rochleder's  proximate  analysis. 
375 
one  another  whose  solution  could  easily  have  been  regarded  as  the  solution 
of  a  single  substance. 
20.  Peroxide  of  lead,  and  peroxide  of  manganese, — Both  these  superoxides, 
in  a  finely  divided  condition^  frequently  produce  an  oxidation  of  one  or  the 
other  constituent  in  the  heated  watery  solutions  of  substances,  while  some 
bodies  are  not  thereby  attacked.  A  few  organic  compounds  withstand  this 
oxidizing  agent  after  the  addition  of  dilute  sulphuric  acid.  Volatile  bodies 
are  often  thereby  formed,  as  formic  acid,  kinone,  &c,  and  frequently 
characteristically-colored  oxidation  products  ;  as,  for  example,  with  strych- 
nine. The  formation  of  colored  oxidation  products  from  many  organic 
bases,  which  are  produced  by  the  action  of  peroxide  of  lead  and  sulphuric 
acid,  depends,  with  the  majority  of  these  substances,  upon  an  illusion. 
Quinetine,  cinchonetine,  morphetine,  &c,  result,  by  the  action  of  sulphuric 
acid  and  peroxide  of  lead,  from  the  bases  concerned,  only  when  the  sul- 
phuric acid  or  the  peroxide  of  lead  is  contaminated  with  nitric  acid.  By 
the  employment  of  pure  reagents  other  decompositions  are  produced. 
21.  Chlorine,  bromine,  and  iodine. — These  three  elements  produce,  partly 
by  the  indirect  oxidation  of  the  substance  in  the  watery  solution  in  which 
they  are  introduced,  and  partly  by  the  substitution  of  hydrogen  which 
they  call  forth,  characteristic  appearances  of  color,  or  precipitations  of  in- 
soluble oxidation  or  substitution  products  ;  consequently,  they  serve  often 
for  the  detection  of  substances  in  mixtures. 
22.  Gelatin. — Solution  of  gelatin  is  employed  for  the  precipitation  of  the 
different  forms  of  tannin.  A  pure  solution  of  gelatin  is  prepared  by  pre- 
cipitating a  solution  of  gelatin,  to  which  a  little  hydrochloric  acid  has 
been  added,  with  alcohol,  washing  the  precipitate  with  spirit,  and  dissolv- 
ing the  washed  gelatin  in  water. 
23.  Infusion  of  galls  serves  for  the  precipitation  of  organic  bases,  as 
well  as  for  throwing  down  some  albuminous  substances  from  their  solu- 
tions. 
The  employment  of  reagents  will  never  lead  to  a  complete  knowledge  of 
the  composition  of  a  vegetable  substance,  because,  from  the  similar  be- 
haviour towards  some  reagents  of  a  body  discovered  with  one  already 
known,  no  conclusion  can  be  drawn  on  their  identity.  Reagents  are  ser- 
viceable for  directing  attention  to  the  presence  of  substances,  and  reagents 
should  show  us  the  way  of  separating  bodies  which  exist  in  a  mixed  state. 
In  the  second  section  it  has  been  explained  how  we  are  to  proceed  with 
different  decoctions  and  infusions  of  a  material  under  examination,  to 
separate  up  to  a  certain  point  the  constituents  from  one  another.  Thereby 
it  was  necessary  to  speak  of  the  application  of  some  reagents.  The  des- 
cription of  the  method  of  separating  mixtures  was  often  interrupted,  and 
it  was  there  stated  that  the  further  examination  must  be  continued  with 
the  aid  of  reagents.  If  we  had  described  in  every  case  the  method  to  its 
termination,  a  numberless  quantity  of  repetitions  would  have  been  un- 
