GENERALIZA TION. 265 



decompose one single grain of water. Lightning was now 

 seen to be electricity of excessively high tension, but 

 extremely small quantity, the difference being somewhat 

 analogous to that between the force of one million gallons 

 of water falling through one foot, and one gallon of water 

 falling through one million feet. Faraday estimated that 

 one grain of water acting on four grains of zinc, would 

 yield electricity enough for a great thunderstorm. 



It was long believed that electrical conductors and in- 

 sulators belonged to two opposed classes of substances. 

 Between the inconceivable rapidity with which the cur- 

 rent passes through pure copper wire, and the apparently 

 complete manner in which it is stopped by a thin parti- 

 tion of gutta-percha or gum-lac, there seemed to be no 

 resemblance. Faraday, again, laboured successfully to show 

 that these were but the extreme cases of a chain of sub- 

 stances varying in all degrees in their powers of conduc- 

 tion. Even the best conductors, such as pure copper or 

 silver offer some resistance to the electric current. The 

 other metals have considerably higher powers of resist- 

 ance, and we pass gradually down through oxides and 

 sulphides. The best insulators, on the other hand, allow 

 of an atomic induction which is the necessary antecedent 

 of conduction. Hence Faraday inferred that whether we 

 can measure the effect or not, ah 1 substances discharge 

 electricity more or less r . One consequence of this doctrine 

 must be, that every discharge of electricity produces an 

 induced current. In the case of the common galvanic 

 current we can readily detect the induced current in any 

 parallel wire or other neighbouring conductor, and can 

 separate the opposite currents which arise at the moments 

 when the original currents begin and end. But a dis- 

 charge of high tension electricity like lightning, though 

 it certainly occupies time and has a beginning and an end, 



r 'Experimental Researches in Electricity,' Series xii. vol. i. p. 420. 



