320  Chili  Saltpeter  Deposits  of  Peru.      { AMj^yu&1;  Sff* 
great  lakes,  there  are  large  quantities  of  it.  The  saltpeter  mines  con- 
sist of  different  strata.  The  surface  of  the  ground  is  composed  of 
silicates,  sandstone  and  pieces  of  lime.  At  a  depth  of  from  8  to  16 
inches,  very  regular  prisms  are  usually  found,  which  sparkle  with  a 
mass  of  very  small  microscopic  crystals ;  the  strata  below  this,  which 
is  of  rocky  hardness,  consists  principally  .of  common  salt,  with  a  lit- 
tle chloride  of  potassium  and  soda  saltpeter,  mixed  with  earth  and 
pieces  of  silicates  and  carbonates,  and  has  a  thickness  of  20  to  25 
inches.  Beneath  this  crust  is  the  pure  soda  saltpeter,  in  more  or  less 
perfect  crystals,  from  20  to  40  inches  long,  and  3  to  7  feet  in  diameter. 
Guano  is  seldom  found  there,  and  only  in  small  quantities ;  and  it 
always  occurs  just  below  a  stratum  of  salt.  It  is  not  in  a  powder, 
like  that  from  the  Chincha  Islands,  but  adheres  together,  and  is  of  a 
brown  color,  containing  the  bones  and  remains  of  birds  and  insects, 
and  has  an  ammoniacal  smell. 
The  chloride  of  sodium  and  lime  present  furnish  mineral  constitu- 
ents required  for  the  formation  of  the  saltpeter.  According  to  Thier- 
celin,  the  guano  furnishes  the  nitrogen ;  but  since  the  guano  is  always 
found  below  the  salt  crust,  Koenig  is  compelled  to  refer  the  nitrogen 
to  some  other  nitrogenous  organic  bodies,  from  whose  decomposition 
ammonia  is  formed,  and  this  in  turn  is  converted  by  the  action  of  the 
air  and  organic  bases  into  nitric  acid.  Besides  the  three  substances 
named,  all  the  conditions  favorable  to  the  formation  of  saltpeter  are 
found  in  that  neighborhood,  namely,  a  pure,  dry  atmosphere,  absence 
of  rain  to  wash  away  the  saltpeter  when  formed,  and  the  regular  night 
fogs.  The  latter,  leaving  the  salt  undissolved,  dissolve  the  saltpeter 
and  filter  it  through  this  stratum,  under  which  it  crystallizes. 
The  search  for  saltpeter  is  conducted  thus :  The  workman  recog- 
nizes its  presence  by  certain  undulatory  elevations  of  the  ground, 
and  numerous  lumps  of  lime  and  disintegrated  sandstone.  He  bores 
a  hole  some  12  to  18  inches  in  diameter,  going  down  till  the  mineral 
is  plainly  visible.  When  the  lowest  layer  is  reached,  the  hole  is 
widened  to  about  three  feet,  filled  with  charcoal  and  sulphur  and  fired. 
The  explosion  breaks  and  tears  up  the  ground  for  twice  that  distance 
around,  and  then  properly  begins  the  bringing  up  of  saltpeter.  The 
crude  article  varies  considerably  in  compactness,  color  and  quality, 
and  is  named  accordingly.  The  so-called  sulphuret,  which  owes  its 
name  to  its  mode  of  manufacture,  is  the  purest.  The  porous,  earthy 
and  the  congealed  are  different  in  quality.    If  the  raw  product  con- 
