PURIFICATION  OF  WATER  SUPPLIED  TO  TOWNS.  433 
In  general  the  wholesomeness  of  water  is  much  more  affected 
by  the  presence  of  organic  substance  than  by  mineral  substance  ; 
and  it  seems  to  be  a  fact  well  established  by  observation,  that 
some  of  the  poisons  producing  epidemic  disease  find  a  congenial 
habitat  in  water  contaminated  with  organic  substance.  More- 
over, when  organic  substance  in  water  undergoes  putrefaction, 
the  sulphates  always  present  in  water  are  decomposed,  and  sul- 
phuretted hydrogen  generated.  The  deleterious  character  of 
the  water  of  the  Niger  was  ascribed  by  the  late  Professor  Da- 
niell  to  this  circumstance. 
The  amount  of  organic  substance  in  water  may  be  very  minute, 
but  it  must  not  on  that  account  be  regarded  as  insignificant. 
The  amount  of  organic  substance  in  the  most  defective  kinds  of 
water  supplied  in  London,  is  very  small  in  proportion  to  the 
mineral  substance ;  but  it  is  generally  considered  by  recognized 
authorities,  that,  under  certain  conditions,  this  organic  substance 
may  acquire  such  a  state  as  to  produce  disease  in  those  drinking 
it  habitually.  In  this  respect  the  cause  of  disease  existing  in 
water  is  analogous  to  that  known  as  sausage-poison,  and  that 
producing  the  frequent  fatal  effects  of  a  cut  with  a  dissecting 
knife,  neither  of  which  appear  to  be  chemically  tangible. 
Investigations  relating  to  the  last  epidemic  of  cholera  have 
shown  that  in  one  district  in  London,  containing  a  population  of 
500,000,  which  were  chiefly  supplied  with  water  by  two  different 
companies,  there  were  over  4000  deaths  from  cholera  during  the 
epidemic.  The  only  recognizable  difference  in  the  conditions 
and  modes  of  the  inhabitants,  was,  that  one  portion  were  sup- 
plied with  water  of  good  quality,  drawn  from  a  point  high  up 
the  Thames  ;  while  the  other  portion  were  supplied  with  water 
drawn  from  a  lower  point  of  the  river,  where  it  was  profusely 
contaminated  with  town-drainage.  It  proved  upon  inquiry  that 
the  mortality  among  the  former  portion  was  37  in  10,000,  while 
among  the  other  portion  it  was  130  in  10,000,  or  three-and-a- 
half  times  as  great  as  in  those  houses  supplied  with  the  better 
water.  Further  inquiry  showed  that  in  the  epidemic  of  1848-49 
the  mortality  was  uniform  throughout  the  district.  There  was 
no  such  difference  between  the  houses  supplied  with  water  by 
the  two  companies,  the  mortality  being  in  one  case  118  and  in 
the  other  125 ;  but  at  that  period  both  companies  drew  their 
28 
