,,the ,,natural  ,,and ,,artificial     
,,disintegration  ,,of ,,the ,,elements 
,,an ,,address ,,by    ,professor ,sir  
,,ernest ,,rutherford    ,kt41 ,d4 ,sc41
 ,,ll4 ,d41 ,ph4 ,d41 ,d4 ,phys41 ,f4   
,r4 ,s4          ,,on ,,the ,,occasion  
,,of ,,the ,,centenary ,,celebration    
,,of ,,the ,,founding ,,of    ,,the     
,,franklin ,,institute    ,,and ,,the   
,,inauguration ,,exercises ,,of ,,the   
,,bartol ,,research ,,foundation        
,,september #ag1 #ah1 #ai1 #aibd        
,,the ,,franklin ,,institute            
,,philadelphia          ,,the ,,natural 
,,and ,,artificial ,,disintegration     
,,of ,,the ,,elements      ,by          
,professor ,sir ,,ernest ,,rutherford1  
,kt41 ,d4 ,sc41    ,,ll4 ,d41 ,ph4 ,d41 
,d4 ,phys41 ,f4 ,r4 ,s4      ,,it is not
 my intention in this paper to give a   
detailed account of the  natural        
disintegration of the radio elements or 
of the methods employed  to effect the  
artificial disintegration of certain    
light elements4 ,i  shall assume that   

you all have a general knowledge of the 
results of  these investigations1 but ,i
 shall confine myself to a consideration
 of  the bearing of these results on our
 knowledge of the structure of the      
nuclei of atoms4    ,there is now a     
general agreement that the atoms of all 
elements have a  similar electrical     
structure1 consisting of a central      
positively  charged nucleus surrounded  
at a distance by the appropriate number 
of  electrons4 ,from a study of the     
scattering of  particles by the atoms of
  matter and from the classical         
researches of ,moseley on ,x-ray        
spectra1 we  know that the resultant    
positive charge on the nucleus of any   
atom1 in  terms of the fundamental unit 
of electronic charge1 is given          
numerically  by the atomic or ordinal   
number of the element1 due allowance    
being made  for missing elements4 ,we   
know that with few exceptions all       
nuclear  charges1 from #a for the       
lightest atom1 hydrogen1 to #ib for the 

heaviest  element1 uranium1 are         
represented by elements found in the    
earth4 ,the  nuclear charge of an       
element controls the number and         
distribution of the  external electrons1
 so that the properties of an atom are  
defined by a  whole number1 representing
 its nuclear charge1 and are only to a  
minor  degree influenced by the mass or 
atomic weight of the atom4    ,this     
minute but massive nucleus is1 in a     
sense1 a world of its own which  is     
little1 if at all1 influenced by the    
ordinary physical and chemical  forces  
at our command4 ,in many respects1 the  
problem of nuclear  structure is much   
more difficult than the corresponding   
problem of the  arrangement and motions 
of the planetary electrons1 where we    
have a  wealth of available information1
 both physical and chemical1 to test the
  adequacy of our theories4 ,the facts  
known about the nucleus are few in      
number and the methods of attack to     
throw light on its structure are        

limited in scope4    ,it is convenient  
to distinguish between the properties   
assigned to the  nucleus and the        
planetary electrons4 ,the movements of  
the outer  electrons are responsible for
 the ,x-ray and optical spectra of the  
elements and their configuration for the
 ordinary physical and chemical         
properties of the element4 ,on the other
 hand1 the phenomena of  radioactivity  
and all properties that depend on the   
mass of the atom are  to be definitely  
assigned to the nucleus4 ,from a study  
of the  radioactive transformations1 we 
know that the nucleus of a heavy atom   
not only contains positively charged    
bodies but also negative electrons1  so 
that the nuclear charge is the excess of
 positive charge over  negative4 ,in    
recent years1 the general idea has      
arisen that there are  two definite     
fundamental units that have to do with  
the building up of  complex nuclei1     
viz41 the light negative electron and   
the relatively  massive hydrogen nucleus

 which is believed to correspond to the 
positive  electron4    ,this view has   
received very strong support from the   
experiments of ,aston  on ,isotopes in  
which he has shown that the masses of   
the various species  of atoms are       
represented nearly by whole numbers in  
terms of ,o  #af4  ,from the general    
electric theory1 it is to be anticipated
 that the mass  of the hydrogen nucleus 
in the nucleus structure will be        
somewhat less  than its value #a4jjgg in
 the free state on account of the very  
close  packing of the charged units in  
the concentrated nucleus4 ,from ,aston's
  experiments1 it appears that the      
average mass of the hydrogen nucleus1   
or proton as it is now generally called1
 is very nearly #a4jjj under  these     
conditions4 ,we should anticipate that  
the whole number rule found  by ,aston  
would hold only to a first              
approximation1 since the mass of the    
proton must be to some extent dependent 
on the detailed structure of the        

nucleus4 ,in the case of tin and xenon  
,aston has already signalized a         
definite departure from the whole number
 rule1 and no doubt a still more        
accurate determination of the masses of 
the atoms will disclose other           
differences of a similar kind4    ,while
 our present evidence indicates that the
 proton and electron are  the           
fundamental constituents of the nucleus1
 it is very probable that  secondary    
combining units play a prominent part in
 nuclear constitution4  ,for example1   
the expulsion of helium nuclei from the 
radioactive bodies  indicates that the  
helium nucleus of mass #d is probably a 
secondary unit  of great importance in  
atom building4 ,on the views outlined1  
we should  expect the helium nucleus of 
charge to be built up of four protons   
and  two electrons4 ,the loss of mass in
 forming this nucleus indicates that a  
large amount of energy must be liberated
 during its formation4 ,if this  be the 
case1 the helium nucleus must be such a 

