A  January Pi9oo.m'}     Contamination  of  Drinking  Water.  31 
In  structure,  a  diatom  is  not  unlike  a  minute  glass  box,  for  it  is 
made  up  of  two  halves,  one  fitting  tightly  within  the  other,  and 
having  its  walls  strongly  silicified.  It  is  this  silica  which  makes  the 
diatomaceous  earth  valuable  for.  polishing  powders.  In  at  least  one 
article  on  the  market  diatom  shells  form  a  considerable  part  of  a 
tooth  powder,  and  the  bad  effect  of  such  hard  material  on  the  teeth 
is  so  obvious  that  it  would  be  well  for  every  one  to  have  a  micro- 
scopical examination  made  of  any  powder  to  be  used  in  this  way. 
Earth  containing  the  remnants  of  diatoms  is  also  used  extensively 
in  the  manufacture  of  dynamite,  and  the  living  marine  forms  consti- 
tute a  valuable  part  of  the  food  of  some  fishes. 
These  plants  have  a  peculiar  method  of  vegetative  multiplication 
which  is  unlike  anything  found  elsewhere  among  the  algae.  The 
two  halves  of  the  "  box,"  which  are  called  valves,  begin  to  separate 
slightly  from  each  other,  and  as  the  contents  divides  into  two  part?, 
there  is  formed  within  two  new  halves,  one  fitting  into  the  larger 
half  of  the  original  cell,  and  the  other  forming  a  new  box  with  the 
smaller  half  of  the  parent  plant.  These  then  separate,  and  thus 
there  are  formed  two  diatoms  of  exactly  the  same  construction  as 
the  mother  cell,  although  one  is  a  trifle  smaller  than  the  other.  In 
addition  to  this  method  of  propagating  the  species,  there  are  various 
ways  by  which  the  plant  forms  a  single  large  resting  spore ;  and 
recently  it  has  been  discovered  (chiefly  through  the  work  of  Cas- 
tracane  and  Murray)  that  it  is  probable  that  the  whole  contents  of 
a  diatom  cell  may  break  up  into  a  number  of  small  spores,  each  one 
of  which  develops  into  a  new  plant. 
There  are  only  a  few  species  which  are  known  to  give  rise  to 
serious  trouble  in  water  supplies,  but  these  occur  quite  frequently 
and  in  great  quantities.  Usually  the  infected  water  has  an  aromatic 
odor,  variously  described  as  resembling  fish  or  geraniums,  and  the 
taste  is  disagreeable  enough  to  render  it  quite  unfit  for  use.  In 
addition  to  this  effect,  however,  diatoms  are  extremely  troublesome 
when  contained  in  water  to  be  used  for  the  manufacture  of  paper 
or  for  laundry  purposes,  because  of  the  greenish-brown  coloring 
matter  they  contain,  and  which  stains  articles  coming  in  contact  with 
it.  Whipple  has  observed  that  the  growth  of  diatoms  seems  to 
depend  upon  certain  definite  conditions  of  the  water;  that  is,  they 
do  not  develop  when  the  bottom  of  the  pond  or  reservoir  is  quiet, 
but  in  spring  and  fall,  when  the  rising  or  lowering  temperature 
