1886.] electric discharge in a uniform electric field. 399 



in the same direction in the neighbourhood of the electrodes. And 

 just as in the non-uniform field the molecules will be more likely 

 to dissociate at the negative electrode than at the positive, for at 

 the positive electrode those molecules which are likely to be dis- 

 sociated are those which are moving away from the electrode, but 

 they must previously have been approaching it, during which time 

 they were being pushed nearer and nearer together, so that at 

 this electrode the molecules which have any tendency to be dis- 

 sociated are those which have been specially prepared to resist this 

 tendenc3 r , and as this is not the case at the negative electrodes the 

 molecules will be dissociated most easily at this electrode. 



Hence the conclusion we arrive at is, that whether the field be 

 uniform or variable, dissociation is more likely to take place at 

 the negative electrode than at the positive, and that the dis- 

 sociation is more likely to take place close to the negative elec- 

 trode than in the body of the gas ; though if the field be very 

 strong or the gas very weak the molecules in the body of the gas 

 may get decomposed. Thus in the experiments described above, 

 though in gases which are electrically strong, such as air and 

 coal-gas, the discharge under certain circumstances could be con- 

 fined to the neighbourhood of the negative electrode, yet in 

 electrically weak gases, such as the vapours of turpentine and 

 alcohol, the gas was under all circumstances (when the pressure 

 was low) decomposed throughout the field, though the greater 

 brightness of the layer near the negative electrode shewed that 

 more gas was decomposed there than in other parts of the field. 



There seems too in the case of the discharge through ordinary 

 vacuum tubes considerable direct evidence that there is a con- 

 siderable amount of decomposition going on near the negative 

 pole, more so than in the rest of the field, for in the first place, 

 the spectrum of the glowing gas surrounding the negative elec- 

 trode generally shews lines, while the spectrum in the rest of 

 the field is a band spectrum, and line spectra are believed to 

 denote a simpler molecular constitution than band spectra : and 

 secondly, the gas near the negative electrode is hotter than that 

 in other parts of the field. 



Let us trace some of the consequences of the gas being 

 decomposed more easily at the negative than at the positive 

 electrode. Since, according to our view, decomposition of the 

 molecules means a spark, it follows that according to this theory 

 a spark ought to pass more easily from a negative than from a 

 positive pole, a result which was long since observed by Faraday 

 {Experimental Researches, § 1501). 



Again, decomposition, and therefore discharge, takes place the 

 more easily the longer the molecules move continuously in the 

 direction of the lines of force ; thus the longer the average time 



