fourth-class degree, but upon his defense it was reclassified as third-class. Later, extensions of this very work would earn him the 1903 Nobel Prize in Chemistry. Arrhenius put forth 56 theses in his 1884 dissertation, most of which would still be accepted today unchanged or with minor modifications. The most important idea in the dissertation was his explanation of the fact that solid crystalline salts disassociate into paired charged particles when dissolved, for which he would win the 1903 Nobel Prize in Chemistry. Arrhenius's explanation was that in forming a solution, the salt disassociates into charged particles that Michael Faraday had given the name ions many years earlier. Faraday's belief had been that ions were produced in the process of electrolysis, that is, an external direct current source of electricity was necessary to form ions. Arrhenius proposed that, even in the absence of an electric current, aqueous solutions of salts contained ions. He thus proposed that chemical reactions in solution were reactions between ions. The dissertation did not impress the professors at Uppsala, but Arrhenius sent it to a number of scientists in Europe who were developing the new science of physical chemistry, such as Rudolf Clausius, Wilhelm Ostwald, and Jacobus Henricus van 't Hoff. They were far more impressed, and Ostwald even came to Uppsala to persuade Arrhenius to join his research team. Arrhenius declined, however, as he preferred to stay in Sweden-Norway for a while (his father was very ill and would die in 1885) and had received an appointment at Uppsala. In an extension of his ionic theory Arrhenius proposed definitions for acids and bases, in 1884. He believed that acids were substances that produce hydrogen ions in solution and that bases were substances that produce hydroxide ions in solution. Middle period In 1885, Arrhenius next received a travel grant from the Swedish Academy of Sciences, which enabled him to study with Ostwald in Riga (now in Latvia), with Friedrich Kohlrausch in Würzburg, Germany, with Ludwig Boltzmann in Graz, Austria, and with Jacobus Henricus van 't Hoff in Amsterdam. In 1889, Arrhenius explained the fact that most reactions require added heat energy to proceed by formulating the concept of activation energy, an energy barrier that must be overcome before two molecules will react. The Arrhenius equation gives the quantitative basis of the relationship between the activation energy and the rate at which a reaction proceeds. In 1891,