,worlds ,)9 ,worlds3                    
,! ,/ory ( ,nucle> ,en]gy               
,volume #a                   ,atomic    
,weights  ,en]gy  ,electricity          
by ,isaac ,asimov                ,u4 ,s4
 ,en]gy ,rese>* & ,developm5t           
,adm9i/ration                           
,(fice ( ,public ,affairs               
,wa%+ton1 ,d4,c4 #bjede                 
,libr>y ( ,congress ,catalog ,c>d       
,numb]3 #ge-ahidgg                      
#aigb    ,no?+ 9 ! hi/ory ( mank9d has  
op5$ our eyes to !  possibilities (     
sci5ce z has ! developm5t ( atomic p[]4 
,9 !  la/ #bjj ye>s1 p h se5 ! com+ ( ! 
/eam eng9e1 !  /eamboat1 ! railroad     
locomotive1 ! automobile1 ! airplane1   
radio1  motion pictures1 television1 !  
ma*9e age 9 g5]al4 ,yet none ( x  seem$ 
q s fanta/ic1 q s unbelievable1 z :at   
man has done  s9ce #aici ) ! atom 444   
!re seem to be almo/ no limits to :at   
may lie ahead3 inexhau/ible en]gy1 new  
worlds1 ev]-wid5+ k  ( ! physical       
univ]se4                                

,isaac ,asimov                          
,illu/ration3 ,photograph ( night sky   
,! ,u4 ,s4 ,en]gy ,rese>* & ,developm5t 
,adm9i/ration publi%es a  s]ies (       
booklets = ! g5]al public4    ,please   
write to ! foll[+ address = a title li/ 
or =  in=mation on a specific subject3  
,,us]da--,te*nical ,in=mation ,c5t]     
,p4 ,o4 ,box #fb    ,oak ,ridge1        
,t5nessee #cghcj                        
,illu/ration3 ,isaac ,asimov            
,,isaac ,,asimov receiv$ 8 academic     
degrees f ,columbia ,univ]sity &  is    
,associate ,pr(essor ( ,bio*emi/ry at ! 
,bo/on ,univ]sity ,s*ool  ( ,m$ic9e4 ,he
 is a prolific au?or :o has writt5 ov]  
#aej books 9  ! pa/ #bj ye>s1 includ+   
ab\t #bj sci5ce fiction works1 & books  
= *ildr54 ,8 _m excell5t sci5ce books = 
! public cov]  subjects 9 ma!matics1    
physics1 a/ronomy1 *emi/ry1 & biology1  
su* z ,! ,g5etic ,code1 ,inside ! ,atom1
 ,build+ ,blocks ( !  ,univ]se1 ,und]/&+
 ,physics1 ,! ,new ,intellig5t ,man's   
,guide to  ,sci5ce1 & ,asimov's         

,biographical ,encyclop$ia ( ,sci5ce &  
,te*nology4    ,9 #aife ,dr4 ,asimov    
receiv$ ! ,james ,t4 ,grady ,aw>d ( !   
,am]ican  ,*emical ,society = 8 major   
contribution 9 report+ sci5ce  progress 
to ! public4                            
,illu/ration3 ,photograph ( night sky   
,,volume #a    ,introduction            
#e    ,atomic ,weights                  
#f    ,electricity                      
#aa        ,units ( ,electricity        
#aa        ,ca?ode ,rays                
#ac        ,radioactivity               
#ag        ,! ,/ructure ( ! ,atom       
#be        ,atomic ,numb]s              
#cj        ,isotopes                    
#ce    ,en]gy                           
#dg        ,! ,law ( ,cons]vation (     
,en]gy                                  
#dg        ,*emical ,en]gy              
#ej        ,electrons & ,en]gy          
#ed        ,! ,en]gy ( ! ,sun           
#ee        ,! ,en]gy ( ,radioactivity   
#eg                                     
,,volume #b    ,mass & ,en]gy           

#fi    ,! ,/ructure ( ! ,nucleus        
#ge        ,! ,proton                   
#ge        ,! ,proton-,electron ,!ory   
#gf        ,protons 9 ,nuclei           
#hj        ,nucle> ,bomb>dm5t           
#hb        ,p>ticle ,accel]ators        
#hf    ,! ,neutron                      
#ib        ,nucle> ,sp9                 
#ib        ,discov]y ( ! ,neutron       
#ie        ,! ,proton-,neutron ,!ory    
#ih        ,! ,nucle> ,int]action       
#aja        ,neutron ,bomb>dm5t         
#ajg                                    
,,volume #c    ,nucle> ,fission         
#aag        ,new ,elem5ts               
#aag        ,! ,discov]y ( ,fission     
#abb        ,! ,nucle> ,*a9 ,reaction   
#abg        ,! ,nucle> ,bomb            
#aca        ,nucle> ,reactors           
#ada    ,nucle> ,fusion                 
#adg        ,! ,en]gy ( ! ,sun          
#adg        ,!rmonucle> ,bombs          
#adi        ,controll$ ,fusion          
#aea    ,beyond ,fusion                 
#aei        ,antimatt]                  

#aei        ,! ,unkn[n                  
#afd    ,read+ ,li/                     
#aff    ,illu/ration3 ,a total eclipse (
 ! sun4                                 
,,introduction      ,9 a way1 nucle>    
en]gy has be5 s]v+ man z long z he has  
exi/$4  ,x has s]v$ all ( life2 x has   
flood$ ! e>? = billions (  ye>s4 ,! sun1
 y see1 is a va/ nucle> eng9e1 & ! w>m? 
&  light t ! sun radiates is ! product (
 nucle> en]gy4    ,9 ord] = man to le>n 
to produce & control nucle> en]gy       
himself1  h[ev] 7some?+ t did n take    
place until ?is c5tury71 ?ree  l9es (   
inve/igation--atoms1 electricity1 &     
en]gy--_h to develop &  meet4    ,we w  
beg9 ) atoms4                           
,,atomic ,,weights      ,z long ago z   
anci5t ,greek times1 !re 7 m5 :o        
suspect$ t  all matt] consi/$ ( t9y     
p>ticles :i* 7 f> too small to see4  ,"u
 ord9>y circum/ances1 !y c\ld n be      
divid$ into any?+  small]1 & !y 7 call$ 
8atoms0 f a ,greek ^w mean+             
8indivisible04    ,x 0 n until #ahjh1   

h[ev]1 t ?is 8atomic !ory0 0 really put 
on a firm f\ndation4 ,9 t ye> ! ,engli% 
*emi/ ,john ,dalton  7#agff-ahdd7       
publi%$ a book 9 :i* he discuss$ atoms 9
 detail4  ,e elem5t1 he sugge/$1 0 made 
up ( its own type ( atoms4 ,!  atoms (  
"o elem5t 7 diff]5t f ! atoms ( e o!r   
elem5t4 ,! *ief diff]5ce betwe5 ! v>i\s 
atoms lay 9 _!  mass1 or weight4#a      
,dalton 0 ! fir/ to try to det]m9e :at  
^! masses might be4 ,he  c\ld n "w out !
 actual masses 9 ounces or grams1 =     
atoms 7  f> too t9y to weigh ) any ( 8  
in/rum5ts4 ,he c\ld1 h[ev]1  det]m9e _! 
relative weights2 t is1 h[ mu* m massive
 "o  k9d ( atom might be ?an ano!r4     
,= in/ance1 he f\nd t a quantity (      
hydrog5 gas inv>iably  comb9$ ) eight   
times its own mass ( oxyg5 gas to =m    
wat]4 ,he  guess$ t wat] consi/$ (      
comb9ations ( #a atom ( hydrog5 )  #a   
atom ( oxyg54 7,a comb9ation ( atoms is 
call$ a 8molecule0 f a  ,greek ^w mean+ 
8a small mass01 & s hydrog5 & oxyg5     
atoms c  be said to comb9e to =m a 8wat]

 molecule047    ,illu/ration3 ,john     
,dalton    ,to acc\nt = ! diff]5ce 9 !  
masses ( ! comb9+ gases1  ,dalton decid$
 t ! oxyg5 atom 0 eight times z massive 
z !  hydrog5 atom4 ,if he set ! mass ( !
 hydrog5 atom at #a 7j =  conv5i5ce7 !n 
! mass ( ! oxyg5 atom ought to be set at
 #h4  ,^! comp>ative1 or relative1      
numb]s 7 said to be 8atomic  weights01 s
 t :at ,dalton 0 sugge/+ 0 t ! atomic   
weight  ( hydrog5 0 #a & ! atomic weight
 ( oxyg5 0 #h4 ,by not+ !  quantity (   
o!r elem5ts t comb9$ ) a fix$ mass (    
oxyg5 or  ( hydrog51 ,dalton c\ld "w out
 ! atomic weights ( ^! elem5ts  z well4 
,dalton's idea 0 "r1 b 8 details 7 wrong
 9 "s cases4 ,=  in/ance1 on clos]      
exam9ation x turn$ out t ! wat] molecule
  0 compos$ ( #a oxyg5 atom & #b hydrog5
 atoms4 ,= ?is reason1 !  wat] molecule 
may be writt5 ,h,o1 :]e ,h is ! *emical 
symbol = a  hydrog5 atom1 & ,o = an     
oxyg5 atom4    ,x is /ill a fact t a    
quantity ( hydrog5 comb9es ) eight times
  its mass ( oxyg51 s ! s+le oxyg5 atom 

mu/ be eight times z  massive z ! #b    
hydrog5 atoms tak5 toge!r4 ,! oxyg5 atom
 mu/  !re=e be sixte5 times z massive z 
a s+le hydrog5 atom4 ,if !  atomic      
weight ( hydrog5 is #a1 !n ! atomic     
weight ( oxyg5 is #af4    ,at fir/ x    
seem$ t ! atomic weights ( ! v>i\s      
elem5ts 7  :ole numb]s & t hydrog5 0 !  
lighte/ "o4 ,x made p>ticul>  s5se1 !n1 
to consid] ! atomic weight ( hydrog5 z  
#a1 because  t made all ! o!r atomic    
weights z small z possible &  !re=e easy
 to h&le4    ,! ,sw$i% *emi/ ,jns ,jakob
 ,b]zelius 7#aggi-ahdh7 cont9u$         
,dalton's  "w & f\nd t elem5ts did n    
comb9e 9 q su* simple  ratios4 ,a giv5  
quantity ( hydrog5 actually comb9$ ) a  
little bit  less ?an eight times its    
mass ( oxyg54 ,!re=e if ! atomic weight 
( hydrog5 7 consid]$ to be #a1 ! atomic 
weight ( oxyg5 w\ld  h to be n #af1 b   
#ae4hg4    ,illu/ration3 ,jns ,jakob    
,b]zelius    ,z x happ5s1 oxyg5 comb9es 
) m elem5ts 7& m easily7 ?an  hydrog5   
does4 ,! ratio ( its atomic weight to t 

