(D. Gi.)
1The circles by Reichenbach, then almost exclusively used in Germany, were read by verniers only.2The diameter of Venus was measured with one of these heliometers at the observatory of Breslau by Brandes in 1820 (Berlin Jahrbuch, 1824, p. 164).3The distances of the optical centres of the segments from the eye-piece are in this method as 1; secant of the angle under measurement. In Bessel’s heliometer this would amount to a difference of15⁄1000th of an inch when an angle of 1° is measured. For 2° the difference would amount to nearly1⁄10th of an inch. Bessel confined his measures to distances considerably less than 1°.4In criticizing Bessel’s choice of methods, and considering the loss of time involved in each, it must be remembered that Fraunhofer provided no means of reading the screws or even the heads from the eye-end. Bessel’s practice was to unclamp in declination, lower and read off the head, and then restore the telescope to its former declination reading, the clockwork meanwhile following the stars in right ascension. The setting of both lenses symmetrically would, under such circumstances, be very tedious.5This most important improvement would permit any two stars under measurement each to be viewed in the optical axis of each segment. The optical centres of the segments would also remain at the same distance from the eye-piece at all angles of separation. Thus, in measuring the largest as well as the smallest angles, the images of both stars would be equally symmetrical and equally well in focus. Modern heliometers made with cylindrical slides measure angles over 2°, the images remaining as sharp and perfect as when the smallest angles are measured.6Bessel found, in course of time, that the original corrections for the errors of his screw were no longer applicable. He considered that the changes were due to wear, which would be much lessened if the screws were protected from dust.7The tube, being of wood, was probably liable to warp and twist in a very uncertain way.8We have been unable to find any published drawing showing how the segments are fitted in their cells.9We have been unable to ascertain the reasons which led Bessel to chooseivoryplanes for the end-bearings of his screws. He actually introduced them in the Königsberg heliometer in 1840, and they were renewed in 1848 and 1850.10A screen of wire gauze, placed in front of the segment through which the fainter star is viewed, was employed by Bessel to equalize the brilliancy of the images under observation. An arrangement, afterwards described, has been fitted in modern heliometers for placing the screen in front of either segment by a handle at the eye-end.11This heliometer resembles Bessel’s, except that its foot is a solid block of granite instead of the ill-conceived wooden structure that supported his instrument. The object-glass is of 7.4 in. aperture and 123 in. focus.12Description de l’observatoire central de Pulkowa, p. 208.13Steinheil applied such motion to a double-image micrometer made for Struve. This instrument suggested to Struve the above-mentioned idea of employing a similar motion for the heliometer.14Manuel Johnson, M.A., Radcliffe observer,Astronomical Observations made at the Radcliffe Observatory, Oxford, in the Year 1850, Introduction, p. iii.15The illumination of these scales is interesting as being the first application of electricity to the illumination of astronomical instruments. Thin platinum wire was rendered incandescent by a voltaic current; a small incandescent electric lamp would now be found more satisfactory.16For a detailed description of this instrument seeDunecht Publications, vol. ii.17Mem. Royal Astronomical Society, xlvi., 1-172.18The primary object was to have the object-glass mounted in steel cells, which more nearly correspond in expansion with glass. It became then desirable to make the head of steel for sake of uniformity of material, and the advantages of steel in lightness and rigidity for the tube then became evident.19For description of the earliest form seeCambridge Phil. Trans.vol. ii., andGreenwich Observations(1840).20Dawes (Monthly Notices, January 1858, andMem. R.A.S.vol. xxxv. p. 150) suggested and used a valuable improvement for producing round images, instead of the elongated images which are otherwise inevitable when the rays pass through a divided lens of which the optical centres are not in coincidence, viz. “the introduction of a diaphragm having two circular apertures touching each other in a point coinciding with the line of collimation of the telescope, and the diameter of each apertureexactly equalto the semidiameter of the cone of rays at the distance of the diaphragm from the local point of the object-glass.” Practically the difficulty of making these diaphragms for the different powers of theexactrequired equality is insuperable; but, if the observer is content to lose a certain amount of light, we see no reason why they may not readily be made slightly less. Dawes found the best method for the purpose in question was to limit the aperture of the object-glass by a diaphragm having a double circular aperture, placing the line joining the centres of the circles approximately in the position angle under measurement. Dawes successfully employed the double circular aperture also with Amici’s micrometer. The present writer has successfully used a similar plan in measuring position angles of a Centauri with the heliometer, viz. by placing circular diaphragms on the two segments of the object-glass.21Dollond provides for changing the power by sliding the lens d nearer to or farther froma.
1The circles by Reichenbach, then almost exclusively used in Germany, were read by verniers only.
2The diameter of Venus was measured with one of these heliometers at the observatory of Breslau by Brandes in 1820 (Berlin Jahrbuch, 1824, p. 164).
3The distances of the optical centres of the segments from the eye-piece are in this method as 1; secant of the angle under measurement. In Bessel’s heliometer this would amount to a difference of15⁄1000th of an inch when an angle of 1° is measured. For 2° the difference would amount to nearly1⁄10th of an inch. Bessel confined his measures to distances considerably less than 1°.
4In criticizing Bessel’s choice of methods, and considering the loss of time involved in each, it must be remembered that Fraunhofer provided no means of reading the screws or even the heads from the eye-end. Bessel’s practice was to unclamp in declination, lower and read off the head, and then restore the telescope to its former declination reading, the clockwork meanwhile following the stars in right ascension. The setting of both lenses symmetrically would, under such circumstances, be very tedious.
5This most important improvement would permit any two stars under measurement each to be viewed in the optical axis of each segment. The optical centres of the segments would also remain at the same distance from the eye-piece at all angles of separation. Thus, in measuring the largest as well as the smallest angles, the images of both stars would be equally symmetrical and equally well in focus. Modern heliometers made with cylindrical slides measure angles over 2°, the images remaining as sharp and perfect as when the smallest angles are measured.
