350         APPARATUS  FOR  EVAPORATION  IN  VACUO,  ETC. 
zation,  in  cases  where  the  heat  of  120°  to  150°  F.  may  be  ap- 
plied. 
It  is  necessary  that  the  distilling  apparatus  be  made  capable 
of  air-tight  closure,  and  that  the  air  be  removed  from  it  to  begin 
with.  Then  the  degree  of  exhaustion  in  the  apparatus  is  in 
direct  ratio  to  the  rapidity  of  condensation  of  the  vapor  pro- 
duced, xind  the  rapidity  of  condensation  is  only  limited  by  the 
degree  and  extent  of  refrigeration  employed,  with  a  given  extent 
of  evaporating  surface  at  a  stated  temperature.  The  air  in  the 
apparatus,  to  begin  with,  may  be  expelled  through  a  suitable 
aperture  by  steam,  which  may  be  generated  in  the  "  receiver"  of 
the  apparatus  or  in  a^n  attachment  thereto. 
Take  two  round-bottomed  glass  flasks,  the  one  having  a  capa- 
city four  to  eight  times  greater  than  the  other.  Adjust  the 
smaller  upon  a  water-bath,  the  larger  at  10  to  15  inches  distance 
from  the  other,  over  a  sink  or  large  basin,  and  connect  the  two 
with  glass  tubing  and  perforated  caoutchouc  stoppers,  so  that  the 
connecting  tube  shall  incline  slightly  downward  from  its  bend 
close  to  the  stopper  of  the  small  flask.  The  stopper  of  the  small 
flask  is  also  to  have  a  second  perforation,  in  which  is  fitted  a 
straight  glass  tube,  2  or  3  inches  long,  its  lower  end  placed  even 
with  the  lower  end  of  the  stopper.  The  upper  end  of  this  tube 
is  very  slightly  drawn  out  for  a  J  of  an  inch,  and  snugly  fitted 
with  IJ  inch  of  firm  rubber  tubing,  the  upper  J  inch  of  which 
is  closed  with  a  piece  of  glass  rod  of  same  diameter  as  the  body 
of  the  tube. 
Now  put  an  ounce  or  two  of  water  in  the  large  flask,  and  the 
material  to  be  evaporated  in  the  small  flask ;  close  the  stoppers 
perfectly,  by  turning  the  flasks  under  them,  and  leave  open  the 
straight  tube.  Apply,  by  the  water-bath,  the  limited  degree  of 
heat  until  it  is  imparted  to  the  contents  of  the  small  flask ;  then 
move  a  lamp  under  the  large  flask  until  the  water  in  it  has  boiled 
briskly  and  the  steam  therefrom  has  escaped  continuously  from 
the  straight  tube  for  some  minutes.  Now  close  the  straight  tube 
with  its  caoutchouc  cap,  at  the  same  time  removing  the  lamp  from 
the  large  flask.  When  the  latter  has  cooled  somewhat,  wrap  it 
smoothly  with  linen  netting  or  gauze,  and  lead  upon  it  a  minute 
stream  of  cold  water,  controlling  the  same  as  required.  The 
