228 



NATURE 



[November 6, 1919 



atoms and are a measure of the rate of 

 transformation. Guided by this theory, the whole 

 sequence of changes in the uranium-radium series, 

 the thorium and actinium series, were investigated 

 in detail, and in a remarkably brief space of time 

 more than thirty new radioactive elements were 

 brought to light and their position in the scheme 

 of radioactive changes determined. Special 

 interest attaches to the discovery by Boltwood of 

 the substance called ionium, which is directly 

 transformed into radium. • This afforded a direct 

 experimental method of determining the average 

 life of radium with a result that is in close accord 

 with the value calculated from the rate of emission 

 of a-particles. The position of actinium in 

 the main scheme of changes has occupied much 

 attention. The constancy of the relative amount 

 of actinium and uranium in uranium minerals 

 showed that it must be derived ultimately from 

 uranium, but the activity of actinium is too 

 small to be in the direct line of succession. This 

 has led to the view that actinium is a branch 

 product at some point of the uranium series where 

 about 6 per cent, is transformed into the actmium 

 branch and 94 per cent, into the main line of 

 descent. The general evidence indicates that this 

 branching occurs near to uranium, and possibly 

 the branch product called uranium-Y by Antonoff 

 is the first member of the family. Recently the 

 intermediate parent substance of actinium itself 

 has been discovered. 



While in the majority of cases the atoms of a 

 radioactive product break up in a very definite 

 fashion and in only one way, certain cases are 

 known where one substance breaks into two 

 chemically distinct substances. Examples of this 

 are : radium C, thorium C, and actinium C. 

 Usually the transformation is mainly in one direc- 

 tion with a small fraction in the side branch. It 

 is quite probable that further study may lead to 

 the discovery of a number of such dual trans- 

 formations. In the violent cataclysm that must 

 accompany the transformation of an atom, it is 

 not unexpected that the constituents of the re- 

 sidual atom may arrange themselves in more than 

 one configuration of temporary equilibrium. 



Much attention has been directed to the proper- 

 ties of the radium emanation — the radioactive gas 

 constantly produced by the transformation of 

 radium atoms. The equilibrium volume of this gas 

 from one gram of pure radium is only six-tenths 

 of a cubic millimetre, but contributes more than 

 three-fourths of the total activity of radium. By 

 concentration of purified emanation into fine glass 

 tubes, very powerful sources of radiation have 

 been obtained, which have proved of great utility 

 both in the laboratory and for therapeutic pur- 

 poses. Although only about one-tenth of a cubic 

 millimetre of purified radium emanation has ordin- 

 arily been available for experiments, methods have 

 been devised to determine its spectrum, molecular 

 weight, freezing and boilmg points. 



We owe to Hahn the discovery of two fairly 

 long-lived products of thorium called mesothorium 

 and radiothorium. The mesothorium,, which is 

 NO. 2610. VOL. 104] 



separated with the radium from ores containing 

 both thorium and uranium, is transformed into 

 radiothorium. These products can be obtained of 

 activity greater than radium for equal weights, and 

 give us another source of powerful radiation. 



The discovery of the production of helium from 

 radium by Ramsay and Soddy was otf great import- 

 ance in emphasising the reality of the transforma- 

 tions occurring in radium. Rutherford showed that 

 the a-rays which are shot out from radium con- 

 sist of positively charged atoms of helium so that 

 all radioactive substances which emit o-rays 

 give rise to helium. The production of helium by 

 radioactive substances explains the occurrence of 

 large quantities of helium in uranium and thorium 

 minerals, and indeed the prediction by Rutherford 

 and Soddy that helium would prove to be a pro- 

 duct of radioactive transformation was based in 

 part on this fact. 



The great majority of radioactive substances are 

 transformed with the expulsion of helium atoms 

 with great velocity, but in a few cases swift elec- 

 trons apjpear. The appearance of helium in so 

 many changes, coupled with the observation that' 

 many of the atomic weights of many known ele- 

 ments differ by four units — the atomic weight of 

 helium — indicates that helium must be one of the 

 secondary units of which many of the' ordinary 

 elements are built up. It is noteworthy that so 

 far no definite evidence has been obtained that 

 hydrogen is a direct product of radioactive trans- 

 formation, although its complete absence would 

 be very surprising. 



The proof by the Curies of the rapid and con- 

 tinuous emission of heat from radium showed 

 clearly the vast amount of energy that must be 

 stored up in radioactive matter and released by its 

 transformation. This heat emission has been 

 shown to be a secondary effect of radioactivity, for 

 it is a measure of the energy of the expelled radia- 

 tions, the greater part being due to the energy 

 of the expelled a-particles. 



The transformation of an atom is the result 

 of an explosion of intense violence in which a 

 part of the atom, whether a helium atom or 

 an electron, is shot out with great speed. In order 

 to produce a-, j8-, or y-rays of equal energy to 

 those emitted by radioactive substances, potential 

 differences of about two million volts applied to a 

 vacuum tube would be necessary. These spon- 

 taneous radiations have been of great utility in 

 studying the ionisation, scattering, and other pro- 

 perties of particles movMng at high speed, while in 

 the very penetrating 7-rays we have a type 

 of X-rays of mjch shorter wave-length than can 

 be produced at present or is likely to be produced 

 by laboratory methods. 



The properties of the o-rays have been very 

 closely studied and their speed and mass have been 

 determined accurately. The definiteness of the 

 range of a-particles, to which Bragg first directed 

 attention, is a matter of remark, and so far the 

 apparent disappearance of the a-particle while 

 still moving with a high velocity has not been 

 adequately explained. The analysis of the )3-rays 



