November 6, 1919] 



NATURE 



229 



has disclosed the presence of groups of elec- 

 trons emitted at a definite velocity, so that the 

 pencil of iS-rays deflected in a magnetic field 

 shows a veritable magnetic spectrum. The 

 presence of these groups of ;3-rays appears to 

 be connected with the emission of characteristic 

 X-radiation from the atom, and the evidence as a 

 whole strongly supports the view that the y-rays 

 from radioactive substances, like the X-rays 

 from a vacuum tube, contain rays of a wide range 

 of frequency in which the characteristic rays from 

 the atom predominate. 



Space does not allow me to do more than men- 

 tion the extraordinary delicacy and definiteness of 

 the electrical methods devised for measuring 

 minute quantities of radioactive matter. By their 

 aid the chemical properties of the numerous radio- 

 active elements have been studied and their posi- 

 tion in the periodic table established. The orderly 

 sequence of changes in the chemical properties of 

 successive elements in the radioactive series has 

 been shown to be intimately connected with 

 the type of radiation, whether a- or )3-ray, 

 emitted by the preceding element. One of the 

 most important fruits of these chemical investiga- 

 tions has been the proof of the existence of non- 

 separable elements, named isotopes by Soddy, 

 which are identical in ordinary physical and chemi- 

 cal properties, but have different atomic weights. 

 In the case of lead, six isotopes are already known 

 which differ from one another either in atomic or 

 radioactive properties. On the nucleus theory 

 of the atom, this Indicates that the charges on the 

 nuclei are the same, but that the masses differ. 

 The proof of the presence of isotopes promises to 

 open up a new and very fundamental field of 

 chemical inquiry which must inevitably exercise 

 a great influence on atomic weight determinations 

 and also on our ideas of atomic constitution. In 

 a recent letter to this journal Merton has indi- 

 cated that the minute change in the wave-length 

 of spectrum lines of isotopes may give us a simple 

 method of attack on this problem. 



While the subject of radioactivity belongs in 

 essence to the border-line of physics and chem- 

 istry, with affiliations to both sciences, it has had 

 numerous connections with other fields of work. 

 The examination of the earth's crust has shown 

 that radioactive matter is very widely distributed, 

 and has disclosed, notably through the work of 

 Strutt and Joly, that the heating effect due to this 

 matter vitiates to a large extent the old arguments 

 of the duration of the earth's heat. While show- 

 ing that the old views are not tenable, radio- 

 activity has at the same time supplied new- 

 methods of estimating the age of minerals and 

 the duration of geological epochs. The minimum 

 age of minerals can be deduced from the helium 

 accumulated from the transformation of radio- 

 active matter, and the maximum age from the 

 accumulated lead which is the product of both 

 uranium and thorium. Xow that the atomic 

 weights of the lead isotopes are well established, 

 the atomic weight of the lead in a uranium 

 mineral should serve as a definite guide to the 

 NO. 2610, VOL. T04] 



fraction of lead present which is due to the trans- 

 formation of uranium and thus give a trustworthy 

 estimate of the age of the mineral. Joly has 

 demonstrated in a striking way that the pleochroic 

 haloes observed in mica are of radioactive origin, 

 and he has also estimated their age. The presence 

 of radioactive matter in the atmosphere has been 

 shown to account for its electrical conductivity. 

 Just before the war, evidence was obtained indi- 

 cating the presence of a very penetrating type 

 of 7-radiation in the upper atmosphere. It is to 

 be hoped that soon a further study will be made 

 to determine the nature and origin of this interest- 

 ing radiation. Finally, numerous inxestigations 

 have been carried out to determine the effects of 

 the radioactive rays on living tissue and on the 

 growth of plants and organisms. \\"ith the in- 

 creased use of radium for therapeutic purposes, it 

 is likely that our knowledge of this important 

 field of inquiry will grow rapidly. 



It is a matter of remark that while the study of 

 radioactivity has disclosed in a striking way the 

 transformation of heavy atoms through a long 

 series of stages, it has at the same time provided 

 us with indubitable proof of the correctness of 

 the old atomic theory of matter. The electric 

 method devised by Rutherford and Geiger of 

 counting single a-particles allows us to count 

 the total number of a-particles projected from 

 one gram of radium per second. By determining 

 the volume of helium produced by the collected 

 a-particles, we have a simple and direct method 

 of determining also the number of molecules in 

 a cubic centimetre of helium at standard pressure 

 and temperature. This number is in good agree- 

 ment with the number found by IVIillikan by 

 measuring the charge on the atom of electricity. 

 On account of the great energy of motion, a single 

 a-particle can be detected in a variety of ways, 

 by the electrical method, by the scintillations pro- 

 duced in zinc sulphide or the diamond, and by its 

 action on a photographic plate. 



The most striking proof of the individuality of 

 the electron, the a-particle, and the ion has 

 been given by C. T. R. Wilson by his beautiful 

 photographs showing the trails of a- and 

 ^-particles through gases. By a sudden expansion, 

 each charged ion produced by the flying particle is 

 rendered visible by becoming the centre of a visi- 

 ble drop of water. In the case of the swift elec- 

 tron, the number of ions per centimetre of path 

 is so small that the number may be directly 

 counted. These photographs bring out in a vivid 

 and concrete way the phenomena accompany- 

 ing the passage of ionising types of radiation 

 through gases, and are, in a sense, the ultimate 

 court of appeal of the accuracy of theories of the 

 properties of these rays. 



The discovery of the electron and of the pro- 

 perty of radioactivity has given a great stimulus 

 to attempts to deduce the structure of the «tom 

 itself, and numerous types of model atoms have 

 been proposed. The great difficulty in these at- 

 tempts is the uncertainty of the relative import- 

 ance of the fdle played by positive and negative 



