June 2, 1904] 



NA TURE 



The Nature of the a Rays emitted by Radio-active 

 Substances. 



The a rays emitted by radium and other radio-active 

 substances have been shown by Rutherford (" Radio- 

 activity," pp. 115-141) to consist of positively charged 

 particles for which e/m=6xio^. They are rapidly 

 absorbed by gases and solids, the absorption coefficient 

 being approximately proportional to the density of the 

 absorbing medium. The value of the absorption coefficient 

 in air divided by the density varies between 350 and 1300 

 for different types of o rays. The velocity of these rays is 

 about i/ioth to i/2oth that of light. 



It is interesting to compare the properties of these rays 

 with those of kathode rays moving with about the same 

 velocity; c; 111 for such rays is about 10", and the value of 

 their absorption coefficient in air at i mm. pressure is 

 085 (Lenard, Ann. der Pliys., Bd. 12, p. 714, 1903) when the 

 velocity is i/ioth that of light and 3-9 when it is 1,20th. 



The absorption coefficient for these rays is also p'O" 

 portional approximately to the density of the absorbing 

 medium. Dividing 085 by the density of air at 1 mm. 

 pressure we get 540,000, and in the same way 39 gives 

 2,500,000. The corresponding numbers for the a rays are 

 about 350 and 1300. Thus we see that the o rays are 

 nearly 2000 times as penetrating as kathode rays moving 

 with the same velocity. 



Assuming that —e for the kathode rays is equal to e for 

 the o rays, we have for the ratio of their inasses 

 io'/6x io' = 1700. It thus appears that the penetrating 

 power of the o rays is to that of kathode rays, moving 

 with the same velocity, approximately as the mass of the 

 a ravs is to the mass of the kathode rays. We may conclude 

 from this that an a particle loses as much energy in 

 colliding w'ith an atom as a kathode-ray particle or cor- 

 puscle. If we regard the a particles as being of atomic 

 dimensions (that is, as having a radius about 10-' cm.), 

 while an electron or corpuscle only has a radius of about 

 10-" cm., it is very difficult to understand this result. 

 On the view that all atoms are assemblies of electrons, the 

 fact that the absorption of kathode rays depends only on 

 the density of the absorbing medium is regarded as in- 

 dicating that the electrons penetrate the atoms and are 

 absorbed by colliding with the electrons which compose the 

 atoms. .Since a particles lose the same amount of energy 

 as electrons in penetrating matter, it seems probable that 

 they also penetrate the atoms and lose energy by colliding 

 with the electrons in exactly the same way. If this view 

 is taken, it becomes difficult to regard an a particle as of 

 atomic dimensions, and w^e may look upon it as a positive 

 electron exactly similar in character to an ordinary negative 

 electron. The mass (m) of an electron is now regarded as 

 being purelv electromagnetic in character, and is given 

 by the formula t»=2c"/3a, where a is its radius and e its 

 charge. For a negative electron this gives a=io-" cm. 

 Regarding an a particle as a positive electron, we get in 

 the same way for its radius about 5X10-'° cm. On this 

 view, therefore, the a particles are enormously smaller than 

 the negative electrons. 



The properties and modes of occurrence of the o particles 

 are in agreement with the view that they are really positive 

 electrons. For example, they are produced like kathode 

 rays in electric discharges at low pressures (being then 

 known as canalstrahlen), and have very similar properties 

 to kathode rays. The writer therefore suggests the view 

 that o particles may be positive electrons having a radius 

 about 2000 times smaller than negative electrons. 



Trinity College, Cambridge. H.irold A. Wilson. 



A Suggested Explanation of Radioactivity. 



I ..\M venturing, in the present note, to add another to 

 the already large number of suggestions as to the meaning 

 of the phenomenon of radio-activity. 



