We thus find that the most exact work on molecular weight determinations in ‹dilute› aqueous solutions agrees excellently, as does the conductivity of such solutions, with the demands of the theory of ionization, a fact which is particularly impressive because osmotic pressure and electrical conductivity are in no wise fundamentally related phenomena, and yet each, as a measure of ionization or electrolytic dissociation, leads independently to the same conclusion.[120] «The Chemical Composition of the Ions of Electrolytes.»—Accepting the theory of Arrhenius, we may now inquire more closely than heretofore, first, what compounds are subject to electrolytic dissociation, and then, what the chemical composition of their ions is and how it is determined. The compounds which are dissociated into ions, by solvents which cause ionization (p. 62), comprise the ‹salts›, the ‹acids› and the ‹bases›; chemists are, in fact, more inclined now to invert the statement and say that those substances which have long been known as salts, acids and bases, owe the essential characteristics, which led to their classification, to the fact that they are ionizable (see pp. 72–82). The composition of the ions, formed from the simpler of these compounds,[121] may be expressed by saying that the metal component or metal-like component (hydrogen, ammonium) forms the positive ion (cation, metal ion) and all the rest of the [p070] molecule forms the negative ion (anion, acid ion[122]). Thus, sodium chloride, nitrate, sulphate, phosphate yield the sodium-ion, Na^{+}, and the chloride (Cl^{−}), nitrate (NO_{3}^{−}), sulphate (SO_{4}^{2−}), or phosphate (PO_{4}^{3−}) ions; cupric nitrate Cu(NO_{3})_{2} dissociates into the cupric ion (Cu^{2+}) and the nitrate ion, calcium sulphate CaSO_{4} into the calcium-ion (Ca^{2+}) and the sulphate-ion, aluminium sulphate Al_{2}(SO_{4})_{3} into the aluminium-ion (Al^{3+}) and the sulphate-ion. But the question arises, as to how we know that the salts mentioned produce ions of the given composition; why, for instance, should sodium nitrate be considered to dissociate into sodium, Na^{+}, and nitrate ions, NO_{3}^{−}, the nitrogen atom carrying all of the oxygen atoms with it in the negative ion? [Illustration: FIG. 13.]