September 2, 1909] 



NA TURE 



281 



In 1885 I closed my address with a reference to the 

 structure of the elements which imphed that their behaviour 

 was that of compound substances ; the feeling that this is 

 the case has long been general among chemists. Our pre- 

 sent attitude towards this problem is a curious one and not 

 altogether satisfactory — it is impossible to deny that we have 

 somewhat lost sense of proportion, even if our methods 

 have not savoured of the unscientific. The discovery of 

 radium appears to have upset our balance — w"e have been 

 carried awav by the altogether mysterious and unprecedented 

 behaviour of this weird and wondrous substance. But may 

 we not ask: Is radium an element? Has it not been too 

 generally, too hastily assumed that it is? Little as we 

 know of it, does not its behaviour straightway outclass it 

 as an element? Surely it does ! Is not the established fact 

 that an emanation proceeds from it, which in turn decom- 

 poses and gives rise to helium, a proof of its compound 

 nature? Again, is the evidence of such a character as to 

 justify us in asserting that uranium is the parent of radium? 

 If it be such, must not uranium also disappear from the 

 list of elements ; must it not indeed be removed on the 

 ground that it gives rise to uranium without any reference 

 to its supposed relationship to radium? 



The answers given to such questions must depend on our 

 definition of an element. At present we seem to be with- 

 out one. 



The conception that the breakdown of radium is spon- 

 taneous and apart from all e.\ternal impulse or control is 

 also one which should be received with caution. There is 

 reason to suppose that in all ordinary cases in which com- 

 pounds undergo decomposition spontaneously, the decom- 

 position is conditioned by an impurity ; the effect, moreover, 

 is usually cumulative. This is true of highly explosive sub- 

 stances, such as chloride of nitrogen and gun-cotton, for 

 example. It might be supposed that something similar would 

 happen in the case of radium — but apparently such is not 

 the case ; it is assumed that occasionally a molecule explodes 

 spontaneously, not only without being incited thereto, but 

 also without in any way affecting its neighbours. 



The alternative explanation that radium in some way 

 acts as a receiver, transforming energy from some e.xternal 

 source to which ordinary substances fail to respond and 

 being thereby stimulated to decompose, is at present out of 

 favour, although perhaps more in accordance w'ith its pecu- 

 liar behaviour.^ 



The liberation of helium as a product of radio-active 

 change is in itself a significant fact, in view of the pos- 

 sibility that helium may be an element of intense activity. 

 Nothing in connection with the problem is more surprising, 

 however, than the apparent production, in course of time, 

 of a whole series of degradation products which differ 



initi.1l oxide of a series, as the proportion of oxygen is increased, the acidic 

 qualities are invariably strengthened. 



The choice of a terminal connoting the elementary radicles which would 

 be applicable generally and also acceptable is very dithcult. If usage do not 

 forbid change, probably our ears will decline to allow us to be systematic. 

 The terminal jftvi is not applicable to many present names. In the interest 

 of euphony, exception may be taken to the a'loption oK ion as a final syllable. 

 In English ears most of the words with this ending have an ugly sound if 

 pronounced so as to make it significant ; moreover, our object is to secure a 

 term which is applicable to the elementary material, whatever its state ; 

 the term ion is suggestive of a particular state— a state of chemical activity ; 

 and at present there is no agreement as to the nature of an ion. The terms 

 atom, radicle (simple and compound), ion and molecule now all have their 

 separate meaning and value and are indispensable. 



The only terminal which seems in any way likely to be generally satis- 

 factory in use is the terminal _v/, which is already applied to organic radicles ; 

 its use might well be extended to radicles generally. 



1 I may here put on record the opinion Lord Kelvin expressed on this 

 question in a letter to me dated September 13, 1906 : — 



" Ever since, nearly four years ago, we heard of the hundred calories per 

 hour given out by radium, I have had on my mind the question of some 

 possible mechanism such as that which you suggest by which energy* from 

 surrounding matter (far or near) could automatically come into radium to 

 supply the energy of the heat which it gives out. The more I think of the 

 question the less I see of that possibility. At present I can see nothing else 

 than that the energy given out is taken from a previously e.xisting store of 

 potential energy of rejjulsive force between separable constituents of radium. 



'■The- disintegration of the radium atom ' is wantonly nonsensical. It is 

 nonsense very misleading and mystifying to the general public, because, if 

 what is at present called radium can be broken into parts, it is not an atom. 



