HALO. 488 HALOPHYTK more commonly hexagonal pyramids. When a ray of light within a prism strikes an inner sur- face at an angle of incidence of about 80.5° it is totally reflected. When a beam of light passes through the two sides of a prism whose angle is 60°, in such a way as to suffer the minimum deviation, the latter will amount to about 21° 50'. Bearing these principles in mind, as also the fact that in an ordinary cloud the prisms have every possible position, we see that the general result will be that crystals that are near the position of minimum deviation will con- spire to refract the light in the same direction; some of the others will send the light in any direction, so the general result will be a bright circle of light surrounding the sun as a centre; its angular radius from the sun will be about 21° 50', or the so-called halo of 22°, which has a dark interior, an inner reddish edge, and a bright exterior. In still air the slender prisms of ice are likely to be suspended more nearly vertically. There- fore, their surfaces reflect a little sunlight as from a vertical mirror to the eye. When the ob- server is in the midst of a cloud or fog of such prisms, he sees a reflection of the sunlight forming a band of white light around the horizon at about half the apparenit angular attitude of the sun. This is called the pnrhelic circle. When other combinations of reflections from snow crystals occur so as to double or treble the brightness of particular spots in this parhelic circle, the.se spots are called mock suns, sun-dogs, or parhelia. A vertical arc may be produced by the reflections from the horizontal surfaces of snowflakes, and this arc may extend for a very considerable dis- tance above and below the sun. The most brilliant attendant of a halo is the tangential arc. which is sometimes seen touching the halo of 4(j° at its summit ; it can only be seen when the sun's altitude is between 12° and 30°, and is due to the reflection of the sunlight through ice needles whose refracting edges are horizontal. The geometrical study of halos was most thoroughly worked out by i?ravais, in his memoir of 1847 in the Jounud de I'Ecolc Poly- technique, vol. xviii. (Paris) : it is also quite fully presented by Maseart, Traite. de Voptique (Paris, 1896). 'The complete application of the theory of interference to the explanation of the phenomena of the supernumerary rings that ac- company halos, and especially rainbows, was given by Dr. Thomas Young in 1S04. but has more recently been presented in both elementary .".nd analytical methods by Dr. J. Pernter, of Vienna. The idea that cloudy particles are not solid small spheres of water, but hollow vessels like a soap-bubble, was abundantly disproved by Clausius, bvit still finds occasional mention in poimlar text-books. However, the arguments for and against this vesicular theory (consult Kober, in Poggendorn's Annalen. Berlin, 1871) show that it can have no standing in science. An exhaustive work on optical meteorology was in course of preparation in 100.'? by Prof. J. Pemter of Vieima. HAIiOANDER. ha'I6-ander, GREGORir.s (1501-.31). A German jurist whose family name was !Meltzer. He was born at Zwickau, and was educated in Xjcipzig. He published, at Nurem- berg, imder the protection and with the help of Wiiibald Pirkheimer: the Pandectce (1529) ; In- slitutiones (1529); Codex Justinianeus (1530); and, in Greek with a Latin version, fiovellce Con- Mitutioncs (1531). He also edited Epictctus's Enchiridion (1529). Consult: Schmidt, fe'j/mboia; (id Vitam Grerforii Haloandri (Leipzig. 1866), and I'lccliseg. (Ircgor Halouiider(Zvickau, 1872). HAI/OGENS (from Gk. fiXs, hah, salt -f -yfw/f, -genes, producing, from yiyvccdai, gig- nesthai, to be born). The name given to the four non-metallic elements, fluorine, chlorine, bro- mine, and iodine. The term was originally used by Berzelius. on account of the ease with which these elements form salts. The halogens combine directly with many of the other elements, much heat being evolved in the process. With hydro- gen, they form the well-known hydrofluoric, hy- drochloric, hydrobromic, and hydriodic acids, respectively. The halogene exhibit an unmistak- able gradation of physical properties. Thus, fluorine is a colorless gas: chlorine is a yellowish- green gas easily condensed to a liquid; bromine is a dark-red volatile liquid : iodine is a lus- trous grayish-violet solid. See Fluorine; Bro- mine: Chlorine: Ioiiixe. HAXOPHILOTJS PLANT (from Gk. aAf, hals, salt -- ip'iAor^ philos, loving). See Halo- PIIYTE. HAL'OPHYTE (from Gk. iT^c, hals. salt + ipvrdi', phi/lun, plant). Plants which grow natu- rally in soils or waters rich in various salts, espe- cially those with alkaline reactions. Common salt is quantitatively much the most important of these substances, although in certain localities alum, saltpetre, and other salts are abundant BALOFHTTES. Longitudinal section of the stem of sampliire. showing obliquely arranged palisade cells, also isolated water tracheids. enough to be of ecological importance. The veg<^- tation of the oceans and salt seas and lakes, as well as that along their marshy shores, represents the greater part of the halophytic vegetation of the world : however, there are great interior halophytic areas in arid climates, where seas or salt lakes have formerly existed. Sometimes salt springs or small areas of salty soil disclose a halophytic vegetation. While a great many species have, perhaps, become perfectly adjusted to soils or waters rich in salt, it is. nevertheless, true that these conditions are unfavorable for the development of the great majority of plants; and experiments have shown that many halophytes may grow as well, if not better, in. soils or waters
