238         ON  THE  EXTRACTION  OF  POTASSA  FROM  MARL. 
ral  days  in  the  cold.  The  potassa  is  thus  obtained  as  a  chlo- 
ride— an  undesirable  form — and  in  mixture  with  other  chlorides, 
the  separation  from  which  is  attended  with  considerable  expense. 
When  green  sand  is  treated  with  sulphuric  acid,  the  alumina 
as  well  as  the  potassa  is  dissolved,  resulting  in  the  formation  of 
alum.  As  the  quantity  thus  obtained  is  considerable,  and  as 
this  substance  has  several  times  already  been  suggested  as  a 
commercial  source  of  alum,  I  have  tried  a  number  of  experi- 
ments to  ascertain  the  comparative  yield.  3500  grains  of  green 
sand  were  treated  with  1750  gr.  sulphuric  acid  (62  per  cent.) 
diluted  with  water.  The  first  crop  of  crystals  weighed  630  gr.  ; 
the  second  200  gr. ;  in  all  830.  The  mother  liquors,  still  be- 
ing quite  acid,  were  mixed  with  1750  grains  more  of  green  sand, 
and  yielded  at  a  third  crop  306  gr.  of  alum.  Again,  adding  a 
fresh  portion  of  green  sand  (1750  gr.)  100  grains  more  were 
obtained,  making  in  all  1236  gr.  of  alum  from  1750  gr.  sul- 
phuric acid,  and  7000  gr.  green  sand  employed.  A  large  quan- 
tity of  the  soft  sulphate  of  the  sesquioxide  of  iron  was  also  ob- 
tained, capable  of  yielding  a  considerable  amount  of  copperas 
on  being  reduced  with  metallic  iron. 
When  the  green  sand  had  been  previously  roasted,  the  yield 
of  alum  was  still  greater.  The  protoxide  of  iron  in  the  green 
sand  being  thus  converted  into  the  more  insoluble  sesquioxide, 
and  thus  rendered  less  capable  of  combining  with  the  sulphuric 
acid.  1750  gr.  sulphuric  acid  (62  per  cent.)  treated  with  suc- 
cessive portions  of  roasted  marl,  yielded  1686  gr.  alum,  and  an 
amount  of  the  sulphate  of  the  sesquioxide  of  iron  sufficient  to 
produce  693  grains  copperas. 
When  the  green  sand  was  washed  from  the  adhering  clay, 
which  in  this  case  constituted  about  9  per  cent,  of  it,  the  yield 
of  alum  was  not  so  great.  A  greenish  crystallo-granular  mass, 
apparently  the  bihydrated  sulphate  of  the  protoxide  of  iron,  not 
observed  in  the  preceding  cases,  was  also  obtained.  1750  gr. 
sulphuric  acid  (62  per  cent.)  treated  with  successive  portions  of 
washed  green  sand,  produced  785  gr.  alum  and  626  gr.  cop- 
peras. 
It  thus  appears  that,  with  a  cheap  source  of  sulphuric  acid, 
the  manufacture  of  alum  from  the  green  sand  of  New  Jersey 
might,  perhaps,  be  profitably  carried  on.  And  it  may  possibly 
hereafter  be  found  that,  within  the  limits  of  this  formation  it* 
