68 
ON  RUBIDIUM  AND  CiESIUM. 
The  sulphate,  RbO.S03,  is  isomorphous  with  that  of  potash. 
At  7°  C.  it  dissolves  in  2*4  times  its  weight  of  water,  and  is, 
consequently,  more  soluble  than  sulphate  of  potash,  which  re- 
quires 104  times  its  weight  of  water  to  dissolve  it.  With  sul- 
phate of  magnesia  it  gives  a  beautiful  octohedral  salt,  and  with 
sulphate  of  cobalt  another  double  salt,  in  fine  crystals,  isomor- 
phous with  C0O.SO3+KO.SO3+6HO. 
The  bisulphate  of  rubidium  can  be  fused  at  a  moderate  tem- 
perature without  decomposition,  but  at  a  higher  temperature  it  is 
reduced  to  the  neutral  salt. 
The  chloride  of  rubidium  is  cubic,  but  crystallises  with  diffi- 
culty. It  is  fusible  and  volatilizable  at  the  extremity  of  a 
platinum  wire.  From  1°  to  7°  C.  it  dissolves  in  1-2  to  1-3 
parts  of  water  ;  KC1  requires  3-4. 
The  double  chloride  of  platinum  and  rubidium,  RbCl.PtCl  is 
a  crystalline  powder  composed  of  microscopic  octohedra.  It 
only  differs  from  the  double  chloride  of  platinum  and  potassium 
by  its  less  solubility ;  it  only  dissolves,  indeed,  in  158  times  its 
wTeight  of  boiling  water,  whereas  the  potassium  salt  only  re- 
quires 19  times  its  weight. 
Caesium  (from  ccesius,  sky-blue,  on  account  of  the  color  of 
the  ray  which  it  produces  in  the  spectrum).  The  double 
chloride  which  it  forms  with  platinum  is  still  less  soluble  than 
that  of  rubidium.  Its  equivalent  ranks  by  the  side  of  those 
of  iodine  and  gold,  128-4,  and  is  consequently,  one  of  the 
highest. 
The  hydrate  of  caesium,  CsO.HO-j-Aq,  is  deliquescent.  When 
heated  it  loses  an  equivalent  of  water,  attacks  platinum,  volati- 
lizes in  a  spirit-flame,  and  easily  dissolves  in  alcohol. 
The  carbonate,  CsO.C02-|-Aq,  crystallizes  badly;  at  19°  C. 
it  dissolves  in  9  times,  and  at  78-4°  C.  in  5  times  its  weight  of 
absolute  alcohol.  It  is  very  caustic,  liquefies  in  the  air,  and  is 
transformed  into  bicarbonate.  It  is  soluble  in  all  proportions  in 
hot  water. 
The  bicarbonate  crystallizes  in  brilliant  prisms. 
The  nitrate  is  anhydrous,  like  that  of  potash,  with  which  it  is 
isomorphous.  It  has  the  taste  of  saltpetre,  as  also  has  the 
nitrate  of  rubidium. 
The  sulphate  of  caesium  is  also  anhydrous,  and  crystallizes 
