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



neous elements held together by an attractive force, varying in 

 the ratio and perhaps in the mode of combination according to 

 the circumstances of the excitement which produced it, we shall 

 be at no loss to understand why electricity appears under such 

 different forms as those called frictional, voltaic, thermal and 

 magnetic, and why these different forms pervade and ai'e con- 

 ducted by the same bodies. It has always been one of the chief 

 arguments of those who contend for the identity of the voltaic 

 and electric agents, that they are transmitted by the same kind 

 of matter. But this is what ought to happen, according to the 

 view here given ; and hence the conduction of the different kinds 

 of electricity by the same bodies is of no force as an argument 

 for identity and against dissimilarity. The passage over the 

 same conductors may very well take place if the elementary con- 

 stituents be the same, although the other circumstances are so 

 different as to impress on the agent the character of total dissi- 

 milarity. It is however to be expected that this difference of 

 circumstances would produce some difference in the effects. 

 That it does is abundantly evident ; so much so, indeed, that 

 there are very few points of real resemblance. 



One very striking difference of properties, which may be fairly 

 attributed to the variation of constitution in the electric agent, 

 is the facility with which statical electricity is conducted by water, 

 and the insuperable difficulty experienced when attempts are 

 made to pass thermo-electricity or thermo-triboelectricity through 

 the smallest portion of even salt water, as if the constituent ele- 

 ment were absent that acts as the vector of all those which in 

 their own nature do not move through conductors. The most 

 powerful frictional electric machine can scarcely be excited if the 

 atmosphere be damp ; and a small jar or large battery will not 

 retain the highest or lowest intensity if the glass be not perfectly 

 dry. But so different is the constitution of electricity furnished 

 by heating two very slender wires of different metals in contact, 

 that although they will cause deflection of the galvanometer wire 

 to 60° (which the most powerful frictional machine and battery 

 will scarcely effect), yet such electricity will not pass through. 

 ^\yth of an inch of salt water interposed between the conducting 

 wires. In an essay not long since read to the Royal Irish Aca- 

 demy, I showed that by causing rapid revolution of a bismuth 

 wheel against a rubber of antimony, each metal being connected 

 with the galvanometer, the needle stood permanently at 60° or 

 70° ; but the interposition of ^th of an inch of salt water be- 

 tween the conducting wires stopped the current, although 40 

 inches of the same water were traversed by the electricity arising 

 from a surface of zinc and a surface of platinum, each half an 

 inch square, the galvanometer needle stauding permanently at 



