354  The  Nature  of  Solution.  {AmjJuKiwm' 
about  50  cm.  of  mercury.  The  arrangement  described  affords  us 
then  a  means  of  measuring  the  pressure  exerted  on  the  solvent  by 
the  particles  of  a  substance  in  solution.  It  is  found  that  this  pres- 
sure is  directly  proportioned  to  the  concentration  of  the  sugar 
solution.  All  the  other  substances  which  have  been  investigated 
follow  this  law — pressure  varies  as  concentration.  It  will  be  noticed 
that  this  law  is  of  exactly  the  same  form  as  that  of  Boyle,  which 
states  that  the  pressure  of  a  gas  varies  inversely  as  the  volume,  i.  e., 
directly  as  the  concentration. 
Again,  the  pressure  of  the  dissolved  substance  increases  as  the 
temperature  rises,  and  for  all  substances  so  far  investigated  the  rate 
of  increase  is  the  same  as  in  the  case  of  gases.  With  gases,  if  the 
volume  be  maintained  constant,  the  pressure  varies  directly  as  the 
temperature  reckoned  from  the  absolute  zero  of  the  air  thermome- 
ter. This  is  Charles'  law,  followed,  as  has  been  said,  by  dissolved 
substances  as  by  gases.  We  see  then  that  the  general  relation  of 
volume,  i.  e., 
I 
concentration, 
to  pressure  and  temperature  is  the  same  for  dissolved  substances  as 
for  gases.    «For  either  case  we  may  write  : 
Volume  varies  as  pressure  X  temperature,  or 
Volume  =  pressure  X  temperature  X  a  constant  quantity. 
The  most  striking  evidence  in  favor  of  the  view  that  substances 
in  solution  are  in  the  gaseous  state  is  afforded  by  the  fact  that  this 
constant  has  the  same  value  for  dissolved  substances  as  for  gases  ; 
in  other  words,  a  dissolved  substance  at  a  certain  temperature  and 
which  exercises  on  the  solvent  a  certain  pressure,  occupies  the 
same  volume  as  it  would  if  under  these  conditions  of  temperature 
and  pressure  it  were  in  the  form  of  a  gas.  An  example  will  make 
this  striking  relation  clear.  At  0°  C.  and  760  mm.  of  mercury 
pressure,  the  molecular  weight  of  any  gas  expressed  in  grams  occu- 
pies 22,380  cc.  The  pressure  of  760  mm.  of  mercury  is  1,033  grams 
per  square  cm.,  which  number  expresses  the  pressure  in  absolute 
units ;  o°  C.  —  273  on  the  scale  of  absolute  temperature ;  there- 
fore the  value  of  the  constant  for  a  gas  is : 
1,033  X  22,380  =  84)700  approximately. 
273 
