122  IMPROVEMENTS  IN  TREATING  FATTY  AND  OILY  MATTERS. 
the  fat  or  oil  in  a  fluid  state  in  the  vessel,  a,  with  from  one-third 
to  one-half  its  bulk  of  warm  water ;  the  disk  or  piston,  b,  per- 
forated with  numerous  small  holes,  being  kept  in  rapid  motion  up 
and  down  in  the  vessel,  a,  causes  the  fat  or  oil  and  water  to  form 
an  emulsion,  or  intimate  mechanical  mixture.  A  force  pump,  c, 
like  those  in  common  use  for  hydraulic  presses,  then  drives  the 
mixture  through  a  long  coil  of  very  strong  iron  tube,  d,  d,  d,  d, 
which  being  placed  in  a  furnace,  e,  e,  is  heated  by  a  fire,  /,  to 
about  the  temperature  of  melting  lead.  From  the  exit  end, 
of  the  heating  tubes,  d,  d,  the  mixture,  which  has  then  become 
converted  into  free  fat  acids  and  solution  of  glycerin,  passes  on 
through  another  coiled  iron  tube,  h,  h,  h,  immersed  in  water,  by 
which  it  is  cooled  down  from  its  high  temperature  to  below  212° 
Fah.,  after  which  it  makes  its  escape  through  the  exit  valve,  i, 
into  the  receiving  vessel.  The  iron  tubes  I  have  employed  and 
found  to  be  convenient  for  this  purpose,  are  about  one  inch  ex- 
ternal diameter,  and  about  half  an  inch  internal  diameter,  being 
such  as  are  in  common  use  for  Perkins's  hot-water  apparatus. 
The  ends  of  the  tubes  are  joined  together  by  welding  to  make 
the  requisite  length  ;  but  where  welding  is  not  practicable  I  em- 
ploy the  kind  of  joints  used  for  Perkins's  hot-water  apparatus, 
which  are  now  well  known.  The  heating  tube,  d,  d,  d,  is  coiled 
several  times  backwards  and  forwards,  so  as  to  arrange  a  con- 
siderable length  of  tube  in  a  moderate  space.  The  different  coils 
of  the  tube  are  kept  about  one-quarter  of  an  inch  apart  from 
each  other,  and  the  interval  between  them  is  filled  up  solid  with 
cast  iron,  which  also  covers  the  outer  coils  or  rows  of  tubes  to 
the  thickness  of  half  or  three-quarters  of  an  inch,  as  shown  in 
fig.  2.  This  casing  of  metal  insures  a  considerable  uniformity  of 
temperature  in  the  different  parts  of  the  coil,  adding  also  to  its 
strength  and  protecting  it  from  injury  by  the  fire. 
The  exit  valve,  i,  is  so  loaded,  that  when  the  heating  tubes, 
d,  d,  d,  are  at  the  desired  working  temperature,  and  the  pump, 
c,  is  not  in  action,  it  will  not  be  opened  by  the  internal  pressure 
produced  by  the  application  of  heat  to  the  mixture,  and  therefore 
when  the  pump,  c,  is  not  in  action,  nothing  escapes  from  the 
valve,  i,  if  the  temperature  be  not  too  high.  But  when  the  pump 
forces  fresh  mixture  into  one  cnd;  j,  of  the  heating  tubes,  d,  d,  d, 
the  exit  valve,  i,  is  forced  open  to  allow  an  equal  amount  of  the 
