144 
DISINFECTION  OF  NITROGENOUS  MANURE. 
into  constituents  of  fixed  compounds.  But  it  often  happens 
that  a  reagent  that  will  fix  one  gas  will  expel  another.  Lime  is 
in  common  use,  because  at  first  it  mostly  destroys  the  odor  by 
decomposing  the  sulphydric  acid  and  forming  sulphide  of  calcium 
and  water,  II  S  -f  Ca  O  =  Ca  S  +  II  0.  But  the  sulphide 
of  calcium  thus  formed  is  liable  to  be  decomposed  by  carbonic 
acid,  which  is  always  present  in  the  mass.  The  lime  also  fixes 
the  carbonic  acid,  though  the  former  expels  ammonia.  More- 
over, the  lime  may  hasten  the  oxydation  of  the  nitrogen,  form- 
ing nitric  acid,  which  latter  may  either  fix  ammonia  or  expel  car- 
bonic acid.    Thus  lime  alone  cannot  be  an  efficient  disinfectant. 
Again  sulphate  of  lime,  (plaster,)  either  calcined  or  simply 
pulverulent,  is  in  use.  In  this  case  the  sulphuric  acid  unites 
with  the  ammonia  of  the  night-soil,  and  the  lime  with  the  carbo- 
nic acid,  N  H4  0,  C  02  +  Ca  0,  S  03  ==  N  H4  0,  S  03  +  Ca 
0,  C  02  The  lime  also,  as  before,  will  decompose  the  suplhy- 
dric  acid,  but  the  resulting  sulphide  of  calcium  is  still  liable  to 
decomposition.  Calcined  plaster  itself  may  evolve  sulphydric 
acid  by  the  simple  addition  of  water,  owing  to  the  presence  of 
some  sulphide  of  calcium.  Therefore  plaster,  though  better 
than  lime,  can  never  be  an  effectual  deodorizer.  In  the  play  of 
chemical  affinities,  when  either  lime  or  plaster  is  used,  some  of 
the  sulphur  must  alternately  unite  with  hydrogen  and  with  cal- 
cium until  it  escapes  in  sulphydric  acid  at  the  surface.  The 
employment  of  bleaching  salt  (so-called  chloride  of  lime)  is  very 
objectionable  on  account  of  the  chlorine  thus  evolved,  and  the 
consequent  impregnation  of  the  air  with  a  poison  often  more  de- 
leterious than  the  effluvia  which  it  is  intended  to  destroy. 
If,  instead  of  lime  or  its  salts,  we  employ  a  small  portion  of 
the  salt  of  a  heavy  metal,  as  sulphate  of  zinc,  iron,  or  of  copper, 
the  sulphydric  acid  is  decomposed,  as  before,  and  fixed  sulphide 
of  the  metal  is  formed.  This  sulphide  is  not  decomposable  by 
any  substance  present.  Here,  then,  we  have  a  complete  remedy 
for  sulphydric  acid.  Sulphate  of  iron  may  be  the  cheapest  salt 
for  this  purpose,  but  it,  as  well  as  salts  of  copper,  forms  a  black 
sulphide,  sometimes  objectionable  on  account  of  the  color.  The 
zinc  salts,  however,  form  a  white  sulphide,  which  can  never  ren- 
der a  city  smutty,  however  freely  used.  Of  a  solution  of  sul- 
phate of  zinc  (of  35°  to  40°  Baume')  it  is  found  necessary  to 
