PYROXAM. 



POLARIZATION OF LIGHT. 



fcfis 



Tliia liquid ia usually insipid and tasteless, but sometimea it has an 

 acid taste. Cullen speaka of this disease as a common complaint in 

 Scotland, and it has been attributed to the employment of farinaceous 

 food, especially oatmeal, aa an article of diet. It usually comes on in 

 the early part of the day when the stomach is empty. The first 

 symptom of it is a pain at the pit of the stomach, with a sense of con- 

 striction as if the stomach were drawn towards the back. The pain is 

 increased by raising the body into an erect poature, and therefore the 

 body is bent forward. The pain is often very severe, and after con- 

 tinuing for some time it brings on an eructation of a thin watery fluid 

 in considerable quantity. These symptoms often occur without any 

 other indications of dyspepsia, although they are not unfrequently 

 accompanied with other indications of derangement of the digestive 

 organs. It sometimes occurs with violent neuralgic pains of the 

 stomach, constituting gastrodynia. Pyrosis also accompanies organic 

 disease of the stomach. 



In the treatment of this disease, the preparations of opium with 

 bismuth and astringents, as kino and catechu, are found to be of 

 most service ; at the same time the bowels require to be kept open, 

 which should be done by means of castor oil, confection of senna, or 

 other mild purgative. 



PYROXAM. [XvLOims.] 



PYROXANTHIN (C^.O. ?). A beautiful yellow crystalline pro- 

 duct of the action of alkalies upon wood-tar. Concentrated sulphuric 

 and hydrochloric acids dissolve it with the production of a dark red 

 colour. 



PYROXYLIC SPIRIT. [METHYL, hydrated oxide of.] 



PYROXYLIN. [Grx COTTON.] 



PYRROL. A volatile oily alkaloid, discovered by Runge in coal- 

 tar. Its composition ia not known. Anderson regards it as a mixture 

 of several substances. 



PYTHIAN GAMES (Pythia, or Pythici Ludi),one of the four great 

 national festivals of the Greeks, were celebrated near Delphi, in honour 

 of Apollo, originally every ninth year, and afterwards every fifth year, 

 in the autumn of the third year of each Olympiad, in the second or 

 third month of the year, according to Clinton. Corsini and others, 

 followed by Boeckh, place them in the spring, in the month Munychion, 

 the tenth of the year. Their origin is assigned by some to Amphic- 

 tyou, the son of Deucalion, or to the Amphictyonic council ; by 

 others to Agamemnon ; by Pausanias to Diomed ; by Strabo to the 

 Delphiana, after the Crissaean war; but most commonly to Apollo, 

 after he had vanquished the serpent Python. (Ovid, ' Met.', i. 445.) 

 There is an account that the gods and heroes contended in the first 

 celebration of these games, when Castor conquered in the horse-race, 

 Pollux in boxing, Calais in the foot-race, Zetes in fighting in armour, 

 Peleus in throwing the quoit, Telamon in wrestling, and Heracles in 

 the Pancratium. But the fact seems to be, as stated by Pausanias 

 (x. 7, 2) and Strabo (ix., p. 421), that the contest was originally in 

 music : the songs (xvOtitol v6poi) were in honour of Apollo, celebrating 

 his victory over the Python ; and the instrument used was the lyre. 

 In the third year of the 48tb Olympiad (B.C. 586), at the close of the 

 C'irrhiean war, the Amphictyons added a content on the flute, which 

 was afterwards discontinued, as the music of the flute was considered 

 too mournful for a joyous festival. In the same year the Amphic- 

 tyons also introduced athletic contests and races (but not with four- 

 horsed chariots), the foot-race being confined to boys ; and the games, 

 according to Strabo, were then for the first time called Pythia ; at all 

 events the subsequent Pythia are computed from this year by Pausa- 

 nias and the Parian marble, though the scholiast on Pindar, and 

 Eusebius, date them from the second celebration, in Ol. 49, 3 : Boeckh 

 and Clinton prefer the former date. Chariot-races were added in the 

 time of Cleisthenes, the tyrant of Sicyon. Statues, pictures, and other 

 works of art, were also exhibited at the Pythia, and prizes adjudged to 

 the most successful artists. The prize in the Pythian games was 

 originally of silver or gold, or something else of intrinsic value ; but 

 afterwards a crown of laurel, or (according to Ovid, ' Met.', i. 449-50) 

 at first, of the bay-oak or beech-tree (cescitlia), for which the laurel 

 was afterwards substituted. The ceremonies observed at these games 

 in common with the three other great festivals, are described under 

 OLTMFIAN GAMES. The attendance at the games was very great, as 

 all Greeks were permitted to be present at them. The Pythia are 

 believed not to have been entirely discontinued till about the same 

 time as the Olympian games, 394 A.D. 



(Pausanias,!. 7; Strabo, ix. ; Potter's Archaoloriia Graca, vol. i. ; 

 Wacbsmuth, lldlenitchc AUertkumtk. ; Clinton, Fasti IltUen. ; Krause, 

 Die Pytkien.) 



