486
Book  V.
Changes
produced
on  the  air
respired.
1.  Part  of  it
is  decomposed. ¬


ANIMAL  FUNCTIONS.
retain  41  cubic  inches  of  air;  after  a  natural  expiration ¬
  they  contain  .  .  .  ..  118  cubic  inches
After  a  natural  inspiration  135
After  a  forced  inspiration  254
By  a  full  forced  expiration,  after  a  forced  inspiration,
he  threw  out  .......  190  cubic  inches
After  a  natural  inspiration  78.5
After  a  natural  expiration  67.5
Let  us  now  endeavour  to  trace  the  changes  produced
by  respiration.  These  are  of  two  kinds,  namely,  1.  The
changes  produced  upon  the  air  respired.  2.  Changes
produced  upon  the  blood  exposed  to  this  air.  Each  of
these  naturally  claims  our  attention.
I.  For  our  knowledge  of  the  changes  produced  upon
the  air  by  respiration,  we  are  chiefly  indebted  to
Priestley,  Cigna,  Menzies,  Lavoisier  and  Seguin,  and
Mr  Davy.  These  changes  are  the  following.  1.  Part
of  the  air  respired  disappears.  2.  It  becomes  impregnated ¬
  with  carbonic  acid.  3.  It  is  loaded  with  water  in
the  state  of  vapour.
1.  From  the  experiments  of  Dr  Menzies,  it  follows
that  one  twentieth  of  the  air  inspired  disappears  in  the
lungs.  This  agrees  pretty  nearly  with  the  experiments
made  with  great  care  by  Lavoisier;  an  account  of  which
he  was  employed  in  drawing  up  when  he  was  unfortunately -
  cut  off  by  orders  of  the  French  government.
Neither  do  the  experiments  published  lately  by  Mr
Davy,  and  which  appear  to  have  been  performed  with
much  precision,  differ  much  from  those  of  Dr  Menzies.
According  to  Davy  about  th  of  the  air  inspired  disappears ¬
  during  respiration.
Concerning  the  portion  of  the  air  which  disappears,
it  has  hitherto  been  the  general  opinion  that  it  is  the