stable structure that the  combined     
energy of four or five of the swiftest  
particles would be  necessary to effect 
its disruption4 ,such a deduction is    
supported by our  failure to observe any
 evidence of disintegration of the swift
 particle  itself1 whether it is used to
 bombard matter or whether the  particle
  is used to bombard other helium atoms4
    ,on these views1 we should          
anticipate that the nucleus of radium of
  atomic number #hh and atomic weight   
#bb4f contains in all #bbf protons of   
mass #a and #ach electrons4 ,while this 
gives us the numerical relation  between
 the two fundamental units1 we have1 at 
present1 no definite  information of    
their arrangement in the minute nuclear 
volume1 nor of  the nature and magnitude
 of the forces that hold them together4 
,we  should anticipate that many of the 
protons and electrons unite to form     
secondary units1 e4 g4 helium nuclei1   
and that the detailed structure  of the 
nucleus may be very different from that 

to be expected if it  consists of a     
conglomeration of free protons and      
electrons4    ,it is thus of great      
importance to obtain definite evidence  
of the nature  and arrangement of the   
components of the nucleus and of the    
forces that  hold them in equilibrium4  
,we shall now consider some of the lines
 of  evidence which throw light on the  
actual dimensions of the nucleus and    
the law of force operative in its       
neighborhood2 the structure and modes   
of vibration of the nucleus1 together   
with the effects observed when  some    
light nuclei are disintegrated by       
bombardment with  particles4            
,,dimensions ,,of ,,the ,,nuclei ,,and  
,,the  ,,law ,,of ,,force      ,the     
conception of the nucleus atom had its  
origin in #aiaa in order to  explain the
 scattering of an  particle through a   
large angle as the  result of a single  
collision4 ,the observation that the    
particle is in  some cases deflected    
through more than a right angle as the  

result of an  encounter with a single   
atom first brought to light the intense 
forces  that exist close to the nucleus4
 ,geiger and ,marsden showed that the   
number of particles scattered through   
different angles was in close  accord   
with the simple theory which supposed   
that1 for the distance  involved1 the   
particle and nucleus behaved like       
charged points1  repelling each other   
according to the law of the inverse     
square4 ,the  accuracy of this law has  
been independently verified by          
,chadwick1 so  that we are now certain  
that in a region close to the nucleus   
the  ordinary laws of force are valid4  
,these scattering experiments also gave 
us the first idea as to the  probable   
dimensions of the nuclei of heavy atoms1
 for it is to be  anticipated that the  
law of the inverse square must break    
down if the   particle approaches       
closely to or actually enters the       
nuclear structure4  ,this variation in  
the law of force would show itself by a 

difference  between the observed and    
calculated numbers of  particles        
scattered  through large angles4 ,geiger
 and ,marsden1 however1 observed no     
certain  variation even when the        
particles of range about #d cms4 were   
scattered through #ajj by a gold        
nucleus4 ,in such an encounter1 the     
closest distance of approach of the     
particle to the center of the  nucleus  
is about #e x #aj-ab cm41 so that it    
would appear that the radius  of the    
gold nucleus1 assumed spherical1 could  
not be much greater than  this value4   
,there is another argument1 based on    
radioactive data1 which gives a  similar
 value for the dimensions of the radius 
of a heavy atom4 ,the   particle        
escaping from the nucleus increases in  
energy as it passes  through the        
repulsive field of the nucleus4 ,to fix 
a minimum limit1  suppose the  particle 
from uranium1 which is the slowest of   
all   particles expelled from a nucleus1
 gains all its energy from the          

electrostatic field4 ,it can be         
calculated on these data that the radius
  of the uranium nucleus cannot be less 
than #f x #aj-ab cm4 ,this is based  on 
the assumption that the forces outside  
the nucleus are repulsive and  purely   
electrostatic4 ,if1 as seems not        
unlikely1 there also exist close  to the
 nucleus strong attractive forces1      
varying more rapidly than an  inverse   
square law1 the actual dimensions may be
 less than the value  calculated above4 
,at this stage of our knowledge it is of
 great importance to test whether  the  
law of force breaks down for the        
distance of closest approach of an      
particle to a nucleus4 ,this can be done
 by comparing the observed  with the    
calculated number of  particles         
scattered through angles of  nearly     
#ahj4 ,it seems almost certain that the 
inverse square law must  break down when
 swift  particles are used4 ,this can be
 seen from the  following argument4 ,if 
an  particle1 of the same speed as that 

ejected  during the transformation of   
uranium1 is fired directly at the       
uranium  nucleus1 it must penetrate into
 the nuclear structure4 ,if a still     
swifter  particle is used1 e4 g4 that   
from radium ,c1 which has about  twice  
the energy of the uranium  particle1 it 
is clear that it must  penetrate still  
more deeply into the nuclear structure4 
,this is based on  the assumption that  
the field due to a nucleus is           
approximately  symmetrical in all       
directions4 ,if this is not true1 it may
 happen that  only a fraction of the    
head-on collisions may be effective in  
penetrating the nucleus4 ,it is hoped   
soon to attack this difficult  problem  
experimentally4    ,we have so far dealt
 with collisions of an  particle with a 
heavy  atom4 ,we know1 however1 from the
 results of ,rutherford1 ,chadwick and  
,bieler that in a collision of an       
particle with the lightest atom1        
hydrogen1 the law of the inverse square 
breaks down entirely when swift         

