480  Romance  of  Chemical  Elements.    { Am'ji?y r"x 9 ihsarm' 
an  element,  for  the  readers  will  all  be  familiar  with  the  changing 
.ideas  concerning  it.  But  I  want  to  point  out  one  great  difference  in 
the  standpoint  of  modern  science  as  compared  with  the  Greek  phi- 
losophers. Since  the  introduction  of  experimental  research,  man- 
kind has  been  enabled  to  accumulate  a  great  deal  of  facts,  which  the 
ancients  did  not  possess.  Every  discovery  of  to-day,  every  technical 
achievement  of  to-day,  in  fact  every  new  invention  is  built  upon  the 
experiences  of  our  forefathers.  There  is  causality  in  the  material 
world. 
With  regard  to  the  chemical  elements  we  have  now  collected  and 
accumulated  enough  data  to  enable  us  to  say  that  there  are  92  chem- 
ical elements,  of  which  5  have  not  yet  been  discovered.  From  the 
results  of  modern  researches  on  the  X-ray  spectra  of  the  elements 
we  are  justified  in  ascribing  an  "  atomic  number  "  to  each  element. 
This  number  indicates  simply  the  relative  position  of  the  spectral 
lines  for  each  element.  But  this  number  increases  strictly  in  ac- 
cordance with  the  atomic  weight.  Table  I  gives  the  atomic  number 
for  each  element  and  shows  at  the  same  time  a  modified  arrange- 
ment of  the  periodic  system. 
Our  series  of  elements  begins  with  hydrogen  No.  1,  and  ends 
with  uranium  No.  92,  and  we  must  assume  that  under  the  present 
cosmical  condition  upon  our  earth  no  more  elements  can  exist.  For 
an  atom  of  uranium  has  become  already  so  unstable  that  it  decom- 
poses and  gives  rise  to  a  series  of  radioactive  substances.  And, 
though  there  are  some  indications  of  elements  lighter  than  hydrogen 
in  the  sun  corona  and  star  nebula,  we  have  every  reason  to  believe 
that  elements  lighter  than  hydrogen  do  not  exist  upon  our  earth. 
Thus  we  have  our  system  of  elements  limited  to  92,  of  which  the 
last  12,  that  is  from  No.  80  on,  form  the  series  of  radioactive  ele- 
ments, whose  atoms  are  so  unstable  that  they  disintegrate  and  break 
apart,  forming  elements  of  lower  atomic  weight,  and  also  producing 
the  so-called  "  isotopes."  For  accuracy's  sake  I  have  given  in  Table 
II  the  disintegration  series  of  uranium,  thorium  and  actinium,  from 
which  the  isotopes  for  each  of  the  radioactive  elements  can  be  seen 
in  the  vertical  column. 
Missing  Elements. 
The  five  missing  elements  are  those  with  atomic  number  43,  61, 
75,  85  and  87.  Of  these  the  element  61  belongs  to  the  rare  earths, 
as  seen  from  the  periodic  table  in  Table  I,  Nos.  43  and  75  are  ele- 
