390  The  U.S. P.  Inorganic  Chemicals.  {AwAtgSkmTm* 
frjm  sulfuric  acid,  various  devices  were  tried,  such  as  the  use  of 
two  or  more  wash-bottles,  the  use  of  lead  oxid,  etc.  None  of  those 
tried  were  sufficient  to  retain  all  the  vapors  of  sulfuric  anhydrid 
which  are  given  in  the  operation  (especially  when  Cu  is  employed) 
and  carried  over  mechanically  with  the  sulfurous  oxid. 
The  chief  objection  to  the  copper  method  is  that  the  metal  be- 
comes quickly  coated  with  an  insoluble  layer  of  sulfates,  when 
reaction  ceases.  This  is  less  marked  with  the  spiral-formed  turnings 
than  with  the  granulated  or  filings.  Charcoal  is  a  cheaper  and  far 
more  convenient  material,  and  when  properly  carried  out  the  method 
will  yield  a  stronger  solution  than  the  copper. 
The  rapid  deterioration  of  sulfurous  acid  solutions  may  be  noted 
on  the  following  two  samples. 
Per  cent.  Per  cent. 
June  17  Sample  I,  6*56  Sample  II,  4*54 
"18   14         6-40   "  4-43 
"23   "        6-o8   "  4-27 
July    1   "        5.92   **  4-20 
The  old  method  of  titrating  by  adding  the  standard  iodin  V.S. 
directly  to  the  diluted  acid,  is  open  to  the  error  introduced  through 
the  reducing  action  of  the  hydriodic  acid  formed  on  the  sulfur  dioxid. 
The  present  method  (Giles  and  Schearer,  your.  Soc.  Chew.  Ind. 
iv,  303)  avoids  this  in  adding  an  excess  of  the  iodin  solution  and 
titrating  back  with  the  thiosulfate  V.S.  The  results  are  very 
accurate  and  concordant. 
Alum. — In  revising  this  text,  the  question  arose  as  to  the  advisa- 
bility of  retaining  the  Potassa  Alum  or  introducing  the  Soda  Alum; 
also  of  rigidly  excluding  the  Ammonia  Alum.  A  letter  directed  to 
one  who  is  thoroughly  informed  upon  this  subject,  brought  the 
following  facts :  "  When  ammonium  sulfate  was  very  much 
cheaper  than  it  is  at  present,  and  before  the  importation  of  the 
potassium  sulfate,  now  obtained  very  cheaply  from  the  Stassfurt 
deposits  in  Germany,  ammonia  alum  was  made ;  but  now  ammonium 
sulfate  is  rarely  used  in  making  alum.  It  has  happened  that 
where  ammonia  alum  is  manufactured  for  special  purposes,  the 
mother  liquors  from  it  were  evaporated  together.  A  small  quantity 
of  alum  of  commerce  may  have  been  found  to  contain  ammonia 
from  the  above-named  cause,  but  the  proportion  of  ammonia  present 
must  have  been  very  small. 
"  Soda  cannot  be  used  for  the  manufacture  of  alum  for  the  reason 
