This term “ion” remained in comparative obscurity for more than half a century, when it was brought into great prominence among chemists by Arrhenius in connection with the ionic theory. _Cannizzaro’s Ideas._—Up to about 1869 chaos reigned among the formulas used by different chemists. Various compound radicals and numerous type-formulas were employed, dualistic and unitary formulas of several kinds were in use, but the worst feature of the situation was the fact that more than one system of atomic weights was in vogue, so that water might be written HO, H̶O, or H_{2}0 and similar discrepancies might appear in nearly all formulas containing elements of different valencies. In 1858, however, an article by the Italian chemist Cannizzaro appeared in which the outlines of a course in chemical philosophy were presented. This acquired wide circulation in the form of a pamphlet at a chemical convention somewhat later, and it dealt so clearly and ably with Avogadro’s principle, Dulong and Petit’s law, and other points in connection with formulas that it led to a rapid and almost universal reform among those who were using unsatisfactory formulas. At about this time also the dualistic formulas of Berzelius were generally abandoned, and hydrogen came to be regarded as the characteristic element of all acids. For instance, CaO.SO_{3}, called “sulphate of lime,” came to be written CaSO_{4} and was called “calcium sulphate,” and while it had been shown as early as 1815 by Davy that “iodic acid,” I_{2}O_{5}, showed no acid reaction until it was combined with water, the accumulation of similar facts led to the formulation of sulphuric acid as H_{2}SO_{4} instead of SO_{3} or H_{2}O.SO_{3}, and that of other “oxygen acids” in a similar way. As a necessary consequence of this view of acids, the bases came to be regarded as compounds of the “hydroxyl” group, OH. Therefore the formula for caustic soda came to be written NaOH instead of Na_{2}O.H_{2}O, and so on. _The Periodic System of the Elements._—The periodicity of the elements in connection with their atomic weights was roughly grasped by Newlands in England, who announced his “law of octaves” in 1863. This was at the time when the atomic weights were being modified and their numerical relations properly shown. The subject was worked out more fully by L. Meyer in Germany a little later, but it was most clearly and elaborately presented by the Russian chemist Mendeléeff in 1869. In order that this subject may be explained to some extent Mendeléeff’s table is given here, with the addition of the recently discovered elements and some other modifications.