particles are used4 ,not only are the   
numbers of ,h nuclei set in swift       
motion much greater than is to be       
expected in the simple-point nucleus    
theory1 but the change of number with   
the velocity of the  particle  varies in
 the opposite way from the simple       
theory4 ,such wide departures  between  
theory and experiment are only          
explicable if we assume either  that the
 nuclei have sensible dimensions or that
 the inverse square law  of repulsion   
entirely breaks down in such close      
collisions4 ,if we  suppose the         
complexity in structure and in laws of  
force is to be  ascribed to the         
particle rather than to the hydrogen    
nucleus1  ,chadwick and ,bieler1 as the 
result of a careful series of           
experiments1  concluded that the        
particle behaved as if it were a        
perfectly elastic  body1 spheroidal in  
shape with its minor axis #d x #aj-ac   
cm4 in the  direction of motion and     
major axis #h x #aj-ac cm4 ,outside this

  spheroidal region the forces fell off 
according to the ordinary inverse       
square law1 but inside this region the  
forces increased so rapidly that  a     
particle was reflected from it as from a
 perfectly elastic body4 ,no  doubt such
 a conception is somewhat artificial1   
but it does serve to  bring out the     
essential points involved in the        
collision1 viz41 that  when the nuclei  
approach within a certain critical      
distance of each  other1 forces come    
into play which vary more rapidly than  
the inverse  square4 ,it is difficult to
 ascribe this break-down of the law of  
force  merely to the finite size or     
complexity of the nuclear structure or  
to  its distortion1 but the results     
rather point to the presence of new and 
unexpected forces which come into play  
at such small distances4 ,this  view has
 been confirmed by some recent          
experiments of ,bieler in the           
,cavendish ,laboratory in which he has  
made1 by scattering methods1 a  detailed

 examination of the law of force in the 
neighborhood of a light  nucleus like   
that of aluminum4 ,for this purpose he  
compared the relative  number of        
particles scattered within the same     
angular limit from  aluminum and from   
gold4 ,for the range of angles employed1
 viz41 up to  #ajj1 it is assumed that  
the scattering of gold follows the      
inverse  square law4 ,he found that the 
ratio of the scattering in aluminum     
compared with that in gold depended on  
the velocity of the  particle4  ,for    
example1 for an  particle of #c4d cms4  
range1 the theoretical ratio  was       
obtained for angles of deflection below 
#dj but was about #g per  cent lower for
 an average angle of deflection of #hj4 
,on the other  hand1 for swifter        
particles of range #f4f cms4 a departure
 from the  theoretical ratio was much   
more marked and amounted to #bi per cent
 for  an angle of #hj4 ,in order to     
account for these results he supposes   
that  close to the aluminum nucleus an  

attractive force is superimposed on the 
ordinary repulsive forces4 ,the results 
agreed best with the assumption  that   
the attractive force varies according to
 the inverse fourth power  of the       
distance and that the forces of         
attraction and repulsion balanced  at   
about #c4d x #aj-ac cm4 from the nuclear
 center4 ,inside this critical  radius  
the forces are entirely attractive2     
outside they are repulsive4    ,while we
 need not lay too much stress on the    
accuracy of the actual  value obtained  
or of the law of attractive force1 we   
shall probably not  be far in error in  
supposing the radius of the aluminum    
nucleus is not  greater than #d x #aj-ac
 cm4 ,it is of interest to note that the
 forces  between an  particle and a     
hydrogen nucleus were found to vary     
rapidly  at about the same distance4    
,it thus seems clear that the dimensions
 of the nuclei of light atoms are       
small1 and almost unexpectedly small in 
the case of aluminum when we  remember  

that #bg protons and #ad electrons are  
concentrated in such a  minute region4  
,the view that the forces between nuclei
 change from  repulsion to attraction   
when they are very close together seems 
very  probable1 for otherwise it is     
exceedingly difficult to understand why 
a  heavy nucleus with a large excess of 
positive charge can hold together  in   
such a confined region4 ,we shall see   
that the evidence from various  other   
directions supports such a conception1  
but it is very unlikely  that the       
attractive forces close to a complex    
nucleus can be expressed  by any simple 
power law4          ,,radioactive       
,,evidence      ,a study of the long    
series of transformations which occur in
 uranium and  thorium provides us with a
 wealth of information on the modes of  
disintegration of atoms1 but            
unfortunately our theories of nuclear   
structure are not sufficiently advanced 
to interpret these data with any        
detail4 ,the expulsion of high speed    

and  particles from the  radioactive    
nucleus gives us some idea of the       
powerful forces resident  in the        
nucleus1 for it can be estimated that   
the energy of emission of  the  particle
 is in some cases greater than the      
energy that would be  acquired if the   
particle fell freely between two points 
differing in  potential by about #d     
million volts4 ,the energies of the  and
  rays  are on a similar scale of       
magnitude4    ,notwithstanding our      
detailed knowledge of the successive    
transformation  of the radio-elements1  
we have not so far been able to obtain  
any  definite idea of their nuclear     
structure1 while the cause of the       
disintegration is still a complete      
enigma4 ,in comparing the uranium1      
thorium1 and actinium series of         
transformations1 one cannot fail to be  
struck by the many points of similarity 
in their modes of  disintegration4 ,not 
only are the radiations similar in type 
and in  energy1 but1 in all cases1 the  

end product is believed to be an isotope
  of lead4 ,this remarkable similarity  
in the modes of transformation is       
especially exemplified in the case of   
the ,c bodies1 each of which is  known  
to break up in at least two distinct    
ways1 giving rise to branch  products4  
,for example1 thorium ,c emits two types
 of  rays1 #fe percent  of range #h4f   
cms4 and #ce per cent of range #d4h     
cms41 and in addition  some  rays4      
,in order to explain these results1 it  
has been suggested that a fraction  of  
the atoms of thorium ,c break up first  
with the expulsion of an   particle and 
the resulting product then emits a      
particle4 ,the other  fraction breaks up
 in a reverse way1 first expelling a    
particle1  while the subsequent product 
emits an  particle4 ,similar dual       
changes  occur in radium ,c and actinium
 ,c1 although the relative number of    
atoms  in each branch varies widely for 
the different elements4    ,this        
remarkable similarity between the ,c    

