84 
rochleder's  proximate  analysis. 
that  every  method  of  vegetable  analysis  which  is  arranged  for  the  present, 
must  be  only  a  provisional  one,  to  be  made  more  comprehensive  as 
goon  as  the  knowledge  of  the  constituents  of  plants  has  been  extended  by 
its  aid — in  other  words,  the  provisional  method  is  the  means  to  arrive  at 
better  methods. 
With  the  majority  of  the  older  analyses  of  vegetables  the  foundation  of 
the  process  was  the  application  of  different  solvents  in  succession.  Ether^ 
alcohol  and  water  were  the  solvents  most  commonly  employed.  In  many 
cases,  the  residues  were  brought  into  contact  with  dilute  acids  and  alkalies, 
generally  with  the  assistance  of  heat,  after  having  been  more  or  less  ex- 
hausted with  the  three  fluids  mentioned.  In  consequence  of  the  facility  with 
which  many  substances  are  transformed  into  others  by  the  action  of 
acids  and  alkalies  in  the  heat,  these  latter  methods  of  treatment  often  gave 
rise  to  incorrect  views  of  the  composition  of  the  plants,  or  those  parts 
under  examination.  The  treatment  of  the  substance  to  be  examined  in 
succession  with  ether,  alcohol,  and  water,  would  have  afforded  much  bet- 
ter results,  as  in  fact  was  mostly  the  case  when  two  conditions  which  did 
not  prevent  a  complete  separation  in  this  way,  were  not  sufficiently  attend- 
ed to  and  calculated  upon.  These  conditions  are  the  following:  the  ex- 
haustion of  the  substance  under  examination  with  one  fluid  must  always  be 
imperfectly  effected  before  the  second  is  allowed  to  act  thereon.  We 
cannot  so  prepare  the  material  that  each  individual  cell  and  its  contents 
are  exposed  to  the  action  of  the  solvent,  because  the  material  reduced  to 
an  impalpably  fine  powder,  and  exhausted  with  a  solvent,  affords  again  to 
the  same  solvent  substances  after  it  has  been  freshly  triturated.  Thus  it 
happens  that  there  are  always  bodies  retained  in  the  substance  under  exam- 
ination after  its  treatment  with  a  solvent  which  are  soluble  therein.  If 
we  now  bring  the  substance  in  contact  with  the  second  solvent,  the  bodies 
not  only  will  dissolve  that  we  intend  therewith  to  extract,  but  often  also 
the  remainder  of  the  bodies  which  the  first  solvent  left  behind.  The  same 
holds  good  with  regard  to  the  third  solvent.  A  solution  of  certain  bodies 
by  a  solvent  will  afford  thereby  no  means  in  many  cases  for  the  separation 
of  other  bodies  which  are  insoluble  in  this  solvent,  because  frequently  sub- 
stances which  are^er  se  insoluble  in  a  liquid,  are  not  insoluble  in  a  solu- 
tion of  other  substances  in  the  same  liquid.  In  this  way  we  obtain,  in  a 
watery  or  alcoholic  extract  of  a  vegetable  substance,  bodies  which  per  se 
are  insoluble  in  water  or  alcohol,  but  which,  by  the  agency  of  other  bodies, 
are  dissolved  therein.  Independently  of  these  detrimental  circumstances, 
which  are  produced  by  an  incomplete  exhaustion  with  one  liquid  before 
the  application  of  a  second  solvent,  there  is  associated  the  condition  that  the 
exhaustion  with  a  liquid,  at  the  same  time,  produces  a  solution  of  bodies 
which  should  not  dissolve,  because  they  are  held  to  be  insoluble  therein. 
But  what  is  termed  insoluble  are,  in  the  majority  of  cases,  only  very  diffi- 
cultly soluble  substances,  that  is,  such  substances  as   require  a  large 
