50        PHOSPHORUS  GLUE  AND  CHLORIDE  OF  AMMONIUM. 
materials  used  for  the  production  of  ferrocyanide.  When  the 
precipitate  has  subsided,  the  solution  of  chloride  of  ammonium 
is  drawn  off,  and  a  fresh  quantity  of  the  hydrochloric  solution 
precipitated  in  the  same  vat.  After  repeating  this  operation 
several  times,  the  precipitate  is  washed  with  water'  to  remove  the 
whole  of  the  chloride  of  ammonium,  and  then  mixed  with  some 
of  the  hydrochloric  solution  of  phosphate  of  lime,  so  as  to  sepa- 
rate the  carbonate  of  lime  mixed  with  the  phosphate.  It  is, 
however,  preferable  to  precipitate  the  phosphate  of  lime  by  means 
of  lime  milk,  because  the  hydrochloric  solution  of  phosphate  of 
lime  is  very  dilute,  and  in  the  evaporation  of  the  dilute  -solution 
of  chloride  of  ammonium  in  iron  pans  the  salt  is  rendered  ferru- 
ginous, and  the  pans  are  eaten  away.  But  when  the  solution  of 
chloride  of  calcium  obtained  in  precipitating  the  phosphate  solu- 
tion by  lime  milk  is  used  for  converting  the  carbonate  of  am- 
monia into  chloride  of  ammonium,  the  solution  may  be  evapo- 
rated in  iron  pans  without  detriment.  The  precipitation  of  the 
carbonate  of  lime  should  be  effected  with  hot  liquids,  so  as  to 
insure  the  complete  precipitation  of  the  lime.  Effervescence 
takes  place  on  mixing  the  liquids,  because  the  carbonate  of  am- 
monia solution  contains  more  carbonic  acid  than  is  equivalent  to 
the  calcium  of  the  chloride  of  calcium.  The  phosphate  of  lime 
obtained  by  precipitating  the  hydrochloric  solution  with  milk  of 
lime  contains  animal  substance.  It  is  dried  in  porous  vessels, 
and  then  gently  ignited  in  a  reverberatory  furnace  until  it  be- 
comes white.  In  this  state  it  is  better  adapted  for  the  prepara- 
tion of  phosphorus  than  burnt  bones,  on  account  of  its  being 
more  easily  decomposed,  and  as  it  does  not  contain  carbonate  of 
lime  there  is  a  proportionate  saving  of  sulphuric  acid. 
The  decomposition  of  the  phosphate  of  lime  by  sulphuric  acid 
is  effected  in  the  usual  way.  The  separation  of  the  phosphoric 
acid  solution  from  the  gypsum  is  effected  in  leaden  vats  with 
perforated  bottoms,  covered  with  layers  of  quartz  fragments  and 
sand.  After  the  acid  solution  has  filtered  off,  the  residue  of 
gypsum  is  washed  by  displacement  with  water.  The  phosphoric 
solution  thus  obtained  is  evaporated  in  leaden  pans,  heated  by 
the  flues  leading  from  the  distillation  furnaces.  The  concen- 
trated residue  of  the  phosphoric  solution,  when  mixed  with  char- 
coal, is  dried  in  iron  pans,  covered  with  a  stone  vault,  with  open- 