6Bessel found, in course of time, that the original corrections for the errors of his screw were no longer applicable. He considered that the changes were due to wear, which would be much lessened if the screws were protected from dust.
7The tube, being of wood, was probably liable to warp and twist in a very uncertain way.
8We have been unable to find any published drawing showing how the segments are fitted in their cells.
9We have been unable to ascertain the reasons which led Bessel to chooseivoryplanes for the end-bearings of his screws. He actually introduced them in the Königsberg heliometer in 1840, and they were renewed in 1848 and 1850.
10A screen of wire gauze, placed in front of the segment through which the fainter star is viewed, was employed by Bessel to equalize the brilliancy of the images under observation. An arrangement, afterwards described, has been fitted in modern heliometers for placing the screen in front of either segment by a handle at the eye-end.
11This heliometer resembles Bessel’s, except that its foot is a solid block of granite instead of the ill-conceived wooden structure that supported his instrument. The object-glass is of 7.4 in. aperture and 123 in. focus.
12Description de l’observatoire central de Pulkowa, p. 208.
13Steinheil applied such motion to a double-image micrometer made for Struve. This instrument suggested to Struve the above-mentioned idea of employing a similar motion for the heliometer.
14Manuel Johnson, M.A., Radcliffe observer,Astronomical Observations made at the Radcliffe Observatory, Oxford, in the Year 1850, Introduction, p. iii.
15The illumination of these scales is interesting as being the first application of electricity to the illumination of astronomical instruments. Thin platinum wire was rendered incandescent by a voltaic current; a small incandescent electric lamp would now be found more satisfactory.
16For a detailed description of this instrument seeDunecht Publications, vol. ii.
17Mem. Royal Astronomical Society, xlvi., 1-172.
18The primary object was to have the object-glass mounted in steel cells, which more nearly correspond in expansion with glass. It became then desirable to make the head of steel for sake of uniformity of material, and the advantages of steel in lightness and rigidity for the tube then became evident.
19For description of the earliest form seeCambridge Phil. Trans.vol. ii., andGreenwich Observations(1840).
20Dawes (Monthly Notices, January 1858, andMem. R.A.S.vol. xxxv. p. 150) suggested and used a valuable improvement for producing round images, instead of the elongated images which are otherwise inevitable when the rays pass through a divided lens of which the optical centres are not in coincidence, viz. “the introduction of a diaphragm having two circular apertures touching each other in a point coinciding with the line of collimation of the telescope, and the diameter of each apertureexactly equalto the semidiameter of the cone of rays at the distance of the diaphragm from the local point of the object-glass.” Practically the difficulty of making these diaphragms for the different powers of theexactrequired equality is insuperable; but, if the observer is content to lose a certain amount of light, we see no reason why they may not readily be made slightly less. Dawes found the best method for the purpose in question was to limit the aperture of the object-glass by a diaphragm having a double circular aperture, placing the line joining the centres of the circles approximately in the position angle under measurement. Dawes successfully employed the double circular aperture also with Amici’s micrometer. The present writer has successfully used a similar plan in measuring position angles of a Centauri with the heliometer, viz. by placing circular diaphragms on the two segments of the object-glass.
21Dollond provides for changing the power by sliding the lens d nearer to or farther froma.
HELIOPOLIS,one of the most ancient cities of Egypt, met with in the Bible under its native name On. It stood 5 m. E. of the Nile at the apex of the Delta. It was the principal seat of sun-worship, and in historic times its importance was entirely religious. There appear to have been two forms of the sun-god at Heliopolis in the New Kingdom—namely, Ra-Harakht, or Rē’-Harmakhis, falcon-headed, and Etōm, human-headed; the former was the sun in his mid-day strength, the latter the evening sun. A sacred bull was worshipped here under the name Mnevis (Eg.Mreu), and was especially connected with Etōm. The sun-god Rē’ (seeEgypt:Religion) was especially the royal god, the ancestor of all the Pharaohs, who therefore held the temple of Heliopolis in great honour. Each dynasty might give the first place to the god of its residence—Ptah of Memphis, Ammon of Thebes, Neith of Sais, Bubastis of Bubastis, but all alike honoured Rē’. His temple became in a special degree a depository for royal records, and Herodotus states that the priests of Heliopolis were the best informed in matters of history of all the Egyptians. The schools of philosophy and astronomy are said to have been frequented by Plato and other Greek philosophers; Strabo, however, found them deserted, and the town itself almost uninhabited, although priests were still there, and cicerones for the curious traveller. The Ptolemies probably took little interest in their “father” Rē’, and Alexandria had eclipsed the learning of Heliopolis; thus with the withdrawal of royal favour Heliopolis quickly dwindled, and the students of native lore deserted it for other temples supported by a wealthy population of pious citizens. In Roman times obelisks were taken from its temples to adorn the northern cities of the Delta, and even across the Mediterranean to Rome. Finally the growth of Fostat and Cairo, only 6 m. to the S.W., caused the ruins to be ransacked for building materials. The site was known to the Arabs as‘Ayin esh shems, “the fountain of the sun,” more recently as Tel Hisn. It has now been brought for the most part under cultivation, but the ancient city walls of crude brick are to be seen in the fields on all sides, and the position of the great temple is marked by an obelisk still standing (the earliest known, being one of a pair set up by Senwosri I., the second king of the Twelfth Dynasty) and a few granite blocks bearing the name of Rameses II.
See Strabo xvii. cap. 1. 27-28; Baedeker’sEgypt.
See Strabo xvii. cap. 1. 27-28; Baedeker’sEgypt.
(F. Ll. G.)
HELIOSTAT(from Gr.ἥλιος, the sun,στατός, fixed, set up), an instrument which will reflect the rays of the sun in a fixed direction notwithstanding the motion of the sun. The optical apparatus generally consists of a mirror mounted on an axis parallel to the axis of the earth, and rotated with the same angular velocity as the sun. This construction assumes that the sun describes daily a small circle about the pole of the celestial sphere, and ignores any diurnal variation in the declination. This variation is, however, so small that it can be neglected for most purposes.
