May 12, 1921] 



NATURE 



337 



exactly. Now the accepted atomic weight of 

 argon is less than 40, so the presence of a lighter 

 isotope was suggested. This was found at 36, and 

 has now been fully substantiated ; its presence to 

 the extent of about 3 per cent, is sufficient to 

 account for the mean atomic weight obtained by 

 density determinations. 



The elements hydrogen and helium presented 

 peculiar difficulties, as their lines were too far 

 removed from the reference lines for direct com- 

 parison. By means of a special "bracketing" 

 method, moderately accurate values were obtained. 

 Helium appears to be exactly 4 on the oxygen 

 scale, but hydrogen is definitely greater than 

 unity. The value obtained agrees very well with 

 that already arrived at by chemical methods — 

 namely, 1008. At the same time, measurements 

 of the 3 line, first observed by Sir J. J. Thomson, 

 were made which came out at 3024, satisfactorily 

 proving it to be due to triatomic hydrogen. 



Krypton and xenon gave surprisingly complex 

 results, the former consisting of six isotopes 78, 

 80, 82, 83, 84, 86. The weights of these could be 

 determined with great accuracy by means of the 

 excellent second- and third-order lines they gave. 

 The first experiments with xenon led to the ob- 

 servation of five isotopes, the provisional values 

 of which were given as one unit too low. Owing 

 to the kindness of Prof. Travers and Dr. Masson, 

 I have recently been enabled to repeat the analysis 

 with gas much richer in xenon. With this the 

 second-order lines could be observed and 

 measured. The five principal isotopes of xenon 

 are 129, 131, 132, 134, 136; there is apparently 

 a faint sixth component at 128, and a doubtful 

 seventh at 130. 



Experiments with boron fluoride indicated that 

 boron has at least two isotopes, 10 and 11, and 

 that fluorine is a simple element of atomic weight 



19- . . 



Silicon is another unmistakably complex 

 element having two isotopes, 28 and 29, with a 

 possible additional one, 30. 



Bromine was of great interest. As it has an 

 atomic weight almost exactly 80, it might reason- 

 ably be expected to be simple and an isobare of 

 one of the kryptons ; actually it consists of equal 

 parts of 79 and 81. 



Sulphur, phosphorus, and arsenic are all appar- 

 ently simple elements. Mercury is certainly com- 

 plex, though its closer components cannot be re- 

 solved with the present apparatus. Its very char- 

 acteristic groups are seen as high as the fifth 

 order, and appear on nearly all the spectra taken. 

 The group consists of a continuous succession of 

 lines forming a band 197 to 200, a strong line at 

 202, and a weak one at 204. Recently at Copen- 

 hagen Bronsted and Hevesy have succeeded in 

 partially separating the isotopes of mercury by a 

 fractional distillation at extremely low pressure. 

 They give as their figures for the densities com- 

 pared with normal mercury as unity : — 



Condensed mercury ... 

 Residual mercury 

 NO. 2689, VOL. J07] 



o- 999980 

 I 00003 I 



The probable error claimed is less than one part in 

 a million. 



Selenium, tellurium, antimony, and tin have all 

 been used in the discharge tube, with no results 

 of any value. This is unfortunate, for the atomic 

 weight of selenium, 792, suggests that one of 

 its isotopes must be an isobare of bromine or 

 krypton ; also the relation between tellurium and 

 iodine is of great interest. 



Iodine, fortunately, gave a very definite result. 

 It is a simple element of atomic weight 127. This 

 is rather surprising, for all the theoretical papers 

 on the isotopic constitution of elements have pre- 

 dicted a complex iodine. Prophecy in physics 

 becomes a difficult trade when experimental results 

 produce these surprises, and apparently the only 

 really trustworthy prediction is that there are 

 plenty more in store for us. 



The following is a list of elements and isotopes 

 determined to date : — 



Table of Elements and Isotopes. 



By far the most important result of these 

 measurements is that, with the exception of 

 hydrogen, the weights of the atoms of all the 

 elements measured, and, therefore, almost cer- 

 tainly of all elements, are whole numbers to the 

 accuracy of experiment — namely, about one part 

 in a thousand. Of course, the error expressed in 

 fractions of a unit increases with the weight 

 measured, but with the lighter elements the diver- 

 gence from the whole-number rule is extremely 

 small. 



This enables the most sweeping simplifications 

 to be made in our ideas of mass. The original 

 hvpothesis of Prout, put forward in 181 5, that 

 ail atoms were themselves built of atoms of pro- 

 tyle, a hypothetical element which he tried to 

 identify with hydrogen, is now re-established, 

 with the modification that the primordial atoms 

 are of two kinds — atoms of positive and negative 

 electricity. 



Although the latter unit has longi been known 