bodies is still further  emphasized by  
the recent discovery of ,bates and      
,rogers that both radium  ,c and thorium
 ,c give rise in small numbers to other 
groups of   particles1 some of them     
moving at very high speeds4    ,it has  
often been a matter of remark that the  
radioactive properties of  the ,c bodies
 seem to depend more on the atomic      
number1 i4 e41 the  nuclear charge1 than
 on the atomic weight4 ,confining our   
attention to  radium ,c and thorium ,c1 
which are best known1 both have a       
nuclear charge  #hc1 but the atomic mass
 of radium ,c is #bad and of thorium ,c 
#bab4 ,the  nucleus of radium ,c thus   
contains two protons and two electrons  
more  than that of thorium ,c4 ,if it   
were supposed that the nuclei of these  
elements consisted of a large number of 
charged units in ceaseless and          
irregular motion1 it is to be           
anticipated that the addition of the    
protons and electrons to the complex    
structure would entirely alter the      

nuclear arrangement and consequently its
 stability and mode of  transformation4 
,on the other hand1 we find that the    
modes of  transformation of these two   
nuclei have striking and unexpected     
points  of resemblance which are in     
entire disaccord with such a            
supposition4 ,we  can1 however1 suggest 
a possible explanation of this anomaly  
by  supposing that the  and  particles  
which are liberated from these  elements
 are not built deep into the nuclear    
structure but exist as  satellites of a 
central core which is common to both    
elements4 ,these  satellites1 if in     
motion1 may be held in equilibrium by   
the attractive  forces arising from the 
core1 and these forces would be the same
 for  both elements4 ,on this view the  
manifestations of radioactivity are to  
be ascribed not to the main core1 but to
 the satellite distribution1  which must
 be somewhat different for the two      
elements although possibly  showing many
 points of similarity4 ,it must be      

admitted that a theory of  this kind is 
highly speculative1 but it does provide 
a useful working  hypothesis1 not only  
to account for the similarity of the    
modes of  transformation of the two     
elements but also immediately suggests a
  possible explanation of the liberation
 of a number of  particles of  different
 ranges from the same element4 ,there   
are two ways of regarding  this         
question4 ,we may in the first place    
suppose that a certain amount  of       
surplus energy has to be liberated in   
the disintegration and that  this energy
 may be given to any one of a number of 
satellites4 ,there  will be a certain   
probability that any particular particle
 will be given  this energy1 and on this
 will depend the relative number of     
particles in  the different  ray groups4
 ,the ultimate energy of ejection of an 
particle will depend on its position in 
the field of force surrounding  the     
inner core at the moment of its         
liberation4 ,on the other hand1 we  may 

suppose that the same  particle is      
always ejected but that the  particle   
may occupy in the atom one of a number  
of stationary  positions analogous to   
the stationary states of the electrons  
in  ,bohr's theory of the outer atom4   
,this rests on the assumption that all  
the atoms will not be identical in      
satellite structure but there will be  a
 number of possible excited states of   
the atom as a consequence of  the       
previous disintegrations4 ,this         
satellite theory is useful in another   
connection4 ,it has been suggested that 
possibly the high frequency   rays from 
a radioactive atom may arise not from   
the movement of the  electrons as       
ordinarily supposed1 but from the       
transfer of  particles  from one level  
to another4 ,in such a case1 the        
difference in energies  between the     
various groups of  particles from radium
 ,c and thorium ,c  should be connected 
by the quantum relation with the        
frequencies of  prominent  rays4 ,the   

evidence at present available is not    
definite  enough to give a final        
decision on this problem1 but points to 
the need  of very accurate measurements 
of the energies of the various groups of
   particles4 ,on account of the        
relatively small number of particles in 
some of the groups1 this is difficult of
 accomplishment4    ,in considering the 
satellite theory in connection with the 
radioactive  bodies1 it is at first     
sight natural to suppose1 since the end 
product  of both the radium and thorium 
series is an isotope of lead1 that one  
of  the isotopes of lead forms the      
central core4 ,it may1 however1 well be 
that the radioactive processes cease    
when there are still a number of        
satellites remaining4 ,if this be so1   
the core may be of smaller nuclear      
charge and mass than that of lead4 ,from
 some considerations1 described  later1 
this core may correspond to an element  
near platinum of number #gg  and mass   
#aib4          ,,frequency ,,of         

,,vibration ,,of ,,the ,,nucleus        
,one of the most interesting and        
important methods of throwing light on  
nuclear structure is the study of the   
very penetrating  rays expelled  by some
 radioactive bodies4 ,the  rays are     
identical in nature with  ,x-rays1 but  
the most penetrating type of rays       
consists of waves of much  higher       
frequency than can be produced in an    
ordinary ,x-ray tube4 ,the  work of the 
last few years has indicated very       
clearly that the major  part of the     
radiation from bodies like radium ,b and
 ,c originates in  the nucleus4 ,a      
determination of the frequencies of the 
rays thus  gives us direct information  
on the modes of vibration of parts of   
the  nuclear structure4 ,the frequency  
of some of the softer  rays excited  by 
radium ,b and radium ,c was measured by 
the crystal method by  ,rutherford and  
,andrade1 but it is difficult1 if not   
impossible1 by this  method to determine
 the frequencies of the very penetrating

