PHYSICAL     EFFECTS     OF     HEAT.  251

vessel  that  contains  the  cream.  The  strong  attrac-
tion between  salt  and  water  causes  the  ice  to  melt
rapidly,  and  the  solid  salt  becomes  liquid  by  solu-
tion. This  rapid  thawing  involves  much  absorption
of  energy,  which  comes  from  the  nearest  objects
whose  temperature  is  higher  than  that  of  the  solu-
tion. The  cream  thus  loses  energy,  its  temperature
becoming  reduced  down  to  the  freezing-point.*

5.  Vaporization.  —  When  heat  is  applied  to  a
liquid  the  temperature  rises  until  the  boiling-point
is  reached,  when  it  stops  and  the  liquid  is  changed
to  vapor  at  that  constant  temperature.  The  vapor
is  nearly  free  from  solids  dissolved  in  the  liquid. —
Example:  Pure  or  distilled  water  is  obtained  by
heating  water  in  a  boiler,  A,  whence  the  steam
passes  through  the  pipe,  (7,  and  the  worm  within  the
condenser,  S,  where  it  is  condensed  and  drops  into
the  vessel,  D.  The  pipe  is  coiled  in  a  spiral  form
within  the  condenser,  and  is  hence  termed  the
worm.  The  condenser  is  kept  full  of  cold  water
from  the  tub  at  the  left.  By  carefully  regulating
the  temperature,  one  liquid  may  be  separated  from
another  by  "fractional"  distillation,  advantage  being
taken  of  the  fact  that  each  liquid  has  its  own  boil-

*  That  freezing  is  a  warming  process  may  be  conclusively  shown  as  fol-
lows :  Gently  melt  some  sodium  sulphate  (a  cheap  salt  that  may  be  ob-
tained from  any  apothecary)  in  a  flask  by  heating  it  over  a  lamp  flame.
