ON  ALUMEN  EXSICCATUM. 
21 
450°  before  the  capsule  was  put  on.  Both  alums  now  were 
rapidly  fused,  puffed  up,  and  soon  became  a  spongy  mass  ;  by 
repeated  weighing  the  loss  was  ascertained.  When  the  ammo- 
nia alum  had  lost  107  grs.  or  44*58  per  cent.,  and  the  potassa 
alum  106  grs.  or  4t-166  per  cent.,  the  dishes  were  removed  from 
the  fire.  The  lower  portion  of  both  cakes  was  a  rather  dense 
mass,  but  the  greater  part  was  of  a  porous  and  light  character : 
the  whole  was  white  and  unobjectionable  in  appearance. 
Twenty  grains  of  the  densest  part  of  the  potassa  alum  was 
dissolved  in  water,  and  precipitated  by  chloride  of  barium ;  the 
sulphate  of  baryta,  after  washing  and  drying,  weighed  35-5  grs., 
corresponding  with  12-24  grs.  S03.  This  part  of  the  alum  is 
therefore  anhydrous ;  for  20  grs.  anhydrous  potassa  alum  con- 
tain 12-31  grs.  S03,  yielding  35-7  grs.  BaO,  S03. 
The  burned  ammonia  alum  was  rubbed  to  a  fine  powder;  20 
grs.  of  the  same  were  treated  as  before,  to  ascertain  the  sulphuric 
acid  present  in  it ;  it  yielded  37  grs.  BaO,  S03  which  con- 
tain 12-758  grs.  S03.  Twenty  grains  of  burned  ammonia  alum 
which  has  left  a  residue  of  55-418  per  cent,  contains  by  calcula- 
tion 12-713  grs.  S03. 
In  neither  of  the  above  cases,  therefore,  has  there  been  any 
sulphuric  acid  expelled,  and  it  now  need  scarcely  be  mentioned 
that  the  test  paper,  suspended  over  the  ammonia  alum  during 
the  exsiccation,  had  in  no  instance  been  changed. 
Crystallized  potassa  alum  contains  45-53  per  cent,  of  water  : 
ammonia  alum,  besides  that  equivalent  of  water  which  is  requisite 
for  the  existence  of  oxide  of  ammonium,  47-67  per  cent,  water  of 
crystallization.  If  we  therefore  expose  either  of  the  two  alums  to 
a  heat  of  450°  until  they  have  lost  from  44  to  45  per  cent,  of 
water,  we  obtain  a  beautiful  light  and  spongy  product,  which 
contains  between  1  and  5  per  cent,  of  its  weight  of  water;  and 
though  not  entirely  anhydrous,  answers  all  the  purposes  for 
which  burned  alum  is  employed ;  while  on  the  other  hand  there 
is  no  danger  of  decomposition  from  too  high  or  too  long  con- 
tinued a  heat. 
It  may  be  considered  unobjectionable  if  its  solution  in  cold 
water  takes  place  slowly,  but  if  6  parts  of  boiling  water  dissolve 
the  whole  of  it  in  a  short  space  of  time. 
Philad'a,  Deo.  1859. 
