September 20, 1906] 



NA TURE 



517 



i>\\x-s at least as much to the chemist as to the 

 physicist. Prof. .Armstrong is almost alone among 

 chemists, as Lord Kelvin is among physicists, in his 

 hostility to the new doctrines. 



Mr. Strutt in two letters (.August 9 and 21) asked 

 wh.it became of the heat generated by the radium 

 .ulinitted to be present in the earth, and recalled the 

 iiKU'pendent evidence of several workers of the con- 

 tinuous renewal of helium from radium. Sir Oliver 

 Lodge directed attention to the magnetic deflection 

 of tlie o-particle as evidence that material particles 

 .ire e.xpellcd from radium, and in his letter laid 

 perhaps undue weight on the evidence, which is still 

 far from complete, that the a-particle is an atom of 

 helium. The vagueness of this argument, and the 

 fact that the letter raised a doubt whether Lord 

 Kelvin had sufficiently examined the published 

 evidence, a doubt which Lord Kelvin himself promptly 

 dispelled, was the subject of a leading article in the 

 Times of August iS. The writer of that article 

 attacked the evidence for the production of helium 

 from radium, using some well-known arguments. The 

 minute quantitv of emanation was graphically likened 

 10 a bubble rising through a glass of whiskey and 

 soda, and it was held that the results were vitiated by 

 the well-known changes the spectra of gases undergo 

 under the prolonged action of the current, due to 

 occlusion by the electrodes and selective conduction 

 rather than to any transmutation. It may be here 

 remarked that the same arguments were set forth in 

 full by Himstedt and Meyer as a preliminary to their 

 experimental examination of the question, yet 

 Himstedt and Meyer, as the result of their own ex- 

 periments, were finally forced to the conclusion that 

 helium is in fact produced from radium. 



Lord Kelvin in his replies (.August 20 and 24) made 

 it clear that he accepted as a fact the continuous 

 evolution of helium from radium, and this admission 

 narrowed very much the issue involved. In reply to 

 a statement of Strutt that if all the helium is re- 

 moved from radium after an interval a further supply 

 can be extracted. Lord Kelvin remarked simply that 

 the if of the statement was wrong. This point was 

 dealt with by the present writer (.August 31), who con- 

 sidered the argument could be definitely answered. 

 For helium is produced from the emanation of radium, 

 about which no question of its being reallv repro- 

 duced can exist. For the removal of the emanation 

 i>. marked by changes in the radio-activitv, notably 

 by the /3 rays, which vanish when the emanation is 

 removed. The recovery of the radio-activity occurs 

 at .a definite rate, and is concomitant to the reproduc- 

 tion of emanation, which can at any time be again 

 extracted as before. As there is no question of the 

 radium creating helium, the only point open for argu- 

 ment is the exact character of the decomposition" by 

 which it and the emanation which gives rise to it 

 are formed. .As there was no further replv to this 

 criticism, it may be taken that the main point of the 

 disintegration theory, that there is a continuous 

 change in the radio-active matter accompanying the 

 radio-activity, is unanswerable. 



On the important question as to the character of 

 the decomposition by which the helium is formed. 

 Lord Kelvin in his later letters favoured a view very 

 different from that of mere occlusion, which the 

 original analogy to cleveite suggested. He quoted a 

 statement of Prof. Rutherford in favour of regarding 

 radium as a chemical compound of helium and other 

 elements, and suggested that radium might be made 

 up of one atom of (?) lead and four of" helium. In 

 a final letter (September 4) Sir Oliver Lodge pointed 

 out that this was the key of the position. Is radium 

 NO._ 1925, VOL. 74] 



a compound or an element? It is satisfactory that, 

 after so much fencing with the question, so simple 

 an alternative has been arrived at. Perhaps the most 

 significant thing about the view that radium is a 

 compound is the silence of the chemists. Surely a 

 chemist might fairly be supposed to know whether a 

 change is what is called a chemical change or not, 

 and the fact that it has been left to a physicist to 

 adopt this view seems fair comment. Not even Prof. 

 Armstrong has yet accepted it. 



On the second point of his challenge, the denial 

 that the heat of the earth is due to radium. Lord 

 Kelvin naturally had an easier task, for matters con- 

 nected with the interior of the earth must necessarily 

 remain speculative. If radium did not decompose 

 under the conditions prevailing in the interior it 

 would emit no heat, and would not tend to diminish 

 in quantity, accounting perhaps, although witli some 

 difficulty in view of the wide distribution of radium 

 in surface rocks, for the continued existence of the 

 substance at the present time. Mr. A. S. Eve, in 

 a vigorous letter (August 28), stated that he had 

 confirmed the estimate of Mr. Strutt of the amount 

 of radium in the earth's crust by a new method, in 

 which the penetrating radiation from the earth's 

 surface was used as the basis of measurement. 

 .Although, of course, in view of the evidence of the 

 independence of radio-active changes upon their 

 environment, it is more of an assumption to suppose 

 that in the interior of the earth radium does not 

 decompose than to take the opposite view, yet clearly 

 here, at any rate, there is plenty of room for legiti- 

 mate differences of opinion. On the other hand, even 

 the opponents of Mr. Strutt's view cannot deny the 

 potentialities of radio-activity, and the part it might 

 play in cosmical processes under favourable conditions. 



The theorv that radium is a compound, waiving 

 the qualification chemical, will no doubt serve 

 sufficiently well for the present as a point of common 

 agreement. As Sir Oliver Lodge remarked, there is 

 no necessity that the question be settled offhand. .As 

 a stepping-stone to further conclusions, it offers 

 advantages to the conservative and cautious. It ex- 

 presses a bare minimum of established fact which 

 even the most sceptical are unable to invalidate. 

 This minimum, briefly stated, is that radium is under- 

 going a continuous change intimately connected with 

 its radio-activity, and that in this change helium is 

 produced, and an enormous but definite amount of 

 energy liberated. Whether anything more is known 

 about transmutation now than formerly, whether 

 lead could change into gold or gold into silver with 

 an emission of energv similar to that evolved 

 from radium, whether this or similar energy plays 

 the large share that has been attributed to it 

 in cosmical processes, are questions which may be 

 legitimately discussed and left open, if only for the 

 reason that thev are far from decided. They are all 

 admittedly steps into the region of hypothesis. 



But what a miserable fraction, even of the known 

 facts, this minimum is ! Ostensibly an explanation 

 of radio-activity, it begins and ends with the fact of 

 the gradual evolution of helium from radium. The 

 numerous other products of radium, the volatile 

 emanation and its non-volatile products, known by 

 their characteristic radio-activity, much as minute 

 quantities of ordinary gases and solids are known by 

 their characteristic spectra, the slower changing later 

 products, of which polonium is one, and is chemically 

 as reminiscent of tellurium as its parent is of barium, 

 remain still to be systematically accounted for. On 

 the important subject of the nature of the a, 0, and 

 y rays, and their origin, the view is silent! The fact 



