Makch 3, 1922] 



SCIENCE 



221 



cation of the electric field of the nucleus by 

 the electric fields of the extra nuclear electrons 

 within the atom. 



II. On the Structure op Atoms 

 Through the advances made by a study of 

 the scattering of a rays and of X rays the 

 attack on the problem of the structure of 

 atoms and the origin of radiations naturally 

 proceeded upon two well-defined lines, namely : 



1. The investigation of the constitution and 

 properties of the nucleus, and 



2. The investigation of the configuration 

 and modes of vibration of extra, nuclear elec- 

 trons in atoms. 



In pursuing this attack it has been assumed, 

 with very good warrant, that the positive elec- 

 tric charges on nuclei are given by Ne where 

 N is the atomic number of the element con- 

 cerned, and that the number of extra nuclear 

 electrons in an atom is N. For example, the 

 number of extra nuclear electrons in various 

 atoms is taken to be as follows : Hydrogen 1, 

 helium 2, lithium 3, carbon 6, fluorine 9, neon 

 10, sodium 11, chlorine 17, argon 18, potas- 

 sium 19, etc. 



III. Positive Rat Analysis 

 This method of analysis was devised by Sir 

 J. J. Thomson^ and consisted in projecting 

 successively through an electric and a magnetic 

 field positively charged atoms or molecules, 

 i. e., those from which one or more extra 

 nuclear electrons had been detached. By this 

 means he was able to show that positive atom 

 ions can be obtained with one, two, three, and, 

 in the ease of mercury, with eight positive 

 elemental charges. 



Among other results he has been able to 

 show that such compounds as CH, CH„ and 

 CHj can exist with a recognisable though 

 transitory existence. He has also shown that 

 a substance having the molecular formula Hj 

 and bearing a single positive elemental charge 

 can be obtained from various sources, a result 

 which has been confirmed by Dempster, who 

 showed that this molecular aggregate can be 

 obtained with a transitory existence when an 



» J. J. Thomson : Ears of positive electricity. 



electric charge is passed through hydrogen. 

 Perhaps the most notable discovery made by 

 Thomson, however, was that neon existed in 

 two forms with identical chemical properties, 

 but with different integral atomic weights, 

 namely, 20 and 22. 



This discovery was of prime importance for 

 it pointed to the probability of the general 

 applicability of the principle which had been 

 alreadj' found by Soddy and others to hold 

 with the radioactive elements, namely, that the 

 atoms of elements consist of isotopes, i. e., 

 that we have atoms of an element with identical 

 chemical properties, but with different atomic 

 masses. This discovery also offered an explan- 

 ation of the non-integral values found by 

 chemical analj'sis for the atomic weights of 

 many of the elements. If it turned out, assum- 

 ing the atomic weight of oxygen to be 16, that 

 the atomic weights of the isotopes of an ele- 

 ment were integers, then the non-integral value 

 found by chemical analysis for the atomic 

 weight of an element would result from the 

 element existing as a mixture of its isotopes. 



IV. Isotopes 

 Aston,^° Dempster,^^ and later G. P. Thom- 

 son'- have recently greatly improved Sir J. J. 

 Thomson's methods of positive ray analysis 

 with the result that they have been able to 

 separate many of the elements of non-integral 

 atomic weight such as chlorine, magnesium, 

 argon and mercury into isotopes, each of which 

 has an integral value for its mass. Chlorine, foi' 

 example, has an atomic weight of 35.5 and 

 can be separated by the positive ray method 

 into an isotope of weight 35, and into one of 

 weight 37. The validity of this result has 

 been confirmed by Harkins,^' who succeeded 



i" Aston, Fhil. Mag., Vol. 38, p. 707, 1919 ; 

 Vol. 39, p. 611, 1920; Vol. 40, p. 628, 1920; 

 Nature, March 17, 1921; May 12, 1921. 



11 Dempster, Fhys. Bev., Vol XI, No. 4, p. 816, 

 1918; Science, Dec. 10, 1920; Apr. 15, 1921; 

 Nov. 25, 1921. 



12 G. P. Thomson, Phil. Mag., Aug., 1920, p. 

 240; Phil. Mag., Nov. 1921, p. 858; Nature, Feb. 

 24, 1921. 



13 Hai-kins, Science, March 19, 1920. 



