NoVeTbef,hi895m-}      History  of  Argon  and  Helium.  553 
ficient  gravitational  attraction,  enough  mass,  to  hold  it  fast.  This, 
he  says,  accounts  for  the  presence  of  helium  and  hydrogen  in  the 
chromosphere,  and  for  the  absence  of  an  atmosphere  and  of  water 
vapor  on  the  moon.  It  would  account  for  the  absence  of  so  light  a 
gas  as  helium  in  our  atmosphere,  and  for  the  presence  of  the  helium 
line  in  the  spectrum  of  the  chromosphere.  If  an  element  forms 
compounds,  or  is  absorbed  by  solids,  it  will  be  found  on  the  earth  ;J 
hence,  we  find  helium  occluded  in  minerals,  while  argon,  which  is 
so  heavy  and  dense,  can  be  held  as  such  about  our  planet.  Argon 
occurs  in  our  atmosphere  because  it  is  so  inert  that  it  does  not  form 
any  compounds,  and  nitrogen  is  there  present  because  it  is  also  inert 
and  rarely  enters  into  combination.  Its  compounds,  too,  are  mostly 
decomposed  by  water,  and  the  excess  of  nitrogen,  hence,  is  in  the 
air.  Oxygen  is  in  the  air  because  there  is  an  excess  over  what  is 
necessary  to  oxidize  everything  on  the  surface  of  the  earth.  If  there 
are  gases  similar  to  argon  in  inertness,  and  whose  density  is  not  less 
than  that  of  nitrogen,  they  may  too  be  looked  for  in  the  air.  The 
density  of  argon  is  too  high,  and  cannot  be  accounted  for  by  assum- 
ing that  it  is  a  mixture  of  A2  and  Av  As  it  clearly  does  not  belong 
where  its  atomic  weight  places  it  in  the  periodic  system,  it  must 
needs  be  a  mixture,  although  its  behavior  near  its  critical  tempera- 
ture indicates  that  it  is  an  element.  There  is,  of  course,  no  place 
for  helium  in  the  periodic  system,  as  it  lies  between  hydrogen  and 
lithium.  But  we  must  not  forget  that  there  is  no  place  for  hydro- 
gen, either,  strictly  speaking,  and  it  may  be  that  there  may  be  an 
eighth  group  series  at  the  beginning  of  our  system,  corresponding 
to  the  iron,  nickel  and  cobalt,  or  the  osmium,  iridium  and  platinum 
series.  If  so,  a  third  gas  of  atomic  weight  between  hydrogen  and 
lithium  is  yet  to  be  discovered.  It  has  been  mentioned  that  argon 
and  helium  have  some  spectral  lines  in  common.  As  no  lines  of  any 
two  elements  have  ever  been  found  to  coincide  as  yet,  we  are  con- 
fronted with  the  possibility  that  both  these  gases  contain  a  common 
ingredient.  The  density  of  helium  is,  however,  already  so  low  that 
it  is  difficult  to  imagine  that  it  contains  as  an  admixture  anything 
much  denser ;  for  to  possess  the  density  it  now  has,  considered  as  a 
mixture,  the  true  helium  would  have  to  be  possessed  of  a  lower  den- 
sity still;  in  other  words,  it  would  have  to  be  lighter  still  than  it  now 
is.  On  the  other  hand,  an  admixture  of  helium  with  argon  would 
tend  to  make  the  density  of  pure  argon,  and  also  its  atomic  weight, 
