January 18, 1906] 



NA TURE 



285 



THE FEES EXT POSITION OF RADIO- 

 ACTIVITY. 1 

 '"THERE are three fundamental conceptions, the atom, 

 the electron, and the ether. The seventy odd different 

 kinds of atoms known, although fundamentally distinct, 

 form a class to themselves in the complexity of matter, 

 si 1 that any discovery fundamentally affecting one must 

 embrace all. The electron expresses for electricity the same 

 idea as the atom does for any one kind of elementary 

 matter, and may be termed the atom of negative electricity. 

 Only one kind of electricity, and only one kind of electron, 

 is known, and this possesses the same essential properties 

 in all its various manifestations. The ether renders possible 

 " action at a distance," and all actions transmitted through 

 tlif ether are of essentially the same character and travel 

 at one speed, namely, the speed of light. 



The electron, although by origin an electrical conception, 

 is in reality a material conception no less than the atom. 

 At rest an electron is a simple charge — an electrostatic 

 phenomenon. In motion it constitutes a current of elec- 

 t lit it y — an electromagnetic phenomenon. When an electron 

 moves from rest to speed and back to rest again, the ether 

 through which it moves at first has no magnetic qualities ; 

 then it acquires an amount of magnetic energy proportional 

 to the speed of the electron, and then it again loses the 

 same amount. Thus the electron cannot move without 

 the expenditure of energy, and cannot be stopped until it 

 has again given up the same amount. According to 

 Newton's laws of motion, therefore, the electron is 

 essentially a material particle. It possesses inertia, or 

 " apparent mass," but it is not yet known whether it obeys 

 the law of gravity, and possesses gravitational mass. 



Since action at a distance travels through the ether at 

 the speed of light, the magnetic field at a point some 

 distance away from the line of motion of an electron cannot 

 instantaneously accommodate itself to a change of motion 

 of the electron, but the disturbance of the magnetic field 

 travels outward from the electron with the speed of light. 

 If the change of motion of the electron is periodic, as in 

 the case of an electron revolving in an orbit within an 

 atom, the disturbance constitutes ordinary light. In the 

 Crookes's tube there is an irregular shower of free-flying 

 independent electrons (kathode rays) upon the anti- 

 kathode. The sudden irregular disturbances in the mag- 

 netic field travelling outward from the anti-kathode at the 

 speed of light constitute the X-rays. The Hertz waves, on 

 the other hand, result when electrons are caused to oscil- 

 late along paths of metrical rather than molecular 

 dimensions, and their wave-length is measured in metres 

 rather than in molecular diameters. 



The apparatus employed largely to generate what are 

 known to medical men as *' high-frequency currents" 

 admirably illustrate the inertia of electrons. Such a current 

 will jump an air gap rather than traverse a spiral rod of 

 copper, and will light a high-resistance incandescent lamp 

 " short-circuited " by loop of bar copper. Lightning 

 possesses the same characteristics, as Sir Oliver Lodge 

 was the first to demonstrate. 



The question arises whether there are two kinds of 

 inertia, essentially similar, the one " material " and the 

 other electromagnetic. If a sufficient number of electrons 

 could be concentrated within a space of atomic dimensions, 

 the total inertia of the aggregate could be made equal to 

 that of an atom. Unfortunately, we know of no means 

 whereby the mutual repulsions of the electrons could be 

 overcome without introducing the hypothetical positive 

 electron or its equivalent. 



The present year is the decennary of M. Henri 

 Becquerel's discovery of the natural radio-activity of matter. 

 Radio-activity has been interpreted as the effect of a 

 process of spontaneous disintegration occurring within the 

 atoms of the radio-element, and already atomic disintegra- 

 tion is recognised as the probable cause of innumerable 

 hitherto isolated phenomena in every branch of knowledge. 

 It is the most fundamental and potent factor of evolution 

 known. The ultimate cause of atomic disintegration, like 

 that of most other common properties of matter, even 

 gravitation, remains quite unknown. The view that radio- 



1 Abridged from the presidential address delivered to the Rontgen Society 

 on January 4 by Mr. Frederick Soddy. 



