July 19, 19 17] 



NATURE 



417 



state. Thus all acids owe their common acidic quality 

 to really free hydrogen, the hydrogen ion, a particle 

 more different from the hydrogen atom than the atom 

 is from the hydrogen molecule. Positive elec- 

 tricity is thus emphatically not the mere absence 

 of electricity, and any electrical theory of matter 

 purporting to explain matter in terms of elec- 

 tricity does so by the palpable sophistry of calling 

 tu'o fundamentally different things by the same name. 

 The dualism remains, whether you speak of matter 

 and electricity, or of positive and negative electricity, 

 and the chemist would do well to stick to his concep- 

 tion of matter until the physicist has got a new name 

 for positive electricity which will not confuse it with 

 the only kind of electricity that can exist apart from 

 matter. 



On the other hand, the theory of the electromagnetic 

 origin of mass or inertia is a true monism. It tries 

 to explain consistentlv two things — the inertia of the 

 electron and the inertia of matter — by the same cause. 

 The inertia of the former being accounted for by the 

 well-known electromagnetic principles of Faraday, by 

 the assumption that the charge on the electron is con- 

 centrated into a sphere of appropriate radius, the 

 two thousand-fold greater inertia of the hydrogen ion, 

 for example, can be accounted for by shrinking the 

 sphere to one two-thousandth of the electronic radius. 



But the electrical dualism remains completely un- 

 explained. Call the electron E and the hydrogen ion 

 H. The facts are that two E's repel one another with 

 the same force and according to the same law as two 

 H's repel each other, or as an H attracts an ^. These 

 very remarkable properties of H and E are not ex- 

 plained by the explanation of the inertia. Are E and 

 H made up of the same stuff or of two different stuffs? 

 We do not know, and certainly have no good reason to 

 assume that matter minus its electrons is made of the 

 same thing as the electron. We have still to reckon 

 with two different things. 



The Chemical Elements not necessarily Homogeneous. 



I pass now to the second and most novel sense in 

 which the elements, or some of them at least, are 

 complex. In their discovery of new radio-active 

 elements M. and Mme. Curie used radio-activity as a 

 method of chemical analysis precisely as Bunsen and 

 Kirchhoff, and later Sir William Crookes, used spec- 

 trum analysis to discover caesium and rubidium, and 

 thallium. The new method yielded at once, from 

 uranium minerals, three new radio-elements — radium, 

 polonium, and actinium. According to the theory of 

 Sir Ernest Rutherford and myself, these elements are 

 intermediate members in a long sequence of changes 

 of the parent element uranium. In a mineral the various 

 members of the series must co-exist in equilibrium, 

 provided none succeed in escaping from the mineral, 

 m quantities inversely proportional to their respective 

 rates of change, or directly proportional to their 

 periods of average life. Radium changes sufficiently 

 slowly to accumulate in small but ponderable quantity 

 in a uranium mineral, and so it was shown to be a 

 new member of the alkaline-earth family of elements, 

 with atomic weight 2260, occupying a vacant place in 

 the periodic table. Polonium changes 4500 times more 

 rapidly, and can only exist to the extent of a few 

 hundredths of a milligram in a ton of uranium mineral. 

 Actinium also, though its life period is still unknown, 

 and very possiblv is quite long, is scarce for another 

 reason : that it is not in the main line of disintegra- 

 tion, but in a branch series which claims only a few 

 per cent, of the uranium atoms disintegrating. In 

 spite of this, polonium and actinium have just as much 

 right to be considered new elements probably as radium 

 has. Polonium has great resemblances in chemical 

 character both to bismuth and tellurium, but was 1 

 NO. 2490, VOL. 99] 



separated from the first by Mme. Curie, and from the 

 second by Marckwald. in the position it occupies as 

 the last member of the sulphur group, bisrnutb and 

 tellurium are its neighbours in the periodic table. 

 Actinium resembles the rare-earth elements, and most 

 closely lanthanum, but an enrichment of the proportion 

 of actinium from lanthanum has been effected by 

 Giesel. The smallness of the quantities alone prevents 

 their complete separation in the form of pure com- 

 pounds, as was done for radium. 



The three gaseous members, the emanations of 

 radium, actinium, and thorium, were put in their 

 proper place in the periodic table almost as soon as 

 radium was, for, being chemically inert gases, their 

 characterisation was simple. They are the last mem- 

 bers of the argon family, and the fact that there are 

 three of about the same atomic weight was probably 

 the first indication, although not clearly appreciated, 

 that more than one chemical element could occupy 

 the same place in the f>eriodic table. 



The extension of the three disintegration series 

 proceeded apace, new members were being continually 

 added, but no other new radio-element — new, that is, 

 in possessing a new chemical character — was dis- 

 covered. Tha four longest lived to be added, radio- 

 lead, or radium-D, as it is now more precisely termed, 

 and ionium in the uranium series, and mesothorium-I 

 and radiothorium in the thorium series, could not be 

 separated from other constituents always present in 

 the minerals — radium-D from lead, ionium and radio- 

 thorium from thorium, and mesothorium-I from 

 radium. An appreciable proportion of the radio- 

 activity of a uranium mineral is due to radium-D and 

 its products, and its separation would have been a 

 valuable technical achievement, but though many 

 attempts have been made, this has never been accom- 

 plished, and, we know now, probably never will be. 



Seven years ago it was the general opinion in the 

 then comparatively undeveloped knowledge of the 

 chemistry of the radio-elements that there was nothing 

 especially remarkable in this. The chemist is familiar 

 with many pairs or groups of elements the separation 

 of which is laborious and difficult, and the radio- 

 chemist had not then fully appreciated the power of 

 radio-ac4ive analysis in detecting a very slight change 

 in the proportions of two elements, one or both of 

 which were radio-active. The case is not at all like 

 that of the rare-earth group of elements, for example, 

 in which the equivalent or atomic weight is used as a 

 guide to the progress of the separation. Here, the 

 total difference in the equivalent of the completely 

 separated elements is only a verv small percentage of 

 the equivalent, and the separation must already have 

 proceeded a long way before it can be ascertained. 



Human nature plays its part in scientific advances, 

 and the chemist is human like the rest. Mv own 

 views on the matter developed with some speed, when, 

 in 1910, I came across a new case of this phenomenon. 

 Trying to find out the chemical character of meso- 

 thorium-I, which had been kept secret for technical 

 reasons, I found it to have precisely the same chemical 

 character as radium, a discovery which was made in 

 the same year by Marckwald, and actually first pub- 

 lished by him. I delayed mv publication some months 

 to complete a very careful fractional crystallisation of 

 the barium-radium-mesothorium-I chloride separated 

 from thorianite. Although a great number of frac- 

 tionations were performed, and the radium was en- 

 riched, with regard to the barium, -several hundred 

 times, the ratio between the radium and mesO- 

 thorium-I was, within the verv small margin of error 

 possible in careful radio-active measurements, not 

 affected by the process. I felt justified in concluding 

 from this case, and its analogy with several other 

 similar cases then known, that radium and meso- 



