,,ernest ,,rutherford                   
,,nobel ,,lecture                       
,december #aa1 #aijh4                   
,,the ,,chemical ,,nature ,,of ,,the    
,,alpha ,,particles                     
,,from ,,radioactive ,,substances       
,the study of the properties of the     
-rays has played a notable part in  the 
development of radioactivity and has    
been instrumental in bringing  to light 
a number of facts and relationships of  
the first importance4  ,with increase of
 experimental knowledge there has been a
 growing  recognition that a large part 
of radioactive phenomena is intimately  
connected with the expulsion of the     
-particles4 ,in this lecture an  attempt
 will be made to give a brief historical
 account of the  development of our     
knowledge of the -rays and to trace the 
long and  arduous path trodden by the   
experimenter in the attempts to solve   
the  difficult question of the chemical 
nature of the -particles4 -rays  were   
first observed in #ahii as a special    

type of radiation and during  the last  
six years there has been a persistent   
attack on this great  problem1 which has
 finally yielded to the assault when the
 resources of  the attack seemed almost 
exhausted4    ,shortly after his        
discovery of the radiating power of     
uranium by the  photographic method1    
,becquerel showed that the radiation    
from uranium  like the ,rntgen-rays     
possessed the property of discharging an
  electrified body4 ,in a detailed      
investigation of this property1 ,i      
examined the effect on the rate of      
discharge by placing successive  layers 
of thin aluminium foil over the surface 
of a layer of uranium  oxide and was led
 to the conclusion that two types of    
radiation of very  different penetrating
 power were present4 ,the conclusions at
 that period  were summed up as follows3
    ,these experiments show that the    
uranium1 radiation is complex and  that 
there are present at least two distinct 
types of radiation--one  that is very   

readily absorbed1 which will be termed  
for convenience the  -radiation1 and the
 other of a more penetrative character1 
which will  be termed the -radiation4#a 
,when other radioactive substances were 
discovered1 it was seen that the types  
of radiation present were  analogous to 
the - and -rays of uranium and when a   
still more  penetrating type of         
radiation from radium was discovered by 
,villard1 the  term -rays was applied to
 them4 ,the names thus given soon came  
into  general use as a convenient       
nomenclature for the three distinct     
types of  radiation emitted from        
uranium1 radium1 thorium1 and actinium4 
,on  account of their insignificant     
penetrating power1 the -rays were at    
first considered of little importance   
and attention was mainly directed  to   
the more penetrating -rays4 ,with the   
advent of active preparations  of       
radium1 ,giesel in #ahii showed that the
 -rays from this substance  were easily 
deflected by a magnetic field in the    

same direction as a  stream of cathode  
rays and consequently appeared to be a  
stream of  projected particles carrying 
a negative charge4 ,the proof of the    
identity of the -particles with the     
electrons constituting the cathode  rays
 was completed in #aijj by ,becquerel1  
who showed that the -particles  from    
radium had about the same small mass as 
the electrons and were  projected at a  
speed comparable with the velocity of   
light4 ,time does  not allow me to enter
 into the later work of ,kaufmann and   
others on this  subject1 which has      
greatly extended our knowledge of the   
constitution  and mass of electrons4    
,in the meantime1 further investigation 
had disclosed that the  -particles      
produced most of the ionization observed
 in the  neighbourhood of an unscreened 
radioactive substance1 and that most of 
the energy radiated was in the form of  
-rays4 ,it was calculated by            
,rutherford and ,mc,clung in #aija that 
one gram of radium radiated a large     

amount of energy in the form of -rays4  
,the increasing recognition of the      
importance of the -rays in  radioactive 
phenomena led to attempts to determine  
the nature of this  easily absorbed type
 of radiation4 ,strutt 7,lord ,rayleigh7
 in #aija and  ,sir ,william ,crookes in
 #aijb suggested that they might        
possibly prove to  be projected         
particles carrying a positive charge4 ,i
 independently  arrived at the same     
conclusion from consideration of a      
variety of  evidence4 ,if this were the 
case1 the -rays should be deflected by a
  magnetic field4 ,preliminary work     
showed that the deflection was very     
slight if it occurred at all4           
,experiments were continued at intervals
  over a period of two years and it was 
not until #aijb1 when a preparation  of 
radium of activity #ai1jjj was          
available1 that ,i was able to show     
conclusively that the particles were    
deflected by a magnetic field1  though  
in a very minute degree compared with   

