ELECTEICAL  DISCHAEGES  AS  OBSEEVED  IN  TOEEICELLIAN  VACUA. 
11 
will  be  found  to  have  become  black  and  rough  (25.),  so  as  to  have  the  appearance  of  being 
corroded ; the  glass  tube  to  a line  with  the  end  of  the  wire  will  be  covered  with  a deposit 
of  platinum,  which  reflects  light  either  from  the  inside  or  outside,  but  when  examined 
by  transmitted  hght  will  be  found  (unless  the  action  has  been  continued  for  a consi- 
derable time)  to  be  translucent,  presenting  a layer  of  a bluish-black  colour. 
38.  Under  certain  conditions,  when  the  discharge  is  heightened  by  increasing  the 
intensity  of  the  battery  to  the  maximum,  but  short  of  the  double  discharge,  in  order  to 
prevent  the  confusion  arising  therefrom,  the  effect  on  the  stratifications  is  much  altered, 
as  in  fig.  8,  Plate  1. : this  experiment,  which  requires  a little  care  in  the  manipulation 
of  the  contact  breaker,  exhibits  a direction  of  force  from  the  positive  to  the  negative, 
and  in  no  way  connected  with  the  passage  of  particles  from  pole  to  pole,  as  in  the  vol- 
taic discharge. 
39.  The  spheroidal  form  of  the  voltaic  arc  has  been  assumed  to  arise  from  the  mutual 
repulsion  of  the  parts  detached  from  each  polar  extremity*:  De  la  Eive  has  shown  that 
the  same  effects  are  produced  by  frictional  electricity ; the  distribution  of  currents  in  the 
hquid  electrolyte  is  also  of  this  character,  as  are  the  forces  of  the  north  and  south  poles 
of  two  magnets  when  placed  in  a line  with  each  other.  In  a moderately  good  vacuum 
the  discharge  from  the  secondary  coil  is  in  the  form  of  a cone,  having  its  apex  at  the 
positive  and  its  base  towards  the  negative,  Plate  I.  fig.  2 B ; and  in  this  medium  it  is 
immaterial  whether  this  positive  discharge  is  taken  from  a point  or  a base : the  discharge 
in  such  a medium  is  always  in  this  fonn. 
40.  That  the  glow  discharge  from  the  secondary  coil  is  not  due  to  the  particles 
emanating  from  the  negative  wire  is  probable,  from  such  particles  being  always  deposited 
in  a lateral  direction  from  the  wire,  and  not  beyond  the  line  on  the  glass  tube  even 
with  the  end  of  the  negative  wire,  from  whence,  to  the  top  of  the  tube,  the  pla- 
tinum is  copiously  deposited ; and  although  the  discharge  would  seem  to  show  by  its 
form  that  its  direction  is  from  the  positive  terminal,  and  that  whatever  may  be  the  sepa- 
ration between  the  positive  and  the  negative  terminal,  the  positive  discharge  or  stratifi- 
cations generally  proceed  to  within  a short  distance  of  the  negative,  still  the  negative 
plays  an  important  part  in  this  form  of  the  electrical  discharge, — witness  the  brilliant 
luminosity  by  which  it  is  surrounded,  and  the  remarkable  heating  of  the  wire  whether 
in  vacuo  or  in  air  (5.);  that  there  is  also  an  apparent  repelling  force  is  shown  by  the 
stratifications  receding  as  the  mercury  ascends,  leaving,  in  some  instances,  3 or  4 inches 
of  the  dark  discharge,  the  stratifications  gradually  collapsing  until  in  a long  tube  they 
disappear. 
41.  In  order  to  test  whether  any  signs  of  interference  could  be  observed  by  making 
the  discharges  of  two  separate  coils  intermingle  with  each  other,  two  instruments  were 
constructed,  Plate  I.  figs.  9 and  10,  in  each  of  which  four  platinum  wires  were  carefully 
sealed : a reference  to  these  figures  will  explain  the  manner  in  which  the  experiment 
was  made;  the  terminals  of  one  coil  were  attached  to  and  of  another  to  hV\  in 
* De  la  Eive’s  ‘Electricity,’  vol.  ii.  p.  296. 
c 2 
