556  Purification  of  Water  Supplies.        { ADecJembe^9oT" 
was  being  lost  in  this  way,  and  it  seemed  worth  while  to  experi- 
ment with  these  beds  and  see  if  it  would  not  be  possible  to  extermi- 
nate the  algal  growth  without  destroying  the  cress.  Consequently, 
a  solution  of  copper  sulphate  (this  metal  being  used  on  account  of 
its  cheapness,  it  having  been  shown  to  be  fully  as  efficient  as  any  of 
the  others)  was  prepared  of  a  strength  of  about  one  to  fifty  million 
parts  of  water,  and  this  was  sprayed  upon  the  algal  mass  in  hopes 
that  it  would  accomplish  the  desired  result.  While  its  use  was 
fairly  satisfactory,  it  was  soon  found  that  the  method  of  application 
would  have  to  be  improved  before  any  success  could  be  attained,  as 
only  that  part  of  the  growth  was  destroyed  which  the  spray  was 
able  to  reach,  and  the  algae  within  the  center  of  the  mass  or  some 
distance  under  the  surface  remained  uninjured.  The  method  was 
then  tried  of  adding  directly  to  the  cress  beds  a  sufficient  amount 
af  copper  sulphate  crystals  to  make  a  solution  of  about  the  same 
strength,  and  this  gave  almost  immediately  most  satisfactory  re- 
sults. In  a  very  short  time  all  of  the  algal  growth  was  extermi- 
nated, and  although  the  first  application  was  made  in  the  fall  of 
1 90 1,  it  has  never  been  necessary  to  apply  copper  more  than  once  a 
year  to  these  beds.  Naturally,  the  degree  of  success  attained  in 
this  work,  while  not  in  any  way  demonstrating  that  a  similar  method 
would  be  efficacious  in  large  reservoirs,  seemed  to  warrant  a  more 
thorough  investigation  of  the  subject,  and  for  this  reason  a  large 
series  of  tests  was  inaugurated  calculated  to  demonstrate  the 
toxicity  of  copper  upon  most  of  the  common  polluting  algae  which 
occur  in  these  supplies.  It  was  soon  found  that  the  toxicity  varied 
very  greatly  for  the  different  plants,  and  that  no  universal  strength 
of  solution  could  be  used  which  would  result  in  the  extermination 
of  all  forms.  However,  the  dilution  necessary  to  kill  any  of  these 
algae  was  so  high  that  it  seemed  to  offer  by  far  the  most  promising 
remedy  which  had  ever  been  devised.  Of  course,  in  hoping  to 
perfect  any  method  of  this  kind,  it  was  necessary  that  the  question 
of  efficiency,  cost  and  harmlessness  to  man  be  kept  prominently  in 
mind.  In  regard  to  the  first  point,  no  amount  of  laboratory  demon- 
stration is  equal  to  the  practical  application  of  a  method  on  a  large 
scale.  For  this  reason,  while  it  might  be  very  interesting  to  give 
you  the  results  of  a  long  series  of  experiments  conducted  on  differ- 
ent organisms  for  the  purpose  of  determining  their  death  points  in 
dilute  solutions  of  copper  sulphate,  I  am  sure  it  would  be  more  con- 
