Am.  Tour.  Pharm. 
May,  191 9. 
Modem  Paint  Vehicles. 
291 
The  complete  answer  to  these  questions  is  very  complex,  but  the 
differences  in  behavior  are  inherent  in  the  chemical  structure  of  the 
individual  oils.  Nature  has  failed  to  complete  the  so-called  drying 
oils,  leaving  them  unbalanced  (or  as  the  chemists  put  it,  "  unsatis- 
fied"), while  the  drying  oils  are  complete  chemical  combinations 
with  their  chemical  "  affinities  "  satisfied.  The  drying  oils  are  there- 
fore ready  to  take  on  additional  atoms  to  make  a  complete  or  satisfied 
compound.  When  hydrogen  is  added,  as  in  the  hydrogenation 
process,  we  obtain  a  fixed  oil  or  fat  (often  edible),  and  when  oxygen 
is  added,  as  in  the  ordinary  process  of  "  drying,"  we  obtain  the 
familiar  pain  or  varnish  film. 
The  absorption  of  oxygen  by  linseed  and  other  drying  oils  is 
progressive  and  continues  long  after  the  oil  would  be  called  "  hard 
dry"  by  the  painter,  although  very  slowly.  The  dried  film  of  lin- 
seed oil  is  known  as  linoxyn  and  contains  about  16  per  cent,  by 
weight  of  oxygen  which  has  been  added  in  the  drying  process,  plus, 
of  course,  the  oxygen  originally  in  the  oil's  composition.  This  in- 
crease in  weight  occurs  in  all  drying  oils,  when  they  are  exposed 
to  the  air  (which  consists  one  fifth  of  oxygen);  and  in  some  is 
greater  than  others. 
The  fats,  waxes  and  the  non-drying  oils  cannot  absorb  oxygen 
because  they  are  "  satisfied  "  compounds  to  start  with ;  while  the. 
name  of  the  semi-drying  oils  explains  their  position  as  intermediate 
between  the  drying  and  the  non-drying  oils. 
In  order  to  determine  the  relative  values  of  various  drying  oils 
we  have  to  determine  the  percentage  of  oxygen  which  they  will 
absorb,  the  greater  the  oxygen  absorption,  the  higher  the  siccative 
or  drying  power  of  the  oil. 
Now  it  is  absolutely  impracticable  to  add  gaseous  oxygen  directly 
to  a  liquid  oil  and  get  it  to  combine  in  a  way  that  can  be  measured. 
This  difficutly  has  been  solved  by  dissolving  the  oil  in  chloroform 
and  adding  a  known  excess  quantity  of  iodine  dissolved  in  acetic 
acid  to  the  oil.  The  (oil  acts  upon  the  iodine  and  combines  with  it, 
just  as  it  does  with  oxygen,  but  much  more  rapidly. 
After  a  definite  length  of  time,  the  chemist  determines  how  much 
iodine  is  free;  that  is  to  say,  has  not  combined  with  the  oil;  the 
difference  between  this  and  the  total  quantity  of  iodine  brought  in 
contact  with  the  oil  represents  the  quantity  added  to  or  absorbed 
by  the  oil. 
This  so-called  "  iodine  number "  or  "  iodine  value,"  therefore. 
