'^""jir^'issr'" }        Organic  Matter  in  Potable  Waters.  47 
occurs.  It  is  j)roposed,  therefore,  to  extend  tlie  time  of  oxidation  to  12 
or  24  hours  at  a  temperature  of  20°,  examinations  of  the  amount  of 
oxidation  being  made  at  intervals  of  1,  3,  6  and  9  hours. 
Effect  on  the  Results  of  the  Different  Processes  by  Varying  the  Extent 
of  Dilution  of  the  same  Organic  Substances  in  Water. — Under  this 
heading  we  find  that  the  weaker  the  sohition  the  greater  is  the  loss  of 
carbon  during  evaporation,  but  the  greater  is  the  gain  in  nitrogen  ; 
hence,  when  applying  Frankland's  ratio  C :  N  to  a  dilute  water,  the 
pollution  would  appear  to  be  of  animal  origin ;  but  the  stronger  the 
water  is,  the  greater  will  be  the  tendency  to  refer  the  contamination  to 
a  vegetable  source.  In  reference  to  Wanklyn's  process,  the  weaker 
the  solutions  are,  the  higher  are  the  results  obtained  for  ammonia  in 
both  forms.  The  influence  of  dilution  on  Tidy's  process  is  far  less 
marked,  but  stronger  solutions  require  somewhat  less  oxygen  than  is 
required  by  calculation.  Among  the  special  conclusions  drawn  con- 
cerning Frankland's  process  we  find  that  the  formation  of  sulphuric 
acid  from  the  sulphurous  acid  added  during  evaporation  is  of  more 
frequent  occurrence  than  is  generally  supposed.  The  combustion  pro- 
cess, in  its  present  form,  cannot  be  considered  as  determining  the 
carbon  and  nitrogen  in  water  absolutely,  as  it  is  but  a  method  of 
approximation ;  but  in  many  cases  its  indications  of  organic  carbon 
are  more  valuable  than  those  of  the  permanganate  process,  and  its 
results  for  organic  nitrogen  more  valuable  than  the  indications  afforded 
by  the  albuminoid  ammonia  process.  The  value  of  Wanklyn's  process 
depends  more  on  watching  the  rate  and  progress  of  evolution  of  the 
ammonia  than  upon  the  determination  of  the  total  amount.  The 
results  obtained  by  Tidy's  process  are  liable  to  variation  with  atmos- 
pheric temperature  at  the  time  of  examination,  and  the  amount  of 
oxygen  consumed  is  not  a  measure  of  the  carbon  present.  The  value 
of  the  results  depends,  as  in  the  ammonia  process,  more  on  watching 
the  rate  and  progress  of  the  oxidation  than  on  the  absolute  amount  of 
oxygen  consumed. 
General  Remarks  on  other  Chemical  Determinations. — The  estimation 
of  total  solids  is  liable  to  great  error  because  of  the  large  effect  pro- 
duced by  slight  differences  in  the  dryness  attained,  or  by  atmospheric 
deposition  on  the  platinum  basin.  Waters  containing  a  high  percent- 
age of  nitrates  generally  contain  but  little  ammonia.  Frankland^s 
view  that  nitrates  are  not  found  in  waters  deficient  in  oxygen  is  in 
accordance  with  the  results  now  obtained,  with  a  few  exceptions ; 
nitrates  are  not  always  formed  by  reduction. 
