Amjine"'I87h6arm*}  Powdered  Drugs  under  the  Microscope.  243 
The  color  of  starch  is  usually  pure  white.  In  some  cases  a  tinge  of 
blue  can  also  be  seen,  as  in  some  wheat-starch,  while  the  starch  from 
the  potato  has  a  slightly  yellowish  cast.  The  fineness  of  the  starch 
powder  depends  on  the  size  of  the  individual  grains,  except  where  the 
grains  are  artificially  agglomerated,  as  in  sago  and  tapioca  ;  the  former 
is  in  small,  round,  brownish  grains,  while  tapioca  is  in  larger,  irregular, 
white  fragments.  The  individual  starch  grains  vary  much  in  size, 
though  they  are  pretty  constant  for  any  given  species.  The  various 
sizes  will  be  described  in  the  special  descriptions  to  follow  ;  here  it  is 
sufficient  to  say  that  oat,  rice  and  rye  starches  represent  the  smallest 
grains,  while  those  of  corn  starch  are  not  much  larger.  The  medium 
size  is  found  in  the  grains  of  wheat  and  arrow-root  starch.  The 
largest  grains  are  found  in  potato  and  canna  starch.  In  the  last  two 
the  unaided  eye  can  distinguish  the  largest  individual  grains,  but  in 
nearly  all  others  they  can  be  seen  only  under  the  microscope.  The 
specific  gravity  is  more  than  that  of  water,  though  it  varies  much  with 
the  state  of  dryness  of  the  starch.  Its  average  is  given,  by  Wiesner, 
at  1*5.  It,  however,  varies  with  the  different  species  of  starch.  The 
grains  contain  considerable  water — as  high  as  30  per  cent,  when  fresh, 
reduced  sometimes  to  7  per  cent.,  when  air  dry. 
The  grains,  when  exposed  under  the  microscope  to  hot  water 
(1400  F.  or  above)  swell,  then  burst,  and  finally  form  a  homogeneous 
paste.  The  same  process  occurs,  and  can  be  easily  observed,  micro- 
scopically, when  they  are  tested  with  dilute  alkalies  or  mineral  acids. 
They  are  insoluble  in  cold  water,  alcohol  and  ether.  The  grains  turn 
blue  when  brought  in  contact  with  a  solution  of  iodine,  a  simple  solu- 
tion of  iodine  in  water  is  sufficient  for  this  very  delicate  test.  This  is 
a  very  satisfactory  test  for  microscopical  use,  as  it  will  betray  at  once 
the  smallest  proportion  of  starch  in  the  tissue  under  examination.  The 
iodine  solution  can  be  applied  in  this  way  :  the  tissue  or  powder  to  be 
examined  is  placed  on  the  glass  slide  in  a  drop  of  water,  and  a  thin 
glass  cover  is  placed  over  it ;  it  is  then  placed  under  the  microscope 
and  focussed,  the  stage  of  the  microscope  being  tilted  a  little,  if  prac- 
ticable ;  a  drop  of  the  iodine  solution  is  then  placed  on  the  slide,  at  the 
upper  edge  of  the  cover.  It  will  gradually  spread  in  the  water  and 
through  the  tissue  examined  ;  meantime  the  eye  at  the  microscope 
watches  the  changes  it  causes.  Should  it  not  draw  through  readily, 
a  bit  of  blotting  paper,  placed  at  the  opposite  edge  of  the  glass  cover, 
