308 
ON  SOME  DOUBLE  SALTS  OF  IRON. 
their  chemical  composition  with  the  true  alums,  they  cannot  be 
regarded  as,  or  termed  alums,  because  their  physical  conforma- 
tion and  properties  do  not  satisfy  the  conditions  required  by  the 
isomorphous  theory.  "With  a  view  of  ascertaining  whether  some 
of  the  metallic  sulphates  might,  in  combination  with  some  other 
persulphate  than  that  already  experimented  upon  by  Kane  and 
others,  form  salts  isomorphous  with  the  true  alums,  I  prepared 
and  examined  the  following  double  salts  of  iron  : 
Persulphate  of  Iron  and  Sulphate  of  Copper. — This  salt  was 
formed  by  adding  in  equivalent  proportion  sulphate  of  copper  to 
a  solution  of  persulphate  of  iron,  containing  an  excess  of  acid, 
and  evaporatiug  the  mixture  until  a  pellicle  formed  on  the  sur- 
face, when  the  solution  was  set  aside  to  crystallize.  By  these 
means  a  bluish-green  salt  was  obtained  in  considerable  quantity, 
whose  crystalline  form  could  only  be  clearly  observed  by  the  aid 
of  the  microscope,  which  proved  that  this  salt  does  not  crystallize 
octohedrons,  but  in  wart-like  masses,  consisting  of  prisms,  and 
therefore  is  not  isomorphous  with  the  true  alums.  This  salt  very 
readily  effloresces  when  exposed  to  the  air  ;  consequently  it  is 
difficult  to  be  preserved,  or  to  be  correctly  analysed,  and  this 
difficulty  is  increased  because  it  is  contaminated  with  free  acid, 
which  is  not  easily  removed,  as  it  cannot  be  crystallized  except 
from  a  strongly  acid  solution.  Notwithstanding  these  disadvan- 
tages I  dried  as  far  as  possible  a  portion  of  the  salt  for  analysis 
by  subjecting  it  to  repeated  strong  pressure,  between  the  folds 
of  bibulous  paper.  2.9  grammes  of  this  salt  thus  treated  yielded 
sulphate  of  baryta  =  .95  gramme  of  anhydrous  sulphuric  acid, 
and  5,6  grammes  gave  .43  gramme  of  oxyde  of  copper  and  .87 
gramme  of  peroxyde  of  iron,  and  its  water  was  estimated  from 
the  loss.  The  following  table  will  show  these  quantities  reck, 
oned  in  per  cent.,  and  also  the  calculated  constitution  of  the 
salt :  — 
Calculated. 
Found. 
Cu  0 
40 
8.07 
7.67 
Fe2  03 
80 
16.13 
15.53 
3  S0o 
160 
32.26 
32.76 
24  HO 
216 
43.54 
44.04 
496        100.00  100.00 