Many forms of heliostats have been devised, the earliest having been described by Wilhelm Jacob s’ Gravesande in the 3rd edition of hisPhysices elementa(1742). One of the simplest consists of a plane mirror rigidly connected with a revolving axis so that the angle between the normal to the mirror and the axis of the instrument equals half the sun’s polar distance, the mirror being adjusted so that the normal has the same right ascension as the sun. It is easily seen that if the mirror be rotated at the same angular velocity as the sun the right ascensions will remain equal throughout the day, and therefore this device reflects the rays in the direction of the earth’s axis; a second fixed mirror reflects them in any other fixed direction. Foucault’s heliostat reflects the rays horizontally in any required direction. The principle of the apparatus may be explained by reference to fig. 1. The axis of rotation AB bears a rigidly attached rod DBC inclined to it at an angle equal to the sun’s polar distance. By adjusting the right ascension of the plane ABC and rotating the axis with the angular velocity of the sun, it follows that BC will be the direction of the solar rays throughout the day. X is the mirror rotating about the point E, and placed so that (if EB is the horizontal direction in which the rays are to be reflected) (1) the normal CE to the mirror is jointed to BC at C and is equal in length to BE, (2) the rod DBC passes through a slot in a rod ED fixed to, and in the plane of, the mirror. Since CE equals BE these directions are equally inclined to, and coplanar with, the normal to the mirror. Hence light incident along the direction BC will be reflected along CE. Silbermann’s heliostat reflects the rays in any direction. The principle may be explained by means of fig. 2. AB is the axis of rotation, BC an adjustable rod as in Foucault’s construction, and BD is another rod which can be set to the direction in which the rays are to be reflected. The rods BC and DB carry two small rods EF, GF jointed at F; at this joint there is a pin which slides in a slot on the rod BH, which is normal to the mirror X. The rods EF, GF are such that BEFG is a rhombus. It is easy to show that rays falling on the mirror in the direction BC will be reflected along BD. One construction of the instrument, described in Jamin’sCours de physique, is shown in fig. 3. The mirrormmis attached to the frameworkpafe, the members of which are parallel to the incident and reflected rays SO, OR, and the diagonalpfis perpendicular to the mirror. The framework is attached to two independent circular arcs Csandrr′ having their centres at O and provided with clamps D and A on the axis F of the instrument. The arc Csis graduated, and is set so that the angle COD equals the complement of the sun’s declination. This can be effected (after setting the axis) by rotating Csuntil a needle indicates true time on the hour dial B. The arcrr′ is set so as to reflect the rays in the required direction. The axis F of the instrument is set at an angle equal to the latitude of the place of observation and in the meridian by means of the screw K, and rotated by clockwork contained in the barrel H. The setting in the meridian is effected by turning the instrument after setting for latitude until a pin-hole aperture s and a small screen P, placed so that Ps is parallel to CO, are in a line with the sun.Many other forms of heliostats have been designed, the chief difference consisting in the mechanical devices for maintaining the constant direction of the reflecting ray. One of the most important applications of the heliostat is as an adjunct to the newer forms of horizontal telescopes (q.v.) and in conjunction with spectroscopic telescopes in observations of eclipses.
Many forms of heliostats have been devised, the earliest having been described by Wilhelm Jacob s’ Gravesande in the 3rd edition of hisPhysices elementa(1742). One of the simplest consists of a plane mirror rigidly connected with a revolving axis so that the angle between the normal to the mirror and the axis of the instrument equals half the sun’s polar distance, the mirror being adjusted so that the normal has the same right ascension as the sun. It is easily seen that if the mirror be rotated at the same angular velocity as the sun the right ascensions will remain equal throughout the day, and therefore this device reflects the rays in the direction of the earth’s axis; a second fixed mirror reflects them in any other fixed direction. Foucault’s heliostat reflects the rays horizontally in any required direction. The principle of the apparatus may be explained by reference to fig. 1. The axis of rotation AB bears a rigidly attached rod DBC inclined to it at an angle equal to the sun’s polar distance. By adjusting the right ascension of the plane ABC and rotating the axis with the angular velocity of the sun, it follows that BC will be the direction of the solar rays throughout the day. X is the mirror rotating about the point E, and placed so that (if EB is the horizontal direction in which the rays are to be reflected) (1) the normal CE to the mirror is jointed to BC at C and is equal in length to BE, (2) the rod DBC passes through a slot in a rod ED fixed to, and in the plane of, the mirror. Since CE equals BE these directions are equally inclined to, and coplanar with, the normal to the mirror. Hence light incident along the direction BC will be reflected along CE. Silbermann’s heliostat reflects the rays in any direction. The principle may be explained by means of fig. 2. AB is the axis of rotation, BC an adjustable rod as in Foucault’s construction, and BD is another rod which can be set to the direction in which the rays are to be reflected. The rods BC and DB carry two small rods EF, GF jointed at F; at this joint there is a pin which slides in a slot on the rod BH, which is normal to the mirror X. The rods EF, GF are such that BEFG is a rhombus. It is easy to show that rays falling on the mirror in the direction BC will be reflected along BD. One construction of the instrument, described in Jamin’sCours de physique, is shown in fig. 3. The mirrormmis attached to the frameworkpafe, the members of which are parallel to the incident and reflected rays SO, OR, and the diagonalpfis perpendicular to the mirror. The framework is attached to two independent circular arcs Csandrr′ having their centres at O and provided with clamps D and A on the axis F of the instrument. The arc Csis graduated, and is set so that the angle COD equals the complement of the sun’s declination. This can be effected (after setting the axis) by rotating Csuntil a needle indicates true time on the hour dial B. The arcrr′ is set so as to reflect the rays in the required direction. The axis F of the instrument is set at an angle equal to the latitude of the place of observation and in the meridian by means of the screw K, and rotated by clockwork contained in the barrel H. The setting in the meridian is effected by turning the instrument after setting for latitude until a pin-hole aperture s and a small screen P, placed so that Ps is parallel to CO, are in a line with the sun.
