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 the chemical party explained it as a consequence of chemical affinity or incipient chemical action between the metals and the surrounding air or moisture. There is also no doubt that the continued activity of a voltaic cell is always accompanied by chemical unions or decompositions; but while the chemical party asserted that these constitute the efficient source of the current, the contact party regarded them as secondary actions, and attributed the continual circulation of electricity to the perpetual tendency of the electromotive force of contact to transfer charge from one substance to another.

One of the most active supporters of the chemical theory among the English physicists immediately preceding Faraday was Peter Mark Roget (b. 1779, d. 1869), to whom are due two of the strongest arguments in its favour. In the first place, carefully distinguishing between the quantity of electricity put. into circulation by a cell and the tension at which this electricity is furnished, he showed that the latter quantity depends on the "energy of the chemical action" —a fact which, when taken together with Faraday's discovery that the quantity of electricity put into circulation depends on the amount of chemicals consumed, places the origin of voltaic activity beyond all question. Roget's principle was afterwards verified by Faraday and by De La Rive ; "the electricity of the voltaic pile is proportionate in its intensity to the intensity of the affinities concerned in its production," said the former in 1834; while De La Rive wrote in 1836, "The intensity of the currents developed in combinations and in decompositions is exactly proportional to the degree of affinity which subsists between the atoms whose combination or separation has given rise to these currents."