426  Dialysis  by  Means  of  Calcium  Sulphate.{Ain^v;^m- 
"  For  example  :  if  a  parchment  paper  filter  be  filled  with  water 
containing  albumen  and  sodium  chloride,  and  its  external  surface  is 
put  in  contact  with  calcined  sulphate  of  calcium  in  powder,  the 
water  and  the  salt  pass  to  the  absorbent,  and  the  albumen  remains 
in  the  dialyzator. 
"  If  the  sulphate  be  substituted  by  fused  chloride  of  calcium,  by 
quick-lime  and  in  general  by  any  hygroscopic  substance,  the  same 
result  is  obtained  as  with  the  first  of  these  absorbents. 
"Taking  as  a  foundation  the  results  gotten  above,  it  can  be  said 
that  the  chemical  affinity  of  the  absorbent  for  the  liquid  is  the  deter- 
minate cause  of  the  phenomenon." 
Practice  of  the  Chemical  Dialysis. — Mr.  Herrera  indicates  first  the 
different  kinds  of  septum  that  can  be  used,  viz :  parchment  paper, 
animal  septum,  the  epidermis  of  American  Agave,  clay  vessels,  slate, 
etc.,  and  he  presents  the  advantages  and  disadvantages  of  each  one, 
but  at  last  recommends  parchment  paper  as  more  convenient  and 
general  in  its  application.  The  calcium  sulphate  should  be  freshly 
calcined,  should  absorb  at  least  two  parts  by  weight  of  water  and 
should  produce  heat  in  the  act  of  being  hydrated.  The  empty  dia- 
lyzator, folded  like  a  filter  and  attached  to  a  rectangular  wooden 
frame,  is  introduced  into  a  vessel  containing  the  calcined  sulphate 
of  calcium  in  powder,  taking  care  that  the  contact  with  the  outer 
surface  of  the  membrane  may  be  as  complete  as  possible,  the  liquid 
is  poured  in  and  the  whole  set  aside  for  some  time.  In  order  to 
increase  the  rapidity  of  the  operation  the  solid  crust  of  hydrated 
sulphate  of  calcium  formed  around  the  filter  is  removed,  powdered 
and  put  again  in  the  vessel.  If  organic  matter  in  a  state  of  decom- 
position be  dialyzed  the  membrane  must  be  changed  frequently.  In 
order  to  recover  the  crystalloid  substance,  the  calcium  sulphate  is 
powdered,  packed  in  a  percolator  and  extracted  with  alcohol. 
Applications. — The  process  may  be  applied  to  the  separation  of 
organic  acids  from  solution,  or  to  the  extraction  of  alkaloids.  The 
latter  is  founded  on  the  general  facts  :  ist,  that  the  salts  of  organic 
bases  belong  to  the  class  of  crystalloid  substances ;  2d,  they  are 
soluble  in  acidulated  water,  and  3d  the  same  salts  are  soluble  in 
alcohol.  He  gives  the  following  method  that  may  be  adopted. 
After  macerating  the  powdered  drug  for  24  hours  in  water  slightly 
acidulated  with  tartaric  acid  the  mixture  is  filtered,  and  the  filtrate 
subjected  to  dialysis  with  the  precautions  given  above.    When  the 