( o!r elem5ts  is also m (t5 a :ole     
numb]4 ,9 work+ out ! atomic weight (   
elem5ts x 0 !re=e m conv5i5t to set !   
atomic weight (  oxyg5 at a :ole numb]  
?an t ( hydrog54 ,b]zelius did ?is1 =   
in/ance1 9 ! table ( atomic weights he  
publi%$ 9 #ahbh4 ,at fir/  he call$ !   
atomic weight ( oxyg5 #ajj4 ,!n he      
decid$ to make !  atomic weights z small
 z possible1 )\t all[+ any atomic weight
  to be less ?an #a4 ,= t reason1 he set
 ! atomic weight ( oxyg5  at exactly #af
 & 9 t case1 ! atomic weight ( hydrog5  
_h to be  plac$ j a trifle high] ?an #a4
 ,! atomic weight ( hydrog5 became      
#a4jjh4 ,?is sy/em 0 reta9$ = ne>ly a   
c5tury & a half4    ,?r\gh\t ! #ai?     
c5tury1 *emi/s kept on work+ out atomic 
weights  m & m c>efully4 ,by ! />t ( !  
#bj? c5tury1 mo/ elem5ts  _h _! atomic  
weights work$ out to two decimal places1
 sometimes  ?ree4    ,a numb] ( elem5ts 
_h atomic weights t 7 ne>ly :ole numb]s 
on ! 8oxyg5  #af0 /&>d4 ,! atomic weight
 ( alum9um 0 j  ab\t #bg1 t ( calcium   

almo/ #dj1 t ( c>bon almo/ #ab1 t (     
gold almo/ #aig1 & s on4    ,on ! o!r   
h&1 "s elem5ts _h atomic weights t 7 f> 
remov$ f :ole numb]s4 ,! atomic weight (
 *lor9e 0 close to  #ce4e1 t ( copp] to 
#fc4e1 t ( iron to #ee4h1 t ( silv] to  
#ajg4i1 & s on4    ,?r\gh\t ! #ai?      
c5tury1 *emi/s did n "k :y s _m atomic  
weights 7 :ole numb]s1 :ile o!rs w]5't4 
,!y simply made _!  measurem5ts &       
record$ :at !y f\nd4 ,= an explanation1 
!y _h  to wait = a l9e ( inve/igation   
into electricity to come to  fruition4  
,,electricity      ,units ( ,electricity
    ,?r\gh ! #ah? c5tury1 sci5ti/s _h   
be5 fasc9at$ by !  prop]ties (          
electricity4 ,electricity seem$1 at !   
"t1 to be a v  f9e fluid t c\ld ext5d   
?r\gh ord9>y matt] )\t tak+ up  any     
room4    ,electricity did m ?an radiate 
?r\gh matt]1 h[ev]4 ,x also  produc$    
important *anges 9 matt]4 ,9 ! fir/ ye>s
 ( ! #ai?  c5tury1 x 0 f\nd t a curr5t (
 electricity c\ld cause  diff]5t atoms  
or diff]5t gr\ps ( atoms to move 9      

opposite  directions ?r\gh a liquid 9   
:i* !y 7 dissolv$4    ,! ,engli% sci5ti/
 ,mi*ael ,f>aday 7#agia-ahfg7 not$ 9    
#ahcb t a  giv5 quantity ( electricity  
seem$ to lib]ate ! same numb] (  atoms (
 a v>iety ( diff]5t elem5ts4 ,9 "s      
cases1 ?\gh1 x  lib]at$ j half ! expect$
 numb] ( atoms2 or ev51 9 a few  cases1 
j a ?ird4    ,sci5ti/s began to         
speculate t electricity1 l matt]1 might 
consi/ ( t9y units4 ,:5 electricity     
broke up a molecule1 p]haps a  unit (   
electricity atta*$ itself to ea* atom4  
,9 t case1 ! same  quantity (           
electricity1 conta9+ ! same numb] (     
units1 w\ld  lib]ate ! same numb] (     
atoms4    ,9 ! case ( "s elem5ts1 ea*   
atom c\ld atta* #b units (  electricity 
to itself1 or p]haps ev5 #c4 ,:5 t      
happ5$ a giv5  quantity ( electricity   
w\ld lib]ate only "o-half1 or only      
"o-?ird1  ! usual numb] ( atoms4 7,?us1 
#ah units ( electricity w\ld lib]ate    
#ah atoms if di/ribut$ #a to an atom2   
only #i atoms if di/ribut$ #b to  an    

atom2 & only #f atoms if di/ribut$ #c to
 an atom47    ,x 0 und]/ood at ! "t t   
electricity exi/$ 9 two v>ieties1  :i* 7
 call$ positive & negative4 ,x appe>$ t 
if an atom  atta*$ a positive unit (    
electricity to itself x w\ld be pull$ 9 
"o direction ?r\gh ! solution by !      
voltage4 ,if x atta*$ a  negative unit (
 electricity to itself x w\ld be pull$ 9
 ! o!r  direction4    ,illu/ration3     
,mi*ael ,f>aday    ,! units (           
electricity 7 a great deal m difficult  
to /udy ?an  ! atomic units ( matt]1 &  
?r\gh\t ! #ai? c5tury !y  rema9$        
elusive4 ,9 #ahia1 ?\gh1 ! ,iri%        
physici/ ,george ,john/one  ,/oney      
7#ahbf-aiaa7 sugge/$ t ! suppos$ unit ( 
electricity be  giv5 a "n at lea/4 ,he  
call$ ! unit an 8electron04      ,ca?ode
 ,rays    ,an electric curr5t fl[s ?r\gh
 a clos$ circuit ( "s conduct+  mat]ial1
 su* z metal wires4 ,x />ts at "o pole (
 a batt]y1 or (  "s o!r electricity     
g5]at+ device1 & ends at ! o!r4 ,! two  
poles are ! positive pole or 8anode0 & !

 negative pole or  8ca?ode04    ,if !re 
is a break 9 ! circuit1 ! curr5t w      
usually n fl[ at  all4 ,if1 h[ev]1 !    
break is n a l>ge "o1 & ! curr5t is "u  
a high driv+ =ce 7:i* is call$ !        
8voltage071 !n ! curr5t  may leap across
 ! break4 ,if two ends ( a wire1 mak+ up
 "p ( a  brok5 circuit1 are br\ght close
 to ea* o!r ) no?+ b air  betwe51 a sp>k
 may leap across ! n>r[+ gap be=e !y    
actually  meet &1 :ile x p]si/s1 !      
curr5t w fl[ despite ! break4    ,!     
light ( ! sp>k1 & ! crackl+ s\nd x      
makes1 are !  results ( ! electric      
curr5t int]act+ ) molecules ( air &     
heat+ !m4 ,nei!r ! light nor ! s\nd is !
 electricity itself4  ,9 ord] to detect 
! electricity1 ! curr5t ought to be =c$ 
across a gap conta9+ no?+1 n ev5 air4   
,9 ord] to d t1 wires w\ld h to be seal$
 into a glass tube  f :i* all 7or almo/ 
all7 ! air 0 )drawn4 ,?is 0 n easy  to d
 & x 0 n until #ahed t ,he9ri* ,geissl] 
7#ahad-ahgi71 a  ,g]man glass-bl[] &    
inv5tor1 accompli%$ ?is feat4 ,! wires  

seal$ into su* a 8,geissl] tube0 c\ld be
 atta*$ to ! poles ( an  electric       
g5]ator1 & if en\gh voltage 0 built up1 
! curr5t  w\ld leap across ! vacuum4    
,illu/ration3 ,a ,geissl] tube4 labell$3
 ,curr5t s\rce    ,su* exp]im5ts 7 fir/ 
p]=m$ by ! ,g]man physici/ ,julius      
,plck] 7#ahja-ahfh74 ,9 #aheh he notic$ 
t :5 ! curr5t fl[$  across ! vacuum !re 
0 a gre5i% gl[ ab\t ! wire t 0  atta*$  
to ! ca?ode ( ! g5]ator4 ,o!rs /udi$ ?is
 gl[ &  f9ally ! ,g]man physici/ ,eug5  
,gold/e9 7#ahej-aica7 decid$ 9 #ahgf  t 
!re 7 rays ( "s sort beg9n+ at ! wire   
atta*$ to !  negatively *>g$ ca?ode &   
end+ at ! "p ( ! tube opposite  !       
ca?ode4 ,he call$ !m 8ca?ode rays04     
,^! ca?ode rays1 x seem$1 might well be 
! electric curr5t  itself1 fre$ f !     
metal wires t usually c>ri$ x4 ,if s1   
det]m9+ ! nature ( ! ca?ode rays might  
reveal a great deal  ab\t ! nature ( !  
electric curr5t4 ,7 ca?ode rays some?+  
l light & 7 !y made up ( t9y waves8 ,or 
7 !y a /ream (  p>ticles possess+ mass8 

,!re 7 physici/s on ea* side ( ! "q4 ,by
 #ahhe1 h[ev]1  ! ,engli% physici/      
,william ,crookes 7#ahcb-aiai7 %[$ t    
ca?ode  rays c\ld be made to turn a     
small :eel :5 !y /ruck t :eel on  "o    
side4 ,?is seem$ to %[ t ! ca?ode rays  
possess$ mass &  7 a /ream ( atom-l     
p>ticles1 r ?an a beam ( mass-less      
light4 ,fur!rmore1 ,crookes %[$ t !     
ca?ode rays c\ld be pu%$  sideways 9 !  
pres5ce ( a magnet4 7,?is effect1 :5    
curr5t fl[s  9 a wire1 is :at makes a   
motor "w47 ,?is meant t1 unlike ei!r    
light or ord9>y atoms1 ! ca?ode rays    
c>ri$ an electric *>ge4    ,illu/ration3
 ,j4 ,j4 ,?omson 9 8 laboratory4 ,on 8  
"r are e>ly  ,x-ray pictures4    ,?is   
view ( ! ca?ode rays z consi/+ ( a /ream
 ( electrically  *>g$ p>ticles 0        
confirm$ by ano!r ,engli% physici/1     
,joseph  ,john ,?omson 7#ahef-aidj74 ,9 
#ahig he %[$ t ! ca?ode rays c\ld  also 
be made to take a curv$ pa? 9 ! pres5ce 
( electrically  *>g$ objects4 ,!        
p>ticles mak+ up ! ca?ode rays 7 *>g$  )

 negative electricity1 judg+ f !        
direction 9 :i* !y 7  made to curve by  
electrically *>g$ objects4    ,?omson _h
 no hesitation 9 ma9ta9+ t ^! p>ticles  
c>ri$  ! units ( electricity t ,f>aday's
 "w _h h9t$ at4 ,ev5tually1  ,/oney's "n
 = ! units ( electricity 0 appli$ to !  
p>ticles  t c>ri$ ^? units4 ,! ca?ode   
rays1 9 o!r words1 7  consid]$ to be    
made up ( /reams ( electrons & ,?omson  
is usually  giv5 cr$it = hav+ discov]$ !
 electron4    ,! ext5t to :i* ca?ode    
rays curv$ 9 ! pres5ce ( a magnet or    
electrically *>g$ objects dep5d$ on !   
size ( ! electric *>ge  on ! electrons &
 on ! mass ( ! electrons4 ,ord9>y atoms 
c\ld  be made to c>ry an electric *>ge &
 by comp>+ _! behavior )  ^? (          
electrons1 "s ( ! prop]ties ( electrons 
c\ld be  det]m9$4    ,!re 71 = in/ance1 
good reasons to suppose t ! electron    
c>ri$ a *>ge ( ! same size z "o t a     
hydrog5 atom c\ld be  made to c>ry4 ,!  
electrons1 h[ev]1 7 mu* easi] to pull   
out (  _! /raight-l9e pa? ?an ! *>g$    

hydrog5 atom 04 ,!  conclusion drawn f  
?is 0 t ! electron _h mu* less mass ?an 
! hydrog5 atom4    ,?omson 0 able to %[1
 inde$1 t ! electron 0 mu* light]  ?an !
 hydrog5 atom1 :i* 0 ! lighte/ ( all !  
atoms4  ,n[adays we "k ! relation%ip q  
exactly4 ,we "k t x w\ld  take #ahcg4aa 
electrons to possess ! mass ( a s+le    
hydrog5 atom4  ,! electron is !re=e a   
8subatomic p>ticle02 ! fir/ ( ?is sort  
to be discov]$4    ,9 #ahig1 !n1 two    
types ( mass-conta9+ p>ticles 7 kn[n4   
,!re  7 ! atoms1 :i* made up ord9>y     
matt]1 & ! electrons1 :i*  made up      
electric curr5t4      ,radioactivity    
,0 !re a connection betwe5 ^! two sets (
 p>ticles--atoms &  electrons8 ,9 #ahig1
 :5 ! electron 0 discov]$1 a l9e (      
rese>*  t 0 to tie ! two k9ds ( p>ticles
 toge!r _h already begun4    ,9 #ahie ! 
,g]man physici/ ,wilhelm ,konrad ,ro5tg5
 7#ahde-aibc7 0  work+ ) ca?ode rays4   
,he f\nd t if he made ! ca?ode rays     
/rike ! glass at ! o!r end ( ! tube1 a  
k9d ( radiation 0  produc$4 ,?is        

radiation 0 capable ( p5etrat+ glass &  
o!r  matt]4 ,ro5tg5 _h no idea z to !   
nature ( ! radiation1 & s  call$ x 8,x  
rays04 ,?is "n1 conta9+ 8,x0 = 8unkn[n01
 0  reta9$ ev5 aft] physici/s work$ out 
! nature ( ,x rays & f\nd  !m to be     
light-l radiation made up ( waves mu*   
%ort] ?an ^?  ( ord9>y light4           
,illu/ration3 ,anto9e ,h5ri ,becqu]el4  
,at once1 physici/s became fasc9at$ ) ,x
 rays & began se>*+  = !m ev]y:]e4 ,"o (
 ^? involv$ 9 ! se>* 0 ! ,fr5*  physici/
 ,anto9e ,h5ri ,becqu]el 7#aheb-aijh74  
,a c]ta9 comp\nd1  potassium uranyl     
sulfate1 gl[$ aft] be+ expos$ to        
sunlight &  ,becqu]el wond]$ if ?is gl[1
 l ! gl[ on ! glass 9  ,ro5tg5's ,x-ray 
tube1 conta9$ ,x rays4                  
,illu/ration3 ,ro5tg5's laboratory      
,illu/ration3 ,wilhelm ,ro5tg5 & 8      
laboratory at ! ,univ]sity (  ,wrzburg4 
,x did1 b :ile inve/igat+ ! problem 9   
#ahif1 ,becqu]el f\nd  t ! comp\nd 0    
giv+ (f invisible p5etrat+ ,x-ray-l     
radiation cont9ually1 :e!r x 0 expos$ to