 rays4  ,fortunately1 due largely to the
 work of ,ellis and ,frulein ,meitner1 a
  new and powerful method has been      
devised for this purpose4 ,it is well   
known that the  rays from radium ,b and 
radium ,c give a veritable  spectrum in 
a magnetic field1 showing the presence  
of a number of groups  of  rays each    
expelled with a definite speed4 ,it is  
clear that each of  the groups of  rays 
arises from conversion of the energy of 
a  ray  of definite frequency into a    
ray in one or other of the electronic   
levels in the outer atom4 ,the energy   
required to move an electron  from one  
of these levels to the outside of the   
atom is known from a  study of ,x-ray   
absorption spectra4 ,the frequency  of  
the  ray is  thus given by the quantum  
relation h  ,e  1 where ,e is the       
measured  energy of the  particle4      
,since each  ray may be converted in any
 one of the known electronic  levels in 
the outer atom1 a single  ray is        
responsible for the  appearance of a    

number of groups of  rays1 corresponding
 to conversion  in the ,k1 ,l1 ,m1 etc41
 levels4 ,in this way1 an analysis of   
the  ray  spectrum allows us to fix the 
frequency of the more intense  rays     
which are emitted from the nucleus4 ,the
 energy of the shortest wave  measured  
in this way by ,ellis corresponds to    
more than two million  volts1 while     
other evidence shows that probably still
 shorter waves are  emitted in small    
quantity from radium ,c4    ,ellis and  
,skinner have shown that the energies of
 these rays show  certain combination   
differences1 such as are so             
characteristic of the  energies of the  
,x-rays arising from the outer          
electrons4 ,a series of  energy levels  
may thus be postulated in the nucleus   
similar in character  to the electron   
levels of the outer atom1 and the  rays 
have their  origin in the fall either of
 an electron or of an  particle between 
these levels4 ,this is a significant and
 important result1 indicating  that the 

quantum dynamics can be applied to the  
nucleus as well as to  the outer        
electronic structure4    ,the           
probability of levels in the nuclear    
structure is most clearly seen  on the  
satellite hypothesis1 but in our        
ignorance of the laws of force  near the
 core we are at the moment unable to    
apply the quantum dynamics  directly to 
the problem4 ,the outlook for further   
advances in this  direction is hopeful1 
but is intimately connected with a      
further  development of our knowledge of
 the laws of force that come into play  
close to the nucleus in the region      
occupied by the satellites4             
,,artificial ,,disintegration ,,of      
,,elements      ,we have seen that it is
 believed that the nuclei of all atoms  
are  composed of protons and electrons  
and that the number of each of these    
units in any nucleus can be deduced from
 its mass and nuclear charge4 ,it  is1  
however1 at first sight rather          
surprising that no evidence of the      

individual existence of protons in a    
nucleus is obtained from a study of  the
 transformations of the radioactive     
elements1 where the processes  occurring
 must be supposed to be of a very       
fundamental character4 ,as far  as our  
observations have gone1 electrons and   
helium nuclei1 but no  protons1 are     
ejected during the long series of       
transformations of  uranium1 thorium and
 actinium4 ,one of the most obvious     
methods for  determining the structure  
of a nucleus is to find a method of     
disintegrating it into its component    
parts4 ,this is done spontaneously  for 
us by nature to a limited extent in the 
case of the heavy  radioactive elements1
 but evidence of this character is not  
available in  the case of the ordinary  
elements4    ,as the swift  particle    
from the radioactive bodies is1 by far1 
the  most energetic projectile known to 
us1 it seemed from the first possible   
that occasionally the nucleus of a light
 atom might be disintegrated as  the    

result of a close collision with an     
particle4 ,on account of the  minute    
size of the nucleus1 it is to be        
anticipated that the chance of  a direct
 hit would be very small and that       
consequently the  disintegration        
effects1 if any1 would be observed only 
on a very minute  scale4 ,during the    
last few years ,dr4 ,chadwick and ,i    
have obtained  definite evidence that   
hydrogen nuclei or protons can be       
removed by  bombardment of  particles   
from the elements boron1 nitrogen1      
fluorine1  sodium1 aluminum and         
phosphorus4 ,in these experiments the   
presence of ,h  nuclei is detected by   
the scintillation method1 and their     
maximum  velocity of ejection can be    
estimated from the thickness of matter  
which  can be penetrated by these       
particles4 ,the number of ,h nuclei     
ejected  even in the most favorable case
 is relatively very small compared with 
the number of bombarding  particles1    
viz41 about one in a million4    ,in    

these experiments the material subject  
to bombardment was placed  immediately  
in front of the source of  particles and
 observations on  the ejected particles 
were made on a zinc sulphide screen     
placed in a  direct line a few          
centimetres away4 ,using radium ,c as a 
source of   rays1 the ranges of         
penetration1 expressed in terms of      
centimetres of  air1 were all in these  
cases greater than the range of free    
nuclei 7#cj  cms4 in air7 set in motion 
in hydrogen by the  particles4 ,by      
inserting  absorbing screens of #cj cms4
 air equivalent in front of the zinc    
sulphide screen the results were quite  
independent of the presence of  either  
free or combined hydrogen as an impurity
 in the bombarded  materials4 ,some of  
the lighter elements were examined for  
absorptions  less than this1 but1 in    
general1 the number of ,h particles due 
to  hydrogen contamination of the source
 and the materials was so large that  no
 confidence could be placed in the      

results4    ,in such experiments many   
scintillations can be observed1 but it  
is very  difficult to decide whether    
these can be ascribed in part to an     
actual  disintegration of the material  
under examination4 ,the presence of     
long-range particles of the  ray type   
from the source of radium ,c  still     
further complicates the question1 since 
in general the number of  such particles
 is large compared with the             
disintegration effect we  usually       
observe4    ,to overcome these          
difficulties1 inherent in the direct    
method of  observation1 ,dr4 ,chadwick  
and ,i have devised a simple method by  
which we  can observe with certainty the
 disintegration of an element when the  
ejected particles have a range of only  
#g cms4 in air4 ,this method is  based  
on the assumption1 verified in our      
previous experiments1 that the          
disintegration particles are emitted in 
all directions relative to the  incident
 rays4 ,a powerful beam of  rays falls  

