ALUM  IN  CHINA. 
69 
method  of  obtaining  its  solution  free  from  ferric  sulphate,  name- 
ly, by  acidulation  with  sulphuric  acid  and  agitation  with  a  little 
pulverized  protosulphid  of  iron,  by  which  the  sesquioxyd  is  im- 
mediately reduced  to  protoxyd. 
To  return  again  to  the  sulphate  of  magnesia,  before  leaving 
the  subject,  I  must  add  that  the  treatment  with  carbonate  of 
baryta,  according  to  a  former  observation  of  my  own,  must  re- 
move also  entirely  the  sulphate  of  lime  which  usually  contami- 
nates commercial  Epsom  salts  ;  for,  as  I  have  there  stated,  car- 
bonate of  baryta  totally  precipitates  gypsum  from  its  solution, 
even  in  the  cold ;  and  I  have  there  proposed  it  as  a  means  of  re- 
moving the  gypsum  from  spring  or  sea  water  which  is  to  be  used 
in  steam  boilers,  (thus  preventing  incrustations,)  as  well  as 
from  the  salt  works.  I  have  since  found  that  carbonate  of  lead 
has  the  same  power  of  precipitating  gypsum,  and  may  possibly 
be  a  cheaper  agent  for  the  purpose  than  carbonate  of  baryta,  con- 
sidering the  facility  with  which  the  lead  may  be  recovered  from 
the  resulting  mixture  of  sulphate  of  lead  and  carbonate  of  lime, 
in  a  metallic  form.— -Am.  Jour.  Science  and  Arts,  Nov.  1858. 
ALUM  IX  CHINA. 
By  Dr.  Macgowan. 
About  eleven  hundred  tons  of  alum  have  been  exported  within 
a  short  period,  chiefly  to  India.  This  mineral  is  largely  em- 
ployed by  the  Chinese  in  dyeing,  and  to  some  extent  in  paper- 
making  as  with  us.  Surgeons  apply  it  variously  after  depriving 
it  of  its  water  of  crystallization,  and  in  domestic  life  it  is  used 
for  precipitating  vegetable  substances  suspended  in  potable  wa- 
ter. It  is  used  also  by  the  Chinese  in  a  manner  peculiar  to  them- 
selves. Fishermen  are  usually  provided  with  it,  and  when  they 
take  one  of  those  huge  Thizostoma  which  abound  on  the  coast, 
they  rub  the  animal  with  the  pulverized  stypic  to  give  a  degree 
of  coherence  to  the  gelatinous  mass.  Architects  employ  it  as  a 
cement  in  those  airy  bridges  which  span  the  watercourses.  It 
is  poured  in  a  molten  state  into  the  interstices  of  the  stones ; 
and  in  structures  not  exposed  to  constant  moisture  the  cohesion 
is  perfect,  but  in  damp  situations  it  becomes  a  hydrate  and  crum- 
bles, a  fact  of  which  the  whole  empire  was  officially  informed  by 
the  government  about  thirty  years  ago.    It  was  discovered  that 
