DISRUPTIVE DISCHARGE AND INSULATION. 95 



ammonia, also when in strong solution (748.). Mr. Harris has mentioned instances* 

 in which the conducting power of metals is seriously altered by a very little alloy. 

 These may have no relation to the former cases, but nevertheless should not be over- 

 looked in the general investigation which the whole question requires. 



1358. Nothing is perhaps more striking in that class of dielectrics which we call 

 electrolytes, than the extraordinary and almost complete suspension of their peculiar 

 mode of effecting discharge when they are rendered solid (380, &c.), even though the 

 intensity of the induction acting through them may be increased a hundred fold or 

 more (419.). It not only establishes a very general relation between the physical 

 properties of these bodies and electricity acting by induction through them, but 

 draws both their physical and chemical relations so near together, as to make us 

 hope we shall shortly arrive at the full comprehension of the influence they mutually 

 possess over each other. 



^ ix. Disruptive discharge and insulation. 



1359. The next form of discharge has been distinguished by the adjective disrup- 

 tive (1319.), as it in every case displaces more or less the particles amongst and across 

 which it suddenly breaks. I include under it, discharge in the form of sparks, 

 brushes, and glow (1405.), but exclude the cases of currents of air, fluids, &c., which, 

 though frequently accompanying the former, are essentially distinct in their nature. 



1360. The conditions requisite for the production of an electric spark in its sim- 

 plest form are well known. An insulating dielectric must be interposed between 

 two conducting surfaces in opposite states of electricity, and then if the actions be 

 continually increased in strength, or otherwise favoured, either by exalting the elec- 

 tric state of the two conductors, or bringing them nearer to each other, or dimi- 

 nishing the density of the dielectric, a sparh at last appears, and the two forces are 

 for the time annihilated, for discharge has occurred. 



1361. The conductors (which may be considered as the termini of the inductive 

 action) are in ordinary cases most generally metals, whilst the dielectrics usually 

 employed are common air and glass. In my view of induction, however, every di- 

 electric becomes of importance, for as the results are considered essentially de- 

 pendent on these bodies, it was to be expected that diff'erences of action never before 

 suspected would be evident upon close examination, and so at once give fresh con- 

 firmation of the theory, and open new doors of discovery into the extensive and 

 varied fields of our science. This hope was especially entertained with respect to the 

 gases, because of their high degree of insulation, their uniformity in physical con- 

 dition, and great difference in chemical properties. 



1362. All the effects prior to the discharge are inductive; and the degree of tension 

 which it is necessary to attain before the spark passes is therefore, in the examination 

 I am now making of the new view of induction, a very important point. It is the 



* Philosophical Transactions, 1827, p. 22. 



