356        Convenient  Apparatus  for  Hot  Filtration.  {^'^■IZ'.'^z^T-'^' 
The  materials  are  found  in  any  ordinary  laboratory.  Select  a  small 
funnel  with  a  long  stem,  and  a  larger  funnel  with  a  wider  throat,  and 
cut  the  stem  of  the  larger  funnel  short  ;  slip  a  piece  of  India-rubber 
tubing  of  the  required  size  over  the  stem  of  the  smaller  funnel,  and 
then  insert  it  in  the  larger  one  so  that  it  fits  water-tight.  The  inner 
funnel  should  project  about  half  a  centimetre  above  the  edge  of  the 
outer,  and  as  much  below  the  stem  of  the  latter  as  it  admits.  We 
have  found  the  three  sizes  named  below  sufficient  for  all  operations  of 
analytical  chemistry,  though  larger  ones  may  be  used  for  preparations. 
Dimensions  given  in  centimeters  ;  the  first  figures  give  the  greatest 
diameter  of  the  funnel,  and  the  second  its  length  including  stem. 
Outer  Funnel.  Inner  Funnel. 
No.  I        .  .  .  .  .7x6^  4  Xio 
No.  2  ,  .  .  .         lojx  9^  6Jxi2i 
No.  3       .  .  .  .  .    13^X13  10  X17 
Steam  generated  in  a  flask  of  about  one  litre  capacity  and  conducted 
by  means  of  a  glass  tube  into  water  filling  the  space  between  the  two 
funnels,  warms  the  filter  on  the  inner  funnel  with  its  contents.  In  one 
experiment  the  water  in  the  outer  funnel  marked  a  temperature  of  97° 
C.  and  the  liquid  in  the  inner  one  76^  C.  The  temperature  in  the 
inner  funnel  may  be  greatly  increased  by  covering  it  with  a  convex 
glass,  or  by  employing  a  saline  solution  in  the  outer  funnel. 
As  a  matter  of  course,  water  condenses  in  the  outer  funnel,  and  must 
be  removed  from  time  to  time.  In  the  case  of  funnels  No.  2  it  accu- 
mulates at  the  rate  of  30  to  35  c.c.  in  half  an  hour  when  boiling  vig- 
orously. This  seems  at  first  sight  to  be  an  objection,  but  the  superflu- 
ous water  can  be  so  readily  removed  with  a  pipette  or  a  siphon  that  it 
does  not  have  much  force.  Or  the  accunmlating  water  may  be  drawn 
back  into  the  steam  generator  through  diminished  pressure  by  simply 
removing  the  lamp  beneath  the  flask.  In  this  case,  the  end  of  the 
tube  should  plunge  but  little  below  the  surface  of  the  water  in  the  outer 
funnel,  else  the  latter  will  be  completely  emptied. 
Actually  the  operator  is  not  at  all  annoyed  by  the  necessity  of  attend- 
ing to  this  point,  for  the  filtration  requires  his  constant  presence. 
Should  the  outer  funnel  be  filled  with  distilled  water  in  the  outset,  an 
overflow  would  not  prove  serious  ;  since  the  inner  funnel  stands  higher 
than  the  outer,  any  disturbance  of  the  precipitate  in  the  former  by 
accumulating  water  is  out  of  the  question. 
The  great  compactness  and  cleanliness  of  this  apparatus  make  it 
