^septTSn'^}      Bimsens  Water-Air  Pamp,  etc.  403 
water.  The  reduction  of  pressure  in  the  apparatus  is  ascertained  by 
a  manometer  tw,  consisting  of  a  bent  glass  tube  containing  mercury, 
and  with  a  scale  in  centimeters  on  one  arm,  a  piece  of  gum  tubing 
connecting  it  with  the  tube  ?^,  joined  into/;  as  the  air  is  removed  from 
the  apparatus,  the  mercury  must  rise  in  the  manometer,  and  by  com- 
parison with  a  good  barometer  hanging  alongside,  indicates  the  rarifica- 
tion  of  the  air ;  suppose  the  barometer  to  show  720  m.  m.  and  the 
manometer  680  m.  m.,  the  rarification  of  the  air  will  be  720 — 680= 
40  m.  m.  mercury  column.  If  vessels  containing  ammoniacal  or  acid 
vapors  are  to  be  exhausted,  a  wash  bottle  containing  sulphuric  acid 
or  an  alkali  may  be  inserted  before  g 
The  apparatus  is  best  made  of  strong  glass  ;  it  is  only  about  1 
meter  high  and  20  cm.  broad,  and  may  be  fastened  upon  a  board 
against  a  wall.  To  avoid  breakage,  in  consequence  of  the  unequal 
expansion  through  change  of  temperature,  the  parts  5,/,  g  and  m  are 
connected  with  each  other  by  gum  tubing,  which  must  be  very  thick, 
including  the  hose  h  ;  with  an  outside  diameter  of  14  m.  m.,  the  bore 
is  only  3  m.  m.,  to  avoid  its  being  compressed  through  the  outside 
pressure  of  the  air.  The  lead  pipe  d  has  an  inside  diameter  of  8  m. 
m.  The  pressure  of  the  air  being  equal  to  the  weight  of  a  column  of 
water  of  about  lOJ  meters,  it  is  evident  that  a  vacuum  cannot  be  pro- 
duced unless  the  water  has  a  fall  of  not  less  than  that  distance,  and 
it  is  advisable,  whenever  possible,  to  exceed  that  height,  if  necessary 
by  running  the  lead  pipe  down  to  the  bottom  of  a  well. 
The  advantages  of  the  water  air-pump  over  others  working  with  a 
piston  are  evident;  it  is  cheap,  automatic  in  its  action,  requires  few 
or  no  repairs,  and  is  not  affected  by  corrosive  vapors.  For  these 
reasons  it  is  particularly  adapted  to  filtration  in  chemical  and  photo- 
graphic laboratories.  To  use  it  for  such  a  purpose,  a  bottle  is  required 
with  a  twice  perforated  rubber  stopper.  One  of  the  perforations  re- 
ceives the  funnel  with  the  filter,  the  other  one  is  connected  with  the 
air-pump  by  means  of  a  bent  glass  tube  and  the  gum  hose  h.  Filter- 
ing paper  being  too  weak  to  withstand  the  pressure,  the  filter  is 
strengthened  by  resting  it  on  a  cone  of  platinum  bent  out  of  one  piece 
of  foil.* 
The  liquor  runs  from  the  filter  almost  in  a  continuous  stream.  The 
precipitates  are  readily  washed  and  finally  obtained  nearly  dry.  Bun- 
*See  also  M.  Tichborne's  paper  oa  page  313  of  this  Journal. 
