ON  ALUMEN  EXSICCATUM. 
19 
the  Pharmacopoeia,  and  the  question  arises  whether  it  is  justi- 
fiable, from  a  pharmaceutical  point  of  view,  to  substitute  one  for 
the  other.  We  know  that  ammonia  alum  has  precisely  the  same 
properties  as  potassa  alum ;  alike  as  they  are  in  their  physical 
appearance,  they  are  also  alike  in  their  general  medicinal  pro- 
perties ;  they  both  lose  their  water  of  crystallization  when 
heated,  and  absorb  it  again  when  brought  into  contact  with 
moisture.  But  there  is  one  material  difference  between  the  two 
alums,  inasmuch  as  sulphate  of  ammonia  is  decomposed  by  heat, 
and  ammonia  alum,  when  heated  sufficiently  high,  leaves  pure 
alumina  behind. 
The  difficulty  with  which  potassa  alum  is  obtained  now-a-days, 
makes  it  desirable  to  be  able  to  use  the  ammonia  alum  in  place 
of  it.  For  preparing  hydrate  and  the  various  salts  of  alumina, 
it  is  superior  to  the  other,  because  the  sulphate  of  ammonia, 
being  easier  soluble,  is  more  readily  washed  away  with  water 
than  sulphate  of  potassa.  But  it  remained  to  be  ascertained 
what  precautions  are  necessary  for  preparing  burned  alum  from 
the  double  sulphate  of  ammonia  and  alumina.  I  have,  there- 
fore, undertaken  some  experiments  with  a  view  to  finding  the 
most  suitable  temperature  for  desiccating  this  alum ;  and  the 
potassa  alum  has  been  likewise  employed  for  some  experiments. 
Before  proceeding  to  the  experiments,  I  may  state  here,  that 
they  have  been  performed  in  a  sand  bath,  heated  by  an  alcohol 
lamp  ;  the  temperature  in  all  cases  applies  to  the  sand  bath ; 
the  alum,  the  weight  of  which  in  each  case  was  six  drachms, 
was  fused  and  desiccated  in  tared  porcelain  capsules,  and  in  the 
case  of  ammonia  alum,  moistened  red  and  blue  litmus  paper  was 
kept  suspended  over  the  dish  so  as  to  notice  any  decomposition 
of  the  sulphate  of  ammonia,  which  at  a  higher  heat  is  decom- 
posed into  water,  nitrogen,  ammonia  and  sulphite  of  ammonia. 
1.  Ammonia  alum  was  heated  to  250° ;  after  three  hours  the 
loss  amounted  to  2  drachms  or  33-3  per  cent. ;  the  product  was 
dense  and  unsatisfactory. 
2.  Ammonia  alum  was  heated  to  350p  for  two  hours ;  the 
whole  loss  was  160  grains  or  444  per  cent.  This  product  was 
likewise  dense,  but  rather  whiter  than  number  one.  If  all  the 
water  had  been  dissipated,  the  loss  ought  to  have  amounted  to 
181-37  grs. 
