294      ELECTRICAL  PROPERTIES  OF  PYROXYLIN  PAPER,  ETC. 
It  is  proper  to  say  that  both  the  paper  and  the  cotton  were 
prepared  nearly  a  year  ago,  and  may  have  undergone  some 
change,  though  nothing  of  the  kind  was  apparent  to  the  eye, 
except  that  one  sheet  of  the  paper  used  seemed  at  one  place  a 
little  discolored. 
The  substances  experimented  with  were  amber,  sealing  wax, 
sulphur,  gum  lac,  pitch,  rosin,  caoutchouc  (native  rubber),  hard 
lubber  (stick  obtained  of  Messrs.  J.  F,  Luhme  &  Co.  of  New 
York),  common  vulcanized  rubber  (as  used  in  forming  gas  bags), 
gutta  percha,  and  various  crystallized  mineral  substances.  But 
these  last,  becoming  always  positive,  will  not  be  further  al- 
luded to.  The  sulphur  by  friction  with  the  gun-cotton  always 
became  positive,  and  also  by  friction  with  different  sheets  of  the 
paper,  except  in  a  single  instance,  when  using  the  paper  which 
was  slightly  discolored,  it  appeared  to  be  feebly  negative. 
Rosin,  pitch,  gum  lac  and  amber,  both  with  the  paper  and  the 
cotton,  became  always  positive,  as  did  also  the  native  rubber, 
by  which  I  mean  the  rubber  as  it  is  imported.  Sealing-wax 
with  the  cotton  becomes  always" positive,  but  with  the  paper  oc- 
casionally negative.  Vulcanized  rubber  (the  kind  used  in 
making  gas  bags)  would  sometimes  become  positive  and  some- 
times negative,  and  the  same  was  true  of  gutta  percha,  two 
different  specimens  being  used.  The  hard  rubber  (from  Luhme 
&  Co.)  became  always  negative,  both  with  the  paper  and  the 
cotton.  This  being  contrary  to  the  results  obtained  by  your- 
self with  this  substance,  I  made  very  many  trials,  but  always 
with  the  same  result. 
In  all  cases  after  friction  with  other  substances,  whether  the 
latter  became  positive  or  negative,  the  paper  and  cotton  would 
be  found  invariably  negative.  Sheets  of  the  paper,  when 
handled,  especially  in  cold,  dry  weather,  often  become  highly 
excited, — always  negatively,  so  far  as  has  been  determined ; 
and  my  son  informs  me  that  sometimes,  when  handling  con- 
siderable quantities  of  the  recently  prepared  paper,  he  was 
even  fearful  that  it  might  become  ignited  by  the  sparks  pro- 
duced ! 
I  will  just  remark  in  closing,' that  in  making  experiments 
like  these,  great  care  is  required  in  order  that  the  result  ar- 
rived at  may  be  satisfactory.    This  is  particularly  the  case  when 