the -rays4 ,this showed  that the -rays 
consisted of projected charged particles
 while the  direction of deflection     
indicated that each particle carried a  
positive  charge4 ,the -particles were  
shown to be deflected also by an        
electric  field and from the magnitude  
of the deflection1 it was deduced that  
the  velocity of the swiftest particles 
was about #b4e x #aji cm per second1 or 
one-twelfth the velocity of light1 while
 the value of e_/m--the ratio  of the   
charge carried by the particle to its   
mass--was found to be  #e1jjj           
electromagnetic units4 ,now it is known 
from the data of the  electrolysis of   
water that the value of e_/m for the    
hydrogen atom is  #i1fej4 ,if the       
-particle carried the same positive     
charge as the unit  fundamental charge  
of the hydrogen atom1 it was seen that  
the mass of  the -particle was about    
twice that of the hydrogen atom4 ,on    
account of  the complexity of the rays  
it was recognized that the results were 

only  approximate1 but the experiments  
indicated clearly that the -particle    
was atomic in mass and might prove      
ultimately to be either a hydrogen or  a
 helium atom or the atom of some unknown
 element of light atomic  weight4 ,these
 experiments were repeated by ,des      
,coudres in #aijc with  similar results1
 while ,becquerel showed the deflection 
of the -rays in  a magnetic field by the
 photographic method4    ,this proof    
that the -particles consisted of actual 
charged atoms of  matter projected with 
an enormous velocity at once threw a    
flood of  light on radioactive          
processes1 in particular upon another   
important  series of investigations     
which were being contemporaneously      
carried on  in the ,laboratory at       
,montreal in conjunction with ,mr4 ,f4  
,soddy4 ,had time  permitted1 it would  
have been of interest to consider in    
some detail the  nature of these        
researches which placed on a firm       
foundation the now  generally accepted  

transformation theory of radioactivity4 
,from a  close examination of the       
substances thorium1 radium1 and uranium1
  ,rutherford and ,soddy had reached the
 conclusion that radioactive bodies     
were in a state of transformation1 as a 
result of which a number of new         
substances were produced entirely       
distinct in chemical and physical       
character from the parent element4 ,from
 the independence of the rate of        
transformation of chemical and physical 
agencies1 it was recognized that  the   
transformation was atomic and not       
molecular in character4 ,each of  these 
new bodies was shown to lose its        
radioactive properties according  to a  
definite law4 ,even before the discovery
 of the material nature of  the -rays1  
it had been considered probable that the
 radiation from any  particular         
substance accompanied the breaking up of
 its atoms4 ,the proof  that the        
-particle was an ejected atom of matter 
at once strengthened  this conclusion   

and at the same time gave a more        
concrete and definite  representation of
 the processes occurring in radioactive 
matter4 ,the  point of view reached by  
us at that time is clearly seen from the
  following quotation1 which with little
 alteration holds good today4  ,the     
results obtained so far point to the    
conclusion that the  beginning of the   
succession of chemical changes taking   
place in  radioactive bodies is due to  
the emission of the -rays1 i4e4 the     
projection of a heavy charged mass from 
the atom4 ,the portion left  behind is  
unstable1 undergoing further chemical   
changes which are again  accompanied by 
the emission of -rays1 and in some cases
 also of -rays4    ,the power possessed 
by the radioactive bodies of apparently 
spontaneously projecting large masses   
with enormous velocities supports  the  
view that the atoms of these substances 
are made up1 in part at  least1 of      
rapidly rotating or oscillating systems 
of heavy charged  bodies1 large compared

 with the electron4 ,the sudden escape  
of these  masses from their orbit may be
 due either to the action of internal   
forces or external forces of which we   
have at present no knowledge4#b         
,consider for a moment the explanation  
of the changes in radium4 ,a minute     
fraction of the radium atoms is supposed
 each second to become unstable1        
breaking up with explosive violence4 ,a 
fragment of the atom--and  -particle--is
 ejected at a high speed1 and the       
residue of the atom1  which has a       
lighter weight than before1 becomes an  
atom of a new  substance1 the radium    
emanation4 ,the atoms of this substance 
are far  more unstable than those of    
radium and explode again with the       
expulsion  of an -particle4 ,as a result
 the atom of radium ,a makes its        
appearance  and the process of          
disintegration thus started continues   
through a long  series of stages4    ,i 
can only refer in passing here to the   
large amount of work done by  various   

