APPARATUS  FOR  EVAPORATION  IN  VACUO,  ETC.  351 
liquid  in  the  small  flask  boils  briskly  (if  aqueous,  boiling  at  120° 
or  150°  F.),  and  the  refrigeration  is  governed  to  prevent  too 
violent  ebullition,  lest  liquid  be  thrown  into  the  connecting  tube  ; 
the  degree  of  applied  heat  is  governed  to  the  same  end. 
An  ordinary  glass  retort  may  be  substituted  for  the  small  flask 
as  an  evaporating  vessel,  and  its  tubule  may  be  fitted  with  a  per- 
forated stopper,  admitting  a  thermometer.  If  there  is  not  room 
in  the  stopper  (of  retort  or  flask)  for  both  the  thermometer  and 
the  steam-escape  tube,  the  latter  may  be  dispensed  with  by  ad- 
justing the  stopper  loose  for  escape  of  steam,  and  pressing  it  tight 
when  the  air  is  expelled.  Flat-bottomed  flasks  favor  equable 
boiling,  but  they  are  liable  to  collapse. 
As  a  condenser^  I  have  used,  instead  of  the  large  flask,  a  cop- 
per vessel,  for  more  ready  application  of  heat  without  danger  of 
breaking,  and  for  more  efficient  refrigeration.  This  copper  re- 
ceiver is  made  of  conical  shape,  with  rounded  bottom,  a  vertical 
diameter  twice  its  horizontal  diameter,  and  a  neck  bent  to  the 
angle  of  about  56°  with  the  vertical  axis  of  the  vessel.  The 
diameter  of  the  neck  is  f  of  an  inch,  to  receive  a  retort  beak, 
the  joint  being  covered  with  a  section  of  caoutchouc  tubing.  Or 
it  may  be  fitted  with  a  perforated  stopper,  to  receive  the  con- 
necting tube  of  the  flask  when  evaporation  is  conducted  in  the 
latter. 
With  linen  netting  to  spread  the  water  over  the  free  surface  of 
the  condensers,  the  evaporation  therefrom  refrigerates  with  a 
comparatively  small  supply  of  water.  Using  a  copper  condenser 
of  the  above  described  shape,  a  vertical  diameter  of  12  inches, 
and  capacity  of  six  pints,  attached  to  an  8-ounce  glass  retort 
containing  distillation  promoters,  I  have  vaporized  4  fluidounces 
of  water  in  sixteen  minutes  at  the  constant  temperature  of  128° 
F.  By  ordinary  care  in  the  expulsion  of  air  and  closure  of  the 
apparatus,  exhaustion  can  be  invariably  secured,  fixing  the 
water-boiling  point  at  below  130°  F.;  that  is,  atmospheric  press- 
ure equal  to  at  least  25  inches  of  mercury  may  be  removed  and 
sustained  by  availing  ourselves  of  the  displacing  effect  of  steam, 
and  the  contraction  of  condensing  vapor,  in  very  simple  appa- 
ratus. 
Notwithstanding  the  illustrations  of  vacuum  by  condensation, 
