\Per`i*od"ic\, a. [Pref. per- + iodic.] (Chem.)
Pertaining to, derived from, or designating, the highest
oxygen acid (HIO?) of iodine.

\Pe`ri*od"ic\, Periodical \Pe`ri*od"ic*al\, a. [L.
periodicus, Gr. ?: cf. F. p['e]riodique.]
1. Of or pertaining to a period or periods, or to division by
   periods.
         The periodicaltimes of all the satellites. --Sir J.
                                               Herschel.
2. Performed in a period, or regular revolution; proceeding
   in a series of successive circuits; as, the periodical
   motion of the planets round the sun.
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. (Rhet.) Of or pertaining to a period; constituting a
   complete sentence.
{Periodic comet} (Astron.), a comet that moves about the sun
   in an elliptic orbit; a comet that has been seen at two of
   its approaches to the sun.
{Periodic function} (Math.), a function whose values recur at
   fixed intervals as the variable uniformly increases. The
   trigonomertic functions, as sin x, tan x, etc., are
   periodic functions. Exponential functions are also
   periodic, having an imaginary period, and the elliptic
   functions have not only a real but an imaginary period,
   and are hence called doubly periodic.
{Periodic law} (Chem.), the generalization 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 following
   tabular arrangement of the atomic weights shows the
   regular recurrence of groups (under I., II., III., IV.,
   etc.), each consisting of members of the same natural
   family. The gaps in the table indicate the probable
   existence of unknown elements.