1874 Prediction of elementary charge or "electron" before its discovery 1826-1911 George Johnstone Stoney [Wikipedia]
1874	Stoney's first presentation regarding the concept of "elementary charge." Geroge Johnstone Stoney On the physical units of nature, in Notices and Abstracts of Miscellaneous Communications to the Sections, Report of the Forty-Fourth Meeting of the British Association for the Advancement of Science held at Belfast in August 1874 (John Murray, London, 1875) p. 22. [Archive]
1881	Stoney's first paper regarding the concept of "elementary charge." He estimated the value of the "definite quantity of electricity" (elementary charge) to be 3 x 10-11 esu. (One order of magnitude smaller than the present value 4.8 x 10-10 esu.) George Johnstone Stoney On the physical units of nature London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 5th Series, Volume 11, pp. 381-390 (1881). [DOI] George Johnstone Stoney On the physical units of nature [Read, February 16th, 1881] in The Scientific Proceedings of The Royal Dublin Society. Vol. III. (New Series). April, 1881. Part II. (The Royal Dublin Society, Dublin, 1881) pp. 51-60. (This Part II is a part of the book Vol. III published on 1883) [Archive] Helmholtz followed Stoney's idea If we accept the hypothesis that the elementary substances are composed of atoms, we cannot avoid concluding that electricity also is divided into definite elementary portions which behave like atoms of electricity. Professor Helmholtz (Hermann Ludwig Ferdinand von Helmholtz) On the modern development of Faraday's conception of electricity Journal of the Chemical Society, Transactions, Volume 39, pp. 277-304 (1881). [DOI]
1894	Stoney named it "electron". George Johnstone Stoney Of the "electron," or atom of electricity London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 5th Series, Volume 38, pp. 418-420 (1894). [DOI]
1833 1834	Stoney has been inspired by Faraday's First Law of Electrolysis. The number of atoms or molecules deposited or liberated at any electrode is directly proportional to the quantity of electricity or charge passed. Since the number of atoms (or molecules) is countable, this law suggests that electricity also can be countable, namely, can be expressed as a sum of elementary charges. Michael Faraday Experimental researches in electricity. Fifth series Philosophical Transactions of the Royal Society of London, Volume 123, pp. 675-710 (1833). [DOI] [JSTOR] Michael Faraday Experimental researches in electricity. Seventh series Philosophical Transactions of the Royal Society of London, Volume 124, pp. 77-122 (1834). [DOI] [JSTOR]