Many other forms of heliostats have been designed, the chief difference consisting in the mechanical devices for maintaining the constant direction of the reflecting ray. One of the most important applications of the heliostat is as an adjunct to the newer forms of horizontal telescopes (q.v.) and in conjunction with spectroscopic telescopes in observations of eclipses.
HELIOTROPE,orTurnsole,Heliotropium(Gr.ἡλιοτρόπιον,i.e.a plant which follows the sun with its flowers or leaves, or, according to Theophrastus (Hist, plant, vii. 15), which flowers at the summer solstice), a genus of usually more or less hairy herbs or undershrubs of the tribeHeliotropieaeof the natural order Boraginaceae, having alternate, rarely almost opposite leaves; small white, lilac or blue flowers, in terminal or lateral one-sided simple or once or twice forked spikes, with a calyx of five deeply divided segments, a salver-shaped, hypogynous, 5-lobed corolla, and entire 4-celled ovary; fruit 2- to 4-sulcate or lobed, at length separable into four 1-seeded nutlets or into two hard 2-celled carpels. The genus contains 220 species indigenous in the temperate and warmer parts of both hemispheres. A few species are natives of Europe, asH. europaeum, which is also a naturalized species in the southern parts of North America.
The common heliotrope of English hothouses,H. peruvianum, popularly known as “cherry-pie,” is on account of the delicious odour of its flowers a great favourite with florists. It was introduced into Europe by the younger Jussieu, who sent seed of it from Peru to the royal garden at Paris. About the year 1757 it was grown in England by Philip Miller from seed obtained from St Germains.H. corymbosum(also a native of Peru), which was grown in Hammersmith nurseries as early as 1812, has larger but less fragant flowers thanH. peruvianum. The species commonly grown in Russian gardens isH. suaveolens, which has white, highly fragrant flowers.
Heliotropes may be propagated either from seed, or, as commonly, by means of cuttings of young growths taken an inch or two in length. Cuttings when sufficiently ripened, are struck in spring or during the summer months; when rooted they should be potted singly into small pots, using as a compost fibry loam, sandy peat and well-decomposed stable manure from an old hotbed. The plants soon require to be shifted into a pot a size larger. To secure early-flowering plants, cuttings should be struck in August, potted off before winter sets in, and kept in a warm greenhouse. In the spring larger pots should be given, and the plants shortened back to make them bushy. They require frequent shiftings during the summer, to induce them to bloom freely.
The heliotrope makes an elegant standard. The plants must in this case be allowed to send up a central shoot, and all the side growths must be pinched off until the necessary height is reached, when the shoot must be stopped and lateral growths will be produced to form the head. During winter they shouldbe kept somewhat dry, and in spring the ball of soil should be reduced and the plants repotted, the shoots being slightly pruned, so as to maintain a symmetrical head. When they are planted out against the walls and pillars of the greenhouse or conservatory an abundance of highly perfumed blossoms will be supplied all the year round. From the end of May till October heliotropes are excellent for massing in beds in the open air by themselves or with other plants. Many florists’ varieties of the common heliotrope are known in cultivation.
Pliny (Nat. hist.xxii. 29) distinguishes two kinds of “heliotropium,” thetricoccum, and a somewhat taller plant, thehelioscopium; the former, it has been supposed, isCroton tinctorium, and the latter theἡλιοτρόπιον μικρόνof Dioscorides orHeliotropium europaeum. The helioscopium, according to Pliny, was variously employed in medicine; thus the juice of the leaves with salt served for the removal of warts, whence the termherba verrucariaapplied to the plant. What, from the perfume of its flowers, is sometimes called winter heliotrope, is the fragrant butterbur, or sweet-scented coltsfoot,Petasites(Tussilago)fragrans, a perennial Composite plant.
Heliotrope, in mineralogy, is the mineral commonly called “bloodstone” (q.v.), and sometimes termed girasol—a name applied also to fire-opal. The name, like those of many ancient names of minerals, seems to have had a fanciful origin. According to Pliny the stone was so called because when thrown into the water it turned the sun’s light falling upon it into a reflection like that of blood.
HELIOZOA,in zoology, a group of the Sarcodina (q.v.) so named by E. Haeckel, 1866. They are characterized by the radiate pseudopods, finely tapering at the apex, springing abruptly from the superficial protoplasm, containing a denser, rather permanent axial rod (figs. 1 (1), 2 (2)); protoplasm without a clear ectoplasm or pellicle, often frothy with large vacuoles, like the alveoli of Radiolaria; nucleus 1 or numerous; skeleton absent, gelatinous or of separate siliceous fibres, plates or spicules, rarely complete and latticed; reproduction by simple fission or by brood-formation, often syngamous; form usually nearly spherical, rarely changing slowly. This group was formerly included with the Rhizopoda; but was separated from it by Haeckel on account of the character of its pseudopods, and its general adaptation to a semipelagic existence correlated with the frothy cytoplasm (fig. 1 (1)).Actinophrys solandActinosphaerium eichhornii(fig. 2), known as sun animalcules to the older microscopists, float freely in stagnant or slow-flowing waters, andMyriophrysis able by an investment of long flagelliform cilia to swim freely. The majority, however, lurk among confervae or the light débris of the bottom ooze; and come under the head of “sapropelic” rather than pelagic organisms. The body is usually of constant spherical form in relation to the floating habit.Nuclearia, however, shows amoeboid changes of general outline. The pseudopods are retractile, the axial filament being absorbed as the filament grows shorter and thicker and disappearing when the pseudopod merges into the ectoplasm, to be reformed at the same time with the pseudopod. There is often a distinction, clear, but never sharp, between the richly vacuolate, almost frothy ectoplasm and the denser endoplasm. One or more contractile vacuoles may protrude from the ectoplasm. The endoplasm contains the nucleus or nuclei. The nucleus when single may be central or excentric: in the latter case, the endoplasm contains a clear central sphere (“centrosome”) on which abut the axial filaments of the pseudopods. The ectoplasm contains, in some species, constantly (Raphidiophrys viridis) or occasionally (Actinosphaerium), green cells belonging to the generaZoochlorellaandSphaerocystis, both probably—the latter certainly—vegetative stages of a Chlamydomonad (Flagellata,q.v.) and of symbiotic significance.
