Am.  Jour.  Pharm. 
July,  1891. 
The  Nature  of  Solution. 
353 
difficulties.  Of  late,  however,  the  question  has  assumed  another 
aspect.  We  have  not  attained,  it  is  true,  to  unanimity  of  opinion 
as  to  what  goes  on  in  the  process  of  solution,  but  there  is  neverthe- 
less a  perfectly  definite  theory  of  the  process  now  current,  which 
has  the  strenuous  support  of  a  prominent  school  of  continental 
chemists.  A  complete  account  of  the  new  theory  has  recently 
appeared  in  an  accessible  form  in  Oswald's  "  General  Chemistry." 
In  this  work  the  pros  rather  than  the  cons  of  the  question  are  dealt 
with.  Exceptions  yet  remain  to  be  explained,  and  fundamental 
experiments  will  have  to  be  repeated  and  confirmed  by  independent 
workers,  before  the  new  theory  can  be  looked  upon  as  firmly  estab- 
lished. Whatever  may  be  the  final  judgment  of  the  chemical  world, 
the  adequacy  of  the  new  theory  to  explain  the  phenomena  of  solu- 
tion is  undoubtedly  one  of  the  great  scientific  questions  of  the  day, 
The  following  is  a  short  resume  of  the  theory  and  the  data  on 
which  it  is  based. 
The  particles  of  a  body  in  solution  are  not  in  the  liquid,  but  the 
gaseous  condition ;  that  is  to  say,  they  obey  laws  having  exactly 
the  same' form  as  the  laws  of  gases.  In  dilute  solutions  the  condi- 
tions are  similar  to  those  in  a  perfect  gas  ;  concentrated  solutions 
show  deviations  from  the  simple  laws  which  are  similar  and  in  the 
same  direction  as  the  deviations  from  the  laws  of  Boyle  and  Charles 
shown  by  gases  near  the  point  of  liquefaction.  If  pure  water  be 
poured  on  the  surface  of  a  solution  of  sugar  the  particles  of  sugar 
rise  against  the  action  of  gravity  and  mix  with  the  pure  water. 
There  are  certain  substances  with  which  we  can  build  up  a  "  semi- 
permeable wall  "  which  will  allow  the  water  to  pass  but  not  the 
dissolved  substance.  A  porous  earthenware  cell  in  and  on  which  is 
deposited  copper  ferrocyanide  forms  such  a  wall.  Its  properties 
are  not  the  same  as  those  of  an  animal  membrane,  such  as  parch- 
ment, which  will  allow  crystalline  substances  in  solution  to  pass 
through  it.  If  a  cell  containing  a  sugar  solution  be  provided  with 
such  a  semi-permeable  wall  and  is  further  connected  with  a  pres- 
sure gauge,  then  on  immersing  the  cell  in  a  vessel  of  pure  water, 
water  passes  slowly  into  the  cell,  the  sugar  does  not  pass  out,  and 
the  particles  of  the  dissolved  substance  exercise  a  pressure  on  the 
solvent  which  is  registered  by  the  manometer.  The  maximum 
pressure  is  only  obtained  slowly,  but  is  of  considerable  magnitude, 
a  one  per  cent,  solution  of  sugar  showing  a  maximum  pressure  of 
