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



battery so charged, " extended to the shoulders with much force ;" 

 but they " had not however the same force as those of the pile 

 from which they had been charged." He estimated the force of 

 the shock given by the Leyden battery, when charged by the 

 200 pairs, as equal to the shock of 100 pairs when taken directly 

 from the pile itself; that is, in all his trials with different num- 

 bers of pairs, the charge communicated to the Leyden battery 

 had but half the power of the pile which afforded it. He then 

 endeavoured to discover the ratio of the charging power of the 

 pile compared with that of a 31-inch plate-machine belonging to 

 the Teylerian Museum (not the large one) ; and after many ex- 

 periments, found that one momentary contact of the pile with 

 the Leyden battery charged the latter to the intensity above- 

 mentioned, while six equally momentary contacts with the con- 

 ductor of the plate-electrical machine, made by an insulated rod, 

 were required to bring the Leyden battery to the same intensity. 

 But as no account is given of the rate at which the plate was re- 

 volving at the instant of contact, the ratio of 1 to 6 taken by 

 itself conveys no information*. 



It appears, however, that the shock from this immense Leyden 

 battery must have been very small. Those who have had expe- 

 rience of piles, are aware of these feeble effects compared with 

 other forms of the voltaic apparatus. The superincumbent 

 weight of the column presses the greater part of the exciting 

 liquid from the interposed cloths ; streams of a good conducting 

 liquid are therefore continually trickling down, while the cloths 

 are left almost dry ; the insulation of the pairs is thus destroyed, 

 and the chemical action on the zinc is rendered feeble. Hence 

 the power of so small a number as 200 pairs to give shocks, 

 when reduced to one-half by being transferred to the Leyden 

 battery, could not be great. This conclusion is supported by 

 the fact that the power of the 200 pairs to produce divergence 

 of the gold-leaf electrometer, through the intervention of the 

 Leyden battery, was equalled by the same battery when charged 

 with six contacts, during a time " as short as possible," with the 

 conductor of a 31-inch plate-machine, the period of each contact 

 being estimated by Van Marum at one-twentieth of a second. 

 Some estimate of the electricity thus thrown in may be formed 

 by comparing the effects of this 31 -inch plate-machine with 

 those produced by a powerful one belonging to the Royal Insti- 

 tution, the plate of which is nearly of three times greater area 

 (viz. 50 inches diameter) than Van Marum's. This large plate, 

 as Professor Faraday informs us, gives ten or twelve sparks of 

 an inch long for each revolution, the revolution occupying four- 

 fifths of a second of time. At this rate, supposing each revolu- 

 * Annates de Chimie, xl. p. 2S ( .K 