on the material to be  examined and the 
liberated particles are observed at an  
average angle of  #ij to the direction  
of the incident  particles4 ,by means of
 screens  it is arranged that no        
particles can fall directly on the zinc 
sulphide screen4    ,this method has    
many advantages4 ,we can now detect     
particles of range  more than #g cms4   
with the same certainty as particles of 
range above #cj  cms4 in our previous   
experiments1 for the presence of        
hydrogen in the  bombarded material has 
no effect4 ,this can be shown at once by
  bombarding a screen of paraffin wax1  
when no particles are observed on  the  
zinc sulphide screen4 ,on account of the
 very great reduction in  number of ,h  
nuclei or  particles by scattering      
through #ij1 the  results are quite     
independent of ,h nuclei from the source
 or of the  long-range  particles4 ,the 
latter are just detectable under our    
experimental conditions when a heavy    
element like gold is used as  scattering

 material1 but are inappreciable for the
 lighter elements4    ,a slight         
modification of the arrangement enables 
us to examine gases as  well as solids4 
,working in this way we have found that 
in addition to the elements  boron1     
nitrogen1 fluorine1 sodium1 aluminum1   
and phosphorus1 which give  ,h particles
 of maximum range in the forward        
direction between #dj and #ij  cms41 the
 following give particles of range above
 #g cms43 neon1  magnesium1 silicon1    
sulphur1 chlorine1 argon1 and potassium4
 ,the numbers  of the particles emitted 
from these elements are small compared  
with the  number from aluminum under the
 same conditions1 varying between  and  
4 ,the ranges of the particles have not 
been determined with  accuracy4 ,neon   
appears to give the shortest range1     
about #af cms41 under  our conditions1  
the ranges of the others lying between  
#ah cms4 and #cj  cms4 ,by the kindness 
of ,dr4 ,rosenhain we were able to make 
experiments  with a sheet of metallic   

beryllium4 ,this gave a small effect1   
about   of that of aluminum1 but we are 
not yet certain that it may  not be due 
to the presence of a small quantity of  
fluorine as an  impurity4 ,the other    
light elements1 hydrogen1 helium1       
lithium1 carbon1  and oxygen1 give no   
detectable effect beyond #g cms4 ,it is 
of interest to  note that while carbon  
and oxygen give no effect1 sulphur1 also
 probably  a pure element of mass #dn1  
gives an effect of nearly one-third that
 of  aluminum4 ,this shows clearly that 
the sulphur nucleus is not built up     
solely of helium nuclei1 a conclusion   
also suggested by its atomic  weight of 
#cb4jg4    ,we have made a preliminary  
examination of the elements from calcium
 to  iron1 but with no definite results1
 owing to the difficulty of obtaining   
these elements free from any of the     
active elements1 in particular1         
nitrogen4 ,for example1 while a piece of
 electrolytic iron gave no  particles   
beyond #g cms41 a piece of ,swedish iron

 gave a large effect1  which was        
undoubtedly due to the presence of      
nitrogen1 for after  prolonged heating  
in vacuo the greater part disappeared4  
,similar  results were experienced with 
the other elements in this region4      
,we have observed no effects from the   
following elements3 nickel1 copper1     
zinc1 selenium1 krypton1 molybdenum1    
palladium1 silver1 tin1 xenon1 gold  and
 uranium4 ,the krypton and xenon were   
kindly lent by ,dr4 ,aston4             
,,examination ,,of ,,light ,,elements   
,,for ,,particles  ,,of ,,range ,,less  
,,than #c ,,cms4 ,,of ,,air      ,when  
particles are scattered from light      
elements1 the simple theory  shows that 
the velocity of the scattered particles 
depends on the angle  of scattering4    
,for example1 using bombarding          
particles of range #g  cms41 the range  
of the  particles scattered through more
 than #ij  cannot be greater than #a4j  
cm4 for lithium 7#g71 #b4j cms4 for     
beryllium  7#aj71 #b4e cms4 for carbon1 

#c4b cms4 for oxygen1 #d4c cms4 for     
aluminum1  and #f4h cms4 for gold4      
,provided we introduce sufficient       
thickness of absorber to stop the       
particles scattered through #ij1 we can 
examine for disintegrated  particles    
from carbon1 for example1 whose range   
exceeds #b4e cms4 ,certain  difficulties
 arise in this type of experiment which 
are absent when the  thickness of       
absorber is greater than #g cms42 any   
heavy element present  as an impurity   
will give scattered  particles of range 
greater than  those from carbon and thus
 complicate the observations4 ,in       
addition1  serious troubles may arise   
due to the volatilization or escape of  
active  matter from the source4 ,this is
 especially marked if the vessel        
containing the radioactive source is    
exhausted4 ,to overcome this            
difficulty1 we have found it desirable  
to cover the source with a thin  layer  
of celluloid of #b or #c mm4 stopping   
power for  rays4 ,by this  procedure we 

have been able to avoid serious         
contamination and to  examine the       
lighter elements by this method4 ,we    
have been unable to  detect any         
appreciable number of particles from    
lithium or carbon for  ranges greater   
than #c cms4 ,if carbon shows any effect
 at all1 it is  certainly less than one 
tenth of the number from aluminum under 
the same  conditions4 ,this is in entire
 disagreement with the work of ,kirsch  
and  ,patterson 7,nature1 ,april #bf1   
#aibd71 who found evidence of a large   
number  of particles from carbon of     
range #f cms4 ,a slight effect was      
observed in  beryllium in accordance    
with our other experiments4 ,no effect  
was noted  in oxygen gas4 ,apart from   
beryllium1 no certain effect has been   
noted  for elements lighter than boron4 
,under the conditions of our experiment1
 it seems clear that neither ,h  nuclei 
nor other particles of range greater    
than #c cms4 can be liberated  in       
appreciable numbers from these elements 

