Electric Discharges in Rarefied Gases. 69 



discharge in rarefied gases. The gas first breaks down along 

 the line of maximum E.M.F. intensity, and a small discharge 

 takes place, producing a supply of dissociated molecules, along 

 which the succeeding discharges can more easily pass. 



Observe discharge with electrodes. When the pressure is 

 about ^ milHmetre of mercury, we see that the cathode is 

 irregularly covered with a velvety light, its distribution depend- 

 ing on the pressure and quantity of current ; then comes 

 Crookes' dark space, after this a luminous column, the negative 

 glow, independent of the position of the electrode, its size 

 depends on the shape of the vessel near the cathode ; next, the 

 Farady dark space, of variable length ; lastly, the luminous 

 column extending to the anode, very regularly striated, and 

 beautiful. Attempts have been made to explain the striae on 

 the hypothesis that the discharge through an exhausted tube is 

 not continuous but intermittent. 



Pliicher first investigated the fluorescence on the walls of the 

 tube near the cathode. Hittorf next discovered that the surface 

 of the electrode is the origin of a motion spreading uniformly 

 through the gas. Goldstein showed that a pointed cathode 

 produces a well-defined shadow on the walls of the tube of a 

 body in front of it, while a cathode of large surface produces a 

 clear but not very sharp shadow, thus proving that the cathode 

 rays, as he called them, came off nearly normally from the 

 cathode, and not like light in all directions. 



In 1879 Crookes wrote his first papers on this subject, and 

 his experiments became popular in this country. The most 

 striking property of cathode rays is their power of producing 

 fluorescence, not only in the gas through which they pass, but 

 also in many substances on which they fall. To show these 

 effects most strikingly we use " solid solutions," which are 

 formed when two salts are simultaneously precipitated from a 

 solution. They are then particularly sensitive to the rays 

 coming from an electric discharge. 



Goldstein discovered that if there are two adjacent cathodes 

 the rays from one are deflected by the other. 



