April \, 1889] 



NATURE 



543 



reached. This constancy of atomic weight, however, only 

 proves that the original body has been split up by the 

 fractionating process into two molecular groupings capable 

 of resisting further decomposition by that identical pro- 

 cess ; but these groupings are not unlikely to break up 

 when a different fractionating process is brought to bear 

 on them, as I found in the separation of didymium and 

 samarium when using dilute ammonia as the fractionating 

 precipitant. In my paper on "Radiant Matter Spectro- 

 scopy" I said ("Part II., Samarium," Phil. Trans. Roy. 

 Soc, Par: 2, p. 129, June 18, 1885) :—" After a time a 

 balance seems to be established between the affinities at 

 work, when the earth would appear in the same propor- 

 tion in the precipitate and the solution. At this stage they 

 were thrown down by ammonia, and the precipitated 

 earths set aside to be worked up by the fusion of their 

 anhydrous nitrates so as to alter the ratio between them, 

 when fractionation by ammonia could be again em- 

 ployed." 



It is obvious that when the balance of affinities here 

 spoken of was reached, the atomic weight of the mixture 

 under treatment would have become constant, and no 

 further fractionation would have caused the atomic weight 

 to alter. 



Atomic weight determinations are valuable in telling 

 when the fractionating operation in use has effected all 

 the separation it can : at this point it becomes constant. 

 The true inference is, not that a new earth has been 

 obtained, but simply that the fractionating operation 

 requires changing for another, which will cleave the 

 group of meta-elements in a different direction. 



Meantime, I have kept strictly in view the question. 

 What is an element, and how shall it be recognized when 

 met.? 



On this subject I beg to submit the following con- 

 siderations, which, primarily referring to didymium, may 

 at any moment apply to other cases : — 



Neodymium and praseodymium are simply the products 

 into which didymium is split up by one particular method 

 of attack. 



It must be remembered that a single operation, be it 

 crystallization, precipitation, fusion, partial solution, &c., 

 can only separate a mixture of several bodies into two 

 parts, just as the addition of a reagent only divides a 

 mixture into two portions, a precipitate and a solution, 

 and these divisions will be effected on different lines 

 according to the reagent employed. We add, e.g., am- 

 monia to a mixture, and at once get a separation into two 

 parts. Or we add, say, oxalic acid to the same original 

 solution, and we then split up the mixture into two other 

 parts differently arranged. 



Thus by crj'stallizing didymium nitrate (in Auer's way) 

 we divide the components into two parts. By fusing 

 didymium nitrate we divide its components in a different 

 way ; but so long as different methods of attack split up 

 a body differently, it is evident that we have not yet got 

 down to " bed rock." 



Further, a compound molecule may easily act as an 

 element. Take the case of didymium, which is certainly 

 a compound, whether the products of Auer's operation be 

 final or not. Didymium has a definite atomic weight ; it 

 has well-defined salts, and has been subjected to the 

 closest scrutiny by some of the ablest chemists in the 

 world. I refer particularly to C16ve's classical memoir. 

 Still the compound molecule known as didymium was 

 too firmly held together to act otherwise than as an 

 element, and as a seeming element it emerged from every 

 trial. The simple operations to which it had been sub- 

 mitted in the preparation of its salts, and in its purifica- 

 tion from other compound molecules, such as samarium 

 and lanthanum, were not sufficient to split it up further. 

 But subjected to a new method of attack it decomposes 

 at once. 



We have, in fact, a certain number of reagents, opera- 



tions, processes, &c., in use. If a body resists all these, 

 and behaves otherwise as a simple substance, we are apt 

 to take it at its own valuation and to call it an element. 

 But for all that it may, as we see, be compound, and as 

 soon as a new and appropriate method of attack is de- 

 vised, we find it can be split up with comparative ease. 

 Still, we must never forget that, however complex, it 

 can hardly be resolved into more than two parts at one 

 operation.' 



From considerations above laid down I do not feel ii> 

 a position to recognize neodymium and praseodymium as- 

 elements. We need some criterion for an element which 

 shall appeal to our reason more clearly than the old 

 untrustworthy characteristic of having not as yet been 

 decomposed ; and to this point I must beg to call the 

 special attention of my colleagues. It may be that what- 

 ever body gives only one absorption band is an element,, 

 but we cannot conversely say that an element may be 

 known by its giving only one absorption band, since most 

 of our elements give no bands at all ! 



Until these important and difficult questions can be 

 decided, I have prefeiTcd to open what may be figura- 

 tively called a suspense account, wherein, as I have 

 previously suggested, we may provisionally enter all these 

 doubtful bodies as " meta-elements." 



But these meta-elements may have more than a mere 

 provisional value. Besides compounds, we have hitherto^ 

 recognized merely ultimate atoms or the aggregations of 

 such atoms into simple molecules. But it becomes more 

 and more probable that between the atom and the com- 

 pound we have a gradation of molecules of different 

 ranks, which, as we have seen, may pass for simple ele- 

 mentary bodies. It might be the easier plan, so soon as 

 a constituent of these earths can be found to be chemically 

 and spectroscopically distinguishable from its next of 

 kin, to give it a name and to claim for it elemental rank \ 

 but it seems to me the duty of a man of science to treat 

 every subject, not in the manner which may earn for him 

 the greatest temporary /cCSor, but in that which will be of 

 most service to science. 



If the study of the rare earths leads us to clearer views 

 on the nature of the elements, neither my colleagues nor 

 myself will, I am sure, regret the months spend in tedious 

 and apparently wearisome fractionations. No one can 

 be more conscious than myself how much ground is yet 

 uncovered, and how many radical questions have received 

 but very inadequate answers. But we can only work on, 

 " unresting, unhasting," trusting that in the end our work 

 will throw some white light upon this deeply interesting 

 department of chemical physics. 



NOTES. 



A MOST important and profoundly interesting letter from Mr. 

 Stanley to the Chairman of the Emin Pasha Relief Committee 

 has been published this week. It is dated Bungangeta Island, 

 Ituri River, or Aruwimi River, August 28, 1888, and record* 

 the adventures of the Expedition from June 28, 1887, until 

 the time when the letter was written. There is no more stirrii^ 

 tale even in the long and romantic history of African explora- 

 tion. On April 29, 1888, Mr. Stanley met Emin Pasha on ttie 

 shores of the Albert Nyanza Lake, and it would be impossible 

 to over-rate the courage, energy, and resource manifested bj 

 the great traveller in grappling with the terrible difficulties which 

 had stood in his way. Having spent some time with Emin, 

 Mr. Stanley returned to the Aruwimi River, and reorganized what 

 remained of Major Barttelot's force. When the letter was. 

 despatched he was on the point of starting again for the Albert 

 Nyanza, and we may have to wait SDnie time for further in- 

 telligence. Next week we hope to give an account of the 

 geographical results of the Expedition, so far as they are now 

 known. 



