3. Happening, by revolution, at a stated time; returning regularly, after a certain period of time; acting, happening, or appearing, at fixed intervals; recurring; as, periodical epidemics. The periodic return of a plant's flowering. (Henslow) To influence opinion through the periodical press. (Courthope)
4. Of or pertaining to a period; constituting a complete sentence. Periodic comet, the generalisation that the properties of the chemical elements are periodic functions of their atomic wieghts. In other words, if the elements are grouped in the order of their atomic weights, it will be found that nearly the same properties recur periodically throughout the entire series. The arrangement of the atomic weights shows the regular recurrence of groups, each consisting of members of the same natural family.
A similar relation had been enunciated in a crude way by Newlands; but the law in its effective form was developed and elaborated by Mendelejeff, whence it is sometimes called Mendelejeff's law. Important extensions of it were also made by L. Meyer. By this means Mendelejeff predicted with remarkable accuracy the hypothetical elements ekaboron, ekaluminium, and ekasilicon, afterwards discovered and named respectively scandium, gallium, and germanium.
Origin: L. Periodicus, Gr., cf. F. Periodique.