PERIHELION 



PERISCOPE 



Perihelion (Gr. peri, around; 

 helios, the sun). In astronomy, the 

 point in the orbit of a planet or 

 comet at which it makes its nearest 

 approach to the sun. It is opposite 

 to aphelion, the farthest distance, 

 the line joining the two points 

 being known as the line of apsides. 

 See Aphelion ; Apsides. 



Perim. Rocky island in the 

 straits of Bab-el-Mandeb under 

 British control. It commands the 

 entrance to, the Red Sea, and is 

 situated 97 m. W. of Aden. It is 

 about 5 sq. m. in area, and is a 

 coaling depot for the mercantile 

 marine. It was held by the British 

 1799-1800, occupied 1857, and is 

 now administered as a dependency 

 of Aden. See Bab-el-Mandeb. 



Perimeter (Gr. peri, around; 

 metron, measure). Line or lines 

 bounding a closed geometrical 

 figure, or any area or surface. 



Perineum. That part of the 

 body which forms the external 

 floor of the pelvis (q.v. ). 



Period (Gr.peri, around; hodos, 

 path). Word used in a number of 

 senses. Historically, it indicates a 

 cycle of years or events, e.g. the 

 Louis XV period, the Common- 

 wealth period, etc. In astronomy, 

 it indicates a definite cycle, as the 

 lunar period or Metonic cycle of 19 

 years. It is also used in astronomy 

 for the time required for a celestial 

 body to complete a revolution in 

 its orbit, e.g. the earth's period is 

 365 days, etc. In mechanics the 

 word is used for the interval of 

 time between the recurrent phases 

 of an oscillation or vibration, e.g. 

 that of a pendulum or a tuning- 

 fork. A periodicity is the regular 

 recurrence of the same phenomena 

 in the same times. A large number 

 of movements in nature are peri- 

 odic, e.g. movements of waves. 



Periodic Law. In chemistry, a 

 basis of classification of the ele- 

 ments. Ohancourtois, in 1862, 

 arranged the elements according 

 to their atomic weights along a 

 spiral line drawn round a cylinder. 

 In 1864 Newlands noticed that 

 when the elements are placed in 

 order of their atomic weights, in 

 seven horizontal series, each con- 

 sisting of eight members, each set 

 of elements thus placed possesses 

 somewhat similar properties. 



Mendeleev, in 1869, independ- 

 ently arrived at a periodic classifi- 

 cation of the elements which he 

 elaborated in considerable detail. 

 This arrangement, brought up to 

 date, is shown in the table. 

 Blank spaces denote the position of 

 elements which may yet be dis- 

 covered. When Mendeleev drew 

 up his table he predicted that 

 other elements with certain pro- 

 ]>erties would be discovered. These 



conditions were fulfilled by gal- 

 lium, discovered in 1875, scan- 

 dium hi 1879, germanium in 1887. 

 Perioeci (Gr. perioikoi, those 

 who dwell around). Intermediate 

 class in the Spartan state, de- 

 scendants of the pre-Dorian popu- 

 lation with an admixture of other 

 elements. They were above the 

 Helots (q.v.) or serfs, could farm 

 their own land and engage in trade, 

 but had not the political rights of 

 the full Spartan citizens, though 

 they were liable for military ser- 

 vice. Pron. Perry-eessl. 



between that of the annelids and 

 the arthropods. It is believed to be 

 related to the original progenitor of 

 all the air-breathing Arthropoda. 

 The cylindrical body is built up of 

 soft rings, and has 17 pairs of short 

 conical feet ending in a pair of 

 hooked claws. The head bears a 

 pair of antennae, two simple eyes, 

 and a mouth provided with swollen 

 lips and four horny jaws. Breath- 

 ing is effected by means of short 

 air tubes, which have their open- 

 ings scattered irregularly over the 

 whole surface. It has paired neph- 



ELEMENTS ARRANGED ACCORDING TO THE PERIODIC LAW 



SHORT PERIODS 



LOSG PERIODS 



NOTE. For explanation of symbols used see under Chemical Signs. Be is 

 the symbol for Beryllium, U for Uranium 



Periostitis (Gr. peri, around; 

 ostea, bones). Inflammation of 

 the periosteum or fibrous mem- 

 brane which invests bones. Acute 

 periostitis is most frequently the 

 result of an injury. It may also 

 develop from inflammation in the 

 neighbourhood of the bone, as, 

 for instance, periostitis of the 

 jaw from a carious tooth. The 

 symptoms are pain, and redness 

 and swelling of the limb over the 

 site of the inflammation. 



Peripatetics (Gr. peripatetikos, 

 walking about). Name given to 

 the followers of Aristotle. They 

 were so called either because their 

 master and his successors were in 

 the habit of delivering their lectures 

 walking about in the Lyceum, or 

 from its walks (peripatoi). The 

 chief Peripatetics are Theophrastus, 

 Eudemus, Aristoxenus, Strato, An- 

 dronicus of Rhodes, and Alexander 

 of Aphrodisias. 



Peripatus. Caterpillar-like ani- 

 mal about three inches in length, 

 whose organization is intermediate 



ridia in each of its segments after 

 the first two, a central nervous 

 system, and a main vascular trunk 

 or heart. Originally it was classed 

 as a slug, owing to its copious 

 ejection of a viscid fluid, which, 

 however, appears to serve for the 

 capture of insects ; later, some of 

 the structural features caused it to 

 be regarded as a worm. It has 

 since been put in a class (Onycho- 

 phora) by itself. The young are 

 produced alive. About 50 species 

 are known, which have been found 

 in S. Africa, S. America, the W. 

 Indies, and New Zealand. 



Peripatus. the caterpillar-like animal 

 of the class Onychophora 



Periscope (Gr. peri, around ; 

 skopein. to watch). Apparatus for 

 observing from a concealed 