"'Energy of an atom' implies a thorough misunderstanding of the 

 meaning of the word energy, which is capacity for doing work. 



" I admire most sincerely and highly the energy of the workers in Radio- 

 activity and the splendid experimental results wliich they have already got 

 by resourceful and inventive experimental skill and laborious devotion. 1 

 feel sure that as things are going on we shall rapidly learn more and more of 

 the real truth about radium." 



greatly in stability — such behaviour is entirely witltout pre- 

 cedent and not at all becoming in eleinents. 



No such remarkable and inspiring problem has ever before 

 been offered for solution. VVe can only wonder at the 

 results and admire the genius which some have displayed 

 in interpreting them, Rutherford in particular. Yet out- 

 siders may well hold judgment in suspense for the present : 

 whilst it is permitted to workers to make use of hypothesis 

 in every possible way in extending inquiry, the public are 

 in no wise called upon to accept such hypothesis as fact. 



But apart from the suggestion that elements may give 

 rise to others spontaneously, we have been entertained of 

 late with stories of elements being converted into others 

 under the influence of the energy let loose by the breakdown 

 of radium. There is reason, however, to suppose that the 

 powers of radium may have been greatly overpainted ; 

 energy of almost any degree of intensity in the form of 

 high-tension electricity is now at our disposal, and the effect 

 which radium produces on living tissues, glass, &c., is of ■ 

 the same character as that effected by the Rbntgen ray 

 discharge, the only difference being that the effect is pro- 

 duced somewhat more rapidly ; it is not to be imagined, 

 therefore, that the discovery of radium has put anv very 

 novel intensity of power into our hatids. 



I pass to the consideration of the classification of the 

 elements. The recognition of certain properties, the asso- 

 ciation of certain ideals with the several elements, is a 

 necessary step in classifying the elements in accordance with 

 Mendelejeff's great generalisation — or rather it may be said 

 to be both involved in and an outcome of Mendelejeff's 

 conception. 



Until recently our difficulty was to understand the rela- 

 tionship of the metallic and the non-metallic elements ; now 

 we are confronted with another problem — that of the exist- 

 ence of inert " paraffinoid " elements. It is commonly- 

 assumed that these are monatomic, but the evidence on 

 which this assumption is based is absolutely unconvincing, 

 and would be generally admitted to be so were we in the 

 habit of looking before we leapt to conclusions. Assuming 

 that the elements are compounds, the formation of inert 

 compounds does not appear to be out of place, in view of 

 the existence of practically inert hydrocarbons. But, on the 

 other hand, in view of the properties of nitrogen, which is 

 one of the most active of substances in the monatomic 

 state, although an inert gas in the diatomic condition, it 

 may well be that the inertness of helium and the other 

 members of the argon group is also simulated. .Sir James 

 Dewar's observations have shown that helium and charcoal 

 have no inconsiderable affinity at the boiling point of the 

 former, which is within five degrees of the absolute zero, 

 the molecular heat of absorption (apart from that due to 

 liquefaction) of helium at that temperature being apparently 

 as high as about sixty calories. The proof he has also 

 given that helium alone does not convey an electric dis- 

 charge is also of significance since the passage of a dis- 

 charge through it under ordinary conditions is an indication 

 that it can be included with other substances in a conducting 

 system. Such evidence as there is therefore points to the 

 elements under discussion being different from the others: 

 only in the degree of stability of their molecules. 



Of late years the difficulty of classifying the elements has 

 been increased rather than diminished, not merely because 

 of the discovery of the inert gases but also on account of 

 the apparent impossibility of ordering the position of an 

 element such as tellurium in accordance with its atomic 

 weight. There appears to be little room left for doubt that 

 the value cannot be far removed from that of iodine ; it 

 should be considerably lower. It may be pointed out that 

 the accepted value of selenium is closer to that of bromine 

 than would be expected if a relationship were maintained 

 corresponding to that between chlorine and sulphur. It 

 would seem that Mendelejeff's original conception of the 

 elements as a simple series in which the properties are 

 periodic functions of the atomic weights must be abandoned 

 in favour of some more comprehensive scheme. From the 

 chemist's point of view, it is impossible to abandon the 

 guiding principle underlying the arrangement in family 

 groups, which dates back to Dumas ; perhaps insufficient 

 attention has been paid in the past to the maintenance of 

 this principle. 



NO. 2079, VOL. 81] 



