DISCHARLE GAsEs. There is reason for believing that a pure gas would be a perfect insulator; hut all gases pan be made conduetors in many ways, in all of which there is evidence that the molecules Of the gas have been broken in to parts—i.e. 'ionized': although the ions in a gas are by no means the same as in a liquid electro lyte. Sonic of the methods for ionizing a gas are the following: (1) Application of high tempera ture, e.g. the gases rising from a Bunsen flame are conductors. (21 Passage of a spark in the neighborhood. (3) Exposure to l'Aitgen rays or to rays from a radio-active substance. such as uranium (q.v.). (4) Exposure to 'cathode rays' t see below). (3) Exposure to 'ultra-violet (See Ltcltr.) One most important fact which seems to be established is that the negative ions of all gases, however these ions are produced. are identical. It can be proved also that the negative ions have less mass than the positive ones. For a full discussion of the recent work on this sub ject. reference must he made to d. Thomson's The Discharge of Elect ricit y Through Gases (London, 1902) and to publications by Professor Thomson and his students in the Philosophical Transact ions ( London. current ).
One of the most interesting illustrations of the passage of electricity through a gas is af forded when the gas is inclosed at low pressure in a glass tube, into which enter two metal wires serving as anode and cathode. When the gas is at a pressure of a few centimeters of mercury. and a discharge of electricity through the tube is produced by an induction coil or by a number of cells, the gas near the anode is apparently ar ranged in colored striations across the tube. while near the cathode there is a dark space and over the cathode itself there is a velvety glow.
The light emitted from the tube is characteristic of the gas inside. See SrEcraoscoev.) As the tube is exhausted further, so that the pressure of the gas decreases. the dark space round the cathode extends through the tube. Finally, if the exhaustion is continued. the character of the phenomenon is changed by the gradual appear ance of the 'cathode rays.' These are streams of minute particles of matter negatively charged, which leave the cathode perpendieularly and pro ecol in straight lines through the gas in the tube. These cat hode rays are themselves invisi ble, but they produce luminous effects where they pass through the gas and where they strike the walls of the tube: at this last place they produce thermal effects also. They produce mechanical motion if they strike any small movable object in the tube. Their path can be deflected by a magnet, proving that they are negatively charged particles moving with great speed—alsalt 25.0011 smiles per second. If at the end of the tube on which the cathode rays strike there is a small opening covered with a Olin sheet of aluminium. similar rays are observed outside the tube. These were first observed by Lenard and are called 'Lenard rays.' Cathode rays affeet ',holographic plates: and, further. NN herever they strike any se lid obstacle-. disturbance, in the ether are pro duced. %%inch in turn vanse other radiation. These were by Rantgen and are called gen rays' er 'S-rays' The best discussion of cathode is given in Les rayons Julu, .s, Villard (Paris, 1900).