 sunlight or n4 ,!  radiation 0 detect$ 
because x w\ld fog a photographic plate 
j z  light w\ld4 ,:at's m1 ! radiation  
w\ld fog ! plate1 ev5 if !  plate 7     
wrapp$ 9 black pap]1 s t x c\ld p5etrate
 matt]  j z ,x rays c\ld4    ,o!rs1 9   
addition to ,becqu]el1 7 soon inve/igat+
 ! new  ph5om5on4 ,9 #ahih ! ,poli%     
7lat] ,fr5*7 physici/ ,m>ie ,sklod[ska  
,curie 7#ahfg-aicd7 %[$ t x 0 ! uranium 
atom t 0 !  s\rce ( ! radiation1 & t any
 comp\nd conta9+ ! uranium  atom w\ld   
give (f ^! p5etrat+ rays4    ,until !n1 
uranium _h n be5 ( mu* int]e/ to *emi/s4
 ,x 0 a  comp>atively r>e metal t 0 fir/
 discov]$ 9 #aghi by ! ,g]man  *emi/    
,m>t9 ,he9ri* ,klapro? 7#agdc-ahag74 ,x 
_h no p>ticul> uses  & rema9$ an obscure
 elem5t4 ,z *emi/s le>n$ to "w out !    
atomic weights ( ! v>i\s elem5ts1 !y    
f\nd1 h[ev]1 t1 (  ! elem5ts !n kn[n1   
uranium _h ! highe/ atomic weight (     
all--#bch4    ,once uranium 0 discov]$  
to be an endless s\rce ( radiation1 x   
ga9$ int]e/ t has ris5 ev] s9ce4 ,madame

 ,curie gave ! "n  8radioactivity0 to   
?is ph5om5on ( cont9u\sly giv+ (f rays4 
,uranium 0 ! fir/ elem5t f\nd to be     
radioactive4    ,x did n rema9 alone1   
h[ev]4 ,x 0 soon %[n t ?orium 0  also   
radioactive4 ,?orium1 :i* _h be5        
discov]$ 9 #ahbi by  ,b]zelius1 0 made  
up ( atoms t 7 ! second mo/ massive kn[n
  at ! "t4 ,?orium's atomic weight is   
#bcb4    ,b :at 0 ! my/]i\s radiation   
emitt$ by uranium & ?orium8    ,almo/ at
 once x 0 le>n$ t :atev] ! radiation 01 
x 0  n uni=m 9 prop]ties4 ,9 #ahii      
,becqu]el 7& o!rs7 %[$ t1  9 ! pres5ce (
 a magnet1 "s ( ! radiation sw]v$ 9 a   
p>ticul> direction4 ,lat] x 0 f\nd t a  
portion ( x sw]v$ 9  ! opposite         
direction4 ,/ill ano!r "p didn't sw]ve  
at all b  mov$ on 9 a /raight l9e4    ,!
 conclusion 0 t uranium & ?orium gave (f
 ?ree k9ds (  radiation4 ,"o c>ri$ a    
positive *>ge ( electricity1 "o a       
negative  *>ge1 & "o no *>ge at all4 ,! 
,new ,zeal&-born physici/ ,erne/        
,ru!r=d 7#ahga-aicg7 call$ ! fir/ two   

k9ds ( radiation 8alpha  rays0 & 8beta  
rays01 aft] ! fir/ two lett]s ( ! ,greek
  alphabet4 ,! ?ird 0 soon call$ 8gamma 
rays0 aft] ! ?ird lett]4                
,illu/ration3 ,erne/ ,ru!r=d            
,illu/ration3 ,m>ie ,curie & h] two     
daught]s1 ,eve 7left7 & ,ir5e1  9 #aijh4
    ,illu/ration3 ,pi]re ,curie dur+ a  
class lecture 9 #aijf1 ! ye> (  8 dea?4 
,! gamma rays ev5tually turn$ out to be 
ano!r light-l =m (  radiation1 ) waves  
ev5 %ort] ?an ^? ( ,x rays4 ,! alpha    
rays  & beta rays1 :i* c>ri$ electric   
*>ges1 seem$ to be /reams (  *>g$       
p>ticles 78alpha p>ticles0 & 8beta      
p>ticles07 j z !  ca?ode rays _h turn$  
out to be4    ,9 #aijj1 inde$1 ,becqu]el
 /udi$ ! beta p>ticles & f\nd !m to  be 
id5tical 9 mass & *>ge ) electrons4 ,!y 
7 electrons4    ,by #aijf ,ru!r=d _h    
work$ out ! nature ( ! alpha p>ticles4  
,!y c>ri$ a positive electric *>ge t 0  
twice z great z !  electron's negative  
*>ge4 ,if an electron c>ri$ a *>ge t    
c\ld  be symboliz$ z -1 !n ! *>ge ( !   

alpha p>ticle 0 4  ,fur!rmore1 ! alpha  
p>ticle 0 mu* m massive ?an ! electron4 
,x 01 inde$1 z massive z a helium atom  
7! second lighte/ kn[n  atom7 & f\r     
times z massive z a hydrog5 atom4       
,nev]!less1 !  alpha p>ticle c p5etrate 
matt] 9 a way 9 :i* atoms _c1 s  t x    
seems mu* small] 9 diamet] ?an atoms    
are4 ,! alpha  p>ticle1 despite its     
mass1 is ano!r subatomic p>ticle4       
,"h1 !n1 is ! meet+ po9t ( electrons & (
 atoms--! p>ticles  ( electricity & (   
matt]4    ,ev] s9ce ,dalton _h fir/     
advanc$ ! atomic !ory ov] a c5tury      
e>li]1 *emi/s _h assum$ t atoms 7 !     
fundam5tal units (  matt]4 ,!y _h assum$
 atoms 7 z small z any?+ c\ld be &  t !y
 c\ld n possibly be brok5 up into any?+ 
small]4 ,!  discov]y ( ! electron1      
h[ev]1 _h %[n t "s p>ticles1 at  lea/1  
might be f> small] ?an any atom4 ,!n1 ! 
inve/igations into  radioactivity _h %[n
 t atoms ( uranium & ?orium spontane\sly
  broke up into small] p>ticles1 includ+
 electrons & alpha  p>ticles4    ,x w\ld

 seem1 !n1 t atoms ( ^! elem5ts &1      
presumably1 (  all elem5ts1 7 made up ( 
/ill small] p>ticles & t among  ^!      
p>ticles 7 electrons4 ,! atom _h a      
/ructure & physici/s  became int]e/$ 9  
discov]+ exactly :at t /ructure 04      
,! ,/ructure ( ! ,atom    ,s9ce         
radioactive atoms gave (f ei!r          
positively *>g$ p>ticles or  negatively 
*>g$ p>ticles1 x seem$ reasonable to    
assume t atoms  g5]ally 7 made up ( bo? 
types ( electricity4 ,fur!rmore1 s9ce  !
 atoms 9 matt] g5]ally c>ri$ no *>ge at 
all1 ! normal  8neutral atom0 mu/ be    
made up ( equal quantities ( positive   
*>ge  & negative *>ge4    ,x turn$ out t
 only radioactive atoms1 su* z ^? (     
uranium &  ?orium1 gave (f positively   
*>g$ alpha p>ticles4 ,_m atoms1  h[ev]1 
t 7 n radioactive1 c\ld be made to give 
(f electrons4  ,9 #ahii ,?omson %[$ t   
c]ta9 p]fectly normal metals ) no  trace
 ( radioactivity gave (f electrons :5   
expos$ to ultraviolet  light4 7,?is is  
call$ ! 8photoelectric effect047    ,x 0

 possible to suppose1 !n1 t ! ma9       
/ructure ( ! atom  0 positively *>g$ &  
g5]ally immovable1 & t !re 7 also       
pres5t light electrons1 :i* c\ld easily 
be deta*$4 ,?omson _h  sugge/$1 z e>ly z
 #ahih1 t ! atom 0 a ball ( matt] c>ry+ 
a positive *>ge & t individual electrons
 7 /uck ?r\gh\t  its sub/ance1 l rais9s 
9 p\nd cake4    ,if some?+ l ! ,?omson  
view 7 correct !n ! numb] (  electrons1 
ea* ) "o unit ( negative electricity1   
w\ld dep5d on  ! total size ( ! positive
 *>ge c>ri$ by ! atom4 ,if ! *>ge  7 #e1
 !re w\ld h to be #e electrons pres5t to
 balance t4 ,!  total *>ge w\ld !n be #j
 & ! atom z a :ole w\ld be  electrically
 neutral4    ,if1 9 su* a case1 an      
electron 7 remov$1 ! atomic *>ge ( #e   
w\ld be balanc$ by only #d electrons ) a
 total *>ge ( -#d4 ,9 t  case1 ! net    
*>ge ( ! atom z a :ole w\ld be #a4 ,on !
 o!r  h&1 if an extra electron 7 =c$    
onto ! atom1 ! *>ge ( #e  w\ld be       
balanc$ by #f electrons ) a total *>ge (
 -#f1 & ! net  *>ge ( ! atom z a :ole   

w\ld be -#a4    ,su* electrically *>g$  
atoms 7 call$ 8ions0 & _! exi/5ce  _h   
be5 suspect$ s9ce ,f>aday's day4 ,f>aday
 _h kn[n t atoms _h  to travel ?r\gh a  
solution "u ! influ5ce ( an electric    
field to  acc\nt = ! way 9 :i* metals & 
gases appe>$ at ! ca?ode  & anode4 ,x 0 
he :o fir/ us$ ! t]m1 ion1 f a ,greek ^w
  mean+ 8travell]04 ,! ^w _h be5 sugge/$
 to him by ! ,engli%  s*ol>1 ,william   
,:ewell 7#agid-ahff74 ,9 #ahhd ! ,sw$i% 
*emi/ ,svante  ,augu/ ,arrh5ius         
7#ahei-aibg7 _h fir/ work$ out a detail$
 !ory  bas$ on ! sugge/ion t ^! ions 7  
atoms or gr\ps ( atoms  t c>ri$ an      
electric *>ge4    ,illu/ration3 ,svante 
,a4 ,arrh5ius    ,by ! close ( ! #ai?   
c5tury1 !n1 ,arrh5ius's sugge/ion seem$ 
correct4 ,!re 7 positive ions made up ( 
atoms or gr\ps ( atoms1  f :i* "o or m (
 ! electrons )9 ! atoms _h be5  remov$4 
,!re 7 negative ions made up ( s+le     
atoms or ( gr\ps  ( atoms1 to :i* "o or 
m extra electrons _h be5 add$4          
,illu/ration3       ,neutral atom       

,ea* unit ( positive *>ge is balanc$ by 
a unit ( negative            *>ge       
,9 ?is case1 total *>ge  #b -#b  #j     
,ioniz$ atom3      ,if an electron is   
remov$1 ! balance is de/roy$      ,9 ?is
 case1 total *>ge  #b -#a  #a    ,al?\gh
 ,?omson's model ( ! atom expla9$ !     
exi/5ce ( ions &  ! fact t atoms c\ld   
give (f electrons or absorb !m1 x 0 n   
satisfactory 9 all ways4 ,fur!r         
inve/igations yield$ results n          
compatible ) ! rais9s-9-!-p\nd-cake     
notion4    ,9 #aijf ,ru!r=d began to    
/udy :at happ5$ :5 massive subatomic    
p>ticles1 su* z alpha p>ticles1 pass$   
?r\gh matt]4 ,:5 alpha  p>ticles pass$  
?r\gh a ?9 film ( gold1 = in/ance1 !y   
rac$  ?r\gh1 = ! mo/ "p1 z ?\gh no?+ 7  
!re4 ,! alpha  p>ticles seem$ to pu% !  
light electrons aside & to act z ?\gh  !
 positively *>g$ ma9 body ( ! atom t    
,?omson _h pictur$  0 n solid1 b 0 s(t &
 spongy4    ,! only tr\ble 0 t e once 9 
a :ile an alpha p>ticle seem$  to /rike 
some?+ 9 ! gold film & b\nce to "o side4

