83  COLORING  PRINCIPLE  OF  VOLATILE  OILS,  ETC. 
otio  or  bergamot  from  the  rind  of  the  fruit,  Citrus  bergamia,  as 
also  otto  of  Ceylon  Lemon-grass,  Andropogon  Schoenantlms, 
yield  by  the  same  treatment  small  portions  of  this  blue  coloring. 
By  repeated  rectification  of  the  blue  fluid,  from  whatever  source 
derived,  I  at  length  render  it  free  from  extraneous  matter  and 
in  a  state  of  purity,  it  then  has  a  fixed  boiling  point  of  576°  F., 
its  sp.  gr.  0-910  ;  when  boiled,  it  produces  a  dense  vapor  of  a 
blue  color,  having  special  optical*  characters.  I  have  named 
this  substance  Azulene,  from  azure — blue.  The  analysis  of 
Azulene  shows  its  formula  to  be : — 
C  16  .  .  82-05  .  .  81-21 
H  13  .  .  1142  .  .  10-95 
0  .       .  6-83     .       .  7-84 
100-00  100-00 
Or,  C16H12+HO.  _ 
The  yellow  coloring-matter  which  imparts  its  tint  to  the 
several  ottoes,  appears  to  be  an  oxidised  portion  of  the  otto  so 
stained.  In  nearly  all  instances,  ottoes  which  are  colorless 
when  first  obtained  from  their  source  become  yellow  by  age,  i.  e., 
oxidation.  This,  however,  is  not  universal,  as  the  otto  of  nut- 
meg remains  colorless  for  a  lengthened  period,  even  when  air  is 
drawn  through  it  by  an  aspirator. 
The  oxidised  portion  of  the  yellow-colored  oils,  when  separa- 
ted from  the  pure  otto  in  which  it  is  dissolved,  are  true  resins ; 
the  majority  of  ottoes  oxidize  during  the  act  of  distillation, 
hence  from  this  cause  they  vary  in  color  from  pale-yellow  to 
red-brown.    When  new,  that  is,  freshly  distilled,  several  essen- 
*  Sir  David  Brewster  lias  optically  examined  two  blue  ottoes  which 
owe  their  color  to  the  presence  of  azulene,  namely  Matricaria  Ghamo- 
milla  and  Achillea  Millefolium.  "  Without  entering,"  says  Sir  David, 
"  into  details  respecting  the  general  action  of  these  oils  upon  the  different 
colored  portions  of  the  spectrum,  I  shall  confine  myself  to  a  slight  notice 
of  their  specific  action  in  which  they  differ  from  air  the  various  bodies 
which  I  have  yet  examined. 
"  Between  the  two  lines  A  and  B,  of  Fraunhofer's  map  of  the  spectrum, 
there  are  two  groups  of  lines  shown  in  that  map.  The  two  oils  absorb 
the  light  in  these  portions  more  powerfully  than  in  the  portions  adjacent 
to  them.  No  other  fluid  or  solid  on  which  I  have  made  experiments  acts, 
in  a  similar  manner;  but  what  is  very  remarkable,  the  earth's  atmosphere 
exercises  a  similar  action  when  the  sunlight  passes  through  its  greatest 
i  ckness  at  sunrise  and  sunset." 
I 
