568 Prof. J. J. Thomson on 



.Properties of the Positive Rays ivhen the Pressure is not 

 exceedingly low. 



The appearance o£ the phosphorescent patch after deflexion 

 in the electric and magnetic fields depends greatly upon the 

 pressure of the gas. I will begin by considering the case 

 when the pressure is comparatively high, say of the order of 

 1/50 of a millimetre. At these pressures, though the walls 

 of the tube in front of the cathode were covered with bright 

 phosphorescence and the dark space extended right up to the 

 walls of the tube and was several centimetres thick, traces of" 

 the positive column could be detected in the neighbourhood 

 of the anode. I will first take the case where the tube was 

 filled with air. Special precautions were taken to free the air 

 from hydrogen ; it was carefully dried, and a sub- 

 sidiary discbarge-tube having a cathode made of -pig. 6. 

 the liquid alloy of sodium and potassium was fused 

 on to the main tube. When the discharge passes 

 from such a cathode it absorbs hydrogen. The dis- 

 charge was sent through this tube at the lowest 

 pressure at which enough light was produced in 

 the gas to give a visible spectrum, until the hydro- 

 gen lines disappeared and the only lines visible 

 were those of nitrogen and mercury vapour. This 

 pressure was a little higher than that used for the 

 investigation of the positive rays, but a pump or 

 two was sufficient to bring the pressure down to 

 this value. The appearance of the phosphorescence 

 on the screen when the rays were deflected by magnetic and 

 electric forces separately and conjointly is shown in fig. 6. 



The deflexion under magnetic force alone is indicated by 

 vertical shading, under electric force alone by horizontal 

 shading, and under the two combined by cross shading. 



The spot of phosphorescence is drawn out into a band on 

 either side of its original position. The upper portion, which 

 is very much the brighter, is deflected in the direction which 

 indicates that the phosphorescence is produced by rays having 

 a positive charge ; the lower portion (indicated by dots in the 

 figure), which though faint is quite perceptible on the Willemite 

 screen, is deflected as if the rays carried a negative charge. The 

 length of the lower portion is somewhat shorter than that of the 

 upper on e, but is quite comparable with it. The intensity of the 

 luminosity in the upper portion is at these pressures quite con- 

 tinuous ; no abrupt variations such as w<ould show themselves 

 as bright patches could be detected, although, as will be seen 

 later on, these make their appearance at lower pressures. 

 Considering for the present the upper portion, the straightness 

 of the edges shows that the velocity of the rays is approxi- 