 ,sometimes  x ev5 b\nc$ directly       
backw>d4 ,x 0 z ?\gh some:]e 9 ea*  atom
 !re 0 some?+ at lea/ z massive z !     
alpha p>ticle4    ,h[ l>ge 0 ?is massive
 portion ( ! atom8 ,x c\ldn't be v  l>ge
 = if x 7 ! alpha p>ticles w\ld hit x   
frequ5tly4  ,in/ead1 ! alpha p>ticles   
made v few hits4 ,?is meant ! massive   
portion 0 v small & t mo/ alpha p>ticles
 tore ?r\gh !  atom )\t com+ any:]e ne> 
x4    ,illu/ration3 ,ru!r=d's alpha     
p>ticle bomb>dm5t app>atus4 ,a  piece ( 
radium 9 ! lead box 7,b7 emits alpha    
p>ticles t g  ?r\gh ! gold foil 7,f74   
,^! p>ticles are scatt]$ at diff]5t     
angles onto ! fluoresc5t scre5 7,s71 :]e
 ! fla%es caus$ by ea*  impact are se5  
?r\gh ! microscope 7,m74 ,bel[1 alpha   
p>ticles are  %[n b\nc+ (f a nucleus 9 !
 gold foil4    ,illu/ration3     ,by    
#aiaa ,ru!r=d ann\nc$ 8 results to ! _w4
 ,he sugge/$ t  j ab\t all ! mass ( !   
atom 0 conc5trat$ into a v t9y1         
positively *>g$ 8nucleus0 at its c5t]4  
,! diamet] ( ! nucleus  0 only ab\t     

#a_/aj1jjj ! diamet] ( ! atom4 ,all !   
re/ ( !  atom 0 fill$ ) ! v light       
electrons4    ,illu/ration3 ,hans ,geig]
 7left7 & ,erne/ ,ru!r=d at ,man*e/]    
,univ]sity ab\t #aiaj4    ,accord+ to   
,ru!r=d's notion1 ! atom consi/$ ( a    
s+le t9y  positively *>g$ lead %ot at ! 
c5t] ( a foam ( electrons4 ,x  0        
,?omson's notion 9 rev]se4 ,/ill1 !     
nucleus c>ri$ a positive  *>ge ( a      
p>ticul> size & 0 balanc$ by negatively 
*>g$  electrons4 ,ru!r=d's model ( !    
atom expla9$ ! exi/5ce (  ions j z      
easily z ,?omson's did & x expla9$ m    
besides4    ,= in/ance1 if all !        
electrons are remov$ s t only ! nucleus 
rema9s1 ?is nucleus is z massive z an   
atom b is s t9y 9 size  t x c p5etrate  
matt]4 ,! alpha p>ticle w\ld be a b>e   
atomic  nucleus f ?is po9t ( view4      
,ru!r=d's model ( ! 8nucle> atom0 is    
/ill accept$ today4      ,atomic ,numb]s
    ,s9ce ! atom consi/$ ( a positively 
*>g$ nucleus at ! c5t]1  & a numb] (    
negatively *>g$ electrons outside1 !    

next /ep 0  to f9d ! exact size ( !     
nucle> *>ge & ! exact numb] (  electrons
 = ! diff]5t v>ieties ( atoms4    ,!    
answ] came ?r\gh a l9e ( rese>* t began 
) ! ,engli%  physici/ ,*>les ,glov]     
,b>kla 7#ahgg-aidd74 ,9 #aiaa he not$ t 
:5  ,x rays pass$ ?r\gh atoms1 "s 7     
absorb$ & "s b\nc$ back4  ,^? t b\nc$   
back _h a c]ta9 ability to p5etrate o!r 
matt]4  ,:5 ! ,x rays /ruck atoms ( high
 atomic weight1 ! ,x rays t  b\nc$ back 
7 p>ticul>ly p5etrat+4 ,9 fact1 ea*     
diff]5t type  ( atom seem$ associat$ )  
reflect$ ,x rays ( a p>ticul>  p5etrat+ 
p[]1 s ,b>kla call$ ^! 8*>act]i/ic ,x   
rays04    ,9 #aiac ano!r ,engli%        
physici/1 ,h5ry ,gwyn-,jeffreys ,moseley
  7#ahhg-aiae71 w5t into ! matt] m      
?or\ghly4 ,he measur$ ! exact  wavel5g? 
( ! *>act]i/ic ,x rays by reflect+ !m f 
c]ta9  cry/als4 ,9 cry/als1 atoms are   
arrang$ 9 regul> ord] & at kn[n         
di/ances f ea* o!r4 ,x rays reflect+ f  
7or m accurately1  diffract+ f7 cry/als 
are b5t out ( _! pa? by ! r[s (  atoms4 

,! long] _! waves1 ! m !y are b5t4 ,f ! 
degree  ( b5d+ ! wavel5g? ( ! waves c be
 det]m9$4    ,illu/ration3 ,*>les ,glov]
 ,b>kla    ,illu/ration3 ,h5ry          
,gwyn-,jeffreys ,moseley    ,moseley    
f\nd t ! great] ! atomic weight ( an    
atom1 ! %ort]  ! waves ( ! *>act]i/ic ,x
 rays associat$ ) x & ! m  p5etrat+ ^?  
,x rays 74 ,!re 0 su* a close           
connection1 9  fact1 t ,moseley c\ld    
arrange ! elem5ts 9 ord] accord+ to !   
wavel5g? ( ! *>act]i/ic ,x rays4    ,=  
"s #dj ye>s prior to ?is1 ! elem5ts _h  
be5 li/$ 9 ord]  ( atomic weight4 ,?is 0
 useful especially s9ce ! ,russian *emi/
  ,dmitri ,i4 ,m5delev 7#ahcd-aijg7 _h  
arrang$ !m 9 a 8p]iodic table0  bas$ on 
! atomic weight ord] 9 su* a way t      
elem5ts ( simil>  prop]ties 7 gr\p$     
toge!r4 ,! elem5ts 9 ?is table 7        
sometimes numb]$ consecutively 78atomic 
numb]07 b ?is 0  inconv5i5t s9ce1 :5 new
 elem5ts 7 discov]$1 ! li/ (  atomic    
numb]s might h to be reorganiz$4        
,illu/ration3 ,dmitri ,m5delev &        

,bohuslav ,braun] 9 ,prague 9 #aijj4    
,braun] 0 a pr(essor ( *emi/ry at !     
,bohemian ,univ]sity 9  ,prague4    ,!  
,dani% physici/ ,niels ,bohr            
7#ahhe-aifb7 _h j advanc$ a !ory  (     
atomic /ructure t made x reasonable to  
suppose t !  wavel5g? ( ! *>act]i/ic ,x 
rays dep5d$ on ! size ( !  nucle> *>ge (
 ! atoms mak+ up a p>ticul> elem5t4     
,moseley  !re=e sugge/$ t ^! ,x rays be 
us$ to det]m9e ! size (  ! positive *>ge
 on its nucleus4 ,! atomic numb] c\ld !n
 be set  equal to t *>ge & be made      
indep5d5t ( new discov]ies (  elem5ts4  
,hydrog51 = in/ance1 has an atomic numb]
 ( #a4 ,its nucleus c>ries a  unit      
positive *>ge1 #a1 & ! hydrog5 atom     
possesses #a electron to  balance ?is4  
,helium1 ) an atomic numb] ( #b1 has a  
nucle> *>ge  ( #b & #b electrons1 ) a   
total *>ge ( -#b1 to balance x4 7,!     
alpha p>ticle releas$ by radioactive    
atoms is id5tical ) a helium  nucleus47 
,! atomic numb] increases z "o goes up !
 l9e ( atoms4 ,oxyg5  atoms1 = in/ance1 

h an atomic numb] ( #h & iron atoms h "o
  ( #bf4 ,at ! upp] end1 ?orium is #ij &
 uranium is #ib4 ,ea* uranium  atom has 
a nucleus be>+ a *>ge ( #ib & conta9s   
#ib electrons to  balance ?is4    ,once 
! notion ( ! atomic numb] 0 work$ out1 x
 became possible  to tell = c]ta9 :e!r  
any elem5ts rema9$ z yet undiscov]$  &1 
if s1 :]e 9 ! li/ !y might be4    ,?us1 
:5 ,moseley fir/ pres5t$ sci5ti/s ) !   
atomic numb] x  turn$ out t !re 7 /ill  
#g elem5ts t 7 n discov]$4 ,at  lea/    
elem5ts ) atomic numb]s ( #dc1 #fa1 #gb1
 #ge1 #he1 #hg1 & #ia  7 /ill n kn[n4   
,by #aide1 all sev5 _h be5 discov]$4    
,x quickly turn$ out t ! atomic numb] 0 
m fundam5tal &  m *>act]i/ic ( a        
p>ticul> elem5t ?an 0 ! atomic weight4  
,illu/ration3 ,niels ,bohr              
,illu/ration3 ,bohr's /udy4    ,s9ce    
,dalton's "t x _h be5 assum$ t all !    
atoms ( a  p>ticul> elem5t 7 ( equal    
atomic weight & t atoms ( two  diff]5t  
elem5ts 7 always ( diff]5t atomic       
weight4 ,! fir/  inkl+ & ! fir/ pro( t  

?is might n be s came ?r\gh !  /udy (   
radioactivity4      ,illu/ration3 %[+   
,helium atom1 ,hydrog5 atom2 ,nucleus1  
,proton1                                
,neutron1 ,electron labell$             
,isotopes    ,9 #aijb ,ru!r=d & 8       
co-work] ,fr$]ick ,soddy 7#ahgg-aief7   
%[$  t :5 uranium atoms gave (f alpha   
p>ticles1 a new k9d ( atom 0  =m$ t 0 n 
uranium at all4 ,x 0 ?is new atom t 0   
ev5tually f\nd to give (f a beta        
p>ticle1 & !n ano!r atom (  /ill ano!r  
elem5t 0 =m$4 ,?is "w ( ,ru!r=d & ,soddy
  began a l9e ( inve/igation t by #aijg 
_h %[n t !re 0 a  :ole radioactive *a9 (
 elem5ts1 ea* "o break+ d[n to ! next  9
 l9e by giv+ (f ei!r an alpha p>ticle or
 a beta p>ticle1 until  f9ally a lead   
atom 0 =m$ t 0 n radioactive4           
,illu/ration3 ,fr$]ick ,soddy    ,!re 01
 9 %ort1 a 8radioactive s]ies0 beg9n+ ) 
uranium  7atomic numb] #ib7 & end+ )    
lead 7atomic numb] #hb74 ,! same 0  true
 ( ?orium 7atomic numb] #ij71 :i* began 
a s]ies t also end$  ) lead4 ,/ill a    

?ird elem5t1 act9ium 7atomic numb] #hi7 
01 at  t "t1 ! fir/ kn[n memb] ( a s]ies
 t also end$ 9 lead4    ,! v>i\s atoms  
=m$ 9 ^! ?ree radioactive s]ies 7 n all 
diff]5t 9 e way4 ,:5 ! uranium atom     
gives (f an alpha  p>ticle1 x =ms an    
atom orig9ally call$ 8uranium ,x04 ,on  
close  inve/igation1 x turn$ out t ?is  
uranium ,x _h ! *emical  prop]ties (    
?orium4 ,uranium ,x1 _h1 h[ev]1         
radioactive prop]ties  diff]5t f ord9>y 
?orium4    ,uranium ,x broke d[n s      
rapidly1 giv+ (f beta p>ticles z x did  
s1 t half ( any giv5 quantity w\ld h    
brok5 d[n 9 #bd days4  ,ano!r way ( say+
 ?is 7:i* 0 introduc$ by ,ru!r=d7 0 t  !
 8half-life0 ( uranium ,x1 is #bd days4 
,ord9>y ?orium1 h[ev]1  gives (f alpha  
p>ticles1 n beta p>ticles1 & does s at  
su* a  sl[ rate1 t its half-life is #ad 
billion ye>s6    ,uranium ,x1 & ord9>y  
?orium 7 9 ! same place 9 ! li/ (       
elem5ts by *emical /&>ds1 & yet !re 0   
cle>ly some?+  diff]5t ab\t ! two4      
,"h is ano!r case4 ,9 #aiac ! ,briti%   

