A^anuar3%Pimm'}     Contamination  of  Drinking  Water.  27 
Certain  of  this  latter  kind  can  frequently  be  observed  in  quiet  or 
stagnant  pools,  and  are  popularly  termed  "  pond  scum,"  "  frog 
spawn,"  etc.  It  is  not  to  these  easily-discerned  plants,  however,  to 
which  we  usually  have  to  look  for  the  cause  of  the  pollution  of  most 
water  ;  while  they  may  cause  considerable  damage  and  trouble  in  a 
mechanical  way,  and  it  is  generally  the  desire  of  the  engineer  to  keep 
them  out  of  his  reservoir,  they  are  probably  a  benefit  rather  than  a 
detriment  so  long  as  they  are  in  active,  healthy  condition.  It  has 
been  found  that  volatile,  fatty  acids,  such  as  butyric  and  valerianic, 
together  with  glucose,  leucin,  tyrosin  and  even  urea,  when  properly 
diluted,  can  be  assimilated  by  such  plants,  and  no  doubt  a  consider- 
able number  of  organic  substances  which  are  carried  into  rivers  by 
drains  are  destroyed  by  the  larger  green  algae  and  diatoms.  It  must 
be  remembered,  however,  that  a  good  share  of  this  work  is  accom- 
plished by  the  so-called  u  water  bacteria,"  which  are  everywhere 
present. 
The  group  of  plants  which  is,  perhaps,  best  known  as  polluting 
drinking  water  is  the  one  containing  the  forms  popularly  called  the 
"  blue-green  algae."  Whether,  in  the  technical  sense,  they  really 
are  algae  is,  perhaps,  a  question,  as  they  are  much  simpler  in  struc- 
ture than  the  true  algae,  and  show  a  great  resemblance  to  the  bac- 
teria, both  vegetatively  and  in  their  methods  of  reproduction.  This 
is  so  evident  that  the  blue-green  forms  have  been  put  with  the  bac- 
teria into  a  separate  group  known  as  the  Schizophyta,  and  termed 
Schizophycece,  while  the  name  Schizomycetes  has  been  applied  to  the 
bacteria.  As  the  common  name  denotes,  these  plants  are  usually  of 
a  bluish-green  color,  but  this  is  not  universally  true,  for  they  may 
assume  various  shades  of  olive,  yellow  and  brown,  even  appearing 
chocolate  or  purplish-red  at  times.  The  blue-green  shades  are 
brought  about  mostly  by  various  mixtures  of"  leaf-green,"  or  chloro- 
phyll, with  a  substance  of  a  nitrogenous  or  proteid  nature,  known  as 
phycocyanin.  Sometimes  there  is  a  coloring  matter  in  the  gelati- 
nous sheath  of  the  plant  called  scytonemin,  which  is  responsible 
for  the  reddish,  violet  and  brown  shades,  but  in  addition  to  this  it 
seems  possible  that  a  few  forms  contain  numerous  gas  vacuoles, which 
line  the  inner  walls  of  the  cells,  producing  an  optical  effect'  which 
makes  them  appear  red  or  chocolate.  This  is  probably  the  cause 
of  the  color  of  Trich odesmium  erythraam,  which  gives  the  name  to 
the  Red  Sea,  and  of  certain  oscillatorias  in  this  country  which  cause 
ponds  and  pools  to  become  a  deep  chocolate  brown. 
