Vacuum Tubes are glass vessels containing rarefied gases, and constructed to exhibit certain electrical phenomena. Platinum wires are fused into the glass, and when the tube has been exhausted to the requisite degree it is hermetically sealed. When the platinum terminals were connected to a source of high-pressure electricity -- such as an influence machine or an induction coil -- the tube is filled with a lambent glow, the colour of which depends, amongst other things, upon the gas with which the tube was filled previous to exhaustion, and upon the pressure of the gas. If the vacuum is very high, the discharge will not pass at all. The stream of light usually exhibits striae -- that is, is not continuous, but divided into a number of sections, separated by dark spaces. Many curious effect may be produced by discharges and very highly exhausted tubes; the discharge then proceeds in straight lines, so that it will not go round a bend in the tube, and on meeting the glass, or an object placed in its path, a bright fluorescent glow is produced, accompanied by considerable heat. Discharges may be produced by induction in exhausted tubes with no electrodes, and in some cases the florescent effect may be produced outside the vessel.