316 
On  the  Power  of  Soils  to  absorb  Manure. 
Lord  Bacon,  in  his  f Sylva  Sylvarum,’  speaks  of  a method  of 
obtaining  fresh  water,  which  was  practised  on  the  coast  of  Bar- 
bary : “ Digge  a hole  on  the  sea-shore  somewhat  above  high- 
water  mark,  and  as  deep  as  low-water  mark,  which  when  the  tide 
cometh  will  be  filled  with  water  fresh  and  potable.”  He  also 
remembers  ‘‘  to  have  read  that  trial  hath  been  made  of  salt  water 
passed  through  earth  through  ten  vessels,  one  within  another,  and 
yet  it  hath  not  lost  its  saltness  as  to  become  potable,”  but  when 
“ drayned  through  twenty  vessels  hath  become  fresh.” 
Dr.  Stephen  Hales,  in  a paper  read  before  the  Royal  Society  in 
1739,  on  “ some  attempts  to  make  sea-water  wholesome,”  men- 
tions, on  the  authority  of  Mr.  Boyle  Godfrey,  that  “ sea  water 
being  filtered  through  stone  cisterns,  the  first  pint  that  runs 
through  will  be  like  pure  water,  having  no  taste  of  the  salt,  but 
the  next  pint  will  be  as  salt  as  usual.”* 
Berzelius  found,  upon  filtering  solutions  of  common  salt  through 
sand,  that  the  first  portions  that  passed  were  quite  free  from  saline 
impregnation.  Professor  Matteucci  extended  this  observation  to 
other  salts,  and  found  that  the  solutions  when  filtered  through 
sand  were  diminished  in  density,  showing  a detention  of  the  sand 
of  certain  quantities  of  the  salt  operated  upon.  Matteucci  ex- 
plains these  phenomena  on  the  principle  of  capillarity.  The 
particles  of  the  sand  have  a surface  attraction  both  for  the  water 
and  the  salt,  but  to  a greater  extent  for  this  latter,  which  is 
therefore  concentrated  and  condensed  on  the  sand,  whilst  the 
liquid  is  proportionably  diminished  in  strength  and  density. 
When  this  attraction  has  been  gratified  the  solution  ceases  to  be 
affected,  and  comes  through  of  its  original  strength,  y 
It  is  to  be  observed  that  this  property  of  sand  to  arrest  and 
separate  saline  substances  from  solution  is  very  limited  in  extent, 
and  requires  careful  arrangements  to  make  it  evident  at  all  by 
experiment.  It  differs  also  fundamentally  from  the  chemical  power, 
which  it  is  the  object  of  the  present  paper  to  explain,  inasmuch 
as  the  physical  action  of  capillarity  is  exerted  on  the  whole  salt, 
whilst  that  we  are  about  to  describe  has  relation  only  to  the  alka- 
line or  earthy  base.  Furthermore,  the  former  property  is  only  the 
resultant  of  two  opposite  forces,  that  of  the  surface  attraction 
of  the  sand  and  of  water  for  the  salt.  It  can  only  therefore 
operate  a condensation  of  the  salt  in  relation  to  the  strength  of  the 
solution,  the  salt  being  continually  shared  in  given  proportions 
between  the  sand  and  the  water,  so  that  eventually  the  whole  is 
washed  away.  Such,  however,  is  not  the  case  with  the  compounds 
which  are  formed  in  the  soil  with  solutions  of  different  alkaline 
* I am  indebted  to  my  friend,  Dr.  Smith,  of  Manchester,  for  calling  my  attention 
to  these  two  recorded  instances  of  the  effect  of  filtration  on  salt  water. 
f ‘ Sur  les  Phenomenes  physiques  des  Corps  vivants,’  p.  2'J.  Paris,  1847. 