*emi/ ,alex&] ,fleck  7#ahhi-    7 /udi$
 8radium ,b0 & 8radium ,d01 ! names giv5
 to two  diff]5t k9ds ( atoms 9 !       
uranium radioactive s]ies4 ,he also     
/udi$ 8?orium ,b0 9 ! ?orium radioactive
 s]ies & 8act9ium ,b0  9 ! act9ium      
radioactive s]ies4 ,all f\r are         
*emically ! same z  ord9>y lead2 all f\r
 are 9 ! same place 9 ! li/ ( elem5ts4  
,yet ea* is diff]5t f ! radioactive     
/&po9t4 ,?\gh all give  (f beta         
p>ticles1 radium ,b has a half-life (   
#bg m9utes1 radium ,d "o  ( #ai ye>s1   
?orium ,b "o ( #aa h\rs1 & act9ium ,b "o
 ( #cf  m9utes4    ,9 #aiac ,soddy call$
 atoms t 7 9 ! same place 9 ! li/ (     
elem5ts1 b :i* _h diff]5t radioactive   
prop]ties1 8isotopes01  f ,greek words  
mean+ 8same place04    ,at fir/1 x seem$
 t ! only diff]5ce betwe5 isotopes might
 be  9 _! radioactive prop]ties & t only
 radioactive atoms 7  involv$4 ,quickly 
t prov$ n to be s4    ,x prov$ t x 0    
possible to h sev]al =ms ( ! same elem5t
  t 7 all diff]5t ev5 ?\gh none ( !m 7  

radioactive4 ,!  uranium s]ies1 ! ?orium
 s]ies1 & ! act9ium s]ies all end$ 9    
lead4 ,9 ea* case ! lead =m$ 0 /able 7n 
radioactive74 ,7  ! lead atoms id5tical 
9 e case8 ,soddy _h work$ out ! way 9   
:i* atomic weights alt]$ e "t an alpha  
p>ticle or a beta  p>ticle 0 giv5 (f by 
an atom4 ,work+ ?r\gh ! ?ree radioactive
  s]ies he decid$ t ! lead atoms _h     
diff]5t atomic weights 9  ea* case4     
,! uranium s]ies ought to end ) lead    
atoms t _h an atomic  weight ( #bjf4 ,! 
?orium s]ies ought to end 9 lead atoms )
 an  atomic weight ( #bjh & ! act9ium   
s]ies 9 lead atoms ) an  atomic weight (
 #bjg4    ,if ?is 7 s1 !re w\ld be #c   
lead isotopes t w\ld diff] n 9          
radioactive prop]ties1 b 9 atomic       
weight4 ,! isotopes c\ld be  ref]r$ to z
 lead-#bjf1 lead-#bjg1 & lead-#bjh4 ,if 
we use ! *emical  symbol = lead 7,pb71  
we c\ld write ! isotopes1 ,pb1 ,pb1 &   
,pb4 7,we read ! symbol ,pb z           
lead-#bjf47 ,atomic weight  measurem5ts 
made 9 #aiad by ,soddy & o!rs support$ t

 !ory4    ,all #c lead isotopes _h !    
same atomic numb] ( #hb4 ,! atoms ( all 
#c  isotopes _h nuclei ) an electric    
*>ge ( #hb & all #c _h #hb  electrons 9 
! atom to balance t positive nucle>     
*>ge4 ,!  diff]5ce 0 9 ! mass ( !       
nucleus only4    ,illu/ration3 ,isotopes
 ( two elem5ts4      ,atomic numb]1 #a  
,hydrog5-#a3 ,mass numb]1 #a2 #a        
,proton1 #a ,electron      ,hydrog5-#b3 
,mass numb]1 #b2 #a ,proton1 #a         
,neutron1 #a ,electron    ,atomic numb]1
 #b      ,helium-#c3 ,mass numb]1 #c2 #b
 ,protons1 #a ,neutron1 #b ,electrons   
,helium-#d3 ,mass numb]1 #d2 #b         
,protons1 #b ,neutrons1 #b ,electrons   
,b :at ( ord9>y lead t exi/$ 9 ! rocks  
f> remov$ f any  radioactive sub/ances &
 t _h presumably be5 /able ?r\gh all  ! 
hi/ory ( e>?8 ,its atomic weight 0      
#bjg4b4    ,0 ! /able lead t _h no      
connection ) radioactivity made up (    
atoms ( /ill ano!r isotope1 "o ) a      
fractional atomic weight8 ,or  c\ld     
/able lead be made up ( a mixture (     

isotopes1 ea* ( a  diff]5t :ole-numb]   
atomic weight & 0 ! ov]all atomic weight
 a  fraction only because x 0 an av]age8
    ,x 0 at ! mom5t difficult to tell 9 
! case ( lead1 b an  answ] came 9       
connection ) ano!r elem5t1 ! r>e gas    
neon  7atomic symbol ,ne71 :i* has an   
atomic weight ( #bj4b4    ,0 t          
fractional atomic weight some?+ t 0     
possess$ by all  neon atoms )\t         
exception or 0 x ! av]age ( "s          
lightweight  atoms & "s heavyweight     
ones8 ,x w\ld be a matt] ( crucial      
importance if isotopes ( neon c\ld be   
f\nd1 = neon _h no?+ to  d ) any ( !    
radioactive s]ies4 ,if neon _h isotopes 
!n any  elem5t might h !m4    ,9 #aiab  
,?omson 0 work+ on neon4 ,he s5t a /ream
 ( ca?ode-ray  electrons ?r\gh neon gas4
 ,! electrons sma%$ into ! neon atoms  &
 knock$ an electron (f "s ( !m4 ,t left 
a neon ion c>ry+  a s+le positive       
*>ge--an ion t c\ld be writt5 ,ne4    ,!
 neon ions move 9 ! electric field z    
electrons d1 b 9 !  opposite direction  

s9ce !y h an opposite *>ge4 ,9 ! comb9$ 
pres5ce ( a magnet & ( an electric      
field1 ! neon ions move 9 a  curv$ pa?4 
,if all ! neon ions _h ! same mass1 all 
w\ld foll[  ! same curve4 ,if "s 7 m    
massive ?an o!rs1 ! m massive  ones w\ld
 curve less4    ,! neon ions end$ on a  
photographic plate1 :i* 0 d>k5$ at !    
po9t ( l&+4 ,!re 7 two regions ( d>k5+1 
because !re  7 neon ions ( two diff]5t  
masses t curv$ 9 two diff]5t  degrees & 
end$ 9 two diff]5t places4 ,?omson %[$1 
f !  am\nt ( curv+1 t !re 0 a neon      
isotope ) an atomic weight  ( #bj & "o )
 an atomic weight ( #bb--,ne & ,ne4     
,:at's m1 f ! int5sity ( d>k5+1 x c\ld  
be se5 t  ord9>y neon 0 made up ( atoms 
t 7 r\ghly #ij ,ne & #aj  ,ne4 ,! ov]all
 atomic weight ( neon1 #bj4b1 0 ! av]age
 atomic  weight ( ^! #b isotopes4       
,?omson's in/rum5t 0 ! fir/ "o capable (
 sep>at+ isotopes  & su* in/rum5ts came 
to be call$ 8mass spectromet]s04 ,! fir/
  to use ! "n 0 ! ,engli% physici/      
,francis ,william ,a/on  7#ahgg-aide71  

:o built ! fir/ effici5t in/rum5t ( ?is 
type 9  #aiai4    ,he us$ x to /udy z _m
 elem5ts z he c\ld4 ,he & ^? :o  foll[$ 
him locat$ _m isotopes & det]m9$ !      
frequ5cy ( _!  occurr5ce ) consid]able  
precision4 ,x turn$ out1 = in/ance1  t  
neon is actually #ij4i ,ne1 & #h4h ,ne4 
,v small quantities  ( /ill a ?ird      
isotope1 ,ne1 are also pres5t1 mak+ up  
#j4c4    ,z = ord9>y lead 9             
nonradioactive rocks1 x is made up (    
#bc4f  ,pb1 #bb4f ,pb1 & #eb4c ,pb4 ,!re
 is /ill a f\r? isotope1  ,pb1 :i* makes
 up ! rema9+ #a4e & :i* is n ! product (
  any radioactive s]ies at all4    ,!   
isotopes always h atomic weights t are  
close to1 b n  q1 :ole numb]s4 ,any     
atomic weight ( an elem5t t dep>ts      
appreciably f an integ] does s only     
because x is an av]age (  diff]5t       
isotopes4 ,= in/ance1 ! atomic weight ( 
*lor9e  7*emical symbol ,cl7 is #ce4e1 b
 ?is is because x is made up ( a        
mixture ( #b isotopes4 ,ab\t "o qu>t] ( 
*lor9e's atoms are ,cl  & ab\t          

?ree-qu>t]s are ,cl4    ,illu/ration3   
,francis ,w4 ,a/on    ,illu/ration3     
,mass spectrograph z us$ by ,?omson &   
,a/on to  measure ! atomic weight (     
neon4    ,to avoid confusion1 ! av]age  
mass ( ! isotopes t make up a  p>ticul> 
elem5t is /ill call$ ! atomic weight ( t
 elem5t4  ,! integ] close/ to ! mass ( !
 individual isotope is spok5 (  z !     
8mass numb]0 ( t isotope4 ,?us1 *lor9e  
is made up (  isotopes ) mass numb]s #ce
 & #cg1 b ! atomic weight ( *lor9e  z x 
is f\nd 9 nature is #ce4e 7or1 to be m  
accurate1 #ce4dec74    ,9 ! same way1   
ord9>y lead is made up ( isotopes ) mass
 numb]s  #bjd1 #bjf1 #bjg1 & #bjh1 & its
 atomic weight is #bjg4ai2 neon is made 
up  ( isotopes ) mass numb]s #bj1 #ba1 &
 #bb1 & its atomic weight is  #bj4ahc1 &
 s on4    ,if ! atomic weight ( "s      
elem5t happ5s to be v close to a :ole   
numb] to beg9 )1 x may consi/ ( a s+le  
k9d ( atom4 ,=  in/ance1 ! gas fluor9e  
7*emical symbol ,f7 has an atomic weight
 (  ne>ly #ai1 :ile t ( ! metal sodium  

7*emical symbol ,na7 is ne>ly  #bc4 ,z x
 turns out1 all ! atoms ( fluor9e are ( 
! s+le v>iety  ,f1 :ile all ! atoms (   
sodium are ,na4    ,sometimes ! atomic  
weight ( an elem5t1 z x occurs 9 nature1
 is  ne>ly a :ole numb] & yet x is made 
up ( m ?an #a isotope4 ,9  t case1 "o ( 
! isotopes makes up v ne>ly all ( x1    
:ile !  o!rs are pres5t 9 su* m9or      
quantities t ! av]age is h>dly  affect$4
    ,helium1 = in/ance 7atomic symbol   
,he7 has an atomic weight ( j  ab\t #d  
&1 inde$1 almo/ all ! atoms mak+ x up   
are ,he4 ,h[ev]1  #j4jjja ( ! atoms1 or 
"o out ( a million1 are ,he4 ,aga91     
#ii4f (  all ! nitrog5 atoms 7atomic    
symbol ,n7 are ,n1 b #j4d are ,n4  ,!n1 
#ih4i ( all c>bon atoms 7atomic symbol  
,c7 are ,c1 b #a4a are  ,c4 ,x is n     
surpris+ t ! atomic weights ( nitrog5 & 
c>bon  are j ab\t #ad & #ab1            
respectively4    ,illu/ration3 ,h>old   
,urey    ,ev5 hydrog5 does n escape4    
,its atomic weight is j ab\t #a &  mo/ (
 its atoms are ,h4 ,! ,am]ican *emi/    

