LEWIS, ON THE ELECTRIC LIGHT. 19 
and discoloration of the surrounding powder took place 
simultaneously. It was expected that if the spark itself were 
actually five-sided, the heat radiated from it would cause the 
discoloured space to be of the same shape, and in some few 
instances such was the case, but in the majority the condi- 
tions were such that no great reliance could be placed upon 
the result as a test. The union between the powder and the 
paper was merely mechanical, so that its separate particles 
were visible when magnified, and were often detached and 
made to dance about on the surface by the vibrations caused 
by the sparks, in addition to which the smoothest paper pro- 
curable was microscopically rough, and its surface was not 
improved in this respect by the amount of rubbing required 
to work the powder into it. Further attemps were then made 
to obtain some impress of the heat of the spark by using 
paper which had been saturated with diluted sulphuric acid 
and dried. Exposure to heat rapidly carbonizes this, and on 
placing it in the path of the sparks the perforations were seen 
to be bordered with black almost as soon as made. The 
action of the acid had also so far rotted the paper that it 
offered comparatively little resistance to the passage of the 
sparks, and the pentagonal shape of the holes was conse- 
quently much more uniform and sharp than in any previous 
experiments. The continuance of the current increased the 
size of the holes much more rapidly than had been the case 
in former instances ; but although the outline of the scorched 
margins for the most part corresponded with that of the holes, 
the varying thickness of the paper obviously prevented it 
from always extending itself equally in every direction. 
Whilst operating upon this paper it was noticed that at the 
instant of perforation, and for some few seconds afterwards, 
the edges of every hole glowed with great brilliancy ; this 
was owing to the heat of the sparks haying first carbonized 
the paper, and next raised it to a state of incandescence, 
until, being entirely consumed, it passed away in a tiny shower 
of microscopic sparks; in fact, for the time, a minature 
electric light had been produced. 
Amongst other substances made use of, the dried leaves of 
plants and trees were tried with some success, especially those 
of the laurel and plane, whose cuticle presented a compara- 
tively smooth surface. Here also scorched borders surrounded 
each perforation, their outlines in many instances closely 
corresponding with each other in shape. The faded leaf of 
the plane tree was rapidly consumed by the continued action 
of the sparks, a strong smell being produced, and two bright 
red bands being added to the spectrum by its incandescent 
