bulb?"

"Yes; we can make what is called the arc light; instead of the
incandescent."

"What is the difference between the two?"

"In the incandescent, such as we talked about making, a thin carbon
filament is enclosed in a glass bulb, from which as much air as possible
has been exhausted, and when a current of electricity passes through
this filament, it is heated up to a white heat."

[Illustration: _Fig. 31. The Electric Arc._]

"Why doesn't it burn out?"

"It does burn out in time. What preserves it, however, for a long time,
is that most of the oxygen has been exhausted from the bulb----"

"Oh, yes; I know, there must be oxygen to support combustion, so that
the carbon is merely heated up?"

"I am glad you remembered that. The arc light, on the other hand,
depends on an entirely different thing. You have seen, no doubt, the
long black pencils used in the large lamps. That is carbon also, made
out of ground coke, molded and compressed into shape."

"What does arc mean!"

"Did you notice that when we put together the two circuiting ends of the
wires in our battery this morning, we could not notice the existence of
a current, but whenever we pulled them apart we had a spark?

"Let us now make a little experiment which will show you the arc. You
see, I am making a sharp point at the end of each wire, and I will
fasten one of the wires so it cannot be moved. Now the other wire will
be placed with its point as close to the other points as possible, and
so fixed to the support that we can adjust it still closer and hold it.
See, the points now touch each other. I will move one of the wires the
slightest distance away from the other. There! see the light?"

"But it goes out in a little while; what is the cause of that?"

"The electricity has been burned off the end of the wire, and the
distance is now too great for the electricity to jump from one to the
other, so they must be moved closer together. That space between the
ends of the two wires is the electric arc. Instead of the two wires the
carbon pencils are used."

"But how are the two carbon pencils kept apart at the right distance at
all times?"

"That is what the invention of the arc light consisted in; to find a
means whereby the current itself makes the adjustment necessary to
furnish a steady, constant light. When we start to make the arc light
the mechanism can be explained."

George's scheme of the electric lamp for the cave had vanished. Bu