,h>old ,clayton ,urey  7#ahic-    7     
detect$ ! exi/5ce ( a m massive isotope1
 ,h4 ,?is  isotope has almo/ twice !    
mass ( ! light] "o4 ,no o!r isotopes  ( 
a p>ticul> atom diff] 9 mass by s l>ge a
 factor4 ,= t  reason ,h & ,h diff] 9   
ord9>y *emical prop]ties m ?an  isotopes
 usually d & ,urey !re=e gave ,h !      
special "n (  8deut]ium0 f a ,greek ^w  
mean+ 8second04    ,illu/ration3 ,w4 ,f4
 ,giauque    ,9 #aibi ! ,am]ican *emi/  
,william ,francis ,giauque 7#ahie-    7 
f\nd  t oxyg5 0 compos$ ( m ?an #a      
isotope4 ,its atomic weight _h  be5 set 
arbitr>ily at #af4jjjj s x 0 a relief t 
#ii4gf ( its  atoms 7 ,o4 ,h[ev]1 #j4bj 
7 ,o1 & #j4jd 7 ,o4    ,z y see1 ,o mu/ 
h a mass numb] ( slightly less ?an      
#af4jjjj  & x mu/ be ! m massive        
isotopes ,o & ,o t pull !  av]age up to 
#af4jjjj4 ,disreg>d+ ?is1 *emi/s clung  
to a /&>d  atomic weight ( #af4jjj =    
oxyg5 z x appe>$ 9 nature1 pref]r+  n to
 conc]n !mselves ) ! sep>ate isotopes4  
,physici/s1 h[ev]1 felt uneasy at us+ an

 av]age z /&>d =  !y 7 m int]e/$ 9 work+
 ) individual isotopes4 ,!y  pref]r$ to 
set ,o at #af4jjjj s t ! av]age atomic  
weight (  oxyg5 0 #af4jjdd & all o!r    
atomic weights rose 9 proportion4       
,atomic weights det]m9$ by ?is sy/em 7  
8physical atomic weights04    ,f9ally1 9
 #aifa1 a compromise 0 /ruck4 ,*emi/s & 
physici/s alike  decid$ to consid] !    
atomic weight ( ,c z exactly #ab & to   
use  t z a /&>d4 ,by ?is sy/em1 ! atomic
 weight ( oxyg5 became  #ae4iiid1 :i* is
 only v slightly less ?an #af4    ,!    
radioactive elem5ts did n escape ?is new
 view ei!r4 ,! atomic  weight ( uranium 
7*emical symbol ,u7 is j ab\t #bch &1   
inde$1  mo/ ( its atoms are ,u4 ,9      
#aice1 h[ev]1 ! ,canadian-,am]ican      
physici/1 ,ar?ur ,jeffrey ,demp/]       
7#ahhf-aiej71 f\nd t #j4g ( its  atoms 7
 a light] isotope1 ,u4    ,^! diff]$    
consid]ably 9 radioactive prop]ties4 ,! 
common  uranium isotope1 ,u1 _h a       
half-life ( #dejj million ye>s1 :ile ,u 
_h a half-life ( only #gjj million ye>s4

 ,fur!rmore ,u broke d[n  9 ?ree /ages  
to act9ium4 ,x 0 ,u1 n act9ium itself1 t
 0  ! beg9n+ ( ! act9ium radioactive    
s]ies4    ,z = ?orium 7atomic symbol ,?7
 ) an atomic weight ( #bcb1 x did  inde$
 turn out t 9 ! naturally occurr+ elem5t
 virtually all  ! atoms 7 ,?4           
,,en]gy      ,! ,law ( ,cons]vation (   
,en]gy    ,we h n[ gone z f> z we       
conv5i5tly c 9 consid]+ !  int]tw9+ /r&s
 ( ! atom & ( electricity4 ,x is "t to  
turn  to ! ?ird /r&--en]gy4    ,to      
physici/s ! concept ( 8"w0 is t ( ex]t+ 
a =ce on a  body & mak+ x move ?r\gh "s 
di/ance4 ,to lift a weight aga9/  ! pull
 ( gravity is "w4 ,to drive a nail into 
wood aga9/ !  friction ( its fib]s is   
"w4    ,any?+ capable ( p]=m+ "w is said
 to possess 8en]gy0 f  ,greek words     
mean+ 8"w )904 ,!re are v>i\s =ms (     
en]gy4  ,any mov+ mass possesses en]gy  
by virtue ( its motion4 ,t is1 a  mov+  
hamm] w drive a nail into wood1 :ile !  
same hamm] held  motionlessly aga9/ !   
nailhead w n d s4 ,heat is a =m (       

en]gy1 s9ce x w exp& /eam t w =ce :eels 
into motion  t c !n d "w4 ,electricity1 
magnetism1 s\nd1 & light c be  made to  
p]=m "w & are =ms ( en]gy4    ,! =ms (  
en]gy are s _m & s v>i\s t sci5ti/s 7   
eag] to f9d "s rule t cov]$ !m all &    
w\ld !re=e s]ve  z a unify+ bond4 ,x did
 n seem impossible t su* a rule might   
exi/1 s9ce "o _h be5 f\nd 9 connection )
 matt] t appe>$  9 ev5 great] v>iety ?an
 en]gy did4    ,all matt]1 :atev] its =m
 & %ape1 possess$ mass1 & 9 !  #aggjs1 !
 ,fr5* *emi/ ,anto9e ,laur5t ,lavoisi]  
7#agdc-agid7  discov]$ t ! quantity (   
mass 0 con/ant4 ,if a sy/em ( matt]  7  
isolat$ & made to und]go complicat$     
*emical reactions1  ev]y?+ ab\t x might 
*ange1 b n its mass4 ,a solid might turn
  into a gas2 a s+le sub/ance might     
*ange into two or ?ree diff]5t          
sub/ances1 b :atev] happ5$1 ! total mass
 at ! end 0 exactly  ! same 7z ne>ly z  
*emi/s c\ld tell7 z at ! beg9n+4 ,none  
0 ei!r creat$ or de/roy$1 h[ev]1 !      
nature ( ! matt] might  *ange4 ,?is 0   

call$ ! 8law ( cons]vation ( mass04     
,illu/ration3 ,lavoisi] 9 8 laboratory  
dur+ 8 /udies on  respiration4 ,f a     
sket* made by ,madame ,lavoisi]4        
,illu/ration3 ,anto9e ,lavoisi] & 8     
wife4    ,naturally1 x w\ld occur to    
sci5ti/s to wond] if a simil> law might 
hold = en]gy4 ,! answ] wasn't easy to   
get4 ,x wasn't z simple to  measure !   
quantity ( en]gy z x 0 to measure !     
quantity (  mass4 ,nor 0 x z simple to  
p5 up a quantity ( en]gy & keep x  f    
escap+ or f ga9+ additional quantity f  
outside1 z x  0 9 ! case ( mass4        
,beg9n+ 9 #ahdj1 h[ev]1 ! ,engli%       
physici/ ,james ,prescott ,j\le         
7#ahah-ahhi7 began a s]ies ( exp]im5ts 9
 :i* he made use ( e  =m ( en]gy he c\ld
 ?9k (4 ,9 ea* case he turn$ x into heat
  & all[$ ! heat to raise ! temp]ature (
 a giv5 quantity (  wat]4 ,he us$ ! rise
 9 temp]ature z a measure ( ! en]gy4 ,by
  #ahdg he 0 conv9c$ t any =m ( en]gy   
c\ld be turn$ into fix$  & pr$ictable   
am\nts ( heat2 t a c]ta9 am\nt ( "w 0   

equival5t to a c]ta9 am\nt ( heat4    ,9
 t same ye>1 ! ,g]man physici/ ,h]mann  
,ludwig ,f]d9& von  ,helmholtz          
7#ahba-ahid7 advanc$ ! g5]al notion t a 
fix$ am\nt (  en]gy 9 "o =m 0 equal to !
 same am\nt ( en]gy 9 any o!r  =m4      
,en]gy might *ange its =m ov] & ov]1 b n
 *ange its  am\nt4 ,none c\ld ei!r be   
de/roy$ or creat$4 ,?is is ! 8law (     
cons]vation ( en]gy04      ,*emical     
,en]gy    ,!re is en]gy 9 a piece (     
wood4 ,left quietly to itself1 x seems  
completely incapable ( br++ ab\t any k9d
 ( "w4 ,set x on fire1  h[ev]1 & ! wood 
plus ! oxyg5 9 ! air w give (f heat &   
light t are cle>ly =ms ( en]gy4 ,! heat 
c\ld help boil wat]  & run a /eam eng9e4
    ,! am\nt ( en]gy 9 burn+ wood c\ld  
be measur$ if x 7 mix$  ) air & all[$ to
 burn 9 a clos$ conta9] t 0 imm]s$ 9  a 
kn[n quantity ( wat]4 ,f ! rise 9       
temp]ature ( ! wat]1  ! quantity ( en]gy
 produc$ c\ld be measur$ 9 units call$  
8calories0 7f a ,lat9 ^w = 8heat074 ,!  
in/rum5t 0 !re=e  call$ a 8calorimet]04 

,9 ! #ahfjs ! ,fr5* *emi/ ,pi]re ,eugne 
,m>cel9 ,b]!lot  7#ahbg-aijg7 c>ri$     
?r\gh hundr$s ( su* det]m9ations4 ,8 "w 
& simil> "w by o!rs made x cle> t su*   
8*emical en]gy0--!  en]gy d]iv$ f       
*emical *anges 9 matt]--fit ! law (     
cons]vation ( en]gy4    ,"h's h[ x look$
 9 ! la/ decades ( ! #ai? c5tury4       
,molecules are compos$ ( comb9ations (  
atoms4 ,)9 ! molecules1  ! atoms /ick   
toge!r m or less tightly4 ,x takes a    
c]ta9 am\nt  ( en]gy to pull a molecule 
ap>t into sep>ate atoms aga9/ !         
resi/ance ( ! =ces hold+ !m toge!r4     
,if1 aft] be+ pull$ ap>t1 ! atoms are   
all[$ to come toge!r  aga91 !y give (f  
en]gy4 ,! am\nt ( en]gy !y give (f 9    
com+ toge!r is exactly equal to ! am\nt 
( en]gy !y _h to  ga9 be=e !y c\ld      
sep>ate4    ,?is is true ( all          
sub/ances4 ,= in/ance1 hydrog5 gas1 z x 
is  f\nd on e>?1 is made up ( molecules 
conta9+ #b hydrog5 atoms ea*  7,h74 ,add
 a c]ta9 am\nt ( en]gy & y pull ! atoms 
ap>t2 all[  ! atoms to come back toge!r 

into pair$ molecules1 & ! add$  en]gy is
 giv5 back aga94 ,! same is true = !    
oxyg5 molecule1  :i* is made up ( #b    
oxyg5 atoms 7,o7 & ( ! wat] molecule    
7,h,o74  ,always ! am\nt ( en]gy absorb$
 9 "o *ange is giv5 (f 9 !  opposite    
*ange4 ,! am\nt absorb$ & ! am\nt giv5  
(f are always  exactly equal4    ,h[ev]1
 ! am\nt ( en]gy involv$ diff]s f       
molecule to  molecule4 ,x is q h>d to   
pull hydrog5 molecules ap>t1 & x is  ev5
 h>d] to pull oxyg5 molecules ap>t4 ,y h
 to supply ab\t #ab  m en]gy to pull an 
oxyg5 molecule ap>t ?an to pull a       
hydrog5  molecule ap>t4 ,naturally1 if y
 let #b oxyg5 atoms come toge!r to  =m  
an oxyg5 molecule1 y get back #ab m     
en]gy ?an if y all[  #b hydrog5 atoms to
 come toge!r to =m a hydrog5 molecule4  
,x takes a consid]ably l>g] am\nt (     
en]gy to pull ap>t a wat]  molecule into
 sep>ate atoms ?an to pull ap>t ei!r    
hydrog5 or  oxyg5 molecules4 ,naturally1
 t great] en]gy is also return$ once  ! 
hydrog5 & oxyg5 atoms are all[$ to come 

back toge!r into  wat] molecules4       
,next1 imag9e pull+ ap>t hydrog5 & oxyg5
 molecules into hydrog5  & oxyg5 atoms &
 !n hav+ ^? atoms come toge!r to =m     
wat] molecules4 ,a c]ta9 am\nt ( en]gy  
is put into ! sy/em to  break up !      
hydrog5 & oxyg5 molecules1 b !n a mu*   
great]  am\nt ( en]gy is giv5 (f :5 !   
wat] molecules =m4    ,x is = t reason t
 a great deal ( en]gy 7mo/ly 9 ! =m (   
heat7 is giv5 (f if a jet ( hydrog5 gas 
& a jet ( oxyg5 gas are  all[$ to mix 9 
su* a way z to =m wat]4    ,j mix+ !    
hydrog5 & oxyg5 isn't en\gh4 ,!         
molecules (  hydrog5 & oxyg5 mu/ be     
sep>at$ & t takes a little en]gy4  ,!   
en]gy 9 a mat* flame is en\gh to raise !
 temp]ature ( !  mixture & to make !    
hydrog5 & oxyg5 molecules move ab\t m   
rapidly & m en]getically4 ,?is increases
 ! *ance t "s  molecules w be brok5 up  
into sep>ate atoms 7?\gh ! actual       
process is r complicat$74 ,an oxyg5 atom
 might !n /rike a  hydrog5 molecule to  
=m wat] 7,o  ,h  ,h,o7 & m en]gy is giv5

