74 



SCIENCE 



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



small indeed as compared with even the 

 minute electrons which circulate about it. 

 We will recall first several of the dis- 

 coveries which have forced us to abandon 

 the idea of an indivisible atom. The funda- 

 mental one was Sir Joseph Thomson's dis- 

 covery of the electron. In studying the 

 nature of the cathode rays he found that 

 they consisted of extraordinarily minute 

 particles all exactly alike, whatever the 

 nature of the gas within the tube might be. 

 In a series of brilliant experimental studies 

 he was able to show that the mass of one of 

 these electrons was only one eighteen hun- 

 dredth that of the lightest known atom. 

 Then came Zeeman's discovery^ that the 

 separate lines of many spectra are broken 

 up into two or more lines by the action of 

 strong magnetic fields. The study of this 

 effect made it quite certain that light radia- 

 tion is caused by the rapid vibration of 

 electrons in the luminous body. Therefore 

 electrons must be present in very many 

 kinds of matter — probably in all. The elec- 

 trons were early proved to carry a negative 

 charge of electricity. Soon they revealed 

 their presence in a great variety of ways 

 and assisted in the explanation of widely 

 different phenomena. But the correspond- 

 ing positive constituent of matter proved 

 singularly elusive although most diligently 

 sought for, and it is only very recently that 

 we seem to have traced it to its hiding-place. 

 Different views regarding the nature of 

 this positive constituent have led to much 

 diversity of opinion regarding the structure 

 of atoms. One of the most successful of 

 these theories is that proposed by Sir 

 Joseph Thomson in 1904.^ He supposed a 

 relatively large positive mass to exist — 

 nearly as large as the atom — with the mi- 

 nute negative electrons distributed through 

 it in such a way as to make the system a 

 stable one. For easy mathematical treat- 

 ment he assumed the electrons at equal 



distances apart in a series of concentric 

 circular rings. To secure stability and 

 illustrate certain atomic properties he 

 supposed these rings to be in rotation. 

 Thomson discussed many such configura- 

 tions and satisfactorily explained many 

 facts regarding the valency, the position in 

 the periodic system, the electropositive or 

 electronegative character and other chem- 

 ical properties of different substances. 



A modification of Thomson's atom was 

 proposed by H. A. Wilson in 1911.* He 

 supposes each negative electron to be situ- 

 ated at the center of a positive sphere of 

 sufficient size to neutralize it electrically, 

 and the atom to be made up of a group of 

 such units, the total number being propor- 

 tional to the atomic weight. In other 

 words, Thomson's one relatively large posi- 

 tive mass is divided up into equal parts, 

 each one containing a single negative elec- 

 tron. The mathematical development of 

 this idea led to the result that the hydrogen 

 atom contains eight such units. The gold 

 atom would therefore contain about six- 

 teen hundred of them. 



In the Thomson and the Wilson atoms, 

 the positive portion is diffused throughout 

 nearly the whole volume of the atom, a 

 region about one hundred millionth of a 

 centimeter in diameter. This type of 

 structure has accounted for many atomic 

 properties but has not been very successful 

 in explaining the position of the lines in 

 light spectra caused by vibrations in the 

 atom. 



I wish to direct your attention to-day 

 more particularly to a type of atom in 

 which the positive charge — equal as before 

 to the sum of the charges of the negative 

 electrons — is highly concentrated at the 

 center of volume of the atom, occupying 

 only an exceedingly small part of the 

 volume. Nagaoka'* had discussed the sta- 

 bility of such an atom in 1904. Sir Ernest 