It seems to be well established that the apparent in- 

 stability of the atoms of radio-active substances is not to 

 any great extent dependent on the temperature of the mass ; 

 the instability, therefore, is not the outcome of inter- 

 molecular collisions. Neither does it seem to arise from 



an excess of the internal energy of the molecule. For the 

 internal agitation of the molecule, so far as is known, shows 

 itself in the emission of light, and this is associated with 

 high mass-temperature. There is, of course, the possi- 

 bility, suggested by Prof. J. J. Thomson, that there are 

 internal degrees of freedom not represented in the spectrum 

 of the gas, and that it is the energy of these which forms 

 the starting point of the radio-active process. On the other 

 hand, it is possible that the atomic instability, not being 

 the result of the agitation of the molecules or of the com- 

 ponent material parts (ions or corpuscles) of which the 

 molecules are composed, must be traced to the agitation of 

 the ultimate constituents of these ions or corpuscles. If, 

 for instance, we take a definite mechanical illustration, and 

 imagine our universe constructed on the model suggested 

 by Prof. Osborne Reynolds, the source of instability must 

 be looked for in the agitation of the " grains " of which he 

 supposes the ether to be constituted. The velocities of these 

 grains follow Maxwell's law of distribution, so that very 

 high velocities, although rare, are not impossible. It is 

 at least thinkable that a grain moving with e.xceptionally 

 high velocity may succeed in breaking down the normal 

 piling in its immediate neighbourhood when this is possible 

 (i.e., probably, when in the immediate proximity of matter), 

 and mav therefore effect a rearrangement of the adjacent 

 ether structure. A process of this kind would be indepen- 

 dent of the mass-temperature ; it would, so to speak, depend 

 solely on the ether temperature, which is supposed, on Prof. 

 Reynolds's hypothesis, to be constant throughout space. It 

 seems probable that the rearrangement would consist of 

 the combination and mutual annihilation of two ether strains 

 of opposite kinds, i.e. in the coalescence of a positive and 

 negative ion, and would therefore result in the disappearance 

 of a certain amount of mass. There would, therefore, be 

 conservation neither of mass nor of material energy ; the 

 process of radio-activity would consist in an increase of 

 material energy at the e.xpense of the destruction of a 

 certain amount of matter. 



Apart, however, from this special mechanical model, it 

 seems probable, on grounds of general dynamics, that the 

 ether does not transmit waves in a perfectly unaltered form, 

 and that there is therefore a continual degradation of the 

 energy of regular waves into an energy of random agitation 

 of the ultimate ether structure. This agitation would afford 

 a suflicient cause for the beginnings of the process which 

 results in the breaking up of the atom. Naturally this 

 agitation would have the best chance of effecting a re- 

 arrangement when the strain is greatest, and therefore 

 when the ions are most closely packed together. A larger 

 energy of agitation w-ould be necessary when the ions were 

 less closely packed. We should, therefore, expect all matter 

 to be radio-active to some e.xtent, but should e.xpect the 

 greatest amount of radio-activity to be shown by the heavier 

 atoms. 



If the instability results from a rearrangement of an 

 ether structure, and not solely of a material structure, we 

 should, a priori, on general grounds of physical dimensions, 

 expect the velocity of the ejection to be comparable with 

 the velocity of waves in the ether, this being the only unit 

 appropriate to the measurement of processes depending on 

 the physical constants of the ether. [Just as, for instance, 

 the velocity of a gas streaming into a vacuum might, 

 a priori, be expected to be comparable with the velocity 

 of sound in the gas.] The suggested cause of instability 

 is therefore in agreement with the observed velocity of the 

 a particles. J. H. Jeans. 



Trinity College, Cambridge. 



The First Record of Glacial Action in Tasmania. 



In a recent paper on the Glacial geology of Tasmania 

 {Quart. Journ. Geol. Soc, vol. Ix. p. 38), I referred to 

 Gould's recognition of Glacial action in Tasmania as not 

 having been directlv published. This view I accepted on 

 the strength of the statement by Mr. R. M. Johnston (" The 

 Glacial Epoch of Australasia," Proc. Roy. Soc. Tasmania, 

 vol. iv., 1893, 1894, pp. 92-3), than whom no one knows 

 better the geological literature of Tasmania, that it was 

 " through verbal communication to a personal friend of my 



NO. 1805, VOL. 70] 