The Heliozoa can move by rolling over on their extended pseudopods;Acanthocystis ludibundatraversing a path of as much as twenty times its diameter in a minute, according to Penard. Several species (e.g.Raphidiophrys elegans) remain associated by the union of their pseudopods, whether into social aggregates (due to approximation) or “colonies” due to lack of separation after fission, is not accurately known. The multinuclear speciesActinosphaerium eichhornii(fig. 2), normally apocytial (i.e.the nuclei divide repeatedly without division of the cytoplasm),may increase in size by the fusion (“plastogamic”) of small individuals. If a large specimen be cut up or fragment itself under irritation, the small ones so produced soon approach one another and fuse completely.
Reproduction.—Binary fission has been repeatedly observed; in some cases one or both of the daughter cells may swim for a time as a biflagellate zoospore (fig. 1 (6, 7)). The process may take place when the cell is naked or after preliminary encystment. Budding has been well studied inAcanthocystis; the cell nucleus divides repeatedly and most of the daughter nuclei pass to the periphery, aggregate part of the cytoplasm, and with it are constricted off as independent cells; one nucleus remains central and the process may be repeated. The detached bud may assume the typical character after a short amoeboid (lobose) stage, sometimes preceded by rest, or it may develop 2 flagella and swim off (fig. 1 (6)).Brood formation is only known here in relation to a syngamic process; this is a sharp contrast to Proteomyxa (q.v.) where brood formation is the commonest mode of reproduction, and plasmodium-formation, rare indeed, is the nearest approach to syngamy observed. Indeed, if we knew the life-history of all the species this difference in the life cycle would be a convenient critical character.Equal conjugation was demonstrated fully by F. Schaudinn inActinophrys; two individuals approach and enter into close contact, and are surrounded by a common cyst wall. The nucleus of either male divides; and one nucleus passes to the surface at either side, and is budded off with a small portion of the cytoplasm as an abortive cell; the two remaining nuclei which are “first cousins” in cellular relationship now fuse, as is the case with the cytoplasts. The resulting coupled cell or zygote divides into two, which again encyst.Actinosphaerium(fig. 2) shows a still more remarkable process, fully studied by R. Hertwig. The large multinucleate animal withdraws its pseudopods, its vacuoles disappear, it encysts and its nuclei diminish in number to about1⁄20th partly by fusion, 2 and 2, probably by digestion of the majority. Within the primary cyst the body is now resolved into nuclear cells, which again surround themselves with secondary cysts. The cell in each secondary cyst divides (by karyokinesis), and these sister cells, or rather their offspring, pair in much the same way as the individual cells ofActinophrys—the chief difference is that after the first division and budding off of a rudimentary cell, a second division of the same character takes place, with the formation of a second rudimentary cell, which is the niece of the first, absolutely in the same way as the 1st and 2nd polar bodies are formed in the maturation of the ovum in Metazoa. The actual pairing cells are thus second cousins, great-granddaughters of the original cell of the secondary cysts. Complete fusion now takes place to form the coupled cell, which is now contracted and forms a gelatinous wall within the siliceous secondary cyst wall (fig. 2 (14)), During a resting stage nuclear divisions occur and finally a brood of young 1-nuclearActinosphaeriumleave the cyst.Classification.Aphrothoraca. Body naked. Actinophrys Ehrb. (fig. 1 (1)) (nucleate), Actinosphaerium Stein plurinucleate (fig. 2 (1)), Camptonema (plurinucleate) Schaud., Dimorpha Gruber (sometimes 2 flagellate).I. Chlamydophora. Investment gelatinous. Astrodiscus.II. Chalarothoraca. Body protected by an investment of spicules or fibre scattered or approximated, never fused into a continuous skeleton.§ 1. Spicules netted or free in the protoplasm. Heterophrys Arch. (fig. 1 (3)), Raphidiophrys Arch. (fig. 1 (4)), Pinacodocystis, Hertw. and Less.§ 2. Spicules approximated radially. Pinaciophora Greeff, Pompholyxophrys Arch., Lithocolla F. E. Schultze, Elaeorhanis Greeff (in the two foregoing genera the spicules represented by sand granules), Acanthocystis Carter (fig. 1 (5)), Pinacocystis (?) Hertw. and Less, Myriophrys Penard. (Astrodisculus).III. Desmothoraca. § 1 attached by a stalk. Clathrulina Cienk. (fig. 1 (2, 7)), Hedriocystis, Hertw. and Less.§ 2. Free Elaster, Grimin, Choanocystis.Literature.—The most important English original papers on this group are those by W. Archer, “On some Freshwater Rhizopoda, new, or little known,”Quarterly Journal of Microscopic Science, N.S. ix.-xi. (1869-1871), and “Résumé of Recent Contributions to the Knowledge of Freshwater Rhizopods,”ibid.xvi., xvii. (1876-1877). See also R. Hertwig and Lesser, “Über Rhizopoda und denselben nahestehenden Organismen,” inArchiv für mikroscopische Anatomie, x. (1874), p. 35; R. Schaudinn, “Heliozoa” inTierreich(1896); E. Penard,Les Héliozoaires d’eau douce(1904); the two last named contain full bibliographies.
