18 LEWIS, ON THE ELECTRIC LIGHT. 
the microscope in the manner described, the observations as 
to the five-sided shape of the spark perforations were not only 
confirmed, but an explanation was given of the causes of some 
variations from that shape which had been previously noticed. 
Chief amongst these, it had been observed that certain holes, 
whilst sharply angular on one side, were rounded on the 
other. Now, in practice it frequently occurred that a spark, 
instead of passing through that portion of the paper lying 
immediately between the terminal points, would perforate 
some adjacent part where the texture probably offered it a 
less degree of resistance, so that its path would be represented 
by the two sides of a triangle, the angular point of which 
being the place of perforation. If, then, the current was 
continued, the succeeding sparks followed each other so 
rapidly as to present the appearance of a quivering thread of 
fire being drawn obliquely through the hole, with the result 
that that side of it only which was nearest to the direct line 
between the points was abraded, and had its angles rounded 
by what may be termed the friction of the stream in endea- 
youring to straighten and thus shorten its course. The samc 
effect was produced when the paper was moved during the 
passage of the sparks, which would continue to pass through 
a hole until its distance from the perpendicular line caused 
the resistance of the atmosphere to exceed that of the paper, 
when a fresh perforation was made. Often, too, when a 
number of holes were in the field of view at the same time, 
the stream of sparks would fly from one to the other without 
apparent cause, in all of which instances the original sym- 
metry of form would be more or less destroyed according to 
the obliquity of the courses taken ; and it will also have been 
anticipated that great irregularities of surface, or the interpo- 
sition of fibres too tough for the spark to break directly 
through, would contribute to the occasional production of 
exceptional shapes. 
During the progress of these experiments it was suggested 
to me that some further test should be applied in order that, 
if possible, more direct proof might be afforded that the 
pentagonal outline of the holes was due to a corresponding 
shape of the spark, and biniodide of mercury was named as 
being well suited to the purpose. This beautiful scarlet 
powder is of so volatile a nature that a moderate degree of 
heat is sufficient to partially decompose it, upon which its 
brilliant colour is instantly changed to a dull greenish-yellow. 
A small quantity of this powder was accordingly rubbed 
down upon paper, and on passing this between the terminals 
whilst the coil was in action the perforation of the paper 
