This simple fact, that the very solutions which give abnormally low molecular weights for the dissolved compounds are also good conductors of electricity, was explained by a theory of ‹electrolytic dissociation› or of ‹ionization›, which Arrhenius had developed[62] from a study of the conductivity of electrolytes. The same fact has aided in establishing this theory which has led to the elucidation of vital problems of ‹electrical conductivity› and to a successful [p041] explanation of the problem of the apparently abnormal ‹osmotic pressures› (and ‹molecular weights›) of electrolyte solutes. It has thus removed the last difficulty in the way of accepting the van 't Hoff-Avogadro Hypothesis (p. 15) as true for all dilute solutions, exactly as the discovery of ‹gaseous dissociation› made it possible to recognize in the original Avogadro Hypothesis a universal truth (p. 36) about gases. And to these results was added, chiefly as the fruit of the work of Ostwald, with the aid of the theory of Arrhenius, the most successful and accurate formulation of the problem of the ‹chemical activity› of electrolytes, known in the history of chemistry. «Main Assumptions of Arrhenius's Theory of Ionization.»—The main assumptions of the theory of electrolytic dissociation or ionization are the following: (1) When an ionogen is dissolved in water, its molecules are immediately, more or less completely, ‹dissociated by the water› into smaller fragments or molecules of unlike composition. (2) These new molecules are charged with ‹electricity›; the molecules of the one product are charged with ‹positive›, the molecules of the other product[63] with ‹negative electricity›, the unit positive charge being equal in quantity, but opposite in kind, to the unit negative charge; the sum of all the positive charges in a solution is equal to the sum of all the negative charges, and the whole solution is electrically neutral. (3) The dissociation is a reversible reaction, and all electrolytes must be considered to be ‹completely ionized at infinite dilution›. (4) Except for the dependence resulting from the electrical charges and the consequent attractions and repulsions between ions, the ions must be considered independent molecules with their own ‹specific chemical› and ‹physical› properties. When a current is passed through the solution of an ionogen, the electrified particles carry their charges to the electrodes (see [p042] below). They are called the ‹ions›[64] of the electrolyte; the positively charged ions are distinguished as ‹cations› from the negatively charged ‹anions›, and the electrode toward which the cations move is called the cathode (negative electrode), and the electrode to which the anions move is called the ‹anode› (positive electrode).