NEGATIVE ELECTRICITY FROM HOT CALCIUM AND FROM LIME. 167 



measure on a milliammeter to 2'8 milli-amperes. These experiments were performed 

 in helium at a pressure of 3'28 millings. Similar results were obtained with lime 

 cathodes. For instance, during the observations recorded in Table VI I L a faint 

 luminosity was noticed round the edges of the anode when the cathode was at 

 1220 C. This luminosity increased in brightness as the temperature was raised, but 

 with a potential difference of 40 volts the glow was always on the anode only. It 

 was found that at 1410 C. it required a potential difference of 74 volts to give a 

 luminosity round the cathode. With 72 volts there was a very bright anode glow, 

 and the current passing was 4*5 milli-amperes. With 74 volts a brilliant cathode 

 glow was obtained, and the current suddenly increased to 0'5 ampere. Further 

 experiments showed that the difference of potential required to give the cathode glow 

 was less the higher the temperature of the cathode. 



The appearance of the luminosity round the cathode has been studied by 

 Professor J. J. THOMSON,* who worked in a slightly different manner from that 

 described above. By keeping the temperature of the cathode constant, and very 

 gradually increasing the potential difference by means of a potential divider, 

 Professor THOMSON found that the luminosity always appeared quite abruptly and 

 was accompanied by a very rapid increase in the negative leak. In one case, at 

 1400 C., an increase in the potential difference of -j-J^ of a volt caused a bright 

 luminosity to appear and increased the current forty-fold. A result similar to this 

 was obtained in the experiments now recorded, when the cathode was at a high 

 temperature and the voltage gradually increased ; but if the temperature of the 

 cathode was not too high, a luminosity round the anode was first obtained, and this at 

 a certain potential difference appeared to leave the anodes and surrounded the cathode. 

 This inversion of the appearance of the discharge was accompanied by a large increase 

 in the current. 



In the present experiments, when a difference of potential of 40 volts was used and 

 the temperature of the cathode gradually increased, the luminosity appeared sometimes 

 round the anodes and sometimes round the cathode, but always quite gradually, except 

 when a cathode newly covered with lime was used. In this case the luminosity did 

 not appear until a temperature higher than usual had been reached. Under these 

 circumstances it generally appeared quite suddenly and then increased in brightness, 

 although the temperature was kept constant or, in some cases, actually lowered. 



Professor THOMSON has concluded from his experiments that the gas becomes 

 luminous in consequence of the internal energy of the atoms increasing, under the 

 bombardment of the corpuscles shot out by the cathode, to such an extent that the 

 equilibrium of the atomic system becomes unstable and an explosion occurs. This 

 explosion results in an expulsion of corpuscles and such a shaking up of those left in 

 the atom that they vibrate so vigorously that the energy radiated is sufficient to 

 produce luminosity. When the luminosity appears abruptly, we must imagine that 

 * Royal Institution Lecture, Friday, Jamiary 19, 1906. 



