20 LEWIS, ON THE ELECTRIC LIGHT. 
particles. Thin lamine of mica resisted attempts to perforate 
them by the means at command, the electricity simply spread- 
ing itself over the surface in a lambent blue flame. Parch- 
ment for the most part allowed sparks to pass through it 
without any marked disruptive effects, and where holes were 
made the heat caused their edges to shrivel or contract in a 
sufficient degree to materially alter their original outlines. 
A similar effect was also observed in the case of the membrane 
which lines the interior of the egg-shell; and when polished 
plates of steel were placed between the terminals their 
surfaces were oxidized by the sparks, but the spots thus 
produced were neither of distinct nor uniform shape. 
As to the results of ‘these experiments, so far as they have 
been conducted, they would appear to lead to the following 
conclusions :—That the true shape of the perforations made 
by the electric spark is pentagonal; that this shape is con- 
stant, without regard to the sources from which the electricity 
is obtained ; that it is not due to the shape of the extremities 
of the terminals or other points from which the sparks are 
discharged, nor to the texture of the substance perforated ; 
and from these conclusions it might be reasonably inferred 
that it is due to the peculiar shape of the spark itself, although 
it would perhaps be considered premature at present, and in 
the absence of further evidence, to insist upon this as a thing 
proved.* 
* Supposing the perforations to be six-sided, as they appear to us, 
and not five-sided, as Mr. Lewis thinks they are, an explanation is not so 
difficult. The shape of the spark itself is, in all probability, that of a 
more or less regular cylinder, whose section is a circle. The resistance 
offered to the passage of the spark by the perforated paper or card, acts at 
the point of passage on its cylindrical form in a manner analogous to that 
in which equal pressure from all sides acts on a solid cylinder, rendering it 
hexagonal; though the spark must not be regarded as anything but a con- 
dition of the atmosphere. The cases of the basaltic pillars of the Giant’s 
Causeway and of agglomerated soap-bubbles are well-known instances of 
this law of pressure.—Kp. 
