RESEARCHES  ON  EVAPORATION. 
263 
layer  of  water  of  one-twelfth  of  an  inch  in  depth,  and  the  second  a 
layer  of  half  an  inch,  the  evaporation  from  the  latter  exceeded 
that  of  the  former  in  the  proportion  of  nearly  4  to  3.  A  similar 
result  was  obtained  with  alcohol.  If  thin  glass  vessels  were  used, 
the  same  increase  of  depth  accelerated  the  evaporation  in  the 
proportion  of  6  to  5.  As  the  author  himself  observes,  this  ap- 
parent influence  of  the  depth  of  a  liquid  on  its  evaporation,  may, 
very  possibly,  be  merely  owing  to  the  greater  facility  with  which 
the  different  layers  are  conveyed,  one  after  the  other,  to  the  sur- 
face, when  the  liquid  is  of  a  certain  depth  than  when  it  is  quite 
shallow. 
4.  Water  containing  a  solution  of  salt  in  about  the  same  pro- 
portion of  sea  water,  evaporates  less  rapidly,  and,  consequently, 
produces  less  cold  than  the  same  quantity  of  distilled  water.  The 
higher  the  temperature  of  the  surrounding  atmosphere,  the  greater 
the  difference  between  the  quantities  of  salt  and  fresh  water 
evaporated  in  a  given  time,  under  similar  circumstances. 
5.  A  given  quantity  of  water,  mixed  with  certain  pulverulent 
substances,  such  as  a  siliceous  sand,  for  the  particles  of  which  it 
has  but  a  slight  adhesion,  evaporates  more  rapidly  than  the  same 
quantity  of  distilled  water  alone.  The  fact  was  ascertained  in 
the  following  manner  : — The  author,  having  procured  two  small 
porcelain  vessels,  exactly  of  the  same  size,  introduced  into  one 
of  them  300  grains  of  distilled  water,  and  into  the  other  a  small 
quantity  of  siliceous  sand,  over  which  300  grains  of  water  were 
poured,  so  as  not  only  to  saturate  the  sand,  but  also  to  leave  a 
layer  of  water  of  about  one- tenth  of  an  inch  in  thickness  over 
and  above  its  surface.  At  the  end  of  five  days,  it  was  observed 
that  the  water  standing  alone  had  lost  184  grains  of  its  previous 
weight,  while  the  water  mixed  with  the  sand  had  lost  no  less  than 
196«grains.  '  The  average  difference,  resulting  from  a  series  of 
experiments,  was  7  J  per  cent,  in  favor  of  the  more  rapid  evapora- 
tion of  water  mixed  with  sand  compared  with  that  of  water  stand- 
ing alone.  If  the  experiment  be  made  with  glass  or  metallic 
vessels,  the  difference  is  only  about  4*  per  cent. 
6.  The  last  result  we  shall  mention,  and  which  may  be  regarded 
as  a  direct  consequence  of  the  preceding  one,  is  the  following : — 
Water  mixed  with  sand  remains  habitually  at  a  slightly  lower 
temperature  than  an  equal  surface  of  water  standing  alone.  The 
difference  varies  to  a  certain  extent,  according  to  the  nature  of 
