The concept of ion gradually emerged in the 19th century. It first appeared when it became necessary to explain the conduction of electricity through electrolytes. Odd so it may seem ions have not been related to atomism despite its renaissance from Dalton’s hypothesis. Indeed, the conception of electricity as fluid was so embedded that no natural philosopher thought it could be discrete. This idea ran during about fifty years, from the very first theory of electrolysis by Grotthuss in 1805 to Hittorf’s theory of migration (1853–1859). Faraday who introduced the word "ion" in 1834 was himself very cautious with the atomic hypothesis and did not relate it to ions. Clausius renewed the image of ions in the middle of the century by introducing atoms in his hypothesis of the existence of free ions within electrolytes. In the 1880’s, two factions fought each other on the very nature of ions: the first side led by J.J. Thomson pictured a discrete form of ions while the second side led by Wilhelm Ostwald held a continuous representation of ions, strongly influenced by energetism.
This case study allows to question the three kinds of discontinuity in sciences that have been selected for the Center for History of Physics conference: 1°) "ions" are alternatively conceived as theoretical entities and experimental traces during the second half of the century; 2°) they are studied by "natural philosophers" who gradually became distinguished into "physicists" and "chemists"; 3°) the transfer of knowledge from "physical sciences" to "biological sciences" occurred quickly after ions had become discrete entities.