ON  SOME  DOUBLE  SALTS  OF  IRON. 
307 
Combine  the  acid  and  sugar,  and  rub  into  a  fine  powder  (if  citric 
acid  is  used  dry  all  the  water  out  over  a  water  bath,)  add  the 
magnesise  citras  and  ol.  limonis,  and  mix  intimately ;  then  add 
the  bicarb,  of  soda,  and  triturate  the  whole  into  a  fine  powder, 
which  must  be  preserved  in  bottles  properly  excluded  from  the 
air.  The  dose  for  an  an  adult  is  from  one  to  three  tablespoonfuls 
mixed  in  a  tumbler  of  water,  and  drank  in  a  state  of  effervescence. 
ON  SOME  DOUBLE  SALTS  OF  IRON,  AND  THE  RELATION  OF 
SIMILAR  SALTS  TO  THE  ALUMS. 
By  Mr.  William  Bastick. 
It  will  be  in  the  recollection  of  many  members  of  the  Society 
that  on  a  recent  occasion  there  was  a  discussion  as  to  the  number 
and  variety  of  double  salts,  properly  included  under  the  generic 
term  alums.  By  some  it  was  argued  that  only  the  double  salts 
formed  by  the  combination  of  persulphate  of  alumina,  persulphate 
of  iron,  persulphate  of  chromium,  or  persulphate  of  manganese, 
with  sulphate  of  potash,  sulphate  of  soda  or  sulphate  of  ammonia, 
crystallized  in  octohedrons  with  twenty-four  equivalents  of  water, 
and  could  therefore  be  regarded  as  alums.  By  others  it  was  as- 
sumed that,  besides  these  double  salts,  the  term  might  be  applied 
with  propriety  to  all  the  combinations  of  the  before-mentioned 
persulphates,  with  the  sulphates  of  magnesia,  manganese,  iron, 
zinc,  copper,  cobalt,  and  nickel.  This  assumption,  although  con- 
trary to  theory,  has  been  supported  by  Sir  Robert  Kane  and 
others,  who  have  described  some  double  salts  formed  of  these 
metallic  and  earthy  sulphates,  in  combination  with  persulphate 
of  alumina,  and  who  have  applied  the  term  alums  to  these  double 
salts,  because  they  possessed  a  similar  constitution  to  the  true 
alums,  notwithstanding  that  their  crystalline  form  belonged  to 
the  prismatic  system.  Now,  I  apprehend,  that  for  any  salts  to 
come  under  the  generic  term  alums,  they  must  be  isomorphous 
with  the  salt  from  which  the  genus  alone  derives  its  name,  and 
that  to  be  thus  isomorphous  it  is  absolutely  necessary  that  the 
salts  should  possess  the  same  crystalline  form,  and  replace  each 
other  in  combination  without  any  material  alteration  in  their 
crystalline  structure.  If  these  positions  be  true,  however  much 
the  so-called  alums  described  by  Kane  and  others  may  agree  in 
