rochleder's  proximate  analysis. 
369 
(Continufd  ftom  page  281.) 
While  fractional  distillation,  or  an  evaporation  undertaken  at  somewhat 
lower  temperatures  in  a  similar  manner,  is  often  the  only  means  of  sepa- 
rating volatile  bodies  which  are  neutral  or  indifferent,  ihe  method  of  par- 
tial combination  is  a  meiJiod  of  separating  volatile  bases  or  volatile  acids  from 
one  another  when  several  bases  or  acids  are  present  at  the  same  time. 
How  the  volatile  acids  are  to  be  separated  from  other  volatile  bodies,  and 
likewise  the  bases  from  other  volatile  compounds,  has  been  above  set  forth. 
The  method  of  partial  combination  was  first  used  by  Liebig  in  separa- 
ting volatile  acids.  When  we  have  prepared  a  fluid  which  contains  the 
volatile  acids  separated  from  the  other  volatile  constituents,  and  after  it 
has  been  measured  off  in  a  graduated  glass,  it  is  divided  into  ten  equal 
parts,  one  part  of  which  is  accurately  saturated  with  a  solution  of  soda  or 
potash,  and  after  the  nine  remaining  parts  have  been  added,  the  fluid  is 
distilled.  The  distillate  is  thereon  divided  into  nine  equal  parts,  one  part 
saturated  with  alkali,  and  the  other  eight  parts  added,  and  the  whole 
again  subjected  to  distillation.  The  distillate  is  now  divided  into  eight  parts, 
one  part  neutralized  with  alkali,  the  remaining  seven  parts  mixed  with  this, 
and  the  fluid  again  distilled.  In  a  similar  way  this  proceeding  is  continued 
until  only  a  tenth  part  of  the  original  solution  of  acids  remains  in  a  free 
condition  ;  consequently  in  this  we  have  the  weakest  acid  in  a  condition 
approximately  pure  ;  in  the  rest  of  the  liquids  the  potash  or  soda  salts  of 
the  other  acids,  in  the  first  residue  of  the  distillation  the  strongest  acid 
either  pure  or  contaminated  with  a  little  of  the  next  weakest  acid,  and  so 
forth.  It  is  clear  that,  in  a  similar  way,  many  volatile  bases  could  be 
separated  from  one  another.  The  only  alteration  required  in  the  method 
given  for  acids  would  be  that  the  neutralization  of  the  parts  of  the  fluid 
must  be  effected  with  dilute  sulphuric  acid  or  phosphoric  acid  instead  of 
soda  or  potash.  If  in  this  way  we  have  obtained  ten  parts  of  the  soda  or 
potash  compounds  of  the  acids,  or  even  as  many  portions  of  the  sulphuric 
acid  or  phosphoric  acid  compounds  with  the  bases,  we  can  procure,  by 
the  decomposition  of  the  alkaline  compounds  with  acids,  or  the  salts  of 
the  bases  with  alkalies  and  distillation,  the  corresponding  acids  and  bases 
in  a  free  condition,  and  then  be  convinced  whether  we  have  had  to  d,o 
with  two  or  more  bases  or  acids.  Of  course  there  still  remains  the  deter- 
mination of  the  atomic  weight  and  elementary  analysis  to  afford  a  guid€ 
whether  the  individual  acid  portions  or  base  portions  require  or  not  a  . fur- 
ther separation  by  the  same  method.  On  this  subject  I  shall  speak  later 
in  the  treatise. 
B. — Examination  of  the  aqueous  decoction. 
We  turn  back  to  a  closer  examination  of  the  watery  decoction  which 
was  obtained  by  the  treatment  of  the  material  under  investigation  with 
boiling  water  in  the  still  at  the  same  time  with  the  watery  distillate.  It  has 
been  already  mentioned  that  b?/  coolirig  of  the  hot  fluid  a  separation  often 
takes  place  of  one  or  other  substance.  If  such  a  separation  is  observed, 
the  separated  body  is  removed  by  filtration  for  a  closer  examination,  and 
24 