NO. 1890, VOL. 73] 



activity is the outward and visible sign of deep-seated 

 material change followed from the elucidation of the 

 nature of the emanations, and of the phenomenon of 

 excited or induced activity. It was shown that the eman- 

 ations and the allied bodies were new forms of matter 

 continuously being produced from the radio-elements, and 

 that they were the products of the changing atoms. 

 Rutherford's discovery that the a radiation consisted of 

 radiant particles, and the gradual accumulation of evidence, 

 amounting to-day to practical proof, that the a particles 

 are radiant atoms of the element helium, enabled the whole 

 process to be simply elucidated. A single radiant atom is 

 within the means of detection, for example, by the spin- 

 thariscope, whereas a million million atoms is not sufficient 

 to be detected by the most delicate and refined spectro- 

 scopic test. The radio-atom suffers successive disintegra- 

 tion, and at each disintegration a single radiant particle 

 is in general expelled. The radium atom successively 

 expels five a particles, so that a residue of atomic weight 

 about 205 should be left if these particles are helium 

 atoms. There is strong indirect evidence for believing that 

 the residue atom is that of lead. In the natural minerals, 

 where the radio-elements occur, are to be found the 

 ultimate products of ages of past accumulation. In the 

 uranium minerals, helium, radium, polonium, and lead 

 have been recognised as the constant companions of the 

 uranium. Direct experiments have established in each 

 case, except lead, the production of these elements during 

 the disintegration. Polonium is the last changing member 

 of the disintegration series, is a higher homologue of 

 tellurium (Marckwald), and is identified with the radium F 

 of Rutherford. The production of lead from polonium has 

 not yet been directly observed, but Boltwood has shown it 

 to be a constant constituent of the uranium minerals. 



There is a comprehensiveness and subtlety in the oper- 

 ations of the laws of nature which the most vivid imagin- 

 ation cannot anticipate. The fact that the proportion of 

 radium in any uranium mineral must be constant, being 

 the ratio between the rate of disintegration of radium and 

 that of uranium (or one to a million), cannot fail to have 

 most important bearings. If to-morrow radium could be 

 imported in quantity from outer space, after a few thousand 

 years the quantity in the earth would be no more and no 

 less than at present. By that time the quantity exhibited 

 to-night will have had its day and ceased to be, but if the 

 rest of the mineral from which it was extracted could again 

 be examined a new amount no less than that originally 

 present would be found to have grown in the interval. 



How far are we justified in extending these ideas to 

 explain analogous phenomena in the case of the inactive 

 elements? We know that radio-activity is a mere accom- 

 paniment by no means essential to the process of atomic 

 disintegration. The evidence available shows clearly that 

 atomic disintegration might be universal and yet beyond 

 the power of direct detection. A discovery fundamentally 

 affecting any one element must embrace the whole class. 

 The internal energy of the atom is merely revealed in 

 radio-activity, in the same way as the internal energy of 

 gun-cotton is revealed only when it explodes. The energy 

 of the disintegration of an element is roughly a million 

 times greater than that of any other change we are 

 acquainted with. The attempt of the alchemist to build 

 up a heavy element like gold from silver was futile, be-" 

 cause, even if it could be done, it could not pay. The 

 energy of some hundreds of tons of coal would have to be 

 put into an ounce of silver to convert it into gold ; but 

 if gold could be formed from the degradation of a heavier 

 element like lead, the gold would be a mere by-product, 

 and the store of energy liberated simultaneously, however 

 reckoned, would be of far greater value than the gold 

 produced. At present we are totally ignorant of any means 

 of altering or affecting in any way the rate of atomic 

 disintegration proceeding spontaneously, or, in other 

 words, we cannot effect artificial transmutation. 



The experimental sciences do not hold out much hope 

 of giving an immediate answer to the question whether 

 atomic disintegration is general, and whether the scarcity 

 or abundance of an element in the earth is a measure of 

 its stability. We are forced back on such indirect evideni e 

 as lies ready to our hand. It is possible to obtain such 

 evidence in the field of economics for the element gold. 



