September 23, 1921] 



SCIENCE 



261 



other original source it is possible that other 

 values of atomic weight would be obtained, 

 exactly as in the case of lead in which the 

 existence of isotopes in the metal found in 

 various radioactive minerals was first con- 

 clusively established. 



ATgon, which has an atomic weight of 

 39.88, was found to consist mainly of an 

 atomic weight of 40, associated to the extent 

 of about 3 per cent, with an isotope of atomic 

 weight 36. Krypton and xenon are far more 

 complex. The former would appear to consist 

 of six isotopes, 78, 80, 82, 83, 84, 86; the 

 latter of five isotopes, 129, 131, 132, 134, 136. 



Fluorine is a simple element of atomic 

 weight 19. Bromine consists of equal quanti- 

 ties of two isotopes, Y9 and 81. Iodine, on 

 the contrary, would appear to be a simple 

 element of atomic weight 127. The case of 

 tellurium is of special interest in view of its 

 periodic relation to iodine, but the results of 

 its examination up to the present are indefi- 

 nite. 



Boron and silicon are complex elements, 

 each consisting of two isotopes, 10 and 11, and 

 28 and 29, respectively. 



Sulphur, phosphorus, and arsenic are ap- 

 parently simple elements. Their accepted 

 atomic weights are practically integers. 



All this work is so recent that there has 

 been little opportunity, as yet, of extending it 

 to any considerable number of the metallic 

 elements. These, as will be obvious from the 

 nature of the methods employed, present 

 special difficulties. It is, however, highly 

 probable that mercury is a mixed element 

 consisting of many isotopes. These have been 

 partially separated by Bronsted anfl Hervesy 

 by fractional distillation at very low pres- 

 sures, and have been shown to vary very slight- 

 ly in density. Lithium is found to consist of 

 two isotopes, 6 and 7. Sodium is simple, 

 potassium and rubidium are complex, each of 

 the two latter elements consisting, apparently, 

 of two isotopes. The accepted atomic weight 

 of caesium, 132.81, would indicate complexity, 

 but the mass spectrum shows only one line 

 at 133. Should this be confirmed caesium 

 would afford an excellent test case. The ac- 



cepted value for the atomic weight is suf- 

 ficiently far removed from a whole number 

 to render further investigation desirable. 



This imperfect summary of Mr. Aston's 

 work is mainly based upon the account he 

 recently gave to the Chemical Society. At 

 the close of his lecture he pointed out the 

 significance of the results in relation to the 

 Periodic Law. It is clear that the order of 

 the chemical or " mean " atomic weights in 

 the periodic table has no practical signifi- 

 cance; anomalous cases such as argon and 

 potassium are simply due to the relative pro- 

 portions of their heavier and lighter isotopes. 

 This does not necessarily invalidate or even 

 weaken the periodic law which still remains 

 the expression of a great natural truth. That 

 the expression as Mendeleeff left it is im- 

 perfect has long been recognized. The new 

 light we have now gained has gone far to clear 

 up much that was anomalous, especially Mose- 

 ley's discovery that the real sequence is the 

 atomic number, not the atomic weight. This 

 is one more illustration of the fact that sci- 

 ence advances by additions to its beliefs 

 rather than by fundamental or revolutionary 

 changes in them. 



The bearing of the electronic theory of 

 matter, too, on Prout's discarded hypothesis 

 that the atoms of all elements were them- 

 selves built up of a primordial atom — ^his 

 protyle which he regarded as probably identi- 

 cal with hydrogen — is too obvious to need 

 pointing out. In a sense Prout's hypothesis 

 may be said to be now reestablished, but with 

 this essential modification — the primordial 

 atoms he imagined are complex and are of 

 two kinds — atoms of positive and negative 

 electricity — respectively known as protons and 

 electrons. These, in Mr. Aston's words, are 

 the standard bricks that nature employs in 

 her operations of element building. 



The true value of any theory consists in 

 its comprehensiveness and sufficiency. As ap- 

 plied to chemistry, this theory of " the inner 

 mechanism of the atom " must explain all its 

 phenomena. We owe to Sir J. J. Thomson 

 its extension to the explanation of the periodic 

 law, the atomic number of an element, and 