  (f ?an 0 absorb$ f ! mat* flame4 ,!   
temp]ature goes up  /ill high] s t fur!r
 breakup among ! oxyg5 & hydrog5        
molecules is enc\rag$4    ,illu/ration3 
,! =mation ( a sodium *loride molecule4 
,?is happ5s ov] & ov] aga9 s t 9 v      
little "t1 !  temp]ature is v high & !  
hydrog5 & oxyg5 are comb9+ to  =m wat]  
at an enorm\s rate4 ,if a great deal (  
hydrog5 & oxyg5  are well-mix$ to beg9  
)1 ! rate ( reaction is s great t an    
explosion occurs4    ,su* a situation1 9
 :i* ea* react+ bit ( ! sy/em adds en]gy
  to ! sy/em by its reaction & br+s ab\t
 m reactions l  itself1 is call$ a 8*a9 
reaction04 ,?us1 a mat* flame put to "o 
corn] ( a l>ge %eet ( pap] w set t corn]
 burn+4 ,! heat  ( ! burn+ w ignite a   
neighbor+ portion ( ! %eet & s on  till 
! entire %eet is burn$4 ,= t matt] a    
s+le smold]+  cig>ette end c s]ve to    
burn d[n an entire =e/ 9 a va/ly        
de/ructive *a9 reaction4      ,electrons
 & ,en]gy    ,! discov]y ( ! /ructure ( 
! atom %>p5$ ! und]/&+  ( *emical en]gy4

    ,9 #aijd ! ,g]man *emi/ ,ri*>d      
,abegg 7#ahfi-aiaj7 fir/ sugge/$  t     
atoms 7 held toge!r ?r\gh ! transf] (   
electrons f "o  atom to ano!r4    ,to   
see h[ ?is work$1 "o began by not+ t    
electrons 9 an atom  exi/$ 9 a s]ies (  
%ells4 ,! inn]mo/ %ell c\ld hold only #b
  electrons1 ! next #h1 ! next #ah & s  
on4 ,x turn$ out t "s  electron         
arrangem5ts 7 m /able ?an o!rs4 ,if only
 !  inn]mo/ %ell conta9$ electrons & x 7
 fill$ ) ! #b  electrons t 7 all x c\ld 
hold1 !n t 0 a /able  arrangem5t4 ,if an
 atom conta9$ electrons 9 m ?an "o %ell 
&  ! out]mo/ %ell t held electrons held 
#h1 t 0 a /able  arrangem5t1 too4       
,?us1 ! helium atom has #b electrons    
only1 fill+ ! inn]mo/ %ell1  & t is s   
/able an arrangem5t t helium und]goes no
 *emical  reactions at all4 ,! neon atom
 has #aj electrons--#b 9 ! inn]mo/      
%ell1 & #h 9 ! next--& x does n react4  
,! argon atom has #ah  electrons--#b1   
#h1 & #h--;& x too is v /able4    ,b :at
 if an atom did n h its electron %ell s 

neatly fill$4  ,! sodium atom has #aa   
electrons--#b1 #h1 & #a--:ile ! fluor9e 
atom has  #i electrons--#b & #g4 ,if !  
sodium atom pass$ "o ( its electrons to 
a  fluor9e atom1 bo? w\ld h ! /able     
configuration ( neon--#b & #h4  ,?is1   
!re=e1 ought to h a great t5d5cy to     
happ54    ,if x did happ51 ?\gh1 !      
sodium atom1 m9us #a electron1 w\ld h  a
 unit positive *>ge & w\ld be ,na1 a    
positively *>g$ ion4  ,fluor9e ) #a     
electron 9 excess w\ld become ,f1 a     
negatively *>g$  ion4 ,! #b ions1 )     
opposite *>ges1 w\ld cl+ toge!r1 s9ce   
opposite *>ges attract1 & ?us ! molecule
 ( sodium fluoride 7,na,f7  w\ld be =m$4
    ,9 #aiaf ! ,am]ican *emi/ ,gilb]t   
,newton ,lewis 7#ahge-aidf7 c>ri$  ?is  
notion f>!r4 ,atoms c\ld cl+ toge!r n   
only z a result (  ! outright transf] ( 
#a or m electrons1 b ?r\gh %>+ pairs  ( 
electrons4 ,?is %>+ c\ld only take place
 if ! atoms rema9$  close neighbors1 & x
 w\ld take en]gy to pull !m ap>t & break
  up ! %>$ pool1 j z x w\ld take en]gy  

to pull #b ions ap>t  aga9/ ! attraction
 ( opposite *>ges4    ,9 ?is way ! vague
 notions ( atoms cl++ toge!r 9 molecules
  & be+ =c$ ap>t gave way to a mu* m    
precise picture (  electrons be+        
transf]r$ or %>$4 ,! electron %ifts c\ld
 be  dealt ) ma!matically by a sy/em t  
came to be call$ 8quantum  me*anics0 &  
*emi/ry 0 ?us made a m exact sci5ce ?an 
x _h  ev] be5 be=e4      ,! ,en]gy ( !  
,sun    ,! mo/ s]i\s problem rais$ by ! 
law ( cons]vation ( en]gy  involv$ !    
sun4 ,until #ahdg1 sci5ti/s did n "q    
sunlight4 ,!  sun radiat$ va/ quantities
 ( en]gy b t app>5tly 0 its  nature & 0 
no m to be puzzl$ ov] ?an ! fact t ! e>?
  rotat$ on its axis4    ,once          
,helmholtz _h /at$ t en]gy c\ld nei!r be
 creat$ nor  de/roy$1 h[ev]1 he 0 b\nd  
to ask :]e ! sun's en]gy came  f4 ,x _h1
 to man's be/ k1 be5 radiat+ heat &     
light1  ) no p]ceptible *ange1 ?r\gh\t !
 hi/ory ( civilization &1  f :at        
biologi/s & geologi/s c\ld d$uce1 =     
c\ntless ages  e>li]4 ,:]e1 !n1 did t   

en]gy come f8    ,! sun gave ! appe>ance
 ( be+ a huge globe ( fire4 ,c\ld x     
actually be t--a l>ge heap ( burn+ fuel1
 turn+ *emical en]gy  into heat & light8
    ,! sun's mass 0 kn[n & its rate (   
en]gy production 0 kn[n4  ,suppose !    
sun's mass 7 a mixture ( hydrog5 & oxyg5
 & x 7  burn+ at a rate suffici5t to    
produce ! en]gy at ! rate x 0  giv+ x   
(f4 ,if t 7 s1 all ! hydrog5 & oxyg5 9  
its mass  w\ld be consum$ 9 #aejj ye>s4 
,no *emical reaction 9 ! sun c\ld       
acc\nt = its hav+ giv5 u heat & light   
s9ce ! days ( !  pyramids1 let alone    
s9ce ! days ( ! d9osaurs4    ,0 !re "s  
s\rce ( en]gy great] ?an *emical en]gy8 
,:at ab\t  ! en]gy ( motion8 ,helmholtz 
sugge/$ t meteors might be fall+  into !
 sun at a /eady rate4 ,! en]gy ( _!     
collisions might !n  be conv]t$ into    
heat & light & ?is c\ld keep ! sun %9+ =
  z long z ! supply ( meteors held      
out--ev5 millions ( ye>s4    ,?is1      
h[ev]1 w\ld mean t ! sun's mass w\ld be 
increas+  /eadily1 & s w\ld ! =ce ( its 

gravitational pull4 ,) !  sun's         
gravitational field increas+ /eadily1 ! 
l5g? ( e>?'s  ye> w\ld be decreas+ at a 
measurable rate--b x wasn't4    ,9 #ahed
 ,helmholtz came up ) some?+ bett]4 ,he 
sugge/$ t !  sun 0 contract+4 ,its      
out]mo/ lay]s 7 fall+ inw>d1 & !  en]gy 
( ?is fall 0 conv]t$ into heat & light4 
,:at's m1 ?is  en]gy w\ld be obta9$ )\t 
any *ange 9 ! mass ( ! sun  :atev]4     
,helmholtz calculat$ t ! sun's          
contraction ov] ! #fjjj ye>s (  record$ 
hi/ory w\ld h r$uc$ its diamet] only    
#efj miles--a *ange  t w\ld n h be5     
noticeable to ! unaid$ eye4 ,s9ce !     
developm5t ( ! telescope1 two & a half  
c5turies e>li]1 !  decrease 9 diamet]   
w\ld h be5 only #bc miles & t 0 n       
measurable by ! be/ te*niques (         
,helmholtz's day4    ,work+ backw>d1    
h[ev]1 x seem$ t #be million ye>s ago1 !
 sun  mu/ h be5 s l>ge z to fill ! e>?'s
 orbit4 ,cle>ly ! e>?  c\ld n !n h      
exi/$4 ,9 t case1 ! maximum age ( ! e>? 
0 only #be million ye>s4    ,geologi/s &

 biologi/s f\nd !mselves di/urb$ by ?is4
 ,! sl[  *anges 9 ! e>?'s cru/ & 9 !    
evolution ( life made x seem  v likely t
 ! e>? mu/ h be5 9 exi/5ce--) ! sun     
deliv]+ heat & light v mu* 9 ! pres5t   
fa%ion--= _m  hundr$s ( millions ( ye>s4
    ,yet !re seem$ absolutely no o!r way
 ( acc\nt+ = ! sun's  en]gy supply4     
,ei!r ! law ( cons]vation ( en]gy 0     
wrong 7:i*  seem$ unlikely71 or !       
pa9fully collect$ evid5ce ( geologi/s & 
biologi/s 0 wrong 7:i* seem$            
unlikely71--or !re 0 "s s\rce  ( en]gy  
great] ?an any kn[n 9 ! #ai? c5tury1 ^: 
exi/5ce  _h someh[ escap$ mank9d 7:i*   
also seem$ unlikely74    ,yet "o ( ^?   
unlikely alt]natives w\ld h to be true4 
,& !n  9 #ahif came ! discov]y (        
radioactivity4      ,! ,en]gy (         
,radioactivity    ,x ev5tually became   
cle> t radioactivity involv$ ! giv+ (f (
  en]gy4 ,uranium emitt$ gamma rays t we
 n[ "k to be a hundr$  ?\s& times z     
en]getic z ord9>y light rays4 ,:at's m1 
alpha  p>ticles 7 be+ emitt$ at         

velocities ( p]haps #cj1jjj kilomet]s   
p] second1 :ile ! light] beta p>ticles 7
 be+ %ot (f at  velocities ( up to      
#bej1jjj kilomet]s p] second 7ab\t #j4h 
times !  velocity ( light74    ,at fir/1
 ! total en]gy giv5 (f by radioactive   
sub/ances seem$ s  small t !re 0 no use 
worry+ ab\t x4 ,! am\nt ( en]gy  lib]at$
 by a gram ( uranium 9 #a second (      
radioactivity 0 an  insignificant       
fraction ( ! en]gy releas$ by a burn+   
c&le4    ,9 a few ye>s1 h[ev]1 some?+   
became app>5t4 ,a lump ( uranium  might 
give (f v little en]gy 9 a second1 b x  
kept on = second  aft] second1 day aft] 
day1 mon? aft] mon?1 & ye> aft] ye> )   
no p]ceptible decrease4 ,! en]gy releas$
 by ! uranium ov] a v  long "t grew to  
be enorm\s4 ,x ev5tually turn$ out t    
:ile !  rate at :i* uranium deliv]$     
en]gy did decl9e1 x did s ) su*         
unbelievable sl[ness t x took #d4e      
billion ye>s 767 = t rate  to decrease  
to half :at x 0 to beg9 )4    ,if all ! 
en]gy deliv]$ by a gram ( uranium 9 !   

