106  Methods  of  Water  Analysis.  {^Sam111, 
The  Beverley  water-supply,  which  became  polluted  with  infected 
sewage  from  an  asylum,  giving  rise  to  a  typhoid  epidemic,  was  pro- 
nounced by  the  chemist  to  be  "  of  a  very  high  degree  of  purity,  and 
eminently  suitable  for  drinking  and  domestic  purposes." 
Analysis  of  water  from  the  sewage-polluted  Trent  showed  that 
"  there  is  no  evidence  of  the  product  of  sewage  contamination." 
The  well-water  supplying  Houghton-le-Spring  became  contami- 
nated with  sewage  from  a  farm, causing  a  sudden  outbreak  of  typhoid 
fever.  The  chemist  who  analyzed  the  water  reported  that  "  this 
water  is  very  free  from  indications  of  organic  impurity.  ...  It 
is  a  good  water  for  drinking  purposes." 
The  reason  for  this  evident  failure  on  the  part  of  the  chemist  to 
detect  dangerous  pollution  is  not  difficult  to  find.  A  generally  pure 
water  may  become  contaminated  with  an  amount  of  sewage  too 
small  to  give  evidence  of  its  presence  when  diluted  with  several 
million  gallons  of  water,  yet  this  small  amount  of  sewage  may  con- 
tain numerous  specific  germs  the  presence  of  which  cannot  be  detected 
by  a  chemical  analysis.  Again,  the  sewage  may  have  undergone 
complete  oxidation  and  the  end  products  taken  up  by  the  plants, 
leaving  no  perceptible  evidence  of  the  pollution,  while  many  of  the 
specific  germs  which  may  have  been  present  in  the  original  sewage 
remain  viable  and  capable  of  causing  disease. 
Before  leaving  this  phase  of  the  subject,  I  wish  to  point  out  the 
value  of  chemical  analysis  in  comparing  different  waters  in  the  same 
locality  or  a  certain  water  at  different  times.  In  this  connection, 
the  data  obtained  by  a  chemical  analysis  are  both  accurate  and 
valuable.  Also  in  the  study  of  filtration,  especially  of  the  slow-sand 
type,  chemical  analysis  of  the  raw  water  and  effluent  made  from 
time  to  time  furnishes  valuable  evidence  of  the  efficiency  of  the  filter 
in  removing  turbidity  and  color,  and  bringing  about  the  nitrification 
of  organic  matter  which  is  the  essential  feature  of  this  process  of 
water-purification. 
BACTERIOLOGICAL  EXAMINATION. 
With  the  advent  of  bacteriology,  and  especially  after  the  intro- 
duction of  Koch's  plate  method  of  isolation  of  bacteria,  the  hope  of 
the  sanitarian  had  been  revived.  It  was  supposed  that  at  last  we 
have  a  method  by  means  of  which  we  may  detect  the  specific  causes 
of  disease  in  water,  and  thus  place  the  examination  of  water  on  the 