PYX, or PIX, the box or casket in which is kept the consecrated 

 host reserved for the sick in the Romish church. Among the ancients 

 the pyxis (iS{is) was the casket in which ladies kept their jewels and 

 other ornaments. It was itself often made of the most costly 

 materials, and enriched with sculpture or with gems. There can be 

 little doubt that it was from the ancient jewel-box that the mediaeval 

 ecclesiastics derived the name of their pyx, and perhaps also the idea 

 of its enrichment. At any rate the pyxes of mediaeval date which still 

 remain are among the costliest examples of the art-workmanship of the 

 middle ages. Some good ones are in the South Kensington Museum ; 

 others are preserved among the plate of some of the colleges of Oxford 

 and Cambridge, but the finest are in the treasuries of continental 



churches. The pyx was placed upon the altar under a covering or canopy, 

 and at its elevation the sacring bell was rung. Among the French 

 especially the pyx seems to have been not unfrequently in the form of a 

 dove, enamelled and enriched with gems. The term pyx was sometimea 

 applied to the casket in which relics were kept. [RELIQUARY.] 



PYXIS NAUTICA (the mariner's compass), a southern constella- 

 tion of Lacaille, placed in Argo. It contains no stars of conspicuous 

 magnitude. 



POLARIZATION OF LIGHT is a peculiar affection of that agent 

 to which the term has been applied because a ray of polarized light 

 possesses properties which have relation to two opposite directions in a 

 plane "perpendicular to the ray. We shall first describe some of the 

 circumstances under which polarized light is formed, and the 

 characters by which it is recognised as polarized, and afterwards 

 explain the view which is taken of its nature in the theory of undula- 

 tions, the only theory which haa given a simple and intelligible account 

 of its complicated phenomena. 



The affection of light now known as polarization was discovered by 

 Huygens in the course of his researches on the laws of double refraction 

 in Iceland spar. A rhomb of this crystal has the property of dividing 

 a ray of light incident upon it into two rays, which are propagated in 

 different directions within the crystal, and produce each an emergent 

 ray parallel to the incident, so that the two emergent rays are 

 separated laterally by a space proportional, cteteris paribus, to the thick- 

 ness of the rhomb, the direction of separation (supposing for simplicity 

 the ray to be incident perpendicularly) being that of a line bisecting 

 an obtuse angle of the rhombic face, and the intensity of the two 

 emergent rays being under ordinary circumstances the same. Such 

 a rhomb gives two images, of equal intensity, of an object on which 

 it ia placed. Now Huygens found that either of the pencils separated 

 by a first rhomb is affected by a second rhumb quite differently from a 

 pencil of common light. When the second rhomb ia placed with its 

 faces parallel to the corresponding faces of the first rhomb, so that the 

 two are in the same relative position as if they formed parts of a 

 larger block of the crystal, each pencil separated by the first rhomb 

 furnishes only a single pencil in the second rhomb, the ordinary only 

 an ordinary, and the extraordinary only an extraordinary, the two 

 being separated on emergence as much as if they had been transmitted 

 through a single block having a thickness equal to the sum of the 

 thicknesses of the two rhombs. If now the second rhomb be turned 

 round the common normal to the adjacent faces of the two rhombs, 

 each pencil transmitted by the first rhomb is immediately divided into 

 two by the second rhomb, the pencil (o) which suffered ordinary 

 refraction in the first rhomb furnishing besides the ordinary pencil (o o) 

 in the second a faint extraordinary pencil (o E), and the pencil (E) which 

 Buffered extraordinary refraction in the first rhomb furnishing besides 

 an extraordinary pencil (E E) in the second a faint ordinary (E o) ; the 

 direction and amount of separation of the pencils o o and o E, and 

 likewise of E o and E E, being those due to the azimuth and thickness 

 of the second rhomb, while the direction and amount of separation of 

 o o and E o, and likewise of o E and E E, are those due to the first 

 rhomb. On continuing to turn, the faint pencils o E and E o become 

 brighter and brighter, and the bright pencils o o and E E fainter and 

 fainter, until the rotation amounts to 90, when the original pencils o o 

 and BE disappear altogether, the whole of the light of o (not counting 

 the small portion lost by reflection) now passing into o E, and the 

 whole of the light of E into E o. On further turning, the pencils o o, 

 E E which vanished reappear, and appropriate to themselves more and 

 more of the light of o, E, respectively, until the rotation amounts to 

 180, when the pencils o E, EO vanish, while o o, E E become as bright 

 as at first, their lateral separation being now, however, that due to the 

 difference instead of the < of the thicknesses of the rhombs. On 

 continuing to turn, the same changes recur periodically, each pair of 

 images vanishing alternately at every quarter of a revolution. 



Now imagine the ordinary pencil o to be isolated by means of a 

 screen, and presented for observation, the apparatus by which it was 

 produced being hidden from the observer. By examining it with a 

 rhomb of Iceland spar he could distinguish it at once from common 

 light, which has no relation to any other direction in space than that 

 of its propagation, whereas the pencil we have supposed possesses 

 different properties with reference to different directions transverse to 

 that of its propagation. By turning the rhomb with which he was 

 furnished into either of the two opposite positions in which the extra- 

 ordinary pencil vanishes, the plane of crystalline symmetry in the 

 rhomb would mark a plane determined toldy by the properties of the pencil 

 of liijht presented for obsei-vation, and which (as well as its rectangular 

 plane) is a plane of symmetry in relation to those properties. The 

 plane of crystalline symmetry passing through the normal to the face 

 of the rhomb is evidently parallel to a line bisecting either obtuse 

 angle of the rhombic face, and contains the crystallographic axis. Any 

 plane paaaiug through this axis in Iceland spar is called in optical 

 language a principal plane, because all such planes are planes of 

 optical, though six of them only of crystallographic, symmetry. 



If our observer were further informed that the light he had been 

 observing was the ordinary ray transmitted through a rhomb of Iceland 

 spar, he would be able to determine the azimuth of the principal plane 

 of that rhomb, which would be the same as the principal plane of the 



