76  Value  of  Vegetable  and  Animal  Oils.  {^"Ver-xSyf 
i-io  of  an  inch.  As  a  rough  rule,  1°  of  gravity  may  be  subtracted 
for  every  2J  per  cent,  excess  of  temperature  above  60°  F. 
The  presence  of  mineral  and  resin  oils  in  a  mixed  oil  must  be  the 
first  point  proved,  and  vv^hen  it  does  exist,  it  increases  the  difficulty  ot 
testing,  for  w^e  have  no  easy  method  of  separating  them  without  actual 
destruction  of  the  fatty  oils.  Saponification  is  not  efficient,  for  mineral 
oil  unites  with  the  soap  produced,  forming  an  emulsion  which  does  not 
separate  after  standing  for  months.  Perhaps  a  lime  soap  might  be  pre- 
pared, pulverized,  and  the  hydrocarbon  extracted  by  some  volatile  sol- 
vent, but  the  most  satisfactory  method  would  be  an  ultimate  chemical 
analysis. 
In  practice,  however,  mineral  oils  can  be  easily  detected  by  two 
characteristic  tests  :  first,  the  fluorescent  properties  it  imparts  to  all 
animal  or  vegetable  oils  ;  second,  the  strongly-marked  aromatic  burn- 
ing flavor  it  communicates  to  mixtures  containing  it.  The  first-men- 
tioned propertv  is  brought  out  by  smearing  a  metallic  surface,  such  as 
tin  plate  or  steel,  with  the  oil,  and  then  viewing  it  at  different  angles  in 
the  open  air  or  sunlight. 
In  examining  a  dark-colored  oil,  it  mav  first  be  necessary  to  refine 
the  sample  by  successive  treatments  with  concentrated  sulphuric  acid 
and  weak  soda  solution  or  lime  water.  As  small  a  quantity  as  2h  per 
cent,  mav  be  detected  by  the  bluish  color  noticed  on  viewing  the  oil  at 
certain  angles  and  by  tasting  it. 
The  absence  of  resin  oil  must  also  be  proved.  Nitric  acid  is  said  to 
be  a  good  test,  as  the  color  developed  is  much  greater  than  in  pure  oils. 
Sometimes  it  may  be  detected  by  the  smell.  The  presence  of  10  per 
cent,  of  resin  or  mineral  oil  in  non-dr)ing  oils  delays  their  solidification 
with  the  nitrate  of  mercury  test. 
Oils  may  be  classified  according  to  their  commercial  value.  The  first 
class  embraces  only  sperm  oil.  The  tests  recommended  by  Mr.  Cole- 
man, for  adulterations  in  this  oil,  are  five  in  number : 
1.  Examine  for  mineral  oil. 
2.  Examine  into  its  drying  properties  by  exposing  some  of  the  oil  for 
some  hours  in  a  thin  layer  to  200°  F. 
3.  Notice  that  other  ^sh  oils  darken  much  more  notably  than  sperm 
oil  when  shaken  up  with  dilute  sulphuric  acid. 
4.  The  most  likely  adulterant  is  African  fish  oil,  which  produces  in- 
tense heat  when  mixed  with  concentrated  sulphuric  acid  ;  thus,  a  mix- 
ture of  I  part  acid  and  4  parts  oil  develops  about  112°  of  heat,  against 
