206     PHYSICAL  CHARACTERISTICS  OF  OFFICINAL  POWDERS. 
The  Pharmacopoeia  does  not  indicate  whether  a  powder  of  a 
specific  grade  is  designed  to  be  composed  of  particles  of  the 
designated  degree  of  fineness  or  coarseness,  or  is  intended  to  be 
a  mixture  of  all  the  powders,  resulting  from  the  process,  which 
will  pass  through  the  prescribed  sieve.  It  could  hardly  have 
been  designed  that  the  powders,  prepared  for  percolation,  should 
consist  of  particles  of  uniform  fineness,  although,  theoretically,  this 
is  the  object  in  view.  The  experiments  upon  which  the  statement 
is  based,  that  a  uniform  degree  of  fineness  in  powders  is  essen- 
tial to  the  most  perfect  exhaustion  by  percolation,  have,  I  believe, 
all  been  made  with  powders  of  uncertain  physical  composition. 
The  investigations  detailed  in  this  Thesis  have  been  chiefly 
directed  to  ascertain  the  difference  between  the  results  of  con- 
tusion and  those  of  trituration,  and  are  calculated  to  prove  this 
obvious  diversity,  and  to  throw  light  upon  the  subject  of  pulver- 
ization generally.  In  order  to  make  these  experiments  corre- 
spond to  the  ordinary  conditions  of  the  dispensing  store,  the 
apparatus  employed  were,  first,  a  quart  iron  mortar  and  pestle 
of  ordinary  shape  ;  second,  a  small  mill  of  usual  construction, 
such  as  are  sold  for  domestic  use  and  commonly  used  in  pharma- 
ceutical stores.  The  sieves  were  of  the  five  grades  prescribed 
by  the  Pharmacopoeia,  viz. :  No.  20,  No.  40,  No.  50,  No.  60 
and  No.  80,  having  respectively  these  number  of  meshes  to  the 
linear  inch.  They  were  five  inches  in  diameter,  with  well  ad- 
justed top  and  receptacle  of  tinned  iron.  The  drugs  were,  first, 
gentian,  suitably  dried  before  weighing ;  second,  the  inner  baik 
of  wild  cherry  ;  third,  decorticated  ginger  ;  fom~th,  ergot ;  fifth, 
Calisaya  bark.  The  contusion  was  accomplished  in  the  ordinary 
way,  removing  the  drug  from  the  mortar  to  the  sieve  at  intervals, 
say  three  times,  until  the  whole  quantity  experimented  with 
passed  the  coarsest  sieve,  care  being  taken  to  observe  uniformity 
throughout  all  the  experiments ;  the  sifted  portions  not  being 
returned  to  the  mortar  after  once  passing  through  the  sieve.  In 
the  use  of  the  mill  the  drug  was  reduced  so  as  to  pass  through 
the  coarsest  sieve  by  two  or  three  grindings  without  previous 
sifting,  the  larger  drugs  being  first  sliced  to  adapt  them  to  the 
mill,  and  the  mill  being  set  to  the  same  point  for  each  operation. 
The  method  pursued  is  believed  to  be  that  most  practiced  in  the 
