Obsei'vations on the Spark Discharge. 



81 



shown in (fig. 2, PI. 4). It is probable that in this figure we are 

 looking at a point on the spark track where a considerable outflow of 

 atoms initiated a divergence of the spark or a bifurcation of it. 



(16.) To investigate the nature of these tracks a slide was experi- 

 mented with when exposed to the reduced air pressure of 10" of 

 mercury. The spark tracks produced at this pressure were found 

 devoid of the side rays. Instead, beads of melted glass had arranged 

 themselves along the centre of the track, some drawn out into long- 

 streaks, others globular. Another slide at 12" pressure showed very 

 rudimentary development of the rays. At 20'' some rays were 

 obtained, but all these slides treated at ordinary pressures rapidly 

 developed side rays, sometimes bordering the old tracks. The low- 

 pressure tracks, it is to be observed, often develop a budded appearance 

 at irregular intervals, as if the accumulated pressure was con- 

 tent with an occasional relief. The bads open outwards from the 

 sides and ends of the track, fanwise. These are often quite smooth, 

 but develop grooved lines or rays when treated at ordinary pressures. 

 They, in fact, experience a more intense outrush of matter at high 

 pressures than at low pressures. 



Why a track can be intensified by repetition of the spark is easily 

 understood. The channels of escape once indicated by the faintest 

 grooving will naturally continue their functions at the passage of each 

 spark, and in this way become deepened. 



(17.) It has frequently been shown that on glass there is a surface 

 condensation of moisture. To find if this had a part in the produc- 

 tion of the rays, slides were made up of glasses which had just cooled 

 after being heated to redness over a Bunsen burner. The results of 

 several experiments showed that the rays were obtained without 

 difficulty, but perhaps not quite so readily as on slides which had only 

 been rubbed clean in the usual way. The effect of the surface layer 

 of moisture is slight, almost inappreciable. 



(18.) The regular spacing of the rays might suggest that the free 

 path of the atoms or molecules might determine in some degree 

 their spacing. To test this question the slides were arranged for 

 experiment in an atmosphere of hydrogen. Great care was taken by 

 a preliminary heating of the glasses composing the slides and sub- 

 sequent frequent exhaustions, after each exhaustion admitting hydro- 

 gen around the slide, to withdraw all air from the space between the 

 glasses. The tracks obtained in this atmosphere of hydrogen turned 

 out to be very similar, generally, to those obtained in air ; in places 

 quite indistinguishable from them. In some places, however, a 

 striking distancing or widening of the side lines is observed. The 

 lines here are thicker, shorter, and gapped, i.e., one ray is not con- 

 tinued unbroken from the axis, but is divided by minute gaps. 

 Measurements showed such lines to be spaced about 250 to the milli- 



