DISCOVERY 



203 



properties, and, with the exception of atomic weight 

 and radio-active properties, the same physical proper- 

 ties. The names of these elements, their radio-active 

 properties, and their atomic weight are given in Table I. 



Nature of Radio-activity. 



In Table II is given a complete list of the elements 

 which Dr. Aston has investigated so far, with their 

 atomic weights and numbers, their isotoi>cs and their 

 atomic weights. 



One may prepare almost pure thorium D in quantity 

 by working up a mineral containing thorium and no 

 uranium, and pure radium G from a mineral containing 

 uranium and no thorium. Each of these substances 

 will seem to be the ordinary element lead in appearance, 

 spectrum, boiling and freezing points, electric con- 

 ductivity, chemical properties, etc., yet one has an 

 atomic weight of two units greater than the other, and 

 each has one unit different from that of lead.' 



Since the conclusion of the war experimental work 

 on isotopes has been mainly confined to the lighter 

 elements. It has been very successful and it is still in 

 progress. The method of investigation is one worked 

 out originally by Sir J. J. Thomson, developed by 

 Dr. F. W. Aston, and known as positive-ray analysis. 

 The essentials of this method are to generate positive 

 rays (which are positively-charged atoms and mole- 

 cules of a gas) in a partially evacuated discharge tube, 

 and then by means of combined electric and magnetic 

 forces to bring these rays to a kind of focus on a photo- 

 graphic plate. If this be done with oxygen gas one 

 line only appears on the plate, and the deduction from 

 that is that oxygen is a single element and not a mixture 

 of two or more isotopes. If chlorine be investigated in 

 a similar way two lines appear on the photographic 

 plate, one four or five times more intense than the 

 other. The interpretation of this is that chlorine is a 

 mixture of two isotopes in a ratio of approximately 4 to 

 I, and from the position of the lines it may be further 

 deduced that the atomic weights of these are exactly 35 

 and 37. Helium, carbon, nitrogen, phosphorus, sulphur, 

 and others are shown by the same method to be " per- 

 fectly pure " elements, that is to say, not mixtures of 

 isotopes. Lithium, neon, silicon, argon, bromine, on the 

 other hand, are " mixed " elements. Like chlorine each 

 has two isotopes. The rare gas .xenon which is found 

 in the atmosphere has five and possibly seven isotopes, 

 and kry^pton, another rare gas, has certainly six. 



1 The chief workers in this field have been Professor Richards 

 of Harvard and Dr. Honigschmid of Vienna 



TABLE II 



This table summarises effectively the present state 

 of knowledge on this subject. Attention may be called 

 to two things. The first is that in every case but one 

 the atomic weights of isotopes are whole numbers, not 

 merely approximately but accurately so. This fact 

 suggests at once that those elements in nature whose 

 atomic weights are not whole numbers, nickel, for ex- 

 ample, are really mixtures of isotopes. 58'68 is a frac- 

 tion not because that is accurately the atomic weight 

 of a particular element, but because that element con- 

 sists of two separate and distinct elements, of atomic 

 weights (accurately) 58 and 60 respectively, mixed in 

 such a proportion that the mean value of the atomic 

 weight happens to be sS'GS ; so for the others. In 

 every case so far investigated (again with the exception 

 of hydrogen) this has proved to be true, and the 

 probability that it holds throughout is very great. 

 The second is a hypothesis which receives a large 

 measure of probability from the above. It is that 

 all the chemical elements are built up of different 

 quantities of the same elementary stuff or stuffs. These 

 are believed to be the elements hydrogen and helium. 

 Whether this hypothesis be true or not is a matter for 

 the future. The experimental attack on the problem 

 has but recently begun. 



In closing, one may point out how theories about the 

 chemical elements have alternated between simplicity 

 and complexity in the last hundred years. When 



