DISRUPTIVE DISCHARGE — SPARK. 105 



also as to the extent to which that polarity must rise before discharge occurs. An 

 analogous difference exists in the specific inductive capacities already pointed out in 

 a few substances (1278.) in the last paper. Such a difference might even account 

 for the various degrees of insulating and conducting power possessed by different 

 bodies, and, if it should be found to exist, would add further strength to the argu- 

 ment in favour of the molecular theory of inductive action. 



1405. Having considered these various cases of sustained insulation in non-con- 

 ducting dielectrics up to the highest point which they can attain, we find that they 

 terminate at last in disruptive discharge ; the peculiar condition of the molecules of 

 the dielectric which was necessaiy to the continuous induction, being equally essen- 

 tial to the occurrence of that effect which closes all the phenomena. This discharge 

 is not only in its appearance and condition different to the former modes by which 

 the lowering of the powers was effected (1320. 1343.), but, whilst really the same in 

 principle, varies much from itself in certain characters, and thus presents us with the 

 forms of spark, brush, and glow (1359.). I will first consider the spark, limiting it 

 for the present to the case of discharge between two oppositely electrified conduct- 

 ing surfaces. 



The electric spark or flash. 



1406. The spark is a discharge or lowering of the polarized inductive state of 

 many dielectric particles, by a particular action of a few of the particles occupying 

 a very small and limited space ; all the previously polarized particles returning to 

 their first or normal condition in the inverse order in which they left it, and uniting 

 their powers meanwhile to produce, or rather to continue, (1417 and 1436.) the dis- 

 charge effect in the place where the subversion of force first occurred. My impres- 

 sion is, that the few particles situated where discharge occurs are not merely pushed 

 apart, but assume a peculiar state, a highly exalted condition for the time, i. e. have 

 thrown upon them all the surrounding forces in succession, and rising up to a pro- 

 portionate intensity of condition, perhaps equal to that of chemically combining 

 atoms, discharge the powers, possibly in the same manner as they do theirs, by some 

 operation at present unknown to us ; and so the end of the whole. The ultimate 

 effect is exactly as if a metallic wire had been put into the place of the discharging 

 particles ; and it does not seem impossible that the principles of action in both cases 

 may, hereafter, prove to be the same. 



1407. The path of the spark, or of the discharge, depends on the degree of tension 

 acquired by the particles in the line of discharge, circumstances, which in every com- 

 mon case are very evident and by the theory easy to understand, rendering it 

 higher in them than in their neighbours, and, by exalting them first to the requisite 

 condition, causing them to determine the course of the discharge. Hence the se- 



MDCCCXXXVIII. p 



