* 
524  ARTIFICIAL  PRODUCTION  OF  SALTPETRE. 
It  is  worth  the  attempt  to  ascertain  whether  the  potash  re- 
quired may  not  be  derived  from  certain  rocks,  especially  granite, 
syenite,  trachyte,  porphyry,  basalt,  &c.  It  is  probable  that  these 
substances,  when  reduced  to  a  coarse  powder  and  mixed  with  a 
proper  proportion  of  caustic  lime,  would  gradually  yield  up  their 
potash,  while  the  silicic  acid  combined  with  the  lime.  The  native 
saltpetre  of  India  is  produced  during  the  decay  of  a  limestone 
containing  potash. 
Ammonia  is  now  so  abundant  a  product  of  technical  processes, 
that  there  is  little  fear  of  obtaining  it  in  sufficient  quantity.  In 
very  many  instances  where  it  is  now  lost  it  would  then  be  worth 
collecting ;  and  if  all  sources  were  exhausted  there  would  be  no 
great  difficulty  in  obtaining  it  from  the  atmospheric  nitrogen. 
Ammoniacal  gas  is  very  copiously  formed  when  a  mixture  of 
nitrogen  and  water  vapor  is  passed  over  a  mixture  of  carbon  and 
carbonate  potash  at  a  red  heat. 
There  are  likewise  two  other  possible  sources  of  nitrate  of  pot- 
ash, which,  although  at  present  possessing  rather  a  theoretical 
than  practical  interest,  ought  not  to  be  passed  over. 
When  ammoniacal  gas,  mixed  with  oxygen,  is  passed  over 
ignited  spongy  platinum,  there  is  formed  a  considerable  quantity 
of  nitrate  of  ammonia,  a  salt  which  may  readily  be  converted 
into  nitrate  of  potash.  It  yet  remains  to  be  ascertained  in  what 
proportion  this  formation  of  nitric  acid  stands  to  the  consump- 
tion of  fuel,  to  the  mass  and  condition  of  the  platinum,  and  the 
duration  of  its  activity,  circumstances  which  will  determine  its 
applicability. 
Ignited  peroxide  of  iron  exercises  a  precisely  similar  action 
upon  ammonia  in  the  presence  of  oxygen,  and  consequently,  on 
account  of  its  greater  abundance,  will  be  more  deserving  of  study 
in  this  respect. 
There  is,  moreover,  a  fact  long  since  observed  by  Cavendish, 
that  by  passing  a  number  of  electrical  discharges  through  a  mix- 
ture of  nitrogen  and  oxygen,  in  the  same  proportion  as  in  atmos- 
pheric air,  a  small  quantity  of  nitric  acid  or  nitrate  of  ammonia 
is  formed.  When  the  gaseous  mixture  contains  hydrogen,  the 
formation  of  nitric  acid  is  remarkably  facilitated,  a  circumstance 
which  appears  to  be  closely  connected  with  the  formation  of  traces 
