264 



NATURE 



[April 29 j 1920 



The Standard of Atomic Weights. 



,Jt is With considerable surprise, as a chemist, that 

 t [see, in Nature of April 22, p. 230, arguments as to 

 tTtie structure of atoms based on the deviations of the 

 atpftiic weights of elements from whole numbers on 

 Pie. standard 0=i6oo The reasons for the use of 

 this arbitrary and inconvenient standard are now 

 niLatters of ancient history, and the values^ of Stas, 

 \yhich \vere regarded as fundamental at the tirne when 

 tjijie standard was adopted, have now been shown by 

 fnany independent lines of experiment to be inexact. 

 It' is almost pathetic to observe modern experimenters 

 wlio have determined equivalents by- the accurate 

 9H?^lysjs of hydrogen compounds, such as hydrogen 

 chloride, methane, hydrogen bromide, ammonia, etc., 

 all. of which are more easily obtained in a state of 

 purity, and analysed, than oxygen compounds, com- 

 pelled to multiply their results by i-oo8 in order to 

 bring them into line with the standard of = 1600. 

 ■, A glance at the International Table of Atomic 

 Weights will show that very few of the elements form 

 ojjygen compounds suitable for analysis, and the state- 

 ment to the contrary, found in most elementary text- 

 books, is clearly inaccurate. A great number of 

 equivalents, on the contrary, have been referred to 

 Ag= 107-88. This number can be brought into ratio 

 with oxygen only through the intermediate link of 

 jTLitrogen, the atomic weight of which has been most 

 accurately determined by the analysis of ammonia. 

 Th^ latter involves the ratio i-oo8 to get the ratio to 

 0=16-00. But the atomic weight of chlorine has been 

 most accurately determined directly to H = i-oo, and 

 the ratio Ag : CI is also very accurately known. From 

 hydrogen to chlorine, from chlorine to silver, and from 

 silver to a large number of other elements seems to 

 bfe the most natural proceeding. Oxygen then comes 

 in from the ratio H : O found, by Morley, Scott, and 

 Burt and Edgar. This is now probably one of the 

 most accurately known atomic weights. The above is 

 one instance -only of the extraordinary branch-chain 

 methods now necessary in order to get the experi- 

 mental numbers referred to oxygen. 



On the theoretical side the advantages of the 

 hydrogen standard are self-evident. No one has ever 

 pretended that the adoption of oxygen as the unit 

 has any theoretical significance ; the retention of the 

 number = i6oo alone is sufficient to prove this. The 

 accumulating evidence on the physical side, such as 

 atomic numbers, the structure of atomic nuclei, the 

 periodic law, and the like, all points unmistakably to 

 the mass of the hydrogen atom as the natural 

 standard. It is no longer correct to say, as is still 

 done in elementary books, and even in other quarters, 

 that the standard of atomic weights is a matter of 

 indifference, and that, apart from experimental 

 reasons, one element is as good as another. We have 

 almost certain evidence that the hydrogen nucleus is 

 a fundamental constituent of all atoms. Prout's 

 hypothesis being thus reinstated, there can be no 

 doubt as to the suitable standard of atomic weights, 

 and Dalton's choice has had a most remarkable vin- 

 dication. 



When, therefore, arguments are advanced based on 

 the standard = i6-oo, it seems time to suggest that 

 some steps should be taken to put an end to the pre- 

 vailing confusion. Physicists have never taken 

 kindly to the oxygen standard, and there is no 

 longer anv reason why chemists should be given need- 

 less trouble. I have, in my elementary lectures, 

 made a practice of using the hydrogen standard, and 

 thus avoiding all the confusion in connection with 

 vapour densities, etc., which comes in with the other 

 system. 



There is on^ other point which seems to me of 



NO. 2635, VOL. 105] 



importance. On the oxygen scale the atomic weights 

 of a number of elements differ by about half a unit 

 from whole numbers. It has been conjectured that 

 these elements are mixtures of isotopes, with atomic 

 weights which are whole numbers. But if there is 

 anything in the theory of isotopes to justify this, it 

 can only rest on the hydrogen nucleus, and the atomic 

 weight of hydrogen should be taken as unity. If this 

 is done, it is found that the suspected elements are 

 replaced by those not at present under any clouds of 

 suspicion. The following table will illustrate this 

 point : 



Atomic Weight Atomic We'ght 



Element 0=i6-co H^foo 



Chlorine 3545 35-i8 



Magnesuim ... 24-32 24-14 



Silicon 28-3 28-1 



Zinc 65-37 64-88 



Copper 63-57 63-10 



It may be that there is some real physical reason 

 for taking 6=1600, and then supposing that, if some 

 elements deviate from the whole number on this 

 basis, they must be mixtures of isotopes, but this 

 reason has so far escaped my attention. 



There seems to me to be a good case for the Com- 

 mittee on Atomic Weights to consider whether the 

 unit 0=i6-oo, adopted largely on account of the per- 

 sistence of Ostwald, is any longer necessary. At best 

 it was a temporary decision, and all the reasons which 

 were advanced in its favour have now lost their force. 

 I am convinced that the arguments in favour of a 

 return to Dalton's unit are so cogent that, once they 

 are clearly realised, they will be admitted. 



J. R. Partington. 

 East London College (University of London), 

 April 23. 



Mortlakes as a Cause of River-windings. 



MoRTLAKE on the Thames has a place-name which 

 not only accords with the natural history of the place, 

 but also supplies a word which might conveniently 

 be brought into common use to signify a process 

 which plays an important part in the development 

 of every river system, just as the River Meander sup- 

 plies a word to signify the windings of any river. 

 The area between Barnes and the Thames was 

 formerlv an island m the river (Fig. i), formed by a 



Fig. I. 



division of the stream into a northern and a southern 

 arm reunited at the down-stream ends. The southern 

 arm is now incomplete ; the part of it remaining is 

 included in the line of the Beverley Brook, which, 

 having come from the south, turns to the east round 

 a bold curve and joins the Thames. This leaves a 

 gap between the convexity of the curve and the river 

 at the point where it previously divided. This gap 

 no\V forms the isthmus of a peninsula into which 

 the island has been converted by the partial efTace- 

 ment of the southern arm of the divided river. Here 

 Mortlake stands. It is on or near to the former line 

 of the stream which has been in part effaced. This 

 part has become a dead stream — a mortlake, the word 

 "lake" having been used in the Middle English sense 



