364 
WHAT  TO  OBSERVE  IN  LICHENS. 
cipitation  of  Prussian  blue  is  prevented,  I  found  that  not  only 
was  this  the  case,  but  also  that  if  the  Prussian  blue  were  first 
precipitated,  the  addition  of  oxalate  of  ammonia  caused  it  to  re- 
dissolve.  I  therefore  prepared  a  quantity  of  Prussian  blue,  which 
was  well  washed  by  decantation,  and  afterwards  on  a  filter.  A 
saturated  solution  of  oxalate  of  ammonia  dissolved  a  considerable 
quantity  of  it  by  the  aid  of  heat,  and  yielded  a  clear  greenish- 
blue  solution.  On  being  evaporated  and  set  aside,  numerous 
crystals  of  oxalate  of  ammonia  were  formed,  but  at  the  same 
time,  at  the  edges  of  the  solution,  were  minute  dark  blue  crystals, 
of  which  a  larger  quantity  was  obtained  by  evaporating  the 
mother  liquor.  They  were  quite  soluble  in  water,  and  the  addi- 
tion of  a  drop  of  acid  caused  a  copious  precipitate  of  Prussian 
blue.  The  quantity  I  obtained  altogether,  however,  was  not 
large  enough  to  enable  me  to  experiment  further  upon  it,  but  I 
am  preparing  the  solution  on  a  larger  scale  for  that  purpose  
Qhem.  Neivs,  London^  Feb.  9,  1861. 
WHAT  TO  OBSERVE  IN  LICHENS. 
There  are  no  plants  so  variable  in  character  as  the  lichens ; 
none  in  which  it  is  consequently  so  difficult  to  decide  w'hat  are 
species  and  what  are  varieties.  In  order  to  a  comprehensive 
knowledge  of  species,  it  is  necessary  to  study  individuals  in  every 
condition  of  growth  and  from  every  possible  habitat.  Hence,  the 
commonest  species  and  varieties  become  of  value — the  more  so  if 
collected  in  countries  comparatively  unexplored  botanically,  for 
lichens  are  no  exceptions  to  the  rule  that  geographical  differ- 
ences are  attended  by  corresponding  differences  in  the  characters 
of  the  same  plants.  Every  collector  of  lichens — however  common 
and  well-known  the  latter  may  be — may  therefore  consider 
himself  as  contributing  towards  a  more  scientific  and  philosophi- 
cal, because  more  comprehensive,  knowledge  of  a  very  Protean, 
but  interesting,  group  of  plants.  If  the  collector  make  a  point 
of  gathering  specimens  of  everything  he  meets  which  belongs 
to  the  lichen  family,  he  will  run  a  good  chance  of  including  some 
novelties,  perhaps  new  species  or  varieties.  New  species  are 
most  likely  to  be  met  with  among  the  very  minute  crustaceous 
