552  History  of  Argon  and  Helium.  {^i^S; 
latter.  The  impurities  of  the  resulting  gas  were  removed  as  follows : 
C02  by  potassium  hydroxide  ;  water  vapor  by  phosphorus  pentoxide  ; 
hydrogen  by  sparking  with  an  excess  of  oxygen,  removing  the 
excess  of  the  latter  by  means  of  pyrogallic  acid.  The  nitrogen  was 
removed  by  repeatedly  passing  the  gases  over  and  through  red-hot 
magnesium  and  its  vapor. 
The  density  of  helium  as  the  mean  of  five  determinations  was 
found  to  be  213  (O  =  16).  The  half  wave-length  of  sound  in 
helium  was  found  to  be  10 15  mm.  and  in  air  36-04  mm.  By  the 
same  calculation  as  was  given  under  argon  above,  it  was  found  that 
helium  is  a  monatomic  gas.  If  the  density  as  compared  with 
H2  is  213,  and  it  is  monatomic,  then  its  atomic  weight  is  4-26.  It 
is  very  slightly  soluble  in  water,  one  volume  absorbing  only  o  0073 
volume  of  the  gas.  This  is  the  lowest  solubility  ever  recorded.  It 
is  insoluble  in  absolute  alcohol  and  in  benzene.  Its  spectrum  is 
made  up  of  several  lines  in  the  red,  orange-red  and  yellow.  Several 
lines  are  identical  with  argon  lines;  thus  two  in  the  red  and  one  in 
the  orange-red,  but  the  bright  red  line  of  argon  is  faint  in  case  of 
helium,  and  the  bright  red  line  of  helium  is  faint  in  case  of  argon. 
The  characteristic  D,  yellow  line  of  helium  is  a  doublet,  one  line  of 
which  is  faint  and  the  other  very  bright,  the  distance  between  them 
being  about  one-fiftieth  of  that  between  the  D1  and  D2  lines  of  the 
solar  spectrum. 
An  analogy  exists  between  argon  and  helium.  Both  are  very 
inert  and  cannot  be  separated  when  mixed,  on  account  of  their  simi- 
larity in  properties.  Their  densities  and  spectra,  however,  plainly 
prove  their  difference.  Both  are  unattacked  by  oxygen  when  sparked 
with  it,  and  both  are  unattacked  by  the  vapor  of  magnesium.  Both 
are  monatomic  and  these  properties  differentiate  them  from  all  other 
elements  A  most  anomalous  property  of  helium  is  its  refractive 
index— which  is  about  one-tenth  that  of  hydrogen,  a  lighter  gas — a 
fact  which  is  as  unlooked  for  as  it  is  inexplicable.  Why  is  argon  in 
the  air,  and  helium  not  there  present,  but  confined  to  minerals? 
Dr.  Johnstone  Stoney1  probably  gives  the  correct  answer  to  this 
question.  He  shows  that,  if  hydrogen  were  present  in  the  atmos- 
phere, it  would  at  once  leave  this  planet  by  virtue  of  the  velocity  of 
its  own  molecular  motion,  and  migrate  to  a  planet  possessed  of  suf- 
1  Chemical  News,  1895,  71,  67. 
