March 20, 1913] 



NATURE 



57 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



The Radio-Elements and the Periodic Law. 



1 am grateful to Prof. Schuster for the opportunity 

 he has afforded by his letter (Nature, March 13) for 

 the discussion of the wide generalisations that have 

 been made with regard to the position of the radio- 

 elements in the periodic table, consequent on the 

 recent experimental work of A. Fleck and of the 

 theoretical suggestions of A. S. Russell and K. 

 Fajans. The whole question is one in which it is 

 important that there should not be any doubt as to 

 the real nature of the evidence adduced. Prof. 

 Schuster's criticism of my views on the subject could 

 scarcely be more sympathetic or helpful, and can only 

 result in a maturer outlook on this important ques- 

 tion. 



Granting' for the sake of argument the possibility 

 of the existence of groups of elements not necessarily 

 of identical atomic mass, with identical chemical pro- 

 perties and spectra, the only known direct manner 

 in which the existence of the members of these groups 

 could be separately recognised is radio-active evidence, 

 in which one member is formed from another, not 

 directly, but through the intermediary of other 

 elements, possessing, necessarily as now appears, 

 completely different chemical properties. Hence it is 

 natural that at first direct evidence should be confined 

 practically to the subject of radio-activity, and much 

 depends upon whether that evidence is considered real 

 evidence approaching experimental proof, or whether 

 it is regarded as merely negative in character. 



In the first place, I admit when I wrote the expres- 

 sion, "non-separable b;' any known process," I had 

 in mind chemical processes. It is unusual and illus- 

 trative of the peculiarities of the problem that the 

 relatively rough and partial means of physical analysis, 

 to which Prof. Schuster refers, may be expected ulti- 

 mately to succeed where the most refined and delicate 

 methods of chemical analysis may be expected to fail. 

 But so it is, and I agree with Prof.' Schuster that it 

 should ultimately be possible partially to separate by 

 purely physical methods certain members of^ these 

 chemically identical groups bv virtue of the slight 

 differences in their molecular masses. In fact, a year 

 ago I commenced an experiment to try to effect a 

 partial separation of the two uraniums by diffusion 

 in solution. This case is an exceptionally favourable 

 one as an alteration in the relative concentration of 

 the two uraniums by only a few per cent, should be 

 detectable without anv uncertainty bv radio-active 

 methods. 



Although the term "non-separable" I think con- 

 notes present inability, without implying, necessarily, 

 anything as regards what may be possible in the 

 future, I do, however, think that there are good 

 grounds for believing that the chemical non- 

 separabilitv of elements occupying the same place in 

 the periodic table is due to the general character of 

 chemical methods rather than the state of refinement 

 and delicacy attained at any particular time. The 

 chemical analysis of matter has given us the periodic 

 law, and there is no case known of two or more 

 ordinary elements with claims to the same place in 

 the periodic table. In this connection the case of 

 the rare-earth group of elements is necessarily ex- 

 cluded, as these elements certainly do not obev the 

 NO. 2264, VOL. 91] 



law without modification. In all other parts of the 

 table the rule is that there is only one element for 

 each place, and each place signifies a separate chem- 

 ical type differentiated in a regular manner from its 

 neighbours. But now the radio-active series have 

 shown that different elements, not necessarily of 

 identical atomic mass, do occupy the same place, and 

 that when this occurs these elements possess identical 

 chemical nature. It is therefore an inference sup- 

 ported by the known facts of chemical analysis that 

 the single place in the periodic classification represents 

 the limits of the analysis of matter by chemical 

 methods, rather than the ultimate analysis into homo- 

 geneous types, such as is usually implied in the 

 conventional view of elements. 



Prof. Schuster admits that the chemical properties 

 of these non-separable groups of radio-elements are 

 probably more nearly equal than those of the longer- 

 known elements, but claims that there is a vast 

 interval between "very similar" and "identical." 1 

 do not like the term "very similar." It is ambiguous, 

 and may mean nothing more than that the experi- 

 mental examination has been neither skilled nor ex- 

 haustive enough to disclose the differences, if any 

 exist. Unless this is the case, I feel that the proper 

 term to use is " identical." Otherwise the word 

 "identical" ought to be expunged from scientific 

 language altogether. Unless there is some reason to 

 foresee a qualification being required by the further 

 progress of knowledge, a definite statement ought to 

 be preferred in science to an ambiguous onej which 

 on account of its vagueness must necessarily remain 

 true for all time. Scientific statements can only ex- 

 press present knowledge, including in this term 

 reasonable inferences from the whole field of such 

 knowledge. 



The term "chemically identical" has not been ap- 

 plied until after an examination, not, of course, in 

 every case, but in every possible case, and in sufficient 

 numbers of cases to reveal the general law, as skilled 

 and exhaustive as the present art of chemical analysis 

 allows, and, what is equally of importance, by the 

 use of methods for detecting changes in relative con- 

 centration as delicate as any that exist. The example 

 quoted of praseodymium and neodymium ought to be 

 more closely examined. These elements proved to be 

 separable as soon as optical methods of revealing their 

 separate existence became known. In the case of the 

 radio-elements the separate radio-active nature of each 

 individual of the group is exactly known, the propor- 

 tion of each in anv mixture can be quantitatively 

 evaluated. Yet they are non-separable. That some 

 mixture to-day may still be classed as a homogeneous 

 element because no means exist for the separate 

 identification of its components does not affect the 

 fact that some mixtures of elements capable of separate 

 identification are chemically non-separable. 



Difficulties of chemical analysis are often not con- 

 nected with the methods of separation at all, but with 

 the means of determining whether or not a separation 

 has been effected, which, in the case of the difficult 

 rare-earth group are relatively crude and sometimi s 

 misleading. 



The suggestion, that in the disintegration process 

 a mass equal to that of the a particle previously lost 

 mav be picked up, is not a probable one, but even if 

 it is admitted, and it is supposed that parent and 

 product have the same mass, it does not affect the 

 view that they are two absolutely distinct types of 

 matter, disintegrating at different speeds and in cer- 

 tain cases with expulsion of different kinds of rays. 

 The attempt to meet this by supposing that the par- 

 ticular instability which determines their future may 

 depend on their past is equivalent to admitting the 



