ON  ALUMEN  EXSICCATTJM. 
17 
water,  slowly  combines  with  the  expelled  water  of  crystalliza- 
tion and  subsequently  dissolves  ;  this  recombination  with  and 
dissolving  in  water  is  effected  in  a  short  time  if  hot  water  is 
employed. 
All  the  Pharmacopoeias  of  this  country,  as  well  as  of  Europe, 
direct  the  employment  of  potassa  alum  for  the  preparation  of 
alumen  exsiccatum.  The  directions  are  in  most  cases  very 
nearly  alike.  The  alum  is  introduced  into  an  earthen  or  iron 
vessel,  or  into  a  dish  or  a  crucible,  and  heated  over  the  fire  till  it 
becomes  dry,  or  till  a  porous  spongy  mass  is  obtained  ;  or  a  con- 
stant agitation  is  ordered  until  vapors  cease  to  be  given  off.  The 
temperature  is  not  indicated  in  any  of  the  Pharmacopoeias ; 
the  French  Codex,  however,  remarks  that  the  temperature  must 
not  be  increased  to  a  red  heat. 
The  whole  product,  as  obtained  by  this  process,  is  allowed  to 
be  used  by  all  the  various  Pharmacopoeias,  with  the  single  ex- 
ception of  that  of  Greece.  All  who  have  prepared  burned  alum 
will  have  noticed  a  denser  mass  on  the  bottom  of  the  vessel  un- 
derneath that  light  spongy  salt,  which  by  the  aqueous  vapors  has 
been  puffed  up  from  the  upper  portions  of  the  fused  alum.  Now, 
this  dense  mass  is  rejected  by  the  Greek  Pharmacopoeia,  which 
allows  only  the  spongy  portion  to  be  used,  probably  from  the 
supposition  that  the  former  was  partly  decomposed.  There  can 
be  no  doubt  that  some  care  is  necessary  to  prevent  such  a  de- 
composition on  that  point  of  the  vessel  which  is  nearest  to  the 
fire ;  but  if  the  heat  is  not  carried  too  high,  no  sulphuric  acid 
will  be  expelled,  and  repeated  experiments  made  at  different 
times,  have  satisfied  me  that  this  portion  is  wholly  soluble  in 
water,  and  is  therefore  alum,  and  not  a  product  of  partial  de- 
composition by  the  influence  of  heat. 
If  burned  alum  is  well  prepared,  that  is,  if  only  the  water 
has  been  expelled,  it  is  very  slowly  soluble  in  cold  water,  but 
dissolves  rapidly  in  hot  water.  This  is  an  easy  test  for  this 
preparation,  and  might  be  regarded  sufficient  for  proving  its 
having  been  well  made.  A  number  of  Pharmacopoeias  give  this 
behaviour  as  the  requisite  test,  while  no  test  at  all  is  given  by 
the  United  States,  the  various  British  and  the  Swedish  Phar- 
macopoeias. The  Pharmacopoeia  Badensis,  of  1841,  has  the 
following  tests :  It  is  white,  tasteless,  at  first  not  soluble  in 
2 
