132 
AN  AUTOMATIC  VACUUM  APPARATUS. 
duce  some  absorbent  into  the  vacuum  chamber.  It  was  observed, 
however,  that  no  material  depression  of  the  mercurial  column 
arose  from  this  cause ;  a  fact  sufficiently  explained  by  the  suppo- 
sition that  the  escape  of  the  water  is  so  rapid  that  none  of  its 
gaseous  constituents  can  be  disengaged. 
The  confined  air  diffuses  itself  throughout  the  entire  apparatus, 
and  as  the  capacity  of  the  vacuum  chamber  is  ten  times  that  of 
the  still,  condensing  pipe  and  receiver  combined,  but  one-eleventh 
of  the  original  atmospheric  pressure  is  present ;  a  repetition  of 
the  process  would  increase  the  tenuity  of  the  air  to  121  times 
that  of  its  normal  condition. 
As  the  temperature  of  the  water  furnished  to  our  cities  proba- 
bly never  exceeds  75°,  and  the  tension  of  its  vapor  at  this  de- 
gree of  heat  is  equal  to  but  *85°  of  an  inch  of  mercury,  this  con- 
trivance is  capable,  under  the  most  unfavorable  circumstances, 
of  producing  a  degree  of  exhaustion  expressed  by  less  than  one 
inch  of  mercury. 
The  vacuum  is  maintained  by  the  condensation  of  the  vapor 
proceeding  from  the  still,  by  an  ample  supply  of  cold  water  to 
the  condenser.  If  the  connections  be  perfect,  distillation  will 
proceed  so  long  as  the  proper  difference  in  the  temperature  of 
the  still  and  condenser  is  preserved. 
The  guage-tube  of  the  receiver  shows  the  quantity  of  the  dis- 
tillate, which  indicates  approximately  when  the  process  should 
cease. 
The  still  could  be  made  more  covenient  by  the  addition  of  a 
syphon  tube  to  the  cock,  p,  for  the  purpose  of  introducing  more 
liquid  without  destroying  the  vacuum  ;  a  "  proof-stick  "  and  a 
thermometer  might  be  inserted  with  equal  advantage.  By  em- 
ploying two  vacuum  chambers,  and  a  suitable  arrangement  of 
floats  and  valves,  the  process  could  be  rendered  continuous  and 
entirely  automatic. 
This  apparatus  would  be  particularly  applicable  to  the  wants 
of  the  manufacturing  pharmaceutist. 
The  fact  that  it  can  only  be  advantageously  employed  in  those 
localities  where  water  works  exist,  detracts  somewhat  from  its 
utility. 
Chicago,  III.,  August,  1865. 
Proc.  of  the  Amer.  Pharm.  Asso.,  1865. 
