420 
satisfactory.  The  mill  consists  of  two  heavy  stones  of  a flint-like 
character  which  are  set  horizontally  so  that  a driving  shaft 
through  the  center  revolves  the  upper  stone  over  the  face  of  the 
lower  one.  The  faces  of  the  stones  are  dressed  in  grooves  run- 
ning in  opposite  directions  so  that  when  the  upper  stone  revolves 
they  come  together  in  the  manner  of  a pair  of  shears.  The  rock 
to  be  crushed  is  fed  between  the  stones  where  it  is  subjected  to  a 
crushing,  tearing  and  grinding  action  all  at  the  same  time. 
COMPUTATION  OF  ACID. 
In  order  to  compute  the  sulphuric  acid  necessary  for  the  forma- 
tion of  acid  phosphate  the  chemical  composition  of  the  rock  must 
be  known.  Very  often  the  complete  analysis  is  not  made,  but 
only  the  more  important  tests  for  phosphoric  anhydrid,  iron 
alumina,  carbonates,  and  sometimes  magnesium  and  flourine. 
From  time  to  time  tables  have  been  made  which  are  supposed 
to  show  at  a glance  how  much  acid  of  a certain  strength  should 
be  used,  but  as  there  are  very  often  other  problems  to  consider 
than  the  removal  of  just  so  many  parts  of  the  bases  involved, 
their  use  is  not  at  all  times  helpful  and  is  certainly  not  economical. 
CHAMBER  ACID. 
What  is  known  as  chamber  acid  is  used  for  acidulating.  It  is 
a dilute  sulphuric  acid  of  from  50°  to  530  Be,  corresponding  to 
a specific  gravity  of  from  1.526  to  1.576  and  containing  about 
65%  of  actual  sulphuric  acid,  the  balance  being  water.  The  rea- 
son for  using  this  dilute  acid  is  that,  the  resulting  products, 
monocalcium  phosphate  and  gypsum,  both  being  hydrated  pro- 
ducts, require  a certain  amount  of  water  for  their  formation.  It 
also  happens  that  chamber  acid  is  the  cheapest  that  can  be  used 
for  the  purpose. 
CHEMICAL  EQUATIONS. 
To  arrive  at 'the  proper  amount  of  acid  necessary  for  acidifying, 
chemical  equations  are  resorted  to  showing  in  what  proportion 
the  acid  and  bases  combine.  Taking  tri-calcium  phosphate,  the 
most  important  phosphate  rock  ingredient  we  have,  for  instance : 
Ca8P208+2H2S04+5H2C>==2CCaS04.2H20)+CaH4P208.H20 
$10  + 196  = 2X172  + 252 
From  the  combining  weights  we  see  that  310  pounds  of  calcium 
phosphate  require  196  pounds  of  monohydrated  sulphuric  acid,  or 
every  pound  of  tricalcium  phosphate  in  the  rock  requires 
i96-:-3io  that  is  .632  pounds  of  concentrated  sulphuric  acid 
