556  Testing  of  Mustard  Oil.  {^"-JoT-iso""' 
It  still  remains  to  be  studied  under  what  conditions  the  formation  of 
the  sulphocyanide  is  most  limited  \  presumably  when  ilie  formation  of 
the  thiosinammin  takes  place  in  the  cold,  which  in  this  case  proceeds 
very  slowly. 
It  would  almost  appear  as  if  allylsulphocyanide  could  be  formed  of 
itself  in  mustard  oil  which  has  been  kept  for  a  long  time,  for  while 
pure  and  fresh  mustard  oil  is  not  changed  by  alcoholic  ferric  chloride, 
I  have  occasionally  observed  with  old  mustard  oil  that  a  red  coloration 
takes  place  upon  the  addition  of  ferric  chloride.  As  is  well  known, 
allylsulphocyanide  boils  at  i6i°,  and  passes  by  simple  distillation  into 
mustard  oil  ;  it  was  therefore  sought  whether  by  the  action  of  a  high 
temperature  the  reverse  reaction  did  not  take  place,  and  the  mustard 
oil  assume  the  property  of  becoming  colored  by  ferric  chloride.  This 
was,  however,  by  no  means  the  case  with  a  specimen  of  mustard  oil 
which  was  heated  in  a  sealed  tube  to  i8o°;  the  oil  became  darker,  but 
mixed,  as  before,  without  change  with  ferric  chloride.  On  the  con- 
trary, the  oil  when  exposed  in  a  sealed  tube  for  three  weeks  to  the 
action  of  sunlight,  assumed  a  darker  color,  showed  on  the  tube  a  brown 
deposit,  and  was  then  colored  red  by  ferric  chloride.  It  remains  a 
peculiar  fact,  that  the  red  colored  compound,  which  is  formed  by  the 
action  of  ferric  chloride  on  old  mustard  oil,  or  on  that  which  has  been 
changed  by  the  action  of  sunlight,  is  insoluble  in  ether,  which  is  not 
the  case  with  the  sulphocyanide  of  iron. 
If  one  will  employ  the  formation  of  the  xanthogenate  of  potassium 
as  a  means  for  the  detection  of  bisulphide  of  carbon  in  mustard  oil, 
the  question  arises,  whether  the  latter  itself  does  not  also  take  part  in 
the  reaction.  This  is  indeed  the  case,  and  with  pure  mustard  oil, 
which  has  been  shaken  with  an  alcoholic  potassa  solution,  a  liquid  is 
obtained  which  may  lead  to  a  deceptive  conclusion,  as  it  also  contains 
■some  potassium  sulphide. 
Indeed  the  behavior  of  the  xanthogenate  of  potassium  to  copper 
salts  is  so  characteristic  that  the  smallest  amounts  of  bisulphide  of  car- 
bon can  be  thus  recognized  ;  but,  if  mustard  oil  be  present  at  the  same 
time,  the  decomposition  products,  proceeding  from  the  latter,  will  also 
give  darkly  colored  precipitates  with  copper  salts.  Dr.  Bertram,  of 
Schimmel  &  Co.,  has  found  that  very  dilute  potassa,  shaken  for  a  very 
short  time  with  mustard  oil,  does  not  furnish  these  deceptive  products 
of  decomposition.  Such  a  solution  is  obtained,  according  to  him,  by 
saturating  absolute  alcohol  with  caustic  potassa,  and  diluting  20  drops 