experimenters in analysing the long     
series of transformations of  radium and
 thorium and actinium2 the linking up of
 radium with uranium  and the discovery 
by ,boltwood of the long looked-for and 
elusive parent  of radium1 viz4 ionium4 
,this phase of the subject is of unusual
 interest  and importance but has only  
an indirect bearing on the subject of my
  lecture4 ,it has been shown that the  
great majority of the transition        
elements produced by the transformation 
of uranium and thorium break up  with   
the expulsion of -particles4 ,a few1    
however1 throw off only  -particles1    
while some are rayless1 i4e4 undergo    
transformation  without the expulsion of
 high-speed - and -particles4 ,it is    
necessary to suppose that in these      
latter cases the atoms break up with    
the expulsion of -particles at a speed  
too low to be detected1 or1 as  is more 
probable1 undergo a process of atomic   
rearrangement without the  expulsion of 
material particles of atomic dimensions4

    ,another striking property of radium
 was soon seen to be connected with  the
 expulsion of -particles4 ,in #aijc ,p4 
,curie and ,laborde showed that  radium 
was a self-heating substance and was    
always above the temperature  of the    
surrounding air4 ,it seemed probable    
from the beginning that the  effect must
 be the result of the heating effect due
 to the impact of the  -particles on the
 radium4 ,consider for a moment a pellet
 of radium  enclosed in a tube4 ,the    
-particles are shot out in great numbers
  equally from all parts of the radium  
and in consequence of their slight      
penetrating power are all stopped in the
 radium itself or by the walls  of the  
tube4 ,the energy of motion of the      
-particles is converted into  heat4 ,on 
this view the radium is subject to a    
fierce and unceasing  bombardment by its
 own particles and is heated by its own 
radiation4  ,this was confirmed by the  
work of ,rutherford and ,barnes in      
#aijc1 who  showed that three quarters  

of the heating effect of radium was not 
directly due to the radium but to its   
product1 the emanation1 and that  each  
of the different substances produced in 
radium gave out heat in  proportion to  
the energy of the -particles expelled   
from it4 ,these  experiments brought    
clearly to light the enormous energy1   
compared with  the weight of matter     
involved1 which was emitted during the  
transformation of the emanation4 ,it can
 readily be calculated that one         
kilogram of the radium-emanation and its
 products would initially emit  energy  
at the rate of #ad1jjj horse-power1 and 
during its life would give  off energy  
corresponding to about #hj1jjj          
horse-power for one day4  ,it was thus  
clear that the heating effect of radium 
was mainly a  secondary phenomenon      
resulting from the bombardment by its   
own  -particles4 ,it was evident also   
that all the radioactive substances     
must emit heat in proportion to the     
number and energy of the -particles     

expelled per second4    ,we must now    
consider another discovery of the first 
importance4 ,in  discussing the         
consequences of the disintegration      
theory1 ,rutherford and  ,soddy drew    
attention to the fact that any stable   
substances produced  during the         
transformation of the radio-elements    
should be present in  quantity in the   
radioactive minerals1 where the         
processes of  transformation have been  
taking place for ages4 ,this suggestion 
was  first put forward in #aijb4c ,in   
the light of these results and the  view
 that has already been put forward of   
the nature of radioactivity1  the       
speculation naturally arises whether the
 presence of helium in  minerals and its
 invariable association with uranium and
 thorium1 may  not be connected with    
their radioactivity1 and again#d4 ,it is
  therefore to be expected that if any  
of the unknown ultimate products of  the
 changes of a radioactive element are   
gaseous1 they would be found  occluded1 

possibly in considerable quantities1 in 
the natural minerals  containing that   
element4 ,this lends support to the     
suggestion already  put forwards1 that  
possibly helium is an ultimate product  
of the  disintegration of one of the    
radioactive elements1 since it is only  
found in radioactive minerals4    ,it   
was at the same time recognized that it 
was quite possible that the  -particle  
itself might prove to be a helium atom4 
,as only weak  preparations were then   
available1 it did not seem feasible at  
that time  to test whether helium was   
produced from radium4 ,about a year     
later1  thanks to ,dr4 ,giesel of       
,braunschweig1 preparations of pure     
radium  bromide were made available to  
experimenters4 ,using #cj milligrams of 
,giesel's preparation1 ,sir ,william    
,ramsay and ,soddy in #aijc were able   
to show conclusively that helium was    
present in radium some months old  and  
that the emanation produced helium4     
,this discovery was of the  greatest    

