Observations on the Spark Discharge. 



83 



and connect; with a coil so that sparks pass. If now a stream of coal- 

 gas be passed across the end of the tube nearest the spark, two-thirds 

 of the length of the spark near this end will become coloured with 

 the blue tint assumed by a spark in coal-gas at atmospheric pressures, 

 the remainder of the spark length remaining as before. Altering the 

 direction of the current does not influence the result. If the coal-gas 

 be passed across the other extremity of the tube, the one-third of the 

 spark length near that end becomes blue, the other two-thirds con- 

 tinuing an air spark. 



Move the spark gap into the middle of the tube and repeat the 

 experiment. Now it is seen that half the spark turns blue when the 

 gas is approached to one end; always that half nearest the end 

 opening into the current of gas. 



If finally, a flame of any kind be approached one end of the tube, 

 at each spark the flame is seen to be blown outwards as if acted on 

 by a blow-pipe. The explanation of the coloration of the coal-gas is 

 to be found in a back suction of gas occurring after each explosion, 

 the succeeding spark becoming coloured according to the distribution 

 of gas in the tube. - This is a question of facility of egress and ingress. 

 When the spark is centrally placed in the tube these are equal at each 

 end, and half the spark will be coloured. In the unsymmetrical 

 position of the spark, outflow and inflow are facilitated at one end 

 and obstructed at the other according as the spark is located nearer 

 the one end and further from the other. 



When a spark is caused to pass in the centre of a tube so long as 

 10 or 12 cm., the puffing out of a flame applied at the end may still 

 be observed, bat there is no coloration of the spark when coal-gas is 

 passed across the end. It fails to penetrate so far along the tube as 

 to reach the spark. 



The partial coloration of the spark, taking place in this very 

 symmetrical way, must evidently depend on a uniform repulsion of 

 matter along the length of the spark. Were the molecules, for 

 example, only repelled close to the positive pole the symmetry of 

 effects would hardly obtain. 



(22.) In addition to the fine rays bordering the spark tracks in 

 more worn tracks, the arrangement of the melted glass is note- 

 worthy. Very often three strings of glass beads are discernible, a 

 central one of large beads or globules, spherical or elongated, and a 

 string of very small spherical globules extending along either side. 

 All are contained in a smooth trough-shaped track. Occasionally 

 the globules are, however, differently arranged. Thus the tiny 

 somewhat \/- sna P e( i marks, all pointing one way, shown in (fig. 1, 

 PI. 5), consist of very minute beads; the largest at the apex, the 

 smaller streaming backwards and outwards like the wash from the 

 bow of an advancing boat. Observations show that this arrangement 



VOL. XLYII. H 



