Am'ju°ner;i887arm'}    Fluorescent  Principle  of  Wild  Cherry  Bark.  287 
continuing  the  percolation  for  some  time  it  was  observed  that  each 
new  portion  of  menstruum  poured  on  the  top  of  the  column  in  the 
percolator  was  instantly  pervaded  by  the  fluorescent  tint  without  ap- 
parent diminution  of  intensity.  This  indication  pointed  to  the  fact 
that  an  almost  vanishing  quantity  of  the  fluorescing  body  could  pro- 
voke its  characteristic  phenomenon  and  also  that  its  total  amount  con- 
tained in  the  bark  was  perhaps  incomparably  small,  but  difficult  of 
extraction. 
On  treating  the  first  half  of  the  percolate  with  diplumbic  acetate  a 
voluminous  light-brown  precipitate  resulted  leaving  with  a  sufficiency 
of  the  reagent  a  colorless  solution,  in  which,  however,  the  fluorescence 
appeared  undiminished.  The  second  half  of  the  percolate  was  much 
darker  brown  than  the  first  half,  and  also  possessed  a  very  decided 
predominance  of  free  ammonia.  On  treatment  with  diplumbic  acetate 
until  the  solution  became  colorless,  a  dark  green  precipitate  resulted. 
The  filtrate  off  this  showed  an  unchanged  fluorescence.  It  was  dis- 
tinctly ammoniacal,  but  absolutely  free  from  lead.  Treated  with 
mercuric  chloride  in  excess,  a  dingy  yellow  precipitate  resulted  whilst 
the  solution  ceased  to  fluoresce.  The  filtrate  gave  with  ammonia  a 
white  precipitate  of  mercurammonium  chloride  and  a  non-fluorescent 
liquor.  This  result  showed  that  the  fluorescing  body  was  now  all 
contained  in  the  yellow  mercuric  chloride  precipitate.  This  precipi- 
tate, after  washing,  readily  dissolved  in  ammonia  to  a  colorless  solu- 
tion only  faintly  fluorescent.  On  spontaneous  evaporation  abundant 
crystals  were  obtained.  These  remained  unaffected  by  either  chloro- 
form or  ether  but  readily  dissolved  in  alcohol,  which,  after  dissipation, 
left  them  in  their  original  form.  Viewed  under  the  microscope  with 
a  half-inch  power,  these  crystals  were  seen  to  be  well  defined  aggre- 
gates of  slender  columns  intercrossing  at  right  angles.  In  such  por- 
tions of  the  field  where  the  accumulation  was  sparser  the  square  ar- 
rangement had  degenerated  into  curves,  owing  probably,  to  inequali- 
ties in  the  surface  of  the  slide.  The  crystals  were  void  of  polares- 
cence.  The  writer  here  uses  the  term  polarescence,  as  indicating  the 
property  of  differentiating  polarized  light.  Of  course,  the  meaning 
of  the  term  stands  in  no  relation  to  fluorescence.  The  color  effect  in 
polarized  light  is  due  to  double  refraction  and  subsequent  interference 
which  is  greatly  aided  by  thin  plates  of  selenite  and  mica.  But 
fluorescence  is  an  absorption  phenomenon. 
Another  portion  of  the  fluorescent  filtrate  from  the  lead  precipitate 