interest and importance1 for it brought 
to light that in  addition to a series  
of transition elements1 radium also gave
 rise in  its transformation to a stable
 form of matter4    ,a fundamental      
question immediately arose as to the    
position of helium in  the scheme of    
transformations of radium4 ,was the     
helium the end or final  product of     
transformation of radium or did it arise
 at some other stage  or stages8 ,in a  
letter to ,nature#e ,i pointed out that 
probably helium  was derived from the   
-particles fired out by the -ray        
products of  radium and made an         
approximate estimate of the rate of     
production of  helium by radium4 ,it was
 calculated that the amount of helium   
produced  per gram of radium should lie 
between #bj and #bjj cubic millimetres  
per  year and probably nearer the latter
 estimate4 ,the data available for      
calculation at that time were imperfect1
 but it is of interest to note  that the
 rate of production of helium recently  

found by ,sir ,james ,dewar1  in #aijh1 
viz4 #acd cubic millimetres per year1 is
 not far from the value  calculated as  
most probable at that time4    ,these   
estimates of the rate of production of  
helium were later modified  as new and  
more accurate data became available4 ,in
 #aije1 ,i measured the  charge carried 
by the -particles from a thin film of   
radium4 ,assuming  that each -particle  
carried the ionic charge measured by    
,j4,j4 ,thomson1  ,i showed that #f4b x 
#ajaj -particles were expelled per      
second per gram  of radium itself and   
four times this number when radium was  
in  equilibrium with its three -ray     
products4 ,the rate of production of    
helium calculated on these data was #bdj
 cubic millimetres per gram per  year4  
,in the meantime1 by the admirable      
researches of ,bragg and ,kleeman in    
#aijd1 our knowledge of the character of
 the absorption of the  -particles by   
matter had been much extended4 ,it had  
long been known  that the absorption of 

-particles by matter was different in   
many  respects from that of the -rays4  
,bragg showed that these differences    
arose from the fact that the -particle1 
on account of its great energy  of      
motion1 was not deflected from its path 
like the -particle1 but  travelled in   
nearly a straight line1 ionizing the    
molecules in its path4  ,from a thin    
film of matter of one kind1 the         
-particles were all  projected at the   
same speed and lost their power of      
producing ionization  suddenly1 after   
traversing a certain definite distance  
of air4 ,the  velocity of the -particles
 in this view were reduced by their     
passage  through matter by equal        
amounts4 ,these conclusions of ,bragg   
were  confirmed by experiments ,i made  
by the photographic method4 ,as a source
  of rays1 a thin film of radium ,c1    
deposited from the radium-emanation on  
a thin wire1 was used4 ,by examining the
 deflection of the rays in a  magnetic  
field1 it was found that the rays were  

homogeneous and were  expelled from the 
surface of the wire at an identical     
speed4 ,by passing  the rays through a  
screen of mica or aluminium1 it was     
found that the  velocity of all the     
-particles were reduced by the same     
amount and the  issuing beam was still  
homogeneous4    ,a remarkable result was
 noted4 ,all -particles apparently lost 
their  characteristic properties of     
ionization1 phosphorescence and         
photographic action1 at exactly the same
 point while they were still  moving at 
a speed of about #i1jjj kilometres per  
second4 ,at this critical  speed1 the   
-particle suddenly vanishes from our ken
 and can no longer  be followed by the  
methods of observation at our command4  
,the use of a homogeneous source of     
-rays like radium ,c at once  suggested 
itself as affording a basis for a more  
accurate determination  of the value of 
e_/m for the -particle and for seeing   
whether the value  was consistent with  
the view that the -particle was a       

