180 
VARIETIES. 
of  half  an  inch  or  less.  If  a  very  perfect  vacuum  is  desired,  this  operation 
may  be  again  repeated ;  and  if  extreme  accuracy  is  required,  it  may  bo 
performed  a  third  time.  It  is  not  likely  that  anything  would  be  gained 
by  carrying  the  process  further.  On  leaving  the  apparatus  to  itself,  the 
carbonic  acid  which  has  displaced  the  residual  air  is  absorbed  by  the  al- 
kaline solution,  and  the  aqueous  vapor  is  afterwards  removed  by  the  sul- 
phuric acid.  The  vacuum  thus  obtained  is  so  perfect,  that  even  after  two 
operations  it  exercises  no  appreciable  tension. 
"To  give  a  Clear  conception  of  the  progress  of  the  absorption,  I  will  de- 
scribe in  detail  one  observation  in  which  the  tension  was  measured  simul- 
taneously by  a  good  syphon-guage  and  by  a  manometer,  formed  of  a  baro- 
metric tube  0.5  inch  in  diameter,  inverted  in  the  same  reservoir  of  mercury 
as  a  similar  tube  communicating  with  the  interior  of  the  receiver.  The 
barometer  had  been  carefully  filled,  and  the  depression  of  the  mercury 
estimated  by  the  method  already  described  at  less  than  l-100,000th  of  an 
inch. 
"  Previous  to  the  admission  of  the  carbonic  acid,  the  exhaustion  was 
carried  only  to  0.4  inch,  it  was  again  carried  to  1  inch,  and  a  third  time 
to  0.5  inch,  after  which  the  apparatus  was  left  to  itself.  The  manometer 
indicated  a  pressure  in— 
15'  of  0.25  inch, 
30'  «  0.17  » 
80'  «  0.10  « 
200'  «  0.02  " 
In  twelve  hours  the  difference  of  level  was  just  perceptible,  when  a  per- 
fectly level  surface  was  brought  down  behind  the  tubes  till  the  light  was 
just  excluded.  In  thirty-six  hours  not  the  slightest  difference  of  level  could 
be  detected.  The  vacuum  has  remained  without  the  slightest  change  for 
fourteen  days. 
It  is  evident  that  the  only  limit  to  the  completeness  of  the  vacuum  ob- 
tained by  this  process,  arises  from  the  difficulty  of  preparing  carbonic  acid 
gas  perfectly  free  from  air.  This  may  be  very  nearly  overcome  by  adopt- 
ing precautions  which  are  well  known  to  practical  chemists.  When  an 
extreme  exhaustion  is  required,  the  gas  holder  should  be  filled  with  recently 
boiled  water,  and  the  first  portions  of  carbonic  auid  that  are  collected  in  it 
should  be  allowed  to  escape. 
The  substitution  of  phosphoric  for  sulphuric  acid  would  remove  the  pos- 
sibility of  either  aqueous  or  acid  vapors  being  present  even  in  the  smallest 
amount,  but  such  a  refinement  will  rarely  be  found  necessary. 
In  the  experiment  just  described,  the  theoretical  residue  of  air  would  be 
1-135, 000th  part  of  the  entire  quantity  in  the  receiver,  which  would  cause  a 
depression  of  l-4500th  of  an  inch.  This  result  must  have  been  nearly  realized. 
If  the  exhaustion  had  been  carried  at  each  time  to  0.2  inch,  the  residue  by 
theory  would  have  been  only  1-3,375, 000th  part.    But  the  experimental 
