VARIETIES. 
179 
On  a  method  of  obtaining  a  perfect  Vacuum  in  the  Receiver  of  an  Air 
Pump.  By  Thomas  Andrews,  M.  D.,  F,  II.  S.  &c. — Mr.  Andrews  published 
a  paper  on  the  above  subject  in  the  Philosophical  Magazine,  which  has  been 
copied  into  the  London  Pharmaceutical  Journal  for  January,  1853.  The  first 
part  of  this  paper  relates  to  the  Torricellian  A^acuum,  which  is  deemed  the 
most  perfect  that  is  capable  of  being  made  by  art.  It  then  details  M. 
Kegnault's  method,  which  consists  in  providing  a  large  glass  globe,  of  4.] 
to  5|  gallons  capacity,  placing  in  it  a  thin  glass  vessel  containing  40  or  50 
grammes  of  sulphuric  acid  and  hermetically  sealed,  and  then  two  or  three 
grammes  of  water,  lie  then  exhausts  the  globe  by  the  pump,  until  the 
water  disappears,  and  the  pump  ceases  to  act,  and  closes  the  cock.  The 
vessel  of  sulphuric  acid  is  then  ruptured  by  agitation,  when  it  soon  absorbs 
the  aqueous  vapor,  which  has  previously  driven  out  the  greater  part  of  the 
residual  air  by  the  action  of  the  machine.  The  receiver  of  the  air  pump, 
in  which  the  desired  vacuum  is  to  be  produced,  is  then  carefully  connected 
with  the  exhausted  globe  ;  as  perfect  a  vacuum  as  can  be  produced  by  the 
machine  is  then  made,  and  the  communication  between  the  globe  and  re- 
ceiver opened,  when  the  small  fraction  of  air  in  the  receiver  distributes 
itself  equally  in  the  two  vessels,  and  is  thereby  reduced  to  a  minute  frac- 
tion in  the  receiver.  A  repetition  of  the  action  of  the  exhausted  globe 
is  necessary  in  the  most  accurate  experiments.  By  this  means  M. 
Kegnault  reduced  the  mercurial  column  in  the  manometer  to  a  small  frac- 
tion of  a  millimetre.  The  following  method  Mr.  Andrews  proposes  as 
affording  a  much  more  perfect  vacuum,  and  with  far  less  trouble  to  the 
operator : 
"  Into  the  receiver  of  an  ordinary  air  pump,  which  it  is  not  required  to  ex- 
haust further  than  to  0.3  or  even  to  0.5  inch,  but  which  must  retain  the 
exhaustion  perfectly  for  any  length  of  time,  two  open  vessels  are  introduced, 
one  of  which  may  be  conveniently  placed  above  the  other;  the  lower  vessel 
containing  concentrated  sulphuric  acid,  the  upper  a  thin  layer  of  a  solution 
of  caustic  potash,  which  has  been  recently  concentrated  by  ebullition.  The 
precise  quantities  of  these  liquids  is  not  a  matter  of  importance,  provided 
they  are  so  adjusted  that  the  acid  is  capable  of  desiccating  completely  the 
potash  solution  without  becoming  itself  notably  diminished  in  strength, 
but  at  the  same  time  does  not  expose  so  large  a  surface,  as  to  convert  the 
potash  into  a  dry  mass  in  less  than  five  or  six  hours  at  the  least.  The 
pump  is  in  the  first  place  worked,  till  the  air  in  the  receiver  has  an  elastic 
force  of  0.3  or  0.4  inch,  and  the  stop-cock  below  the  plate  is  then  closed. 
A  communication  is  now  established  between  the  tube  for  admitting  air 
below  the  valves  and  a  gas  holder  containing  carbonic  acid,  which  has 
been  carefully  prepared  so  as  to  exclude  the  presence  of  atmospheric  air. 
After  all  the  air  has  been  completely  removed  from  the  connecting  tubes 
by  alternately  exhausting  and  admitting  the  carbonic  acid,  the  stop-cock 
below  the  plate  is  opened,  and  the  carbonic  acid  allowed  to  pass  into  the 
receiver.    The  exhaustion  is  again  quickly  performed  to  about  the  extent 
