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



There may be some advantage in the change which I do not 

 perceive ; but if my view of the inefficiency of quantity be cor- 

 rect, it does not appear what may be the use of its quick passage. 



To all the foregoing reasonings it might be replied, that it is 

 in vain to search too minutely into the modus operandi of a na- 

 tural cause. The argument would be a good one if it were 

 proved that quantity is the natural cause of the phsenomenon ; 

 but this is the point at issue. Taking leave of the question 

 whether quantity can produce the effects attributed to it, let us 

 turn to the more profitable one of whether it does produce them. 



Those who sought to establish identity had long been embar- 

 rassed by the failure of all efforts to produce deviation of the 

 galvanometer needle by means of common electricity, although 

 it is so easily effected through the agency of voltaic. M . Col- 

 ladon of Geneva, imagining that this want of success was occa- 

 sioned by an insufficient supply of the electric fluid, or by im- 

 perfect insulation of the coil of the galvanometer, employed a 

 charged Leyden battery of 4000 square inches of coated surface, 

 and a galvanometer coil of 100 turns covered with double silk, 

 and oiled silk interposed between the layers. He armed each 

 extremity of his coil with a sharp point, and applied one of them 

 to the external coating of the battery. The other point being 

 approached, by means of a glass handle, to the ball connected 

 with the inside coating of the jars, the needle deviated 23° ; the 

 experiment was often repeated. What I conceive to be an im- 

 portant result of these trials is, that the deviation of the needle 

 increased with the intensity of the charge and the proximity of the 

 point to the ball of the battery. Sometimes the deviation amounted 

 to 40°, but the average was 20° to 30°. The direction of the 

 needle was determined by that of the current, according to a 

 well-known law. 



Laying aside the battery, he employed a Nairne's electrical 

 machine, furnished with a positive and negative conductor, one 

 extremity of the galvanometer being attached to each. When 

 the cylinder was made to revolve, he obtained a deflection of 3° 

 or 4° only ; but recollecting that the charge of a Leyden phial 

 passes, with little reduction of power, through a wire many 

 thousand metres in length, he made a galvanometer with a coil 

 of 500 turns, doubly covered with silk, the layers of the coil 

 being separated from each other by varnished silk. He knew 

 that, in the case of common electricity, the greater the number 

 of turns the greater would be the deflection, which however is 

 not the case with voltaic electricity. On connecting one end of 

 this coil with the rubber of a plate electric machine of 5 feet 

 diameter, and approaching the other end to the positive con- 

 ductor, a deflection ensued, which varied with the distance of 



