262 
RESEARCHES  ON  EVAPORATION. 
20°,  water  is,  on  the  average,  0*3°  warmer  in  an  open  metallic 
vessel  than  in  a  similar  one  of  polished  porcelain,  and  0-2°  warmer 
than  in  a  similar  one  of  glass.  It  is  the  same  with  alcohol. 
Again,  both  water  and  alcohol  evaporate  more  rapidly  from  a 
porcelain  vessel  than  from  a  metallic  or  glass  vessel  of  precisely 
the  same  size.  For  example — three  similar  vessels,  one  of  metal, 
the  second  of  porcelain,  and  the  third  of  glass,  containing  each 
600  grains  of  water,  having  been  exposed  to  evaporation  during 
seven  days,  the  temperature  of  the  surrounding  atmosphere  vary- 
ing from  20°  to  25°,  it  was  found,  that  at  the  end  of  that  time, 
the  porcelain  vessel  had  lost  303  grains  of  its  previous  weight, 
the  metallic  one  277,  and  the  glass  vessel  275-5  grains  only.  The 
author  enters  into  considerable  detail  as  to  the  precautions  he 
took  to  make  sure  that  these  differences  could  not  be  attributed 
to  any  difference  in  the  radiating  or  conducting  powers  of  the 
vessels  employed. 
The  differences  observed  in  the  temperature  of  liquids,  accord- 
ing to  the  nature  of  the  vessels  in  which  they  are  contained, 
depends,  no  doubt,  _  on  the  property  with  which  these  vessels 
appear  to  be  endowed,  of  accelerating  or  delaying  evaporation. 
It  is  evident,  that  in  each  case  the  quantity  of  sensible  heat  sub- 
tracted from  the  liquid,  or,  in  other  words,  the  diminution  of  its 
temperature,  must  be  in  proportion  to  the  quantity  of  vapor 
formed.  For  instance,  the  fact  that  water  and  alcohol  are  con- 
stantly colder  in  a  porcelain  vessel  than  in  a  similar  vessel  of 
metal  or  glass,  is  the  natural  result  of  the  more  rapid  evaporation 
of  these  liquids  from  the  former  of  these  vessels  than  from  the 
two  latter.  The  reason  why  a  porcelain  vessel  evaporates  more 
freely  than  a  metallic  or  glass  one  is  far  less  evident.  The 
author  has  proved,  by  placing  a  hermetically-closed  bottle  of  por- 
celain, containing  water,  under  the  vacuum  of  the  air  pump,  that 
it  cannot  be  owing  to  any  perviousness  of  the  sides  of  the  vessel, 
as  he  was  at  first  inclined  to  suspect. 
3.  The  influence  of  the  mass  or  depth  of  a  liquid  was  next 
examined.  The  author's  experiments  appear  to  lead  to  the  curi- 
ous fact,  that  the  rapidity  with  which  any  given  liquor  evaporates 
depends  not  only  on  the  extent  of  its  surface,  but  also,  within 
certain  limits,  on  its  depth.  He  found,  for  instance,  that  with 
two  similar  cylindrical  porcelain  vessels,  containing,  the  first  a 
