SO^  -f  Sn  '=  SnS  +  HHH.  [243]

Although  the  sulphur  is  removed  by  the  tin,  the  volume  of  the
gas  does  not  change.  Hydric  sulphide  is  not  unfrequently
formed  in  nature  from  calcic  sulphate,  which  in  contact  with
decaying  animal  or  vegetable  matter  loses  its  oxygen,  when
the  carbonic  acid  of  the  atmosphere,  acting  on  the  resulting
calcic  sulphide,  sets  free  the  compound  in  question.  It  is  thus
that  the  soluble  sulphides  in  many  mineral  springs  probably
originate.
Hydric  sulphide  is  one  of  the  most  important  chemical  re-
agents, and  is  used  to  convert  into  sulphides  various  metallic
hydrates  and  other  salts.
§239.]  SULPHUR.  311

1.  Action  on  alkaline  hydrates.
(K-0-H+  H2S  +  Aq)  —  (K-S-H+  ff20  +  Aq).  [244]

(K-S-H+  K-0-H+  Aq)  =  (K2S  +  H20  +  Aq).  [245]

Thus  may  also  be  formed  Na-Hs,  Na2=S,  NH^-Hs,


(38.)

2.  Action  on  salts  of  the  more  electro-negative  metals.
(CdS04  +  ff2S+Aq)  =  CdS+  (H2SO,  +  Aq).  [246]
So  also  may  be  precipitated  from  acid  solutions  of  their  salts


Sb2S3,       Sb2S5,       SnS,       SnS*       PtS2,       Au4S»

Yellow.  Bed.  Orange.  Brown.  Yellow.  Brown.  Black.
all  of  which  are  soluble  in  alkaline  sulphides,  and

CdS,       CuS,       J3i2S3,      Ag2S,        HgS,      [fffrlS,      PbS,

Yellow.  Black.  Black.  Black.  Black.  Black.  Black.
all  of  which  are  insoluble  in  alkaline  sulphides.
3.  Action  on  salts  of  the  more  electro-positive  metals.     The
following  sulphides,  although  not  precipitated  from  acid  solu-
tions, are  precipitated  when  sufficient  ammonia  is  added  to  neu-
tralize all  the  acids  present,  or  when  an  alkaline  sulphide  is
used  in  place  of  ffaS.
ZnS,  MnS,  FeS,  NiS,  CoS.
White.  Pink.  Black.  Black.  Black.