charged atom of  helium4 ,in the course 
of a long series of experiments1 ,i     
proved that the  -particles1 whether    
expelled from radium1 thorium or        
actinium1 were  identical in mass and   
must consist of the same kind of matter4
    ,the velocity of expulsion of the   
-particles from different kinds of      
active matter varied over comparatively 
narrow limits but the value of  e_/m was
 constant and equal to #e1jgj4 ,this    
value was not very different  from the  
one originally found4 ,a difficulty at  
once arose in  interpreting this result4
 ,we have seen that the value of e_/m   
for the  hydrogen atom is #i1fej4 ,if   
the -particle carried the same positive 
charge as the hydrogen atom1 the value  
of e_/m for the -particle would         
indicate that its mass was twice that of
 the hydrogen atom1 i4e4 equal  to the  
mass of a hydrogen molecule4 ,it seemed 
very improbable that  hydrogen should be
 ejected in a molecular and not an      
atomic state as a  result of the atomic 

explosion4 ,if1 however1 the -particle  
carried a  charge equal to twice that of
 the hydrogen atom1 the mass of the     
-particle would work out at nearly four1
 i4e4 a mass nearly equal to  that of   
the atom of helium4    ,i suggested     
that1 in all probability1 the -particle 
was a helium atom  which carried two    
unit charges4 ,on this view1 every      
radioactive  substance which emitted    
-particles must give rise to helium4    
,this at  once offered an explanation of
 the fact observed by ,debierne that    
actinium as well as radium produced     
helium4 ,it was pointed out that the    
presence of a double charge of          
helium-atom was not altogether          
improbable  for reasons to be given     
later4    ,while the evidence as a whole
 strongly supported the view that the   
-particle was a helium atom1 it was     
found exceedingly difficult to  obtain a
 decisive experimental proof of the     
relation4 ,if it could be  shown        
experimentally that the -particle did in

 reality carry two unit  charges1 the   
proof of the relation would be greatly  
strengthened4 ,for  this purpose an     
electrical method was devised by        
,rutherford and ,geiger  for counting   
directly the -particles expelled from a 
radioactive  substance4 ,the ionization 
produced in a gas by a single -particle 
is  exceedingly small and would be      
difficult to detect electrically except 
by a very refined method4 ,recourse was 
had to an automatic method of           
magnifying the ionization produced by an
 -particle4 ,for this purpose it  was   
arranged that the -particles should be  
fired through a small  opening into a   
vessel containing air or other gas at a 
low pressure1  exposed to an electric   
field near the sparking value4 ,under   
these  conditions the ions produced by  
the passage of the -particle through    
the gas generate a large number of fresh
 ions by collision4 ,in this way  it was
 found possible to magnify the          
electrical effect due to an  -particle  

several thousand times4 ,the entrance of
 an -particle into  the testing vessel  
was then indicated by a sudden          
deflection of the  electrometer needle4 
,this method was developed into an      
accurate method  of counting the number 
of -particles fired in a known time     
through the  small aperture of the      
testing vessel4 ,from this was deduced  
the total  number of -particles expelled
 per second from any thin film of       
radioactive matter4 ,in this way it was 
shown that #c4d x #ajaj -particles  are 
expelled per second from one gram of    
radium itself and from each of  its -ray
 products in equilibrium with it4       
,the correctness of this method was     
indicated by another1 quite distinct    
method of counting4 ,sir ,william       
,crookes and ,elster and ,geitel had    
shown  that the -particles falling on a 
screen of phosphorescent zinc sulphide  
produced a number of scintillations4    
,using specially prepared screens1      
,rutherford and ,geiger counted the     

number of these scintillations per      
second with the aid of a microscope4 ,it
 was found that1 within the limit  of   
experimental error1 the number of       
scintillations per second on a  screen  
agreed with the number of -particles    
impinging on it1 counted by  the        
electrical method4 ,it was thus clear   
that each -particle produced a  visible 
scintillation on the screen1 and that   
either the electrical or  the optical   
method could be used for counting the   
-particles4 ,apart  from the purpose for
 which these experiments were made1 the 
results are  of great interest and      
importance1 for it is the first time    
that it has  been found possible to     
detect a single atom of matter by its   
electrical  and optical effect4 ,this is
 of course only possible because of the 
great  velocity of the -particle4       
,knowing the number of -particles       
expelled from radium from the counting  
experiment1 the charge carried by each  
-particle was determined by  measuring  