c\rse ( its  radioactivity ov] _m       
billions ( ye>s 0 totall$1 x 0          
enorm\sly great] ?an ! en]gy produc$ by 
! burn+ ( a c&le  ) a mass equal to t ( 
uranium4    ,let's put x ano!r way4 ,we 
might ?9k ( a s+le uranium atom  break+ 
d[n & %oot+ (f an alpha p>ticle4 ,we    
might also ?9k (  a s+le c>bon atom     
comb9+ ) #b oxyg5 atoms to =m c>bon     
dioxide4 ,! uranium atom w\ld give (f   
#b1jjj1jjj times z mu* en]gy  9 break+  
d[n1 z ! c>bon atom w\ld 9 comb9+4    ,!
 en]gy ( radioactivity is millions (    
times z int5se z !  en]gy releas$ by    
*emical reactions4 ,! reason mank9d _h  
rema9$  unaw>e ( radioactivity & v aw>e 
( *emical reactions 01  fir/1 t ! mo/   
common radioactive processes are s sl[ t
 _!  great en]gies 7 /ret*$ ov] su*     
enorm\s blocks ( "t z to be             
insignificant on a p] second basis4     
,secondly1 *emical reactions are easily 
controll$ by *ang+  quantities1         
conc5trations1 temp]atures1 pressures1  
/ates ( mixtures1  & s on1 & ?is makes  

!m easy to take note ( & to /udy4 ,!    
rate ( radioactive *anges1 h[ev]1 c\ld n
 app>5tly be alt]$4  ,! e>ly            
inve/igators quickly f\nd t ! breakd[n (
 uranium-#bch1  = in/ance1 c\ld n be    
ha/5$ by heat1 pressure1 *anges 9       
*emical comb9ation1 or1 inde$1 any?+    
else !y c\ld ?9k (4 ,x  rema9$ incr$ibly
 sl[4    ,b despite all ?is1            
radioactivity _h at la/ be5 discov]$ & !
  int5sity ( its en]gies 0 recogniz$ &  
po9t$ out 9 #aijb by  ,m>ie ,curie & h] 
husb& ,pi]re ,curie 7#ahei-aijf74       
,:]e1 !n1 did ! en]gy come f8 ,c\ld x   
come f ! outside8  ,c\ld ! radioactive  
atoms someh[ collect en]gy f _!         
surr\nd+s1 conc5trate x sev]al          
million-fold1 & !n let x out  all at    
once8    ,to conc5trate en]gy 9 ?is     
fa%ion w\ld violate some?+ call$  8!    
second law ( !rmodynamics04 ,?is 0 fir/ 
propos$ 9 #ahej by  ! ,g]man physici/   
,rudolf ,julius ,emmanuel ,clausius     
7#ahbb-ahhh7 & _h  prov$ s useful t     
physici/s did n l to ab&on x unless !y  

absolutely _h to4    ,ano!r possibility 
0 t radioactive atoms 7 creat+ en]gy out
  ( no?+4 ,?is1 ( c\rse1 violat$ ! law (
 cons]vation ( en]gy  7also call$ 8!    
fir/ law ( !rmodynamics07 & physici/s   
pref]r$  n to d t ei!r4    ,! only ?+ t 
seem$ to rema9 0 to suppose t some:]e   
)9 ! atom 0 a s\rce ( en]gy t _h nev]   
made itself  evid5t to humanity until ! 
discov]y ( radioactivity4 ,becqu]el 0   
"o ( ! fir/ to sugge/ ?is4    ,x might h
 seem$ at fir/ t only radioactive       
elem5ts _h ?is  supply ( en]gy some:]e  
)9 ! atom1 b 9 #aijc ,ru!r=d  sugge/$ t 
all atoms _h a va/ en]gy supply hidd5 )9
  !mselves4 ,! supply 9 uranium & ?orium
 leak$ slightly1 s to  speak1 & t 0 all 
t made !m diff]5t4    ,illu/ration3 ,!  
room 9 :i* ! ,curies discov]$ radium4   
,pi]re  ,curie's writ+ is on !          
blackbo>d4    ,b if a va/ supply ( en]gy
 exi/$ 9 atoms1 x 0 possible t  !       
solution to ! puzzle ( ! sun's en]gy    
might re/ !re4 ,z  e>ly z #ahii !       
,am]ican geologi/ ,?omas ,*r[d] ,*amb]l9

  7#ahdc-aibh7 0 already speculat+ ab\t 
a possible connection betwe5            
radioactivity & ! sun's en]gy4    ,if x 
7 "s v>iety ( ?is newly discov]$ s\rce (
 en]gy 7n  necess>ily ord9>y            
radioactivity1 ( c\rse7 t p[]$ !        
sun--millions ( times z int5se z *emical
 en]gy--!n ! sun might  be p\r+ out     
en]gy = hundr$s ( millions ( ye>s )\t   
p]ceptible physical *ange--j z uranium  
w\ld %[ sc>cely any  *ange ev5 9 s      
mighty a "t span4 ,! sun w\ld n h to be 
contract+2 x w\ld n h _h to fill ! e>?'s
 orbit #be1jjj1jjj  ye>s ago4    ,?is 0 
all excit+1 b 9 #aijj ! /ructure ( !    
atom _h n yet  be5 work$ out & ?is new  
en]gy 0 j a vague supposition4 ,no "o   
_h any idea ( :at x actually might be or
 :]e 9 ! atom x might  be locat$4 ,x    
c\ld only be spok5 ( z exi/+ 8)9 ! atom0
 &  0 !re=e call$ 8atomic en]gy04 ,?r\gh
 long habit1 x is /ill  call$ t mu* ( ! 
"t4 ,& yet 8atomic en]gy0 is n a good   
"n4 ,9 ! fir/ c\ple ( decades ( ! #bj?  
c5tury1 x became  app>5t t ord9>y       

*emical en]gy involv$ electron %ifts &  
^? electrons 7 c]ta9ly compon5ts (      
atoms4 ,?is meant t a  wood fire 0 a k9d
 ( atomic en]gy4    ,! electrons1 h[ev]1
 exi/$ only 9 ! out] regions ( ! atom4  
,once ,ru!r=d work$ out ! !ory ( !      
nucle> atom1 x became  app>5t t ! en]gy 
involv$ 9 radioactivity & 9 sol>        
radiation _h to involve compon5ts ( !   
atom t 7 m massive  & m en]getic ?an !  
light electrons4 ,! en]gy _h to come1   
someh[1 f ! atomic nucleus4    ,:at is  
involv$ !n 9 radioactivity & 9 ! sun is 
8nucle>  en]gy04 ,t is ! prop] "n = x & 
9 ! next section we w  consid] !        
subsequ5t hi/ory ( ! nucle> en]gy t     
broke ^u  ! />tl$ consci\sness (        
sci5ti/s z ! #bj? c5tury op5$ &  :i*1   
less ?an half a c5tury lat]1 0 to face  
mank9d ) untold  consequ5ces = good & = 
evil4                                   
,,footnotes      #a8,mass0 is ! correct 
t]m1 b 8weight01 :i* is a some:at       
diff]5t ?+1 is s commonly us$ in/ead t 9
 ?is book ,i      won't try to make any 

di/9ction4                              
,,quotation ,,cr$it        ,inside front
 cov]    ,copyright  by ,abel>d-,%uman1 
,ltd41 ,new ,york4                      
,repr9t$ by p]mission f ,inside ! ,atom1
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,asimov1 #aiff4                         
,,photo ,,cr$its        ,cov]           
,! ,metropolitan ,museum ( ,art    ,page
 fac+ inside    ,! 8,horsehead0 ,nebula 
9 ,orion4 ,hale    cov]                 
,obs]vatories4    ,au?or's ,photo       
,jay ,k4 ,kle9    ,cont5ts page         
,lick ,obs]vatory    page #d    ,page   
#g                     ,new ,york       
,public ,libr>y    #i                   
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,weeks1                                 
,*emical ,education ,publi%+ ,company1  
#aifh4    #ab                    ,libr>y
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,new ,york ,public ,libr>y    #ba       
,copyright  #aife by ,b>b>a ,lovett     

,cl9e1                          repr9t$ 
f h] volume ,! ,que/ion]s3              
,physici/s & ! ,quantum ,!ory by        
p]mission                          (    
,?omas ,y4 ,cr[ell ,company1 ,inc41 ,new
 ,york4    #bb  #bc               ,curie
 ,f\ndation1 ,in/itute ( ,radium    #bf 
,academic ,press1 ,inc4    #bi          
,van ,no/r& ,re9hold ,company    #ca    
,top1 ,nobel ,in/itute2 bottom1 f       
,discov]y (                          !  
,elem5ts1 ,m>y ,e4 ,weeks1 ,*emical     
,education                              
,publi%+ ,company1 #aifh4    #cb        
,f ,discov]y ( ! ,elem5ts1 ,m>y ,e4     
,weeks1                                 
,*emical ,education ,publi%+ ,company1  
#aifh4    #cd                    ,top1  
,nobel ,in/itute2 bottom1 ,niels ,bohr  
,in/itute4    #cf1 #db1 #dd1  #de       
,nobel ,in/itute    #dh                 
,academic ,press1 ,inc4    #di          
,f ,discov]y ( ! ,elem5ts1 ,m>y ,e4     
,weeks1                                 
,*emical ,education ,publi%+ ,company1  

#aifh4    #fj                    ,curie 
,f\ndation1 ,in/itute ( ,radium         
,u4,s4 ,,gov]nm5t ,,pr9t+ ,,(fice3      
#aige--fdj--bhe_/ac      ,a ^w ab\t     
,,erda4444    ,! mission ( ! ,u4 ,s4    
,en]gy ,rese>*  ,developm5t             
,adm9i/ration  7,,erda7 is to develop   
all en]gy s\rces1 to make ! ,nation     
basically  self-suffici5t 9 en]gy1 & to 
protect public heal? & welf>e &  !      
environm5t4 ,,erda programs are divid$  
into six major categories3              
,,cons]vation ,,( ,,en]gy--,m effici5t  
use ( exi/+ en]gy s\rces1  developm5t ( 
alt]nate fuels & eng9es = automobiles to
 r$uce  dep5d5ce on petroleum1 &        
elim9ation ( wa/eful habits ( en]gy     
consumption4     ,,fossil               
,,en]gy--,expansion ( coal production & 
! developm5t (  te*nologies = conv]t+   
coal to syn!tic gas & liquid fuels1     
improvem5t ( oil drill+ me?ods & (      
te*niques = conv]t+  %ale deposits to   
usable oil4     ,,sol>1 ,,geo!rmal1 ,,& 
,,advanc$ ,,en]gy ,,sy/ems--,rese>* on  

sol>  en]gy to heat1 cool1 & ev5tually  
electrify build+s1 on conv]sion  (      
und]gr\nd heat s\rces to gas &          
electricity1 & on fusion  reactors = !  
g5]ation ( electricity4     ,,environm5t
 ,,& ,,safety--,inve/igation ( heal?1   
safety1 &  environm5tal effects ( !     
developm5t ( en]gy te*nologies1 &       
rese>* on managem5t ( wa/es f en]gy     
production4     ,,nucle> ,,en]gy--,exp&+
 m$ical1 indu/rial & rese>* applications
  & upgrad+ reactor te*nologies = !     
g5]ation ( electricity1  p>ticul>ly us+ 
! bre$] concept4     ,,national         
,,security--,production & adm9i/ration (
 nucle> mat]ials  s]v+ bo? civilian &   
milit>y ne$s4    ,,erda programs are    
c>ri$ out by contract & coop]ation )    
indu/ry1  univ]sity communities1 & o!r  
gov]nm5t ag5cies4 ,= m  in=mation1 write
 to ,,us]da--,te*nical ,in=mation ,c5t]1
 ,p4 ,o4 ,box #fb1  ,oak ,ridge1        
,t5nessee #cghcj4        ,illu/ration3  
,,en]gy ,,rese>*  ,,developm5t          
,,adm9i/ration ,,usa                    

,unit$ ,/ates               ,en]gy      
,rese>* & ,developm5t ,adm9i/ration     
,(fice ( ,public ,affairs               
,wa%+ton1 ,d4,c4 #bjede                 
,transcrib]'s ,notes      --,reta9$     
publication in=mation f ! pr9t$ edition3
 ?is e,book    is public-doma9 9 !      
c\ntry ( publication4    --,9 ! text    
v]sion only1 und]l9$ or italiciz$ text  
is delimit$    by und]scores4    --,:]e 
possible1 ,,utf sup]script & subscript  
numb]s are us$2 "s    e-read] fonts may 
n support ^! *>act]s4    --,9 ! text    
v]sions only1 o!r sup]script text is    
prec$$ by c>et    & delimit$ by         
brackets4    --,9 ! text v]sions only1  
o!r subscript$ text is prec$$ by        
und]score & delimit$ by brackets4