The periodicaltimes of all the satellites. Sir J. Herschel.

The periodic return of a plant's flowering. Henslow.

To influence opinion through the periodical press. Courthope.

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.

--------------------------------------------------------------
   ]I.   II.  III.  IV.  V.    VI.  VII.  VIII.
   ]                RH4  RH3   RH3  RH         
   ]R2O  RO   R3O3  RO2  R2O5  RO3  R2O7  RO4  
 --------------------------------------------------------------

Li 7

Na 23

K 39

(Cu) 63

Rb 85.2

(Ag) (108)

Cs 133

(-)

(-)

(Au) (197)

(-)

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* 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.

-- Periodic star (Astron.), a variable star whose changes of brightness recur at fixed periods. -- Periodic time of a heavenly body (Astron.), the time of a complete revolution of the body about the sun, or of a satellite about its primary.