590 Professor' J. J. Thomson [March 22, 



difference of 100 volts or less is able to produce positive rays. The 

 interest of the experiments at very low pressures lies in the fact that 

 in this case the rays are the same whatever gas may be used to fill 

 the tube ; the characteristic rays of the gas disappear, and we get the 

 same kind of carriers for all substances. 



I would especially call attention to the simplicity of the effects 

 produced at these low pressures ; only two patches of phosphorescence 

 are visible. This is, I think, an important matter in connection with 

 the interpretation of these results ; for at these low pressures we have 

 to deal, not only with the gas with which the tube was originally filled, 

 but also with the gas which is given off by the electrodes and the 

 walls of the tube during the discharge ; and it might be urged that 

 at these low pressures the tube contained nothing but hydrogen given 

 out by the electrodes. I do not think this explanation is feasible, for 

 the following reasons : — 



(1) The gas developed during the discharge is not wholly hy- 

 drogen : if the discharge is kept passing long enough to develop so 

 much gas that the discharge through the gas is sufficiently luminous 

 to be observed by a spectroscope, the spectrum always showed, in addi- 

 tion to the hydrogen lines, the nitrogen bands ; indeed, the latter 

 were generally the most conspicuous part of the spectrum. If the 

 phosphorescent screen on which the positive rays impinge is observed 

 during the time this is being given off, the changes which take place in 

 the appearance of the screen are as follows : If, to begin with, the 

 pressure is so slow that the phosphorescent patches are reduced to 

 two bright spots, then, as the pressure begins to go up owing to the 

 evolution of the gas, the deflection of the spots increases. This is 

 owing to the reduction in the velocity of the rays consequent upon 

 the reduction of the potential difference between the terminals of the 

 tube, as at this stage an increase in the pressure facilitates the passage 

 of the discharge. In addition to the increase in the displacement 

 there is an increase in the area of the spots giving a greater range of 



values of ^ ; this is owing to the increase in the number of colhsions 



m 



made by the particles in the rays on their way to the screen. As 

 more and more gas is evolved the patches get larger, and finally 

 overlap ; the existence of the second patch being indicated by a 

 diminution in the brightness of the phosphorescence at places outside 

 its boundary. As the pressure increases the luminosity gets more and 

 more continuous, and we finally get to the continuous band, as shown 

 in Fig. 6. At this stage it is probable that there may be enough 

 luminosity to give a spectrum showing the nitrogen lines, indicating 

 that a considerable part of the gas in the tube is air. It is especially 

 to be noted that during this process, when gas was coming into the 

 tube, there has been no development of patches in the phosphorescence 

 indicating the presence of new rays ; on the contrary, one type of 



