). 



7. E. Rutherford, Phil. Mag., 21, 669-688 

 (1911). 



8. H. Geiger, Proc. Man. Lit. and Phil. Soc, 

 55, Pt. II., p. XX (1911) ; Phil. Mag., 25, 604-623 

 (1913). 



9. H. Barkla, Phil. Mag., 21, 648 (1911). 



10. C. I. E. Wilson, Proc. Boy. Soc. A, 87, 277 

 (1912). 



11. E. Marsden, Phil. Mag., 27, 824-830 (May, 

 1914). 



12. E. Eutherford, Phil. Mag., 27, 488-499 

 (Mar., 1914). 



13. J. W. Nicholson, Boy. Astron. Soc. M. N., 

 72, 49-64 (1911). 



14. M. Wolf, Nature, 89, 70 (Mar. 21, 1912). 



15. J. W. Nicholson, Boy. Astron. Soc. M. N., 

 72, 139-150 (1911). 



16. N. Bohr, Phil. Mag., 26, 1-25 (July, 