Reproduction.—Binary fission has been repeatedly observed; in some cases one or both of the daughter cells may swim for a time as a biflagellate zoospore (fig. 1 (6, 7)). The process may take place when the cell is naked or after preliminary encystment. Budding has been well studied inAcanthocystis; the cell nucleus divides repeatedly and most of the daughter nuclei pass to the periphery, aggregate part of the cytoplasm, and with it are constricted off as independent cells; one nucleus remains central and the process may be repeated. The detached bud may assume the typical character after a short amoeboid (lobose) stage, sometimes preceded by rest, or it may develop 2 flagella and swim off (fig. 1 (6)).
Brood formation is only known here in relation to a syngamic process; this is a sharp contrast to Proteomyxa (q.v.) where brood formation is the commonest mode of reproduction, and plasmodium-formation, rare indeed, is the nearest approach to syngamy observed. Indeed, if we knew the life-history of all the species this difference in the life cycle would be a convenient critical character.
Equal conjugation was demonstrated fully by F. Schaudinn inActinophrys; two individuals approach and enter into close contact, and are surrounded by a common cyst wall. The nucleus of either male divides; and one nucleus passes to the surface at either side, and is budded off with a small portion of the cytoplasm as an abortive cell; the two remaining nuclei which are “first cousins” in cellular relationship now fuse, as is the case with the cytoplasts. The resulting coupled cell or zygote divides into two, which again encyst.
Actinosphaerium(fig. 2) shows a still more remarkable process, fully studied by R. Hertwig. The large multinucleate animal withdraws its pseudopods, its vacuoles disappear, it encysts and its nuclei diminish in number to about1⁄20th partly by fusion, 2 and 2, probably by digestion of the majority. Within the primary cyst the body is now resolved into nuclear cells, which again surround themselves with secondary cysts. The cell in each secondary cyst divides (by karyokinesis), and these sister cells, or rather their offspring, pair in much the same way as the individual cells ofActinophrys—the chief difference is that after the first division and budding off of a rudimentary cell, a second division of the same character takes place, with the formation of a second rudimentary cell, which is the niece of the first, absolutely in the same way as the 1st and 2nd polar bodies are formed in the maturation of the ovum in Metazoa. The actual pairing cells are thus second cousins, great-granddaughters of the original cell of the secondary cysts. Complete fusion now takes place to form the coupled cell, which is now contracted and forms a gelatinous wall within the siliceous secondary cyst wall (fig. 2 (14)), During a resting stage nuclear divisions occur and finally a brood of young 1-nuclearActinosphaeriumleave the cyst.
Classification.
Aphrothoraca. Body naked. Actinophrys Ehrb. (fig. 1 (1)) (nucleate), Actinosphaerium Stein plurinucleate (fig. 2 (1)), Camptonema (plurinucleate) Schaud., Dimorpha Gruber (sometimes 2 flagellate).
Aphrothoraca. Body naked. Actinophrys Ehrb. (fig. 1 (1)) (nucleate), Actinosphaerium Stein plurinucleate (fig. 2 (1)), Camptonema (plurinucleate) Schaud., Dimorpha Gruber (sometimes 2 flagellate).
I. Chlamydophora. Investment gelatinous. Astrodiscus.II. Chalarothoraca. Body protected by an investment of spicules or fibre scattered or approximated, never fused into a continuous skeleton.§ 1. Spicules netted or free in the protoplasm. Heterophrys Arch. (fig. 1 (3)), Raphidiophrys Arch. (fig. 1 (4)), Pinacodocystis, Hertw. and Less.§ 2. Spicules approximated radially. Pinaciophora Greeff, Pompholyxophrys Arch., Lithocolla F. E. Schultze, Elaeorhanis Greeff (in the two foregoing genera the spicules represented by sand granules), Acanthocystis Carter (fig. 1 (5)), Pinacocystis (?) Hertw. and Less, Myriophrys Penard. (Astrodisculus).III. Desmothoraca. § 1 attached by a stalk. Clathrulina Cienk. (fig. 1 (2, 7)), Hedriocystis, Hertw. and Less.§ 2. Free Elaster, Grimin, Choanocystis.
I. Chlamydophora. Investment gelatinous. Astrodiscus.
II. Chalarothoraca. Body protected by an investment of spicules or fibre scattered or approximated, never fused into a continuous skeleton.
§ 1. Spicules netted or free in the protoplasm. Heterophrys Arch. (fig. 1 (3)), Raphidiophrys Arch. (fig. 1 (4)), Pinacodocystis, Hertw. and Less.
§ 2. Spicules approximated radially. Pinaciophora Greeff, Pompholyxophrys Arch., Lithocolla F. E. Schultze, Elaeorhanis Greeff (in the two foregoing genera the spicules represented by sand granules), Acanthocystis Carter (fig. 1 (5)), Pinacocystis (?) Hertw. and Less, Myriophrys Penard. (Astrodisculus).
III. Desmothoraca. § 1 attached by a stalk. Clathrulina Cienk. (fig. 1 (2, 7)), Hedriocystis, Hertw. and Less.
§ 2. Free Elaster, Grimin, Choanocystis.
Literature.—The most important English original papers on this group are those by W. Archer, “On some Freshwater Rhizopoda, new, or little known,”Quarterly Journal of Microscopic Science, N.S. ix.-xi. (1869-1871), and “Résumé of Recent Contributions to the Knowledge of Freshwater Rhizopods,”ibid.xvi., xvii. (1876-1877). See also R. Hertwig and Lesser, “Über Rhizopoda und denselben nahestehenden Organismen,” inArchiv für mikroscopische Anatomie, x. (1874), p. 35; R. Schaudinn, “Heliozoa” inTierreich(1896); E. Penard,Les Héliozoaires d’eau douce(1904); the two last named contain full bibliographies.
(M. Ha.)
