264 
ON  THE  STRENGTH  OF  FRENCH  ESSENCES. 
reaction  with  iodide  of  potassium  is  exhibited  in  the  following 
equation  : 
3KI+KO,  2Cr08+7HCl=  P+Cr2  CP+7HO. 
The  action  is  immediate,  and  provided  that  the  solutions  are 
kept  cool,  no  secondary  result  is  formed.  The  precipitated 
iodine  speedily  subsides,  leaving  the  supernatant  liquid  quite 
clear,  though  slightly  colored,  with  a  few  floating  particles  of 
iodine  on  the  surface.  When  the  solution  of  the  bichromate  is 
overdosed  with  the  iodide,  it  becomes  dark  red,  from  a  portion 
of  the  iodine  being  dissolved  by  the  excess  of  the  iodide.  This 
change  of  color  is  useful  as  indicating  when  the  addition  of  the 
iodide  has  been  carried  too  far.* 
ON  THE  ESTIMATION  OF  THE  STRENGTH  OF  FRENCH 
ESSENCES. 
By  Mr.  Thomas  Jackson. 
Wishing  to  estimate  the  comparative  value  of  two  samples  of 
"  Extrait  triple  au  Jasmin"  in  a  more  positive  and  satisfactory 
manner  than  that  founded  on  their  specific  gravities,  or  their  odors, 
and  having  searched  in  vain  through  the  works  at  my  command 
for  some  acknowledged  mode  of  operating,  I  took  into  considera- 
tion the  possibility  of  isolating  the  essential  principles  present, 
modified  by  heat  or  by  combination. 
A  preliminary  experiment,  performed  with  one  drachm  of  es- 
sence added  to  two  ounces  of  a  strong  solution  of  chloride  of  so- 
dium slightly  acidulated,  exhibited  on  the  surface  of  the  liquid  a 
film  of  essential  oil,  reddened  by  the  free  acid,  after  a  night's  re- 
pose. With  the  view  of  obtaining  a  similar  but  more  complete 
result,  I  substituted  chloride  of  calcium  for  common  salt.  Com- 
mercial chloride  of  calcium  was  dissolved  in  rectified  spirit,  and 
the  solution  filtered  and  evaporated  to  dryness.  To  half-an-ounce 
of  essence,  contained  in  a  graduated  tube,  the  dry  chloride  of  cal- 
cium was  added  to  saturation,  then  two  drachms  of  water,  or  as 
much  as  was  required  to  render  the  mixture  sufficiently  fluid  for 
the  essential  oil  to  separate.  The  solution,  previously  to  the  ad- 
dition of  the  water,  was  clear,  but  when  the  water  was  added  it 
*  Those  who  may  desire  the  details  of  this  paper,  are  referred  to  the 
Chemical  Gazette,  vol.  x.  p.  392,  from  whence  it  is  taken. — Ed.  Am.  Jour. 
Pharm. 