in a direction at right  angles to the  
bombarding  rays4 ,this is1 in a sense1 
a disappointing  result1 for1 unless    
these elements are very firmly bound    
structures1 it  was to be anticipated   
that an  particle bombardment would     
resolve them  into their constituent    
particles4    ,we hope to examine this  
whole question still more thoroughly1 as
 it is a  matter of great importance to 
the theory of nuclear constitution to be
  certain whether or not the light      
elements can be disintegrated by swift  
particles4    ,in considering the       
results of our new and old observations1
 some points  of striking interest      
emerge4 ,in the first place1 all the    
elements from  fluorine to potassium    
inclusive suffer disintegration under   
ray  bombardment4 ,as far as our        
observations have gone1 there seems     
little  doubt that the particles ejected
 from all these elements are ,h nuclei4 
,the odd elements1 ,b1 ,n1 ,f1 ,na1 ,al1
 ,p1 all give long-range particles      

varying in range from #dj cms4 to #ij   
cms4 in the forward direction1 the  even
 elements1 ,c1 ,o1 ,ne1 ,mg1 ,si1 ,s1   
either give few particles or none at    
all as in the case of ,c and ,o1 or give
 particles of much less range than  the 
adjacent odd numbered elements4 ,the    
differences between the ranges  of      
even-odd elements become much less      
marked for elements heavier than        
phosphorus4    ,this obvious difference 
in velocity of expulsion of the ,h      
nuclei from  even and odd elements is a 
matter of great interest4 ,such a       
distinction  can be paralleled by other 
observations of an entirely different   
character4 ,harkins has shown that      
elements of even atomic number are  much
 more abundant in the earth's crust than
 elements of odd atomic  number4 ,in his
 study of ,isotopes1 ,aston has shown   
that in general odd  numbered elements  
have only two isotopes differing in mass
 by two units1  while even numbered     
elements in some cases contain a large  

number of  isotopes4 ,this remarkable   
distinction between even and odd        
elements  cannot but excite a lively    
curiosity1 but we can at present only   
speculate on its underlying cause4      
,,velocity ,,of ,,escape ,,of ,,hydrogen
 ,,nuclei      ,we have seen that the   
experiments of ,bieler on the scattering
 of  rays  by aluminum and magnesium    
indicate that a powerful attractive     
force  comes into play very close to the
 nuclei of these atoms4 ,if this be the 
case1 the forces of attraction and      
repulsion must balance at a certain     
distance from the nucleus4 ,outside this
 critical point the forces on a         
positively charged body are entirely    
repulsive4 ,certain important           
consequences follow from this general   
view of nuclear forces4 ,suppose1  for  
example1 that1 due to a collision with a
 swift  particle1 a  hydrogen nucleus is
 liberated from the nuclear structure4  
,after passing  across the critical     
surface1 it will acquire energy in      

passing through  the repulsive field4   
,it is clear1 on this view1 that the    
energy of a  charged particle after     
escape from the atom cannot be less than
 the  energy acquired in the repulsive  
field2 consequently we should expect to 
find evidence that there is a minimum   
velocity of escape of a  disintegration 
particle4 ,we have obtained definite    
evidence of such an  effect both in     
aluminum and sulphur by examining the   
absorption of ,h  nuclei from these     
elements4 ,the number of scintillations 
for a thin film  was found to be nearly 
constant for absorption between #g and  
#ab cms41  but falls off rapidly for    
greater thicknesses4 ,this is exactly   
what is  to be expected on the views    
outlined4 ,no doubt the limiting        
velocity  varies somewhat for the       
different elements1 but a large amount  
of  experiment will be required to fix  
this limit with accuracy4 ,from these   
results it is possible to form a rough  
estimate of the potential of the  field 

at the critical surface1 and this comes 
out to be about #c million  volts for   
aluminum4 ,the value for sulphur is     
somewhat greater4 ,this  brings out in a
 striking way the extraordinary         
smallness of the nuclei  of these       
elements1 for it can be calculated that 
the critical surface  cannot be distant 
more than #f x #aj-ac cm4 from the      
centre of the  nucleus4 ,these          
deductions of the critical distance are 
in excellent  accord with those made by 
,bieler from observations of the        
scattering of   particles4    ,another  
important consequence follows4 ,it is   
clear that an  particle  fired at the   
nucleus will not be able to cross this  
critical surface and  thus be in a      
position to produce disintegration1     
unless its velocity  exceeds that       
corresponding to the critical potential4
 ,in an experiment  made a few years    
ago1 we found that the number of ,h     
nuclei liberated  from aluminum fell off
 rapidly with diminution of the velocity

 of the   particle and was too small in 
number to detect when the range of the  
particle was less than #d4i cms4 ,this  
corresponds to the energy of an         
particle falling between about #c       
million volts--a value in good accord   
with that calculated from the escape of 
,h nuclei4    ,further experiments are  
required with other elements to test if 
this  relation between the minimum      
velocity of ,h nuclei and the minimum   
velocity of the  particle to produce    
disintegration holds generally2  but the
 results as far as they go are certainly
 very suggestive4    ,it is of interest 
to note that these results afford a     
definite proof of  the nuclear          
conception of the atom and give us some 
hope that we may  determine the         
magnitude of the critical potential for 
a number of the  light elements4        
,,evolution ,,of ,,nuclei      ,in      
concluding1 ,i would like to make a few 
remarks of a more speculative  character
 dealing with the fundamental problem of

