562 
VARIETIES. 
The  spring  in  question  appears  on  the  highest  point  of  a 
narrow  ravine,  known  as  "  The  Dark  Hollow,"  in  the  hills 
forming  the  boundary  of  Allegheny  county.  A  layer  of  kies 
and  sand,  from  50  to  60  feet  in  height,  rests  upon  slate  rock, 
which  is  impenetrable  for  water.  The  water  filters  through  the 
kies  and  sand,  and  rushes  from  between  the  rock  and  the  sandy 
stratum  in  a  constant  stream,— collecting  in  a  small  basin,  and 
falls  over  the  slate  rock  about  25  feet  into  another  basin. 
After  heavy  rains  the  spring  water  contains  a  trace  of  carbonic 
acid,  which,  however,  is  lost  in  falling  over  the  precipice,  and 
the  water  below  the  falls  is  always  chemically  pure.  Blue  and 
red  litmus  paper  is  not  altered  by  it ;  nitrate  of  silver  and 
chloride  of  barium  occasion  no  turbidity  ;  and  the  vapors  of  the 
water,  when  conducted  into  a  solution  of  nitrate  of  baryta, 
rendered  alkaline  by  ammonia,  do  not  disturb  the  same  ;  boiled 
with  solution  of  indigo,  no  discolorization  takes  place  ;  five 
gallons  of  it,  when  evaporated,  left  no  solid  residue. 
In  answer  to  the  lecture  of  a  Mr.  Clemson,  before  the  Smith- 
sonian Institution,  in  Washington,  D.  C,  which  was  published 
in  the  National  Intelligencer,  the  author  wrote  a  paper,  to 
which  Mr.  Clemson  replied,  that  it  could  not  be  chemically 
pure  ;  but  must  contain  oxygen  from  the  atmosphere.  But, 
inasmuch  as  the  author  did  not  state  the  spring  to  be  a  hot 
one,  and  as  cold  distilled  water  absorbs  air  when  left;  in  contact 
with  it,  the  author  has  proved  it  to  be  as  pure  as  cold  distilled 
water.  All  his  reagents  are  dissolved  in  this  water,  and  he 
never  observed  any  alteration  in  these  solutions.- — Wittsteins 
V.  Lehr.  ix.  370-373.— j.  m.  m. 
Pennsylvania  Bock  OiL — In  most  countries,  a  troublesome  process  must 
be  undergone  to  extract  oil  from  mineral  substances,  such  as  from  coal  and  m 
asphalt ;  but  Pennsylvania  seems  to  be  so  favorably  dealt  with  by  Dame 
Nature  that  the  very  rocks  distil  oil  into  her  lap.  The  north-western  part 
of  that  State  seems  to  contain  a  number  of  subterranean  springs  which 
yield  a  limpid  oil,  some  of  which  we  have  examined.  It  appears  that  the 
Pennsylvania  Rock  Oil  Company  purchased  the  spring  of  Brewer,  William 
&  Co.,  for  the  sum  of  5000  dollars  ;  and,  in  1858,  leased  it.  to  Mr.  E.  L. 