the total positive charge carried by all
 the -particles  expelled4 ,it was found
 that each -particle carried a positive 
charge of  #i4c x #aj-aj electrostatic  
units4 ,from a consideration of the     
experimental evidence of the charge     
carried by the ions in gases1 it was    
concluded that the -particle did carry  
two unit charges1 and that the  unit    
charge carried by the hydrogen atom was 
equal to #d4fe x #aj-aj  units4 ,from a 
comparison of the known value of e_/m   
for the -particle  with that of the     
hydrogen atom1 it follows that an       
-particle is a  projected atom of helium
 carrying two charges1 or1 to express it
 in  another way1 the -particle1 after  
its charge is neutralized1 is a  helium 
atom4    ,the data obtained from the    
counting experiments allow us to        
calculate  simply the magnitude of a    
number of important radioactive         
quantities4 ,it  was found that the     
calculated values of the life of radium1
 of the  volume of the emanation1 and of

 the heating effect of radium were in   
excellent agreement with the values     
found experimentally4 ,a test of the    
correctness of these methods of         
calculation was forthcoming shortly     
after the publication of these results4 
,rutherford and ,geiger  calculated1 on 
the assumption that the -particle was a 
helium atom1  that one gram of radium in
 equilibrium should produce a volume of 
#aeh  cubic millimetres of helium per   
year4 ,sir ,james ,dewar in #aijh       
carried  out a long experimental        
investigation on the rate of production 
of  helium by radium1 and showed that   
one gram of radium in equilibrium       
produced about #acd cubic millimetres   
per year4 ,considering the  difficulty  
of the investigation1 the agreement     
between the experimental  and calculated
 values is very good and is strong      
evidence in support of  the identity of 
the -particle with a helium atom4       
,while the whole train of evidence we   
have considered indicates with  little  

room for doubt that the -particle is a  
projected helium atom1  there was still 
wanting a decisive and incontrovertible 
proof of the  relationship4 ,it might be
 argued1 for example1 that the helium   
atom  appeared as a result of the       
disintegration of the radium atom in the
  same way as the atom of the emanation 
and had no direct connection with  the  
-particle4 ,if one helium atom were     
liberated at the same time that  an     
-particle was expelled1 experiment and  
calculation might still  agree and yet  
the -particle might be an atom of       
hydrogen or of some  unknown substance4 
,in order to remove this possible       
objection1 it is  necessary to show that
 the -particles1 collected quite        
independently  of the active matter from
 which they are expelled1 give rise to  
helium4  ,with this purpose in view some
 experiments were recently 7#aijh7 made 
by  ,rutherford and ,royds4 ,a large    
quantity of emanation was forced into a 
glass tube which had walls so thin that 

the -particles were fired  right through
 them1 though the walls were impervious 
to the emanation  itself4 ,the          
-particles were projected into the glass
 walls of an outer  sealed vessel and   
were gradually released into the        
exhausted space  between the emanation  
tube and the outer vessel4 ,after some  
days a  bright spectrum of helium was   
observed in the outer vessel4 ,there is1
  however1 one objection to this        
experiment4 ,it might be possible that  
the  helium observed had diffused       
through the thin glass walls from the   
emanation4 ,this objection was removed  
by showing that no trace of helium      
appeared1 when the emanation was        
replaced by a larger volume of helium   
itself4 ,we may thus confidently        
conclude that the -particles  themselves
 give rise to helium1 and are atoms of  
helium4 ,further  experiments showed    
that when the -particles were fired     
through the  glass walls into a thin    
sheet of lead or tin1 helium could      

always be  obtained from the metals     
after a few hours' bombardment4         
,considering the evidence together1 we  
conclude that the -particle is a        
projected atom of helium1 which has1 or 
in some way during its flight  acquires1
 two unit charges of positive           
electricity4 ,it is somewhat  unexpected
 that the atom of a monatomic gas like  
helium should carry a  double charge4   
,it must not however be forgotten that  
the -particle is  released at a high    
speed as a result of an intense atomic  
explosion1 and  plunges through the     
molecules of matter in its path4 ,such  
conditions are  exceptionably favourable
 to the release of loosely attached     
electrons  from the atomic system4 ,if  
the -particle can lose two electrons in 
this  way1 the double positive charge is
 explained4    ,we have seen that there 
is every reason to believe that the     
-particles1  so freely expelled from the
 great majority of radioactive          
substances1  are identical in mass and  

