PHYSICAL  CHARACTERISTICS  OF  OFFICINAL  POWDERS.  205 
I  then  tried  plain  filters  off  the  same  sheet  against  each  other 
in  like  funnels  ;  usually  the  results  varied  but  a  few  per  cent., 
though  sometimes  much  more  ;  the  greatest  difference  noticed 
was  2  to  1 ;  several  times  the  results  corresponded  exactly  on  re- 
peated runs  of  500  cubic  centimetres. 
Experiments  were  made  to  determine  the  difference  in  effi- 
ciency between  the  single  and  the  triple  sides  of  filters.  No.  1, 
had  its  triple  side  covered  with  paraffin,  leaving  the  single  side 
free.  No.  2  had  the  triple  side  free,  while  the  single  was 
covered ;  with  paraffin  the  result  was  175  :  100  ;  glycerin  was 
then  tried  with  the  result  200  :  100,  showing  the  additional 
paper  considerably  retarded  the  flow. 
I  thought,  since  the  adhesion  of  the  water  to  the  glass  is  the 
cause  of  slow  filtration,  I  might  increase  the  flow  by  coating  the 
funnels  on  the  inside  with  paraffin,  to  which  water  does  not 
adhere.    No.  1,  being  coated,  No.  2,  left  clean,  I  got 
i.     ii.  r.  ii. 
1st  trial,       200  :  100  3d  trial,       100  :  100. 
2d    "  84  :  100  4th  "  13T  :  100 
The  filters  in  the  third  and  fourth  trials  were  the  same,  but  the 
funnels  were  changed  about. 
The  outside  or  skeleton  filters,  above  described,  may  be  cut 
the  same  size  as  the  inner  filter  ;  if  much  smaller  the  upper  part 
of  the  inner  filter  clings  to  the  glass;  if  larger  a  part  of  the 
precipitate  is  liable  to  adhere  to  the  outer  filter,  and  even 
with  great  care  a  part  of  the  precipitate  would  creep  up  and 
be  lost. 
Boston,  Feb.  1868. 
ON  THE  PHYSICAL  CHARACTERISTICS  OF  THE  OFFI- 
CINAL POWDERS. 
By  Clemmons  Parrish. 
[An  Inaugural  Essay,  presented  to  the  Philadelphia  College  of  Pharmacy.] 
The  new  feature  of  the  Pharmacopoeia  of  the  United  States, 
by  which  powders  are  classified  with  reference  to  their  degree  of 
fineness,  has  not,  I  believe,  been  the  subject  of  much  experiment 
with  a  view  to  determine  the  actual  physical  condition  of  differ- 
ent drugs  when  brought  to  the  officinal  standard  of  pulverization, 
