344  OILS—AROMATIC—UNCTUOUS—EMPYREUMATIC.
To  treat  of  the  oils  more  particularly,  they  must  be  distinguished
into  two  principal  kinds,  the  ARoMATIC  and  the  UNCTuoUS  ;  to  which
it  is  uſual  to  ſubjoin  a  third  ſection,  named  EMPYREUMATIC.
The  aromatic  oils  are  all  formed  by  nature,  and  chiefly  in  vegetables. ¬
  In  these  they  are  fecreted  juices,  lodged  in  particular  parts  or
repositories  in  the  structure  of  the  plant.  All  the  aromatio  oils  make
a  strong  impression  on  the  organs  of  taste  and  smell,  exciting  on  the
tongue  the  feeling  of  heat  and  acrimony.  And  they  have  not  that
ſmoothneſs  and  slipperiness,  when  felt  between  the  fingers,  which  is
named  unctuofity.  They  are  alfo  all  capable  of  rising  in  vapour,  in
the  heat  of  boiling  water,  or  at  least  along  with  the  vapour  of  water.
And  they  are  very  inflammable.  The  wicks  of  candles  are  often  prepared -
  for  being  quickly  lighted  up,  by  putting  a  small  quantity  of  some
of  theſe  oils  on  the  extremity  of  them.
The  unctuous  oils  are  also  natural  productions,  and  are  found  in
vegetables  and  animals,  in  both  of  which  they  are  also  secreted  juices,
or  are  formed  and  lodged  in  particular  parts  or  repositories.
When  not  corrupted,  they  have  not  any  pungent  taste,  or  remarkable -
  odour,  but  have  unctuosity  in  a  great  degree,  and  for  that  reason
are  much  employed  to  diminish  friction  in  machines  of  all  kinds.
The  heat  of  boiling  water  is  not  fufficient  to  convert  them  into  vapour, ¬
  and  they  are  not  fo  readily  and  quickly  inflamed  as  the  aromatic ¬
  oils.
The  empyreumatic  oils  refemble  the  aromatic  so  much  by  many
of  their  chemical  qualities,  that  they  are  not  distinguishable  from  them
by  any  general  difference,  except  the  circumstance  of  their  origin.
None  of  them  are  natural  productions.  They  are  all  produced  by
art,  and  are  either  oils,  changed  from  their  natural  state  by  the  action
of  violent  heats,  or  are  entirely  produced  by  heat  from  vegetable  or
animal  fubstances  which  do  not  contain  a  formed  oil  before  the  heat
is  applied  to  them.  We  may  also  add,  as  an  article  in  the  character  of
the  empyreumatic  oils,  that  the  odour  of  them  is  in  general  offensive.