

Faraday. 195 



and did not profess to have proved, that this process consists in 

 the actual movement of a quasi-substance, electricity, from one 

 plate of the condenser to the other, or of two quasi-substances, 

 the resinous and vitreous electricities, in opposite directions. 

 The process had been pictured in this way by many of his 

 predecessors, notably by Volta; and it has since been so 

 pictured by most of his successors : but from such assumptions 

 Faraday himself carefully abstained. 



What is common to all theories, and is universally conceded, 

 is that the rate of increase in the total quantity of electrostatic 

 charge within any volume-element is equal to the excess of the 

 influx over the efflux of current from it. This statement may 

 be represented by the equation 



|+divi = 0, (1) 



where p denotes the volume-density of electrostatic charge, 

 and i the current, at the place (x, y, z) at the time t. Volta's 

 assumption is really one way of interpreting this equation 

 physically: it presents itself when we compare equation (1) 

 with the equation 



which is the equation of continuity for a fluid of density p and 

 velocity v : we may identify the two equations by supposing i 

 to be of the same physical nature as the product /v; and 

 this is precisely what is done by those who accept Volta's 

 assumption. 



But other assumptions might be made which would equally 

 well furnish physical interpretations to equation (1). For 

 instance, if we suppose p to be the convergence of any vector of 

 which i is the time-flux,* equation (1) is satisfied automatically ; 



* In symbols, 



div 8 = - , 



where s denotes the vector in question. 



02 