constitution and must consist of atoms  
of  helium4 ,we are consequently driven 
to the conclusion that the atoms of  the
 primary radioactive elements like      
uranium and thorium must be built  up in
 part at least of atoms of helium4      
,these atoms are released at  definite  
stages of the transformations at a rate 
independent of control  by laboratory   
forces4 ,there is good reason to believe
 that in the  majority of cases1 a      
single helium atom is expelled during   
the atomic  explosion4 ,this is         
certainly the case for radium itself and
 its series  of products4 ,on the other 
hand1 ,bronson has drawn attention to   
certain  cases1 viz4 the emanations of  
actinium and of thorium1 where          
apparently  two and three atoms of      
helium respectively are expelled at one 
time4 ,no  doubt these exceptions will  
receive careful investigation in the    
future4  ,it is of interest to note that
 uranium itself appears to expel two    
-particles for one from each of its     

products4 ,knowing the number of  atoms 
of helium expelled from the atom of each
 product1 we can at once  calculate the 
atomic weights of the products4 ,for    
example1 in the  uranium-ionium-radium  
series1 uranium expels two -particles   
and each of  the six following -ray     
products one1 i4e4 eight in all4 ,taking
 the  atomic weight of uranium as       
#bch4e1 the atomic weight of ionium     
should be  #bcj4e1 of radium #bbf4e1 of 
the emanation #bbb4e1 and so on4 ,it is 
of  interest to note that the atomic    
weight of radium deduced in this way is 
in close agreement with the latest      
experimental values4 ,the atomic  weight
 of the end-product of radium1 resulting
 from the transformation  of radium ,f  
7polonium7 should be #bch4e - #h x #d   
#bjf4e1 or a value  close to that for   
lead4 ,long ago1 ,boltwood suggested    
from examination of  analyses of old    
uranium minerals1 that lead was in all  
probability a  transformation product of
 the uranium-radium series4 ,the        

coincidence of  numbers is certainly    
striking1 but a direct proof of the     
production of  lead from radium will be 
required before this conclusion can be  
considered as definitely established4   
,it is very remarkable that a chemically
 inert element like helium should  play 
such a prominent part in the            
constitution of the atomic systems of   
uranium and thorium and radium4 ,it may 
well be that this property of  helium of
 forming complex atoms is in some way   
connected with its  inability to enter  
into ordinary chemical combinations4 ,it
 must not be  forgotten that uranium and
 thorium and each of their              
transformation  products must be        
regarded as distinct chemical elements  
in the ordinary  sense4 ,they differ    
from ordinary elements in the           
comparative instability  of their atomic
 systems4 ,the atoms break up           
spontaneously with great  violence1     
expelling in many cases an atom of      
helium at a high speed4 ,all  the       

evidence is against the view that       
uranium or thorium or radium can  be    
regarded as an ordinary molecular       
compound of helium with some known  or  
unknown element1 which breaks up into   
helium4 ,the character of the           
radioactive transformations and their   
independence of temperature and  other  
agencies have no analogy in ordinary    
chemical changes4    ,apart from their  
radioactivity and high atomic weight1   
uranium1 thorium1  and radium show no   
specially distinctive chemical          
behaviour4 ,radium for  example is      
closely allied in general chemical      
properties to barium4 ,it  is           
consequently not unreasonable to suppose
 that other elements may be  built up in
 part of helium1 although the absence of
 radioactivity may  prevent us from     
obtaining any definite proof4 ,on this  
view1 it may prove  significant that the
 atomic weights of many elements differ 
by four--the  atomic weight of          
helium--or a multiple of four4 ,time is 

too limited  to discuss in greater      
detail these and other interesting      
questions which  have been raised by the
 proof of the chemical nature of the    
-particle4      ,footnote #a3 ,e4       
,rutherford1 ,uranium radiation and the 
electrical  conduction produced by it1  
,phil4 ,mag41 #dg 7#ahii7 #aaf4         
,footnote #b3 ,e4 ,rutherford and ,f4   
,soddy1 ,phil4 ,mag41 #e 7#aijc71 #ajf4 
,footnote #c3 ,e4 ,rutherford and ,f4   
,soddy1 ,phil4 ,mag41 #d 7#aijb71 #ehb4 
,footnote #d3 ,e4 ,rutherford and ,f4   
,soddy1 ,phil4 ,mag41 #e 7#aijc71 #dec4 
,footnote #e3 ,e4 ,rutherford1 letter in
 ,nature1 #fi 7,aug4 #bj1 #aijc74