The Constitution of the Electric Spark. 335 



Dr. Feddersen was led through his researches to the conclusion that 

 the metallic particles after being once torn off from the electrodes by 

 the discharge took no further part in it, but were thrown irregularly into 

 the space surrounding the electrodes quite independently of the electric 

 current. Although in some cases, and especially with magnesium 

 poles, there is some evidence that this is partly true, we are led to take 

 the following modified view of the matter. 



The initial discharge of the jar takes place through the air ; it must 

 do so because there is at first no metallic vapour present. The intense 

 heat generated by the electric current volatilises the metal, which then 

 begins to diffuse away from the poles ; the subsequent oscillations of 

 the discharge take place through the metallic vapours and not through 

 the air. We find confirmation of this view in a striking experiment 

 which is easily repeated. If a coil of wire be inserted in the spark 

 circuit of a Leyden jar, which may be charged either by a Wimshurst 

 machine or an induction coil, the air lines disappear almost completely, 

 the metallic lines alone remaining. According to our view we should 

 explain the experiment by saying that the coil which adds self-induc- 

 tion lengthens the duration of the discharge, and allows time for the 

 metallic molecules to diffuse properly into the spark gap. A great 

 part of the energy of the current may then do useful work by heating 

 up the metallic molecules instead of those of air. Mr. Hemsalech is 

 at present engaged in investigating the changes in the metallic spectra 

 which accompany the insertion of self-induction. 



The first spark passing through the air will give rise to a sound 

 wave which, during the complete time of the discharge, will only travel 

 a few millimetres. We may therefore consider that the mass of metal- 

 lic vapours suddenly set free is driven by its own pressure into the 

 partial vacuum formed by the heated air. It would seem more correct 

 to liken the process to that of a gas under pressure flowing into a 

 vacuum than to that of a pure thermal diffusion. There is not much 

 difference between these views, and we may take it that in our experi- 

 ment we have approximately measured the velocity of sound in the 

 metallic vapours. This gives a relation between their temperature and 

 density. If we neglect the differences in the ratio of specific heat we 

 find approximately 



V = 80 



where T is the absolute temperature and p the vapour density referred 

 to hydrogen. Thus for cadmium the average molecular velocity found 

 was 560, and substituting p = 56 we obtain T = 2700, which seems a 

 possible value. Hence we conclude that the molecule of cadmium in 

 the spark cannot have a mass which is much smaller than that directly 

 determined near the boiling point of the metal. 



In conclusion we have also taken some photographs in which the slit 



