rochleder's  proximate  analysis/  373 
hydrogen  gas,  in  the  manner  explained  in  the  treatment  of  certain  bodies 
with  baryta  (page  42.) 
The  disposition  of  many  organic  substances  to  oxydize  with  oxygen  in 
contact  with  alkalies,  can  likewise  be  used  for  the  detection  of  bodies.  la 
these  processes  of  oxidation  changes  of  color  generally  appear  which  are 
not  easily  overlooked, 
16.  Ammonia. — In  its  method  of  action,  ammonia  is  closely  allied  to 
the  bases  above  mentioned.  In  many  cases  it  possesses  a  preference  over 
them,  in  so  far  as  an  excess  of  it  can  be  easily  removed.  Its  action  is  gen- 
erally less  energetic  than  that  of  the  fixed  alkalies  and  baryta.  Ammonia 
frequently  acts  in  a  peculiar  way  on  organic  substances,  for  many  organic 
substances  combine  therewith  under  the  absorption  of  oxygen  and  the 
separation  of  water,  and  produce  sometimes  brilliantly  colored  compounds. 
In  contact  with  many  compounds,  ammonia  produces  immediately  char- 
acteristic colorations.  The  treatment  of  dry  substances  with  dry  aramo- 
niacal  gas,  sometimes  at  ordinary  and  sometimes  at  higher  temperatures, 
serves  frequently  for  the  detection  of  bodies  which  would  otherwise  re- 
main unobserved,  and  for  the  recognition  of  bodies  in  other  respects 
similar. 
17.  Nitric  acid. — The  acid  acts  in  a  concentrated  condition  very  vehe- 
mently on  almost  all  organic  bodies.  Their  oxidation  products,  or  sub- 
stitution products,  which  contain  N04  in  the  place  of  hydrogen,  are  thus 
produced.  The  body  often  splits  up,  and  the  products  of  this  decomposi- 
tion then  undergo  further  changes.  By  the  action  of  nitric  acid  many 
bodies  assume  very  intense  characteristic  colorations,  others  do  not ;  so 
that  this  acid  can  be  used  by  this  property  for  the  recognition  of  bodies  in 
mixtures  with  others.  Nitric  acid  mixed  with  sulphuric  is  particularly 
suitable  for  the  development  of  nitro-compounds. 
18.  Sulphuric  acid  is  employed  in  a  diluted  as  well  as  a  concentrated 
condition.  The  heated  dilute  acid  splits  up  many  bodies  into  decomposi- 
tion products,  although  in  this  respect  hydrochloric  acid  is  much  to  be  pre- 
ferred. But  it  may  be  more  readily  and  quickly  separated  from  a  liquid 
than  hydrochloric  acid,  which  in  many  cases  demands  care.  In  a  mode- 
rately concentrated  state  it  precipitates  many  forms  of  tannin  from  con. 
centrated  aqueous  solutions,  particularly  when  no  acetic  acid  is  present* 
It  gives  crystallizable  salts  with  many  organic  bases,  which,  with  the  ex- 
ception of  the  salts  of  methylamine,  are  soluble  in  alcohol.  The  dilute 
acids  often  effect  in  the  heat,  with  some  organic  substances,  an  absorbtion 
of  water,  or  rather  of  its  elements  ;  with  others,  an  expulsion  of  hydrogen 
and  oxygen  in  equal  equivalents.  In  this  respect  its  action  has  much  re- 
semblance to  the  action  of  alkalies. 
The  concentrated  acid  dissolves  many  organic  substances  without  exer- 
cising any  influence  on  their  composition.  When  the  substance  is  in- 
soluble ic  water,  it  is  often  precipitated  unchanged  from  such  solutions  by 
