18 
ON  ALUMEN  EXSICCATUM. 
water,  but  dissolves  after  some  time  when  left  in  contact  with  the 
air.  The  Prussian  and  the  Danish  Pharmacopoeias  allow  a 
partially  decomposed  burned  alum,  for  they  state,  as  a  test,  that 
it  is  almost  wholly  soluble  in  water. 
As  far  as  the  taste  is  regarded,  it  is  true  that  none  is  ob- 
served the  first  moment  that  the  burned  alum  is  applied  to  the 
tongue ;  but  the  presence  of  moisture  there  and  the  heat  of  that 
organ  almost  immediately  render  a  portion  soluble,  and  the 
peculiar  sweetish  astringent  taste  of  alum  is  perceptible.  This 
behaviour  is  observed  with  all  burned  alum,  provided  that  no  sul- 
phuric acid  has  been  expelled,  in  which  latter  case  the  tasteless- 
ness  is  soon  replaced  by  the  mild  saline  taste  peculiar  to  sulphate 
of  potassa.  Even  alum,  which  by  continued  exposure  to  heat 
has  been  deprived  entirely  of  its  water  of  crystallization,  soon 
imparts  to  the  tongue  this  taste.  Tastelessness,  therefore, 
is  no  criterion  for  the  faultless  preparation  of  burned  alum ; 
but,  on  the  contrary,  is  a  sign  that  the  heat  employed  has  been 
high  enough  to  decompose  the  double  salt  and  drive  off  a  por- 
tion of  the  sulphuric  acid. 
A  mark  for  the  same  faulty  preparation  is  a  partial  solubility. 
It  is  presumed  that  the  action  of  burned  alum  consists  essen- 
tially in  its  power  to  absorb  a  considerable  proportion  of  water, 
after  which  its  astringent  powers  come  into  play.  If  the  alum 
has  been  exsiccated  insufficiently,  the  former  quality  cannot  be 
fully  developed,  and  if  it  has  been  decomposed  by  too  high  a  heat, 
the  absorbing  power  and  the  astringency  have  been  wholly  or 
partially  destroyed.  The  observance  of  a  certain  temperature 
during  desiccation  is,  therefore,  essential ;  but  we  possess  no 
experiments  which  would  point  out  to  the  pharmaceutist  the 
necessary  precautions  to  be  observed.  We  have  been  so  far 
satisfied  with  the  statement  of  chemists,  that  potassa  alum  fuses 
when  heated,  and  loses  its  water  of  crystallization,  and  that  at  a 
dull  red  heat  also  sulphuric  acid  is  driven  off  from  the  alumina. 
Of  latter  years  potassa  alum  has  been  scarce,  and  commanded 
a  high  price,  and  ammonia  alum  has  been  substituted  in  com- 
merce for  the  former.  There  can  be  no  doubt  that  ammonia 
alum  has  been  extensively  used  in  the  preparation  of  burned 
alum,  and  as  no  complaints  have  found  their  way  into  the  jour- 
nals, it  is  to  be  assumed  with  success.   It  is  a  deviation  from 
