200 Mr. M. Donovan on the supposed Identity of the Agent 



facts is, that electricity, whatever may he its source, is identical 

 in its nature. The phenomena in the five kinds or species quoted 

 differ, not in their character, but only in degree ; and in that 

 respect vary in proportion to the variable circumstances of quan- 

 tity and intensity*." 



In this expression of opinion I believe most, if not all, philo- 

 sophers of the present day agree, whether they view the electric 

 fluid as homogeneous or not. The same view is given by Davy, 

 Becquercl, Roget, and other eminent authorities, all following 

 in the footsteps of Cavendish ; I shall therefore devote my chief 

 attention to the consideration of it. 



It is, in the first place, necessary to understand precisely what 

 is meant by these terms quantity and intensity ; they have been 

 always used in explaining the difference between frictional and 

 voltaic electricity ; but, as it appears to me, without giving any 

 satisfactory account of how these conditions of the electric fluid 

 act. Dr. Faraday thus expresses himself : — "The term quantity 

 in electricity is perhaps sufficiently definite as to sense ; the term 

 intensity is more difficult to define strictly. I am using the 

 terms in their ordinary and accepted meaning." To understand 

 his views, it is therefore necessary to inquire what the " ordinary 

 and accepted meaning " of the word intensity is. 



Professor Hare gives a definition which I believe is a concise 

 enunciation of the opinions of most philosophers on this subject. 

 He asks, " What does intensity mean as applied to a fluid ? Is it 

 not expressed by the ratio of quantity to space ? If there be 

 twice as much electricity within one cubic inch as within another, 

 is there not twice the intensityf?" Elsewhere he says, he is 

 unable to form any other idea of intensity than " that of the 

 ratio of quantity to space." Sir H. Davy gives his opinion to the 

 same effect J. Dr. Roget, in his excellent essay on galvanism in 

 the Encyclojxedia Metropolitana,^. 208, says, " that when all the 

 circumstances relative to the conductors and the surrounding 

 bodies are the same, the intensity will increase in a certain ratio 

 with the absolute quantity of electricity that has been given to 

 the conductor by the machine." Mr. Snow Harris's opinion 

 may be resolved into the same meaning. Pouillet says that in- 

 tensity is " precisely in the inverse ratio to the length of the 

 circuit §." The same opinion is conveyed by Gmelin in the 

 following sentence : — " The difference between the effects of the 

 voltaic pile and those of the electric machine consists of the two 

 following points : 1, by means of the latter a large quantity of 



* Researches, par. 360. 

 t Philosophical Magazine, 1821, p. 292. 

 X Elements of Chemical Philosophy, p. 137. 

 § Elem. de Physique, i. 633 et seq. 