HELIUM(from Gr.ἥλιος, the sun), a gaseous chemical element, the modern discovery of which followed closely on that of argon (q.v.). The Investigations of Lord Rayleigh and Sir William Ramsay had shown that indifference to chemical reagents did not sufficiently characterize an unknown gas as nitrogen, and it became necessary to reinvestigate other cases of the occurrence of “nitrogen” in nature. H. Miers drew Ramsay’s attention to the work of W. F. Hillebrand, who had noticed, in examining the mineral uraninite, that an inert gas was evolved when the mineral was decomposed with acid. Ramsay, repeating these experiments, found that the inert gas emitted refusedto oxidize when sparked with oxygen, and on examining it spectroscopically he saw that the spectrum was not that of argon, but was characterized by a bright yellow line near to, but not identical with, the D line of sodium. This was afterwards identified with the D3line of the solar chromosphere, observed in 1868 by Sir J. Norman Lockyer, and ascribed by him to a hypothetical elementhelium. This name was adopted for the new gas.
Helium is relatively abundant in many minerals, all of which are radioactive, and contain uranium or thorium as important constituents. (For the significance of this fact seeRadioactivity.) The richest known source is thorianite, which consists mainly of thorium oxide, and contains 9.5 cc. of helium per gram. Monazite, a phosphate of thorium and other rare earths, contains on the average about 1 cc. per gram. Cleveite, samarskite and fergusonite contain a little more than monazite. The gas also occurs in minute quantities in the common minerals of the earth’s crust. In this case too it is associated with radioactive matter, which is almost ubiquitous. In two cases, however, it has been found in the absence of appreciable quantities of uranium and thorium compounds, namely in beryl, and in sylvine (potassium chloride). Helium is contained almost universally in the gases which bubble up with the water of thermal springs. The proportion varies greatly. In the hot springs of Bath it amounts to about one-thousandth part of the gas evolved. Much larger percentages have been recorded in some French springs (Compt. rend., 1906, 143, p. 795, and 146, p. 435), and considerable quantities occur in some natural gas (Journ. Amer. Chem. Soc.29, p. 1524). R. J. Strutt has suggested that helium in hot springs may be derived from the disintegration of common rocks at great depths.
Helium is present in the atmosphere, of which it constitutes four parts in a million. It is conspicuous by its absorption spectrum in many of the white stars. Certain stars and nebulae show a bright line helium spectrum.
Much the best practical source of helium is thorianite, a mineral imported from Ceylon for the manufacture of thoria. It dissolves readily in strong nitric acid, and the helium contained is thus liberated. The gas contains a certain amount of hydrogen and oxides of carbon, also traces of nitrogen. In order to get rid of hydrogen, some oxygen is added to the helium, and the mixture exploded by an electric spark. All remaining impurities, including the excess of oxygen, can then be taken out of the gas by Sir James Dewar’s ingenious method of absorption with charcoal cooled in liquid air. Helium alone refuses to be absorbed, and it can be pumped off from the charcoal in a state of absolute purity. In the absence of liquid air the helium must be purified by the methods employed for argon (q.v.). If thorianite cannot be obtained, monazite, which is more abundant, may be utilized. A part of the helium contained in minerals can be extracted by heat or by grinding (J. A. Gray,Proc. Roy. Soc., 1909, 82A, p. 301).
Properties.—All attempts to make helium enter into stable chemical union have hitherto proved unsuccessful. The gas is in all probability only mechanically retained in the minerals in which it is found. Jacquerod and Perrot have found that quartz-glass is freely permeable to helium below a red-heat (Compt. rend., 1904, 139, p. 789). The effect is even perceptible at a temperature as low as 220° C. Hydrogen, and, in a much less degree, oxygen and nitrogen, will also permeate silica, but only at higher temperatures. They have made this observation the basis of a practical method of separating helium from the other inert gases. M. Travers has suggested that it may explain the liberation of helium from minerals by heat, the gas being enabled to permeate the siliceous materials in which it is enclosed. Thorianite, however, contains no silica, and until it is shown that metallic oxides behave in the same way this explanation must be accepted with reserve.
The density of helium has been determined by Ramsay and Travers as 1.98. Its ratio of specific heats has very nearly the ideal value 1.666, appropriate to a monatomic molecule. The accepted atomic weight is accordingly double the density,i.e.approximately four times that of hydrogen. The refractivity of helium is 0.1238 (air = 1). The solubility in water is the lowest known, being, at 18.2°, only .0073 vols. per unit volume of water. The viscosity is .96 (air = 1).
The spectrum of helium as observed in a discharge tube is distinguished by a moderate number of brilliant lines, distributed over the whole visual spectrum. The following are the approximate wave-lengths of the most brilliant lines:
When the discharge passes through helium at a pressure of several millimetres, the yellow line 5876 is prominent. At lower pressures the green line 4922 becomes more conspicuous. At atmospheric pressure the discharge is able to pass through a far greater distance in helium than in the common gases.
M. Travers, G. Senter and A. Jacquerod (Phil. Trans.A. 1903, 200, p. 105) carefully examined thebehaviourof a constant volume gas thermometer filled with helium. For the pressure coefficient per degree, between 0° and 100° C., they give the value .00366255, when the initial pressure is 700 mm. This value is indistinguishable from that which they find for hydrogen. Thus at high temperatures a helium thermometer is of no special advantage. At low temperatures, on the other hand, they find, using an initial pressure of 1000 mm., that the temperatures on the helium scale are measurably higher than on the hydrogen scale, owing to the more perfectly gaseous condition of helium. This difference amounts to about1⁄10° at the temperature of liquid oxygen, and about1⁄5° at that of liquid hydrogen.
The liquefaction of helium was achieved by H. Kamerlingh Onnes at Leiden in 1908. According to him its boiling point is 4.3° abs. (−268.7° C.), the density of the liquid 0.154, the critical temperature 5° abs., and the critical pressure 2.3 atmospheres (Communications from the Physical Laboratory at Leiden, No. 108; see alsoLiquid Gases).
