576 
Romance  of  Chemical  Elements.    (Am-  J°ur-  Phanrm- 
J  L     August,  1 918. 
the  Greek  term  for  lazy,  indicates  its  inertness.  The  difficulty  arose 
how  to  place  these  elements  in  the  periodic  system,  and  a  "  zero  " 
group  was  added.  Now  we  know  that  these  elements  form,  so  to 
speak,  the  "  missing  link  "  in  the  system,  for  they  form  the  transition 
from  a  highly  electro-negative  group  of  elements  to  a  highly  electro- 
positive group.  From  the  halogens  to  the  alkali  metals.  So  they 
became  of  great  theoretical  importance  in  chemistry. 
Table  VII. 
The  Family  Tree  of  the  Noble  or  Inert  Gases. 
Year. 
Discoverer. 
Air 
(1,000  liters) 
Oxygen 
(209.9  1) 
Nitrogen 
I  
Nitrogen 
(780  3  1) 
Xumber  8 
Symbol  o 
At.  W.  16 
7 
N 
14 
Helium 
(.004) 
2 
He 
4 
Argon 
(9.4  1) 
Rutherford 
Priestley, 
Scheele,  etc. 
Ramsay  and 
Rayleigh 
Ramsay.  Cleve 
Neon 
.012 
I 
Argon 
9  4 
I 
Krypton 
.00005 
i 
Xenon 
.000006 
I 
10 
18 
36 
54 
Ne 
A 
Kr 
Xe 
20 
40 
83 
130 
Travers 
N.  B.  The  amount  of  each  gas  by  volume  is  given  in  parenthesis.  Thus 
1,000  liters  of  air  contain  about  0.004  liters  of  helium,  etc. 
Radioactive  Substances. 
Following  this  epoch-making  discovery  there  came  another  one 
of  equal,  perhaps  still  greater  importance,  namely,  the  radio-active 
substances.  The  time  was  ripe  and  the  stage  set  for  this  discovery. 
It  came  after  physics  had  settled  down  and  declared  that  there  could 
no  more  be  anything  new  in  physics,  and  that  the  work  of  the  physi- 
cist simply  consisted  in  working  out  the  details.  Then  came  the 
discovery  of  the  X-rays  by  Professor  Rontgen,  and  with  it  an 
entirely  new  field  had  been  opened  up.  Everyone  began  to  work 
with  "  rays  "  of  some  kind  or  other.    Becquerel  in  Paris  studied 
