2.  The  SILICATED  OAEBON  PILTEE,  in  which
the  medium  is  a  compact  substance,  formed  of
animal  charcoal  and  the  ashes  of  Boghead  coal.
Of  the  many  forms  of  this  filter,  we  may
mention  the  '  Syphon  Filter  for  Travellers ; '
by  means  of  which  wholesome  water  may  be
drunk  from  any  pond  or  stream  by  simply  im-
mersing the  filter  therein  and  drawing  the
water  through  the  tube  by  suction.  Of  the
'Silicated. Carbon  Filter,'  Professor  Wanklyn
says  that  it  will  render  river  water  containing
a  considerable  amount  of  free  and  albuminoid
ammonia  as  pure  as  deep  spring  water.
8.  BisoHOPP's  Patent  Sponsy-ieoh  pil-
TEE. — This  differs  from  one  invented  many
years  ago  by  Dr  Medlock,  in  bringing  the
•water  into  contact  with  spongy  iron  instead  of
thin  iron  rods,  and  thus  effecting  filtration
much  more  rapidly.    Tlledlock  believed  that

'  A  very  similar  filter  to  this  was  invented  by  tiie  late
Mr  George  Uobins,  the  celebrated  auctioneer.  Mr
Robins'  filter  differed  from  the  above  in  havina  a  lid  with
a  hole  in  the  centre  in  which  a  sponge  was  placed ;  an
arrangement  which  by  keeping  back  the  suspended  mat-'
ter  contained  in  the  water,  prevented  the  flltf  r  from  being
'Clogged  up.
•  JB'rankland  and  Byrne  have  shown  that  animal  is
greatly  superior  to  vegetable  charcoal  when  eraplojed  (or
'water-filtera.
the  iron  rods  brought  about  the  oxidation  of
the  nitrogenous  organic  matter  and  its  con-
sequent conversion  into  nitrites  and  nitrates.
Bischoff  states  that  he  has  experimentally
investigated  the  properties  of  spongy  iron,  and
finds  that  it —

a.  Decomposes  even  distilled  water,  which
has  been  previously  boiled.
h.  That  it  reduces  nitric  acid  to  ammonia.
e.  That  the  amounts  of  organic  nitrogen  and
albuininoid  ammonia  are  always  much  reduced
after  filtration  through  spongy  iron.
