ON  TIN  ORE  AT  DURANGO  IN  MEXICO. 
295 
it  is  necessary  to  rub  a  substance,  as  a  roll  of  sulphur  or  a 
stick  of  sealing-wax,  successively  with  different  substances, 
some  of  which  give  the  positive  and  others  the  negative  elec- 
tricity. Occasionally  it  will  be  found  that  the  substance  will 
be  positive  at  some  points  and  negative  at  others ;  and  in  such 
cases  the  only  way  is  to  lay  the  particular  specimens  aside 
until  they  shall  have  returned  to  their  natural  condition  at  all 
points.  8ometimes*a  substance  when  first  rubbed,  after  having 
remained  undisturbed  twenty-four  hours  or  more,  will  take  on 
one  electricity,  but,  by  continuing  the  friction  a  very  little 
time,  it  will  take  on  the  other.  Thus,  a  stick  of  sealing-wax 
in  its  natural  state,  when  gently  rubbed,  one  or  two  strokes, 
with  a  silk  handkerchief,  will  often  be  found  decidedly  positive, 
but  by  a  few  strokes  more  it  will  become  as  decidedly  nega- 
tive ;  and  it  cannot  be  made  positive  again  by  friction  with 
silk  until  allowed  by  repose  first  to  return  to  its  natural  state. 
— Am.  Jour.  Sci.  and  Arts,  May,  1865. 
ON  TIN  ORE  AT  DURANGO  IN  MEXICO. 
By  Prof.  C.  F.  Chandler. 
I  have  recently  examined  a  sample  of  1450  grams  of  tin 
ore  from  Durango,  and  find  it  to  be  a  handsome  "wood  tin" 
in  pebbles  and  fibrous  crusts,  some  of  which  are  an  inch  in 
their  longest  diameter.  The  color  varies  from  a  very  light 
brown  to  black. 
Associated  with  the  cassiterite,  there  are  brilliant  crystals  of 
topaz,  some  of  which  are  half  an  inch  long.  They  vary  from 
transparent  to  opaque,  and  from  colorless  to  deep  brown.  I 
have  not  been  ablo  to  examine  them  very  closely,  but  have 
noticed  the  planes  0,  2,  v£,  and  2.  Owing  to  the  development 
of  the  planes  vi  and  2,  and  their  rich  brown  color,  some  of 
the  crystals  might  easily  be  mistaken  for  cassiterite. 
The  sample  examined  gave — 
Tin  (by  crucible  assay).       -        -        -        -  50-90 
Topaz  crystals  easily  separated  by  the  process,    -  3-10 
Other  topaz  crystals,  too  small  to  be  easilv  separated,  l-00(?) 
Oxygen  and  impurities  (by  difference),        -       -  45-00 
100-00 
