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



the thinnest foil, exposed a surface of only one-hundredth of a 

 square inch. The platinum plate was immersed in a mixture of 

 equal parts of nitric acid and water ; the zinc was then intro- 

 duced, when instantly the needle whirled round the circle com- 

 pletely. In order to estimate what was the equivalent of the 

 voltaic power that produced this effect, I arranged a couronne des 

 tasses of twenty pairs of copper and zinc plates, each three- 

 quarters of a square inch in surface, the exciting liquid being 

 dilute sulphuric acid. The polar wires were connected with an 

 electrometer, consisting of two gold leaves detached and insu- 

 lated from each other, and so contrived that they could be 

 made to recede or approach. By causing the gold leaves con- 

 nected with the polar wires to approach very gradually, it was 

 found that when they were within about one-tenth of an inch of 

 each other, an attraction was observable. Each gold-leaf proved 

 by calculation to weigh one-fiftieth of a grain ; consequently 

 each was 3000 times lighter than the galvanometer needle. The 

 couronne des tasses produced its electrical effects on the gold 

 leaves by a surface of metal more than 1000 times greater than 

 the pair of minute plates which whirled the needle round. If 

 then the electrical influence on the gold leaves was barely ob- 

 servable, the electrical effect of the minute pair of plates of pla- 

 tinum and zinc must have been the one-thousandth of a barely 

 observable influence, that is, practically speaking, no electrical 

 effect at all ; yet a violent impulse was communicated to a needle 

 3000 times heavier than each of the gold leaves. How is it 

 possible that the agent which influenced the gold leaves and the 

 galvanometer needle could be precisely the same ? 



In coming to the conclusion that the agent was different in 

 each case, we are relieved from all embarrassment relative to any 

 supposed influence of intensity or quantity, as it was voltaic 

 electricity that acted in both instances. I have represented the 

 effects here according to the condition most favourable to the 

 common hypothesis. I have taken the power of the couronne 

 des tasses in its state of highest intensity, that is, when the cir- 

 cuit was open, in order tbat the electrical power of the minute 

 pair of zinc and platinum plates might be fairly estimated. It 

 may be supposed that the quantity of electricity of the couronne 

 des tasses would have been greater had the circuit been closed, 

 but then there could have been no appreciable intensity. 



There is a familiar fact, which seems to give no small support 

 to the notion of the compound nature of the electric fluid, and 

 the different ratio in which its constituent elements exist under 

 various circumstances. Connect a zinc and silver plate by means 

 of a straight copper wire placed horizontally. Let a sharp poiut 

 of a sewing-needle be made to stand erect on the copper wire, 



