NOTE  ON  VARIOUS  PHENOMENA  OF  OXYGENATION.  137 
leaving  only  that  of  the  turpentine.  In  this  reaction,  which  ap- 
pears to  be  facilitated  by  the  solar  radiation,  formation  of  sul- 
phuric acid  takes  place  at  the  expense  of  the  oxygen  absorbed  by 
the  turpentine,  which  is  taken  from  it  by  the  sulphurous  acid  be- 
fore it  has  time  to  appropriate  it  in  a  more  permanent  manner. 
If  sulphurous  acid  gas  be  passed  into  a  moist  glass  globe  con- 
taining the  vapour  of  an  oxygenated  essential  oil,  the  sulphurous 
acid  disappears  by  degrees ;  on  the  other  hand,  if  a  mixture  of 
an  aqueous  solution  of  sulphurous  acid  and  an  aerated  essential 
oil  be  allowed  to  become  concentrated  in  contact  with  the  air, 
the  sulphuric  acid  formed  carbonizes  the  essential  oil  without  the 
necessity  of  raising  the  temperature  of  the  mixture. 
The  oxygenating  action  of  the  aerated  essential  oil  is  not 
confined  to  sulphurous  acid  ;  it  extends  also  to  other  acids,  such 
as  hyposulphurous  acid,  the  sulphites,  arsenious  acid,  &c. 
Peculiar  Reactions  of  the  Essential  Oils  in  Painting. 
The  essential  oils,  from  the  nature  of  their  constituent  prin- 
ciples, may  be  regarded  as  possessing  naturally,  and  especially 
under  the  influence  of  heat  or  of  the  sun,  a  reductive  power  which 
acts  slowly  upon  white  lead  and  the  colored  oxides.  However 
this  may  be,  the  resinifiable  essential  oils  temporarily  possess 
another  property  of  an  opposite  nature,  as  I  have  just  shown  ; 
and  this  deserves  to  be  taken  into  consideration  in  the  study  of 
the  modifications  undergone  by  paintings  in  oil,  namely,  that  of 
absorbing  oxygen  by  mere  contact  with  the  air.  The  result  of 
this  is,  that  at  the  moment  of  their  employment  the  essential  oils 
may  exercise  an  oxidizing  action,  tending  to  destroy  vegetable 
colors  and  to  modify  some  mineral  colors.  Thus 
Litharge,  heated  with  aerated  oil  of  turpentine  furnishes  the 
puce-colored  oxide  of  lead. 
If  oil  of  turpentine  be  agitated  at  the  ordinary  temperature 
with  the  hydrated  protoxides  of  iron,  tin  and  manganese,  these 
oxides  pass  to  a  higher  degree  of  oxidation.  With  a  solution  of 
protosulphate  of  iron,  basic  sesquisulphate  is  produced,  which 
separates  from  the  liquid.  The  white  precipitate  formed  by 
ferrocyanide  of  potassium  with  a  protosalt  of  iron  immediately 
acquires  the  intense  color  of  prussian  blue. 
Blue  and  red  flowers,  decolorized  by  sulphurous  acid,  reacquire 
