NOTE  ON  COLLODION. 
105 
the  taste  ;  nor  do  they  resemble  each  other  very  closely  if  com- 
pared. Soap  bark  is  more  fibrous ;  the  inner  surface  is  seldom 
so  white,  it  has,  in  fact,  when  old,  a  decided  brownish  tinge ; 
the  outer  bark  is  much  thicker  and  rougher  and  has  a  reddish 
brown  color,  not  grey.  Another  remarkable  characteristic  of 
Soap  bark  is  the  appearance  of  minute,  glistening  crystals, 
which  cover  the  whole  fibrous  structure,  and  are  seen  when  it  is 
fractured.  Its  acrid  taste,  and  the  frothing  of  its  infusion,  are 
highly  characteristic. 
NOTE  ON  COLLODION. 
By  William  Procter,  Jr. 
Notwithstanding  so  much  has  been  written  on  the  subject  of 
collodion,  and  it  has  become  an  officinal  preparation  many  con- 
tinue to  find  difficulty  in  obtaining  a  gun  cotton  that  will  dis- 
solve readily  and  produce  each  time  a  uniform  preparation  of 
equable  consistency.  Usually  the  Pharmacopoeia  process,  strictly 
followed,  affords  a  cotton  of  which  the  greater  part  is  taken  up 
by  alcoholic  ether,  leaving  an  undissolved  residue  which  needs 
to  be  strained  out.  Sometimes  it  happens  that  the  cotton  loses 
its  fibrous  texture  and  coalesces  into  a  gummy  mass,  appearing 
like  tragacanth  mucilage,  which  does  not  properly  dissolve. 
Again  it  will  be  like  quince  mucilage  in  consistence,  whilst  fre- 
quently a  perfect  solution  is  obtained.  The  action  of  nitric 
acid  on  cotton  is  now  well  understood  to  be  the  substitution  of 
nitrous  acid  for  hydrogen,  or  perhaps  nitric  acid  for  water  in 
the  formula  of  cotton.  Soluble  cotton  is  considered  to  be  less 
nitricised  than  the  true  gun-cotton,  the  formula  of  which,  ac- 
cording to  Porret  and  Teschemacher  (Gregory's  Chemistry, 
fourth  edition),  is  C12  H8  O8  +  4N05  or  C12  H8  O12  4NO*. 
Acording  to  this  formula  two  equivalents  of  water  are  replaced  by 
two  eq.  of  NO5,  and  then  two  other  eqs.  of  NO5  are  taken  into 
the  composition  of  the  gun-cotton  :  so  that  the  anhydrous  cotton 
is  to  the  nitric  acid  as  144  to  216.  Other  chemists  have  given 
the  formula  for  gun-cotton  with  a  less  proportion  of  nitric  or  of 
nitrous  acid.  The  soluble  cotton  has  been  stated  to  contain  3 
equivalents  of  NO5  to  2  equivalents  of  anhydrous  cotton  C2i  H17 
O17,  3N04  (Gladston  in  Pharm.  Jour,  xi.,  481).  Be  this  as  it  may, 
