Mixed  Gases :  Aqueous  Tension.  —  Two  gases  at  the  same
temperature,  provided  they  do  not  interact  chemically,  do  not  inter-
fere with  each  other's  pressures  when  mixed.  Thus,  if  they  are  forced
into  the  same  volume,  the  pressure  of  the  mixture  is  equal  to  the
sum  of  those  of  the  components  (Dalton's  law,  1807).  The  gases
are  therefore  still  thought  of  individually,  and  the  share  which  each
gas  has  in  the  total  pressure  is  called  its  partial  pressure.  This,  like
any  other  gaseous  pressure,  is  proportional  to  the  concentration  of
the  particular  gas  in  the  mixture.
For  example,  a  gas  measured  over  water  contains  water  vapor.
The  partial  pressure  of  this,  called  the  aqueous  tension  (q.v.),  which
is  definite  for  each  temperature,  must  be  subtracted  from  the  total
pressure.  The  remainder  is  the  partial  pressure  of  the  gas  being
measured,  and  this  remainder  is  used  as  the  pressure  of  this  gas  in
any  calculation.  Thus,  in  a  gas  measured  over  water  at  22°,  the
total  pressure  includes  19.7  mm.  (the  aqueous  tension  at  22°)  pressure
of  water  vapor.  Hence  150  c.c.  of  gas  over  water  at  22°  and  750
mm.  is  the  same  in  amount  as  150  c.c.  of  the  same  gas  in  dry  condi-
tion at  22°  and  730.3  mm.  (there  being  simply  150  c.c.  of  water
vapor  at  19.7  mm.  mixed  with  it).  To  obtain  the  volume  of  dry  gas
at  0°  and  760  mm.  we  have  the  expression  150  x  f  H  x  *f&3.
MEASUREMENT   OF   QUANTITY  IN   GASES  61

Densities  of  Gases.*  —  The  density  of  a  gas  is  the  mass  of  1  c.c.
