The  vapor  pressure  of  the  ice  takes  the  place  of
4.5  mm.  of  mercury  in  balancing  the  atmospheric
pressure,  and  so  the  mercury  column  falls  by  this
amount.  Similarly,  water  at  10°  causes  a  fall  of
9.1  mm.  and  at  20°  of  17.4  mm.,  so  that  these
represent  the  mercury-height  values  of  the
aqueous  tension  at  these  temperatures.  The
quantity  of  water  used  makes  no  difference,  so
long  as  a  little  more  is  present  than  is  required
to  fill  the  available  space  with  vapor.  With
ether,  instead  of  water,  at  10°  the  fall  is  28.7  mm.
With  water  at  higher  temperatures  the  fall  of
the  mercury  column  becomes  much  greater.  At  50°  it  is  92  mm.,
at  70°  it  is  233.3  mm.,  at  90°  it  is  525.5  mm.,  and  at  100°  it  is  760


FIG.  25.
80  COLLEGE   CHEMISTRY

mm.,  or  one  atmosphere.  At  121°  the  aqueous  tension  is  two
atmospheres,  at  180°  it  is  ten  atmospheres.
When  water  at  a  certain  temperature  has  given  the  full  amount  of
water  vapor  to  the  space  above  it  that  its  aqueous  tension  permits,
we  say  that  the  space  is  saturated  with  vapor.  That  concentration  of
vapor  which  constitutes  saturation  varies  with  the  temperature  of
the  water  and  depends  therefore  solely  on  the  power  of  the  water  to
give  off  vapor.  It  has  nothing  to  do  with  the  size  of  the  space,  and
is  even  independent  of  other  gases  the  space  may  already  contain
(p.  60,  also  pp.  90-91.  See  footnote  to  p.  24).
