20 
ON  ALUMEN  EXSICCATUM. 
3.  Potassa  alum  was  fused  at  250°;  it  soon  became  tough 
again,  and  the  heat  was  therefore  raised  to  330°;  after  two 
hours,  the  residue  was  a  coherent,  dense  mass,  weighing,  after 
subtracting  the  weight  of  the  capsule,  3  drs.  40  grs.  It  wag 
broken  into  pieces  and  heated  for  half  an  hour;  its  weight  now 
was  3  drs.  37  grs.;  is  was  rubbed  to  powder,  and  after  another 
half  hour  its  weight  still  amounted  to  3  drs.  34  grs.  The  fire 
was  now  removed,  and  the  capsule  left  over  night  in  the  sand 
bath ;  next  morning  its  weight  was  ascertained  to  be  the  same 
as  the  night  previous ;  the  whole  loss  therefore  was  146  grains, 
or  40-55  per  cent.  The  product  was  grey,  dense  and  not 
satisfactory. 
4.  Potassa  alum  was  at  once  heated  to  350°;  after  two  hours 
it  weighed  3  drs.  22  grs.  It  was  now  powdered  and  heated 
again  for  the  space  of  one  hour,  when  its  weight  was  ascertained 
to  be  3  drs.  19  grs.  The  whole  loss  therefore  amounted  to  161 
grains  or  44-72  per  cent.;  the  product  resembled  the  foregoing 
and  was  not  fully  satisfactory.  If  entirely  anhydrous  the  loss 
ought  to  have  been  163-36. 
The  last  made  burned  alum  was  analyzed  for  its  sulphuric 
acid;  twenty  grains  were  shaken  with  one  ounce  of  distilled 
water ;  after  seven  hours  it  was  not  wholly  dissolved  yet ;  the 
solution  was  perfect  after  twenty -four  hours,  but  it  contained 
impurities  from  the  alum,  amounting,  when  filtered  off,  washed 
and  dried,  to  one  quarter  of  a  grain ;  if  the  impurities  were 
evenly  diffused  through  the  burned  alum,  they  would  amount  to 
grs.  for  the  whole  weight  of  199  grs.,  and  prove  the  alum  to 
be  anhydrous. 
The  alum  solution  when  treated  with  chloride  of  barium,  yield- 
ed a  precipitate  of  sulphate  of  baryta,  which,  after  washing  and 
drying  weighed,  34-5  grs.,  corresponding  with  11-9  grains  of 
S03.  19-75  grs.  Al2  03,  KO,  4S03  would  contain  12-15  grs. 
sulphuric  acid. 
The  above  experiments,  though  yielding  an  anhydrous  alum, 
did  not  give  satisfaction,  on  account  of  the  product  not  being 
light  enough,  in  which  state  it  is  generally  preferred.  Experi- 
ments were  therefore  undertaken  at  a  higher  temperature  and 
particular  attention  paid  to  the  loss.  Four  drachms  of  alum  were 
used,  and  the  heat  of  the  sand  bath  was  previously  raised  to 
