ON  GALLIC  AND  GALLHUMIC  (METAGALLIC)   ACID.  79 
Moreover  the  perfumes  properly  so  called,  as  musk  and  the 
aromatic  essences,  rose,  lemon,  orange,  bergamot,  lavender,  &c. 
are  eminently  hydrogen  compounds.  They  are  not  all  volatile, 
and  some  of  them  may  be  exposed  to  the  air  for  years,  exhal- 
ing odor  all  the  time,  with  no  sensible  loss  of  weight.  Among 
these  are  perfumes  isolated  by  Milon  in  1856.*  The  cause  of 
odors  is  not  referable  exclusively  to  the  phenomena  of  volatility, 
although  as  a  general  thing  the  odor  of  most  bodies  is  developed 
when  they  are  volatilized. 
Hydrogen  must  be  considered,  par  excellence,  the  exciting 
cause  of  odors.  This  element  possesses  above  all  other  sub- 
stances the  peculiar  property  of  developing  odors  even  with  odor- 
less bodies,  as  N,  C,  Se,  Te,  P,  &c,  and  a  great  number  of  com- 
pounds, of  these  and  other  elements. 
Oxygen,  on  the  other  hand,  appears  to  act  the  chief  part  in  the 
perception  of  odors  ;  it  seems  indeed  proved  that  odors  are  not 
recognizable  where  there  is  not  oxygen  in  the  air  to  bathe  the 
olfactory  membranes.] — Am.  Jour.  Sci.  and  Arts,  Nov.  1859. 
ON  GALLIC  AND  GALLHUMIC  (METAGALLIC)  ACID. 
By  Dr.  F.  Mahla,  Ph.D. ,  Chicago. 
It  is  mentioned  among  the  reactions  of  gallic  acid  in  almost 
every  handbook  of  chemistry,  that  its  solution  produces  a  deep 
bluish-black  color  with  a  solution  of  the  salts  of  the  sesquioxyd 
of  iron,  which  disappears  when  the  solution  is  heated.  As  I 
have  nowhere  found  an  explanation  of  this  fact,  I  have  tried  to 
investigate  it  by  some  experiments. 
When  the  solutions  of  the  sesquioxyd  of  iron  and  gallic  acid 
are  used  in  a  diluted  state,  the  resulting  mixture  appears  only 
slightly  colored ;  but  if  they  are  concentrated,  it  assumes,  after 
being  heated  to  ebullition,  a  dark  brown  tint,  and  then  causes 
black  spots  on  the  skin,  which  can  be  washed  away  only  with 
the  greatest  difficulty.  Such  a  solution  might  perhaps  be  used 
advantageously  as  a  hair  dye. 
If  the  iron-solution  was  not  'added  in  too  large  proportion, 
liquid  ammonia  no  longer  precipitates  hydrated  sesquioxyd  of 
*This  Journal,  July,  1856,  p.  109. 
