AM  AUTOMATIC  VACUUM  APPARATUS. 
131 
i 
expand  its  closed  extremity  to  a  larger  chamber,  and  the  dis- 
covery of  Torricelli  is  again  available  for  a  practical  use. 
By  consulting  the  annexed  drawing  and.  references,  the  appa- 
ratus will  be  readily  understood;  its  operation  is  as  follows: 
The  stopcock,  j,  being  closed,  cocks,  c  and  are  opened,  (as 
shown  in  the  drawing)  and  water  enters  from  the  hydrant  pipe,  b. 
When  the  vacuum  chamber  has  become  full,  water  flows  through 
the  pipe,  <i,into  the  reservoir,  where  it  raises  the  float,  /*,  and  shuts 
the  cock,  c.  The  . liquid  to  be  evaporated  having  been  introduced 
into  the  still,  and  the  connections  being  properly  made,  cocks,  j 
and  m,  are  opened.  The  water  immediately  descends  through 
the  pipe,  i,  and  at  the  same  time  it  enters  the  vacuum  chamber 
from  the  reservoir;  the  float,  meanwhile,  falls,  leaving  the  cock 
c,  closed,  (as  shown  by  the  dotted  lines)  and  carrying  with  it  the 
rod,  g.  The  spring,  /,  in  its  descent,  fastens  upon  the  lever  of 
the  cock,  e,  closing  it,  (shown  by  dotted  lines.) 
Heat  is  now  applied  to  the  water-bath,  and,  after  the  lapse  of 
a  few  minutes,  the  stopcock,  m,  is  shut.  A  plentiful  supply  of 
cold  water  is  made  to  circulate  through  the  condenser,  until  the 
completion  of  the  process. 
The  flow  of  water,  when  admitted  to  the  vacuum  chamber,  is 
regulated  by  the  " ball-cock  "  contrivance,  which  allows  the  vessel 
to  be.  kept  constantly  full  and  ready  for  use,  without  attention. 
The  reservoir,  while  it  serves  as  a  cistern  for  the  float,  provides 
for  the  filling  of  the  pipe,  i,  at  the  commencement  of  the  opera- 
tion, without  the  admission  of  air  to  the  vacuum  chamber.  When 
the  cock,  is  opened,  water  ascends  the  pipe,  d,  on  the  principle 
of  the  syphon,  and,  as  the  capacity  of  the  reservoir  is  about 
double  that  of  the  pipe,  i,  the  latter  will  have  been  completely 
filled  before  the  cock,  e,  is  closed  by  the  fall  of  the  float. 
The  pipe,  i,  being  full,  the  water  will  escape  until  it  reaches  a 
height  corresponding  to  the  atmospheric  pressure ;  this  point  is, 
however,  some  distance  below  the  vacuum  chamber,  as  the  column 
of  water,  thus  supported,  would  rarely  exceed  thirty-four  feet. 
An  absolute  vacuum  is  not  obtained  by  this  means,  because  of 
the  ready  diffusibility  of  aqueous  vapor,  and  the  escape  of  air 
and  absorbed  gases,  from  the  water. 
It  was  feared  that  the  large  amount  of  carbonic  acid  gas  fre- 
quently present  in  this  fluid  would  render  it  necessary  to  intro- 
