The dissociation of hydrogen chloride may be expressed, in the terms of the assumptions made, in the following equation: HCl ⇄ H^{+} + Cl^{−}; that is, hydrogen chloride is dissociated, to a greater or smaller extent and in reversible fashion, into positively charged hydrogen ions H^{+}, and negatively charged chloride ions Cl^{−}, and the charge on each chloride ion is equal in quantity to the positive charge on each hydrogen ion. Zinc chloride is dissociated according to the equation ZnCl_{2} ⇄ Zn^{2+} + 2 Cl^{−}, and, according to (2), the charge on each zinc ion is twice as great in quantity as the charge on each chloride ion, and therefore twice as great also as the charge on each hydrogen ion (see below, p. 58). It is practically certain, according to more recent results, that the ions are combined with water to form hydrates, such as H^{+}(H_{2}O)_{‹x›} and Cl^{−}(H_{2}O)_{‹y›}.[65] This does not modify, essentially, the fundamental assumptions of the theory, but contributes rather to a satisfactory explanation of the rôle of water as an ionizing agent, a question to which we shall return later. «The Theory of Ionization and the Electron Theory of Electricity and of Matter.»[66]—According to the views held by many leading physicists at the present time, ‹negative electricity› consists of ultimate particles of matter called electrons or corpuscles. The mass of an electron is about 1 / 1000 the mass of an atom of hydrogen, and the electric charge of the electron is equal to that carried by a chloride ion in solution.[67] The atoms of the elements are considered to consist of aggregations of large numbers of electrons in a kind of "shell" or "body" of positive electricity. This positive electricity, in a given atom, is equal in quantity to the total negative charge of the electrons in the atom, the atoms as such [p043] containing no excess of either positive or negative electricity. The number of electrons in the atom of an element is considered to be definite and constant for that element, but the number varies as we go from the atoms of one element to those of a second element, the number increasing with the atomic weight of the element. One of the most fundamental and most characteristic properties of elements is considered to be the ‹affinity which their atoms show for electrons›; thus, the atoms of metals like sodium and potassium, which are generally called "electropositive" elements,[68] show an enormous tendency ‹to lose one electron› each and to form positively charged particles[69] Na^{-ε} (= Na^{+}) and K^{-ε} (= K^{+}).[70] The atoms of strongly electronegative elements, like chlorine, have a tremendous tendency for ‹gaining› and ‹holding electrons› beyond the number originally in such atoms. Thus, chlorine atoms tend to ‹assume an electron› each; they thereby become ‹negatively charged› particles, Cl^{+ε} ( = Cl^{−}).