36 
SOBM  PYROPHOSPHAS. 
sition,  and  consequently  -without  impoverishing  or  changing  the 
gastric  secretions. 
The  process  for  its  preparation  involves  a  process  for  Pyro- 
phosphate of  Soda. 
Sodce  Pt/ropTiosphas. 
Take  of  Crystallized  Phosphate  of  Soda,  sixteen  parts. 
Melt  the  salt  in  a  porcelain  capsule  by  means  of  heat,  and 
boil  it  down  to  dryness.  Collect  the  dry  salt  in  a  sand  crucible 
and  expose  it  to  a  cherry  red  heat  until  it  fuses.  Finally,  when 
cold,  break  the  crucible,  and  having  collected  the  fused  salt, 
powder  it  and  keep  it  in  a  stoppered  bottle.  The  yield  is  five 
and  a  half  parts. 
The  Phosphate  of  Soda  readily  fuses  or  melts  in  its  water  of 
crystallization,  and  is  easily  evaporated  to  dryness  over  a  gas 
flame.  When  nearly  dry  it  should  be  stirred  to  avoid  loss  by 
splashing,  and  render  it  more  easily  detached  from  the  capsule. 
The  dry  phosphate  from  sixteen  parts  weighs  seven  and  a  quar- 
ter parts.  The  heating  in  the  crucible  to  drive  off  the  equiva- 
lent of  basic  wTater  requires  much  care  and  attention,  because,  if 
the  heat  goes  beyond  cherry-redness  a  portion  of  the  salt  is  de- 
composed, probably  by  the  silica  of  the  crucible, — the  crucible  is 
perforated,  and  the  portion  in  the  lower  part  escapes  into  the 
fire  before  the  upper  portions  are  fused.  The  writer  has  repeat- 
edly lost  portions  of  the  salt  in  this  way.  Cast  iron  vessels,  or 
crucibles  are  no  better,  but  are  even  more  easily  burned  through. 
The  crucible  should  be  heated  slowly  and  with  constant  watch- 
fulness, allowing  plenty  of  time,  and  the  salt  should  be  pushed 
down  in  the  crucible  as  the  lower  portions  fuse.  It  does  not 
fuse  short  of  a  full  cherry-red  heat.  In  cooling  the  salt  con- 
tracts and  becomes  crystalline,  so  that  in  breaking  the  crucible 
it  is  not  very  difficult  to  detach  it  with  a  hammer,  without  dis- 
turbing the  thin  film  that  lines  the  crucible.  This  latter  should 
not  be  detached,  because  it  is  partially  decomposed  and  contam- 
inated with  the  material  of  the  crucible.  The  fused  salt  from 
sixteen  parts  weighs  five  and  seven-eighth  parts,  but  by  reject- 
ing that  portion  which  is  unfit  for  use,  only  five  and  a  quarter 
parts  are  obtained.  As  this  salt  is  of  difficult  solubility  it 
should  be  kept  powdered  for  use.  # 
