Note 10 — SECTION II. ON FORCE, AND THE RELATIONS BETWEEN FORCE AND MATTER. (continued 2)

They are called vacuum tubes, and are filled while open by putting one end in communication with the vessel in which the gas is generated, and the other end in communication with an air pump. As soon as the atmospheric air is pumped out, the gas rushes in and fills the tube, the communication with the vessel containing the gas is cut off by fusing up that end of the tube, and as soon as the gas is sufficiently rarefied the other end is fused up also. An electrical discharge that will not pass through one inch of air, will pass through thirty or forty inches in a vacuum tube.