MANUFACTURE  OF  ALUM.
149
some  alkali,  fixed  or  volatile,  to  prepare  it  for  affording  good  crystals
of  alum  by  evaporation.  The  uſe  of  the  alkali  is  partly  to  precipitate
some  calx  of  iron,  which  is  commonly  diffolved  in  this  liquor  along
with  the  alum,  and  partly,  as  is  supposed,  to  saturate  a  portion  of
superfluous  sulphuric  acid.  The  crystals  are  firſt  obtained  of  a  moderate ¬
  fize,  but  are  afterwards  united  into  large  masses  by  roaching.
Mr.  Chaptal’s  process  is  very  refined  and  artificial.  He  observed
that  a  gentle  roaſting  of  clay  dispoſes  it  to  a  more  ready  union  with  the
acid,  especially  when  this  is  in  the  form  of  steam.  He  therefore  makes
up  the  clay  into  ſmall  balls,  and  roaſts  them  in  an  oven  or  kiln.  One
effect  of  this  must  be  the  driving  out  the  water,  and  leaving  the  whole
maſs  very  porous  and  accessible  to  the  steam  of  the  sulphurous  acid.
The  balls  are  now  spread  out  on  the  floor  and  ſhelves  of  a  great  room,
whoſe  walls  and  cieling  are  covered  with  a  fat  luting  impenetrable  by
the  fumes.  There  is  a  furnace  constructed  in  a  corner  of  the  room,  in
which  ſulphur  is  made  to  burn  in  a  manner  which  disperses  it  through
the  room  in  steams  of  the  suffocating  acid  :  The  balls  are  penetrated,
and  the  clay  unites  with  it.  When  judged  to  be  superficially  saturated -
  with  it,  and  alum  formed  in  them,  which  requires  some  days,  the
room  is  opened,  and  the  balls  taken  out  and  exposed  to  the  air  undei
cover.  The  volatile  acid  becomes  sulphuric  by  extracting  oxygen
from  the  air,  and  diffolves  the  clay,  and  the  balls  are  then  lixiviated.
This  manufacture  is  faid  to  be  very  profitable.  (Ann.  de  Chem.
vol.  3.)
The  moſt  inſtructive  dissertation  that  I  have  ſeen  on  this  manufacture ¬
  is,  Disquisitio  Chemica  de  Confectione  Aluminis,  Auctore  Gustavo
Suedilio,  Upsal,  1767.
The  appearance  and  more  obvious  qualities  of  alum  are  more  or
leſs  known  to  you.  It  is  exposed  to  ſale  in  large  cryſtalline  maſſes
containing  much  water.  By  the  application  of  heat  it  melts,  swells,
boils,  and  dries,  and  is  then  called  burnt  alum,—alumen  ustum,  which
has  often  a  good  effect  on  foul  ulcers.  In  a  greater  heat,  it  does  not