2()2 
Rapid  Filtration. 
f  Am.  Jour.  Pharaoh 
\      July,  1876. 
is  selected  and  perforated  for  the  reception  of  tube  i.  The  latter  has- 
about  -j\  inch  internal  diameter  by  9  inches  in  length.  Its  lower  end 
is  cut  off  obliquely  and  provided  with  marks  showing  J  inches,  and 
beginning  at  the  obtuse  angle  of  the  oblique  cut.  Tube  B  is  filled  up 
with  mercury  to  a  height  of  about  2  inches,  then  2  inches  of  water 
are  given  on  the  top  of  this.  The  working  of  this  valve  is  easily 
understood.  By  moving  tube  /,  downwards  or  upwards,  its  lower  end 
is,  to  a  larger  or  smaller  extent,  immersed  in  the  quicksilver,  and  if 
suction  is  applied  to  one  of  the  side  tubes,  the  other  being  connected 
with  a  closed  vessel,  the  rarefaction  of  air  in  B?  and  in  the  vessel  con- 
nected with  it,  will  reach  a  certain  maximum  degree,  beyond  which  it 
cannot  go ;  air  entering  by  way  of  tube  /,  and  penetrating  the  mer- 
cury, it  maintains  that  status  in  the  apparatus  to  which  the  valve  has 
been  adjusted.  If  the  end  of  tube  i  is  cut  off  obliquely,  as  directed, 
the  passage  of  the  air  through  the  quicksilver  goes  on  more  quiet,  not 
so  eruption-like,  and  the  supernatant  water  prevents  entirely  the  spat- 
tering of  the  metal.  Several  physical  causes  unite  to  give  the  gradua- 
tion on  tube  /  only  an  empirical  value.  Under  the  circumstances 
named,  f  inch  immersion  of  tube  /  will  create  in  C  a  water-column  of 
about  15  inches  height. 
4.  The  connection  of  the  single  parts  is  effected  by  rubber  tubing,, 
which  need  not  be  of  the  heaviest  kind,  as  it  is  not  exposed  to  any 
high  external  pressure.  Aspirator  and  mercury  valve  are  best  mounted 
on  a  board  (as  seen  in  the  sketch)  and  hung  in  a  proper  place  on  the 
wall,  within  reach  of  the  hand. 
If  this  apparatus  is  to  be  constructed  out  of  material  on  hand  in 
every  laboratory,  we  may  take  a  chloride  of  calcium  tube  for  the 
aspirator  (Scheibler),  and  a  large  test-tube,  or  any  wide-mouthed  bot- 
tle, for  the  regulator,  in  which  case  the  communication  of  the  single 
parts  is  brought  about  by  perforations  in  the  corks. 
In  case  it  should  be  preferred  to  make  use  of  Bunserfs  platinum 
funnels,  we  can  proceed  a  little  different  from  Bunsen's  method,  as 
such  an  exact  fit  as  needed  in  high  pressure  is  not  wanted  in  our  case. 
Divide  a  round  piece  of  moderately  thin  platinum  foil,  about  \\  inch 
in  diameter,  into  two  equal  halves,  and  shape  them  into  cones,  the  centre 
of  the  circle  forming  the  apex  of  the  cone.  In  funnels  of  less  than 
60  degrees  the  sides  overlap,  in  funnels  of  60  degrees  they  meet 
exactly,  but  can  even  then  be  soldered  together  in  one  point  about  the 
