280  Microscopical  Examination  of  Powders.  {Amjune^'Sfrm 
he  found  that  the  oil  merely  brightens  the  reddish  yellow  color  of 
the  pure  rhubarb,  without  extracting  it,  while  the  smallest  speck  of 
turmeric  was  surrounded  by  a  broad  halo  of  bright  yellow,  besides 
acquiring  itself  a  purely  yellow  color.  This  will  be  noticed  already 
on  mixing  the  powder  with  the  oil  on  the  slide. 
Silicate  of  sodium  as  a  medium. — The  writer  has  found  soluble 
glass  (water  glass)  to  be  an  excellent  medium  for  permanent  mounts, 
possessing  several  advantages.  It  clears  well  and  dries  ("  sets  ") 
very  quickly.  Scarcely  fifteen  minutes,  after  having  adjusted  the 
cover  glass,  the  slide  may  be  scrubbed  with  a  nail  brush,  without 
dislodging  the  cover.  "  Ringing  "  is  not  necessary.  Its  disadvan- 
tage is  that  sooner  or  later  flakes  appear  here  and  there  in  the 
mount,  this  may  be  obviated  by  adding  glycerin  in  the  proportion 
of  I  volume  of  glycerin  to  4  volumes  of  soluble  glass  (but  then  it 
takes  a  longer  time  to  dry) ;  the  mixture  at  first  quite  turbid,  clears 
very  soon.  Another  disadvantage  is  that  after  some  time  it 
becomes  next  to  impossible  to  remove  the  cover  glass,  and  when 
removed,  the  slide  will  be  found  roughened.  Soluble  glass  is 
incompatible  with  acids  (even  very  weak),  alcohol,  ethereal  liquids, 
collodium,  essential  oils,  carbolic  acid,  gum  arabic  mucilage,  all  of 
which  precipitate  the  silicic  acid  in  the  well-known  gelatinous 
form.  Its  alkalinity  will,  of  course,  cause  it  to  alter  the  shade  of 
most  of  the  stains — carmine,  for  instance,  gets  an  orange  shade, . 
and  the  purplish-blue  color  of  hematoxylin  stain  turns  sepia-brown 
— and  color  lignified  tissue  more  or  less  yellow ;  but  this  is  not 
exactly  a  disadvantage. 
Dark  colored  powders  may  be  rendered  a  good  deal  lighter  in 
color  (some  quite  bleached)  by  a  24  hours'  previous  maceration  in 
moderately  strong  water  of  ammonia  and  subsequent  washing  with 
water  ;  as  far  as  can  be  judged,  no  alteration  beyond  the  removal 
of  color  takes  place,  not  even  the  individual  starch  grains  are 
altered  by  this  treatment. 
Fineness  of  Powder. — In  conclusion  the.  writer  would  call  atten- 
tion to  the  fact  that  the  three  different  degrees  of  powder  in  com- 
merce— very  fine,  fine  and  moderately  coarse — quite  seldom  give 
identical  slides.  In  "  very  fine  "  are  often  found  structures  which 
are  mostly  wanting  in  "  fine  "  and  especially  in  "moderately  coarse" 
and  vice  versa.    There  are,  though,  several  firms  who  make  a  point 