 the origin and  evolution of the       
elements from the two fundamental       
building units1 the  positive and       
negative electrons4 ,it must be         
confessed that there is  little         
information to guide us with the        
exception of our knowledge of  the      
nuclear charges and masses of the       
various species of elements which       
survive to-day4 ,it has always been a   
matter of great difficulty to  imagine  
how the more complex nuclei can be built
 up by the successive  additions of     
protons and electrons1 since the proton 
must be endowed  with a very high speed 
to approach closely to the charged      
nucleus4 ,i  have already discussed in  
this paper the evidence that powerful   
attractive forces varying very rapidly  
with the distance are present  close to 
the nuclear structure and it seems      
probable that these forces  must        
ultimately be ascribed to the           
constituent proton4 ,in such a case it  
may be possible for an electron and     

proton to form a very close             
combination1 or neutron1 as ,i have     
termed it4 ,the probable distance       
between the centre of this doublet is of
 the order of #c x #aj-ac cm4 ,the      
forces between two neutrons would be    
very small except for distance of       
approach of this order of magnitude1 and
 it is probable that the  neutrons would
 collect together in much the same      
fashion as a number of  small movable   
magnets would tend to form a coherent   
group held together  by their mutual    
forces4    ,in considering the origin of
 the elements1 we may for simplicity    
suppose  a large diffused mass of       
hydrogen which is gradually heated by   
its  gravitational condensation4 ,at    
high temperatures the gas would consist 
mainly of free hydrogen nuclei and      
electrons1 and some of these would in   
course of time combine to form neutrons1
 emitting energy in the process4  ,these
 neutrons would collect together in     
nuclear masses of all kinds of          

complexity4 ,now the tendency of the    
groups of neutrons would be to form     
more stable nuclear combinations1 such  
as helium nuclei of mass four1  and     
possibly intermediate stages of masses  
two and three4 ,energy would  be emitted
 in these processes probably in the form
 of swift surplus  electrons which were 
not necessary for the stability of the  
system4 ,in a  sense1 all these nuclear 
masses would be radioactive1 but some of
 them  in their transformation may reach
 a stable configuration which would     
represent the nucleus of one of our     
surviving elements4 ,if we suppose  that
 nuclear masses over a wide range of    
mass can be formed before  serious      
transformation occurs1 it is easy to see
 how every possible type  of stable     
element will gradually emerge4 ,if we   
take the helium nucleus  as a combining 
unit which emits in its formation the   
greatest amount of  energy1 we should   
ultimately expect many of the neutrons  
in a heavy  nucleus to form helium      

nuclei4 ,these helium nuclei would tend 
to collect  together and form definite  
systems and it seems not unlikely that  
they  will group themselves into orderly
 structures1 analogous in some  respects
 to the regular arrangement of atoms to 
form crystals1 but with  much smaller   
distances between the structural units4 
,in such a case1  some of the elements  
may consist of a central crystal type of
 structure  of helium nuclei surrounded 
by positive and negatively charged      
satellites in motion round this central 
core4 ,assuming that such orderly       
arrangements of helium nuclei are       
possible1 it is of interest to note     
that the observed relations between     
atomic charge and atomic mass for  the  
elements can be approximately obtained  
on a very simple assumption4  ,suppose  
that helium nuclei form a point centred 
cubic lattice with an  electron at the  
centre of a crystal unit of eight helium
 nuclei4 ,a few  of the possible types  
of grouping are given in the following  

table1 with  corresponding masses and   
nuclear charges4 ,the structure #d4 #c4 
#b4 means  a rectangular arrangement    
with sides containing #d4 #c4 #b4 nuclei
  respectively4 ,it will thus contain   
#bd helium nuclei1 have a mass #if1 and 
will contain #f intranuclear electrons4 
,its nuclear charge will therefore  be  
#dh - #f  #da4      ,structural         
arrangement of    ,calculated           
,calculated  ,known element of          
helium nuclei       nuclear charge      
,mass       equal charge    #c4 #b4 #b4 
#bb              #dh      ,ti #dh    #c4
 #c4 #b4                        #cb     
#gb      ,ge #gd1 #gb1 #gj    #c4 #c4   
#c4                        #df          
#ajh      ,pd #ajf4g    #d4 #b4 #b4     
#bi              #fd      ,cu #fc4ce    
#d4 #c4 #b4                        #db  
#if      ,mo #if    #d4 #c4 #c4         
#fj             #add      ,nd #add      
#d4 #d4 #c4                        #gh  
#aib      ,pt #aie      ,while the      
agreement is far from perfect for all   

these structures1 there  is a general   
accord with observation4 ,if we take the
 view that some of  these structures can
 grow by the addition of satellites1    
there is room  for adjustment of masses 
and to include the intervening elements4
 ,this  point of view is admittedly very
 speculative and there may well be other
  types of structure involved4 ,at the  
same time1 the general evidence         
suggests that there are some basal      
structures on which the heavier atoms   
are progressively built up4 ,the failure
 of the whole number rule for the  mass 
of isotopes1 observed in some cases by  
,aston1 e4g41 between tin  and xenon1   
certainly supports such a conception4   
,from a study of the  artificial        
disintegration of the elements we have  
seen that carbon and  oxygen represent  
very stable structures probably composed
 of helium  nuclei4 ,it is possible that
 oxygen nuclei1 for example1 may be the 
structural basis of some of the elements
 following oxygen1 but our  information 

is at present too meagre to be at all   
certain on this point4    ,i think1     
however1 it will be clear from this     
lecture what a difficult  but           
fascinating problem is involved in the  
structure of nuclei4 ,before  we can    
hope to make much advance1 it is        
essential to know more of the  nature of
 the forces operative close to protons  
and electrons1 and we  may hope to      
acquire much information by a detailed  
study of the  scattering of swift  rays 
and  rays by nuclei4 ,fortunately1 there
 is  now a number of distinct lines of  
attack on this problem1 and from a      
combination of the results obtained we  
may hope to make steady1 if not  rapid1 
progress in the solution of this1 the   
greatest problem in  ,physics4