LEWIS, ON THE ELECTRIC LIGHT. 15 
either pointed or blunt, or even when the sparks were passed 
between two smooth brass knobs. 
With a view to ascertain whether the texture of the ma- 
terial had any influence in determining the shape of the 
holes, I next procured specimens of various kinds of paper and 
card, and perforated them by sparks from 4 inch to 5 inches 
in length, still further varying the experiments by using dif- 
ferent induction-coils, and by obtaining the inducing-currents 
from the action of batteries of different kinds, in all of which 
cases the results tended strongly to confirm the observations 
previously made. When sparks of great intensity but small 
quantity were employed, the perforations were generally well 
defined at their edges, and were made without any indication 
of a raised burr being formed on either side of the paper ; and 
when many sparks were permitted to pass through the same 
hole, it was gradually enlarged by their action, but preserved 
its original shape for some time after the heat had begun to 
scorch its margin. In common blue-laid or wove post and in 
varnished cardboard the continued action of the sparks cal- 
cined the fibre, and a quantity of ash remained in the holes, 
but the pentagonal shape was well defined in almost every 
instance. In thick, white, unglazed paper somewhat less ash 
‘was left; but in highly finished, thick, cream-laid note-paper 
no trace of it could be found—the diameter of the holes by 
sparks from 2 inches to 5 inches in length being from {5th 
of an inch to ;4,th of aninch. Sparks from 1th of an inch 
to l inch through cream-laid note-paper gave equally clear 
results, especially in those instances where the fewest number 
of sparks had passed through, the diameter of the holes being 
from =;th of an inch to =,',,th of an inch. In the case of 
thick card some difficulty was experienced, owing to the very 
oblique and sometimes zigzag course which sparks frequently 
took in passing through; but the pentagonal form was even 
more distinct than in paper, and neither ash nor burr were 
present in any instance, the diameter of the holes varying 
from ~1,th of an inch to =,th of an inch. 
By way of accumulating the electricity, and obtaining 
sparks of much greater ‘‘ quantity,” a small Leyden jar was 
then included in the circuit, and the terminal wires respec- 
tively connected with its inner and outer coatings. The effect 
of these condensed sparks upon card was very violent, a large 
raised burr being formed on both sides round every hole, 
whilst the disrupted fibre was heaped up in such a way as to 
obscure their outline; it was therefore necessary to make 
thin transverse sections of the card, in order to ascertain the 
true shape of the perforations. The continued action of these 
