98 
ON  SULPHITE  OF  MAGNESIA. 
of  the  two  salts  taken,  and  the  yield  was  much  less  than  that 
stated,  in  the  writer's  hands.  Having  had  the  same  experience 
with  regard  to  saturating  the  carbonate,  suspended  in  water, 
with  sulphurous  acid  gas  a  somewhat  different  plan  was  adopted. 
Five  grammes  Jenning's  Magnesia  (re-calcined)  was  made  into 
a  thick  smooth  paste,  with  10  c.c.  of  distilled  water ;  to  this  was 
added  slowly  with  stirring  102  c.c.  of  aqueous  sulphurous  acid, 
sp.  gr.  1*037.  The  liquid  on  the  surface  now  showed  an  acid 
reaction  and  had  a  yellow  color  ;  after  standing  a  few  minutes 
the  supernatant  liquid  was  decanted;  the  crystals  were  then 
placed  on  a  tared  filter  and  washed  with  distilled  water  until  the 
washings  came  through  colorless,  and  a  small  portion,  on  being 
tested  with  chloride  of  barium,  produced  a  precipitate  which  was 
almost  entirely  dissolved  by  hydrochloric  acid.  The  filter  and 
contents  were  then  dried  at  a  temperature  not  exceeding  100° 
F.,  and  weighed  15-310  grammes,  which  was  3-06  times  the 
weight  of  the  magnesia  used,  or  306*2  per  cent.  ;  the  theoretical 
yield  should  have  been  3*95  times  the  quantity  of  magnesia 
started  with.*  The  loss  is  owing  principally  to  oversaturation, 
as  sulphite  of  magnesia  is  very  soluble  in  aqueous  sulphurous 
acid,  but  it  was  thought  to  be  best  to  secure  the  conversion  of 
the  whole  of  the  magnesia  by  oversaturating,  since  the  excess  of 
sulphurous  acid  can  easily  be  disposed  of  by  decantation,  and, 
after  draining  and  washing,  the  crystals  are  of  course  free  from 
acid.  The  sulphite  of  magnesia  as  obtained  was  in  very  small 
white  crystals,  having  the  peculiar  taste  of  the  sulphites, 
though,  on  account  of  its  insolubility,  the  taste  was  not  so  disa- 
greeable as  that  of  the  more  soluble  sulphites  of  soda  and  po- 
tassa.  The  process  was  tried  on  a  somewhat  larger  scale  and 
succeeded.  Eight  ounces  a  v.  of  Jenning's  Calcined  Magnesia 
was  made  into  a  smooth  paste  with  a  pint  of  distilled  water  and 
aqueous  sulphurous  acid  U.  S.  P.,  sp.  gr.  1*035,  was  added  with 
stirring,  until  the  liquid  gave  a  slight  acid  reaction  ;  the  crystals 
formed  were  then  allowed  to  subside,  and  the  clear  liquid  was 
decanted  ;  the  sulphite  of  magnesia  was  then  drained  on  a  muslin 
*  This. is  the  average  of  several  experiments,  all  conducted  on  the  same 
plan  and,  with  the  same  quantities,  and  represents  about  what  the  results 
would  he.  in  ordinary  practice. 
