392 
PERMANGANATE  OF  POTASSA. 
pounds  of  the  binoxide,  thus  economizing  the  expense  for  ma- 
terial. Then,  if  the  residuary  binoxide  could  be  powdered  and 
used  over  again,  the  expense  would  be  still  farther  reduced. 
The  principal  items  of  cost,  however,  are  the  skill  time  and 
labor. 
The  evaporation  of  the  clear  solution  should  be  conducted 
with  care  or  by  steam  heat,  particularly  toward  the  close,  as  too 
much  heat  then,  appears  to  decompose  a  portion  of  the  salt,  with 
the  evolution  of  gas — probably  oxygen.  If  the  evaporation 
be  carried  beyond  the  point  indicated  there  is  much  loss. 
Whilst,  if  it  stop  short  of  that,  an  unnecessary  proportion  of  the 
salt  is  carried  on  in  the  mother  liquor  to  the  next  portion  made. 
The  treatment  of  the  crude  crystals  and  their  purification  by 
recrystallization  is  simple  enough  and  needs  no  comment. 
The  crystals  are  usually  obtained  in  fine  slender  needles,  and 
are  purest  when  small,  because  then  the  washing  is  more  effect- 
ual in  freeing  them  from  potassa.  These  crystals  in  drying 
always  undergo  a  slight  amount  of  decomposition  from  the 
organic  matter  in  the  atmosphere,  and  consequently,  they  always 
give  a  slight  deposit  of  hydrated  binoxide  of  manganese  upon 
being  dissolved.  The  amount  however  is  practically  insignifi- 
cant when  the  salt  has  been  carefully  prepared.  The  presence 
of  caustic  potassa  and  chloride  of  potassium  frequently  present, 
is  a  much  more  serious  objection  to  the  salt  containing  them 
because  both  are  active  irritants.  The  salt  should  always  be 
well  crystallized  and  dry,  and  should  stand  the  sulphate  of  iron 
test. 
How  the  oxidation  is  accomplished  in  the  above  given  pro- 
cess,— or  rather  whence  the  oxygen  is  obtained,  the  writer  can- 
not tell.  At  first  it  was  supposed  to  be  by  decomposition  of 
the  water  of  aspersion,  but  afterwards  it  was  thought  more 
probable  that  it  came  from  the  air,  and  that  the  agency  of  the 
water  was  merely  that  its  vapor  or  steam  displaced  the  nitrogen 
left  in  the  interstices  of  the  powder  after  the  oxygen  of  the  air 
had  been  absorbed,  and  then  as  this  steam  was  driven  off,  it 
gave  place  to  fresh  supplies  of  air  for  the  oxidation.  The 
powder  before  aspersion  is  greenish  black,  and  gradually 
changes  with  each  aspersion  to  a  rich  deep  chocolate  or  purplish 
