78 



Mr. J. Joly. 



hire of energy is confined to the immediate surface of the poles. In 

 the spark the expenditure of energy is continuous from pole to pole. 

 The abundance with which ions are liberated precludes the idea that 

 they are liberated only at the poles. This distinction is doubtless 

 ascribed partly to the free motion and isolation of the molecules dis- 

 integrated by the spark, partly owing to the explosive disturbance 

 and dispersion of the atoms. For heating, repulsion, and chemical 

 dissociation will unite in setting up a violent rush of atoms and 

 molecules from the axis of the spark outwards, in fact, conferring its 

 explosive character upon it. 



The following observations for the most part find their explanation 

 in the mechanical conditions obtaining in the spark path and relate 

 to the behaviour of the spark in passing through confined spaces. 



(14.) Two leads of very thin platinum foil are laid down on a slip 

 of glass. The leads are about 30 mm. in length by 1 mm. in width. 

 The glass slip may be an ordinary microscope slip. The leads are laid 

 along the slip in the same straight line, with their ends (preferably 

 cut to points) separated by about 20 mm. at the centre. A piece of 

 thin cover glass is now cut into a rectangular shape, 40 X 20 mm. q.p., 

 warmed and touched on one side at each corner with shellac. This is 

 laid down centrally on the slip, covering the gap between the leads, 

 and the whole is held over a flame till the shellac between the cover 

 glass and the slip begins to melt. It is then placed on a smooth table 

 and the cover pressed and rubbed down till Newton's rings appear in 

 the space separating the leads. This is easily accomplished if the 

 surfaces have been wiped clean before putting them together. The 

 rubbing is continued till the black spot is produced afc the centre of 

 the rings. 



If wires from a R-uhmkorff coil are now brought to touch the leads 

 where they extend beyond the cover glass, it will be seen that the 

 spark in its passage across the confined space between the cover 

 glass and the slip refuses to cross the centre of the rings. It makes 

 a detour, curving round at a distance of four or more rings from the 

 black spot, that is, it will sometimes pass in the fourth ring, some- 

 times in the sixth or eighth. It will often divide into several sparks, 

 some going one side, some going the other. These sparks show no 

 difference in appearance from free sparks. The rings will be seen 

 to be disturbed by the sparks, generally widening as if the glasses 

 came closer. In some cases, however, they soon become destroyed. 

 It is observable, however, that sparks passed through such a narrow 

 space will often produce the rings where none at first existed. Such 

 rings persist for many hours, so that the effect ]*3 hardly due to heat- 

 ing, but probably due to an electrostatic straining of the glass. 



It appears from this experiment that the spark experiences a higher 

 resistance near the centre of the rings, either because the molecules 



