Hydrogen  Persulphide,  H2S2(?). — The  sulphides  of  cer-
tain metals,  particularly  the  so-called  alkali  metals,  so-
dium and  potassium,  combine  with  sulphur  to  form  the
polysulphides,  examples  of  which  are  K2S2,  K2S3,  K2S4,
and  K2S5.  When  these  are  decomposed  with  dilute  acids
compounds  of  hydrogen  and  sulphur  are  formed.  It
has  thus  far,  however,  been  impossible  to  determine
whether  more  than  one  such  compound  is  formed,  for
the  reason  that  there  is  no  means  of  deciding  whether
the  substances  formed  are  chemical  compounds  or  mere
mixtures  of  sulphur  and  some  one  compound  of  sulphur
and  hydrogen.  Hydrogen  persulphide  is  a  liquid  with  a
very  disagreeable  odor.  Just  as  hydrogen  dioxide  de-
composes readily  into  water  and  oxygen,  so  hydrogen
persulphide  decomposes  readily  into  hydrogen  sulphide
and  sulphur.
Concerning  the  constitution  of  hydrogen  persulphide
nothing  definite  is  known.  If  the  constitution  of  hydro-
gen dioxide  is  H-O-O-H,  and  hydrogen  persulphide  is
a  disulphide,  it  seems  probable  that  it  has  the  constitu-
tion H-S-S-H,  but  there  is  no  evidence  bearing  upon
this  point.  The  fact  that  the  sulphides  can  take  up  four
and  only  four  atoms  of  sulphur,  just  as  they  can  take  up
four  and  only  four  atoms  of  oxygen,  taken  together  with
the  general  similarity  between  the  conduct  of  oxygen  and
that  of  sulphur,  suggests  that  these  two  reactions  may
be  of  the  same  kind :

K2S  +  4S  =  K2SS4 ;
K2S  +  4O  =  K2S04.
But  for  reasons  which  will  be  more  fully  considered
under  Sulphuric  Acid  (which  see),  it  is  generally  believed
that  in  this  acid  two  of  the  'oxygen  atoms  are  in  direct
combination  with  sulphur  alone,  while  two  are  in  com-
bination with  sulphur  and  hydrogen.  If  the  polysul-


202  INORGANIC  CHEMISTRY.
