AfffOfSO  +  (H20  +  Aq)  =

Ag-Ag  +  (H2=02=S02  +  Aq).  [275]

245.  Hyposulphites.  —  Hyposulphurous  acid  has  never  been
isolated ;  but  several  hyposulphites  may  be  obtained  by  passing
a  stream  of  S02  through  solutions  of  the  corresponding  sul-
phides, or  digesting  a  solution  of  the  sulphite  on  powdered
sulphur.
8  +  (Na2=02=SO  +  Aq)  =  (NafOf(S-0-S)  +  Aq.  [276]

Calcic  hyposulphite  is  formed  spontaneously  in  large  quanti-
ties, both  in  the  refuse  lime  taken  from  the  purifiers  of  the  gas-
works, and  in  the  refuse  after  the  lixiviation  of  the  black-ball  at
the  alkali  works,  and  from  this  source  sodic  hyposulphite  is
now  obtained.  It  is  the  only  hyposulphite  of  practical  value,
and  is  not  only  used  in  photography,  but  also  for  removing  the
last  traces  of  chlorine  from  the  bleached  pulp  used  in  paper-
making,  and  in  the  treatment  of  silver  ores.
246.  Sulphuric  Anhydride.  S03.  —  Soft,  white,  silky-looking
crystalline  solid,  melting  at  25°,  and  volatilizing  at  35°.     May
be  obtained  either  by  distillation  from  the  Nordhausen  acid  or


316  SULPHUR.  [§247.
from  sodic  disulphate,  or  else  by  passing  a  mixture  of  S02  and
0=0  through  a  heated  tube  filled  with  platinum  sponge.
02=(S02-  0  -S02)  =  Bf  02=S02  +  ^©3.       [277]
02=(S02-0-S02]  =  Na2=  OfSOt  +  SCDj.     [278]
2^©2  +  ©=©  =  2S©8.  [279]

It  unites  with  many  metallic  oxides  to  form  sulphates,  and
baryta  burns  in  its  vapor.
BaO  +  ^©3  =  BaO,SO3.  [280]

It  has  an  intense  affinity  for  water,  and  the  heat  developed  by
the  union  is  so  great  that  the  solid  hisses  like  red-hot  iron  when
dropped  into  the  liquid.  The  product  is  common  sulphuric
acid.
