George thought he saw a way to make the light necessary for the cave. "Why can't we rig up an electric light now and explore the cave?"
"We might do that, but we have several things to do before we can have light from that source."
"Haven't we the electricity for it?"
"Do you know how many years electricity was known before electric light was discovered? Before we can utilize this agency for lighting purposes, we must make a machine which will produce a vacuum; we must make glass; we must learn to carbonize threads; and the art of blowing glass would be a necessary accomplishment."
"As usual," said Harry, "something must be made to make something that makes something else."
"But can we make electric light without putting it in a glass 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."