218         INVISIBLE  RADIATION  OP  THE  ELECTRIC  LIGHT. 
salt  cell.  The  vessel  intended  for  a  solution  of  alum,  which 
usually  accompanies  the  lamp  of  Duboscq,  and  the  sides  of 
which  are  of  glass,  answers  admirably.  It  is  not,  however,  quite 
deep  enough  for  the  several  tests  to  which  I  have  subjected  it, 
and  in  crucial  experiments  I  employ  a  deeper  vessel  with  rock- 
salt  sides. 
With  the  eight-inch  mirror  just  referred  to  behind  the  electric 
light,  the  opaque  solution  in  front,  and  the  focus  of  invisible 
rays  about  six  inches  distant  from  the  electric  light,  the  follow- 
ing effects  have  been  obtained  : 
1.  Wood,  painted  black,  when  brought  into  the  dark  focus, 
emits  copious  volumes  of  smoke,  and  is  soon  kindled  at  the  two 
spots  on  which  the  images  of  the  two  coal-points  fall. 
2.  A  piece  of  brown  paper  placed  near  the  focus  soon  shows 
a  burning  surface,  which  spreads  over  a  considerable  space,  the 
paper  finally  bursting  into  flame. 
3.  Black  paper  brought  into  the  focus  is  immediately  in- 
flamed. 
4.  The  wood  of  a  hat-box  similarly  placed  is  rapidly  burnt 
through,  and  usually  bursts  into  flame. 
5.  The  end  of  a  cigar,  placed  at  the  dark  focus,  is  instantly 
ignited. 
6.  Disks  of  charred  paper,  placed  in  the  focus,  are  raised  to 
brilliant  incandescence,  surfaces  of  considerable  extent  being 
brought  to  a  vivid  glow.    Charcoal  is  also  ignited. 
7.  A  piece  of  charcoal,  suspended  in  a  receiver  of  oxygen,  is 
ignited  in  the  dark  focus  and  caused  to  burn  brilliantly,  the 
rays  after  crossing  the  glass  of  the  receiver  being  still  sufficiently 
powerful  to  heat  the  coal  up  to  incandescence. 
8.  A  mixture  of  oxygen  and  hydrogen  is  exploded  in  the  dark 
focus  by  the  ignition  of  its  envelope. 
9.  A  piece  of  zinc-foil,  blackened  on  one  side  to  diminish  re- 
flection, is  pierced  and  inflamed.  By  gradually  drawing  the 
strip,  once  inflamed,  across  the  focus,  it  may  be  kept  blazing  for 
a  considerable  length  of  time.  This  is  a  particularly  beautiful 
experiment. 
10.  Magnesium  wire,  presented  suitably  to  the  focus,  burns 
with  its  intensely  luminous  flame. 