References.—A bibliography and summary of the earlier work on helium will be found in a paper by Ramsay,Ann. chim. phys.(1898) [7], 13, p. 433. See also M. Travers,The Study of Gases(1901).
References.—A bibliography and summary of the earlier work on helium will be found in a paper by Ramsay,Ann. chim. phys.(1898) [7], 13, p. 433. See also M. Travers,The Study of Gases(1901).
(R. J. S.)
HELIX(Gr.ἕλιξ, a spiral or twist), an architectural term for the spiral tendril which is carried up to support the angles of the abacus of the Corinthian capital; from the same stalk springs a second helix rising to the centre of the capital, its junction with one on the opposite side being sometimes marked by a flower. Sometimes the term “volute” is given to the angle helix, which is incorrect, as it is of a different design and rises from the same stalk as the central helices. Its origin is probably metallic, that is to say, it was copied from the conventional treatment in Corinthian bronze of the tendrils of a plant.
HELL(O. Eng.hel, a Teutonic word from a root meaning “to cover,” cf. Ger.Hölle, Dutchhel), the word used in English both of the place of departed spirits and of the place of torment of the wicked after death. It is used in the Old Testament to translate the HebrewSheol, and in the New Testament the Greekᾃδης, Hades, andγεέννα, HebrewGehenna(seeEschatology).
HELLANICUSof Lesbos, Greek logographer, flourished during the latter half of the 5th centuryB.C.According to Suidas, he lived for some time at the court of one of the kings of Macedon, and died at Perperene, a town on the gulf of Adramyttium opposite Lesbos. Some thirty works are attributed to him—chronological, historical and episodical. Mention may be made of:The Priestesses of Hera at Argos, a chronological compilation, arranged according to the order of succession of these functionaries; theCarneonikae, a list of the victors in the Carnean games (the chief Spartan musical festival), including notices of literary events; anAtthis, giving the history of Attica from 683 to the end of the Peloponnesian War (404), which is referred to by Thucydides (i. 97), who says that he treated the events of the years 480-431 briefly and superficially, and withlittle regard to chronological sequence:Phoronis, chiefly genealogical, with short notices of events from the times of Phoroneus the Argive “first man” to the return of the Heraclidae;TroicaandPersica, histories of Troy and Persia.
Hellanicus marks a real step in the development of historiography. He transcended the narrow local limits of the older logographers, and was not content to repeat the traditions that had gained general acceptation through the poets. He tried to give the traditions as they were locally current, and availed himself of the few national or priestly registers that presented something like contemporary registration. He endeavoured to lay the foundations of a scientific chronology, based primarily on the list of the Argive priestesses of Hera, and secondarily on genealogies, lists of magistrates (e.g.the archons at Athens), and Oriental dates, in place of the old reckoning by generations. But his materials were insufficient and he often had recourse to the older methods. On account of his deviations from common tradition, Hellanicus is often called an untrustworthy writer by the ancients themselves, and it is a curious fact that he appears to have made no systematic use of the many inscriptions which were ready to hand. Dionysius of Halicarnassus censures him for arranging his history, not according to the natural connexion of events, but according to the locality or the nation he was describing; and undoubtedly he never, like his contemporary Herodotus, rose to the conception of a single current of events wider than the local distinction of race. His style, like that of the older logographers, was dry and bald.
Fragments in Müller,Fragmenta historicorum Graecorum, i. and iv.; see among older works L. Preller,De Hellanico Lesbio historico(1840); Mure,History of Greek Literature, iv.; late criticism in H. Kullmer, “Hellanikos” inJahrbücher für klass. Philologie(Supplementband, xxvii. 455 sqq.) (1902), which contains new edition and arrangement of fragments; C. F. Lehmann-Haupt, “Hellanikos, Herodot, Thukydides,” inKliovi. 127 sqq. (1906); J. B. Bury,Ancient Greek Historians(1909), pp. 27 sqq.
Fragments in Müller,Fragmenta historicorum Graecorum, i. and iv.; see among older works L. Preller,De Hellanico Lesbio historico(1840); Mure,History of Greek Literature, iv.; late criticism in H. Kullmer, “Hellanikos” inJahrbücher für klass. Philologie(Supplementband, xxvii. 455 sqq.) (1902), which contains new edition and arrangement of fragments; C. F. Lehmann-Haupt, “Hellanikos, Herodot, Thukydides,” inKliovi. 127 sqq. (1906); J. B. Bury,Ancient Greek Historians(1909), pp. 27 sqq.
HELLEBORE(Gr.ἑλλέβορος: mod. Gr. alsoσκάφη: Ger.Nieswurz,Christwurz; Fr.hellébore, and in the district of Avranche,herbe enragée), a genus (Helleborus) of plants of the natural order Ranunculaceae, natives of Europe and western Asia. They are coarse perennial herbs with palmately or pedately lobed leaves. The flowers have five persistent petaloid sepals, within the circle of which are placed the minute honey-containing tubular petals of the form of a horn with an irregular opening. The stamens are very numerous, and are spirally arranged; and the carpels are variable in number, sessile or stipitate and slightly united at the base and dehisce by ventral suture.
Helleborus niger, black hellebore, or, as from blooming in mid-winter it is termed the Christmas rose (Ger.Schwarze Nieswurz; Fr.,rose de Noëlorrose d’hiver), is found in southern and central Europe, and with other species was cultivated in the time of Gerard (seeHerball, p. 977, ed. Johnson, 1633) in English gardens. Its knotty root-stock is blackish-brown externally, and, as with other species, gives origin to numerous straight roots. The leaves spring from the top of the root-stock, and are smooth, distinctly pedate, dark-green above, and lighter below, with 7 to 9 segments and long petioles. The scapes, which end the branches of the rhizome, have a loose entire bract at the base, and terminate in a single flower, with two bracts, from the axis of one of which a second flower may be developed. The flowers have 5 white or pale-rose, eventually greenish sepals, 15 to 18 lines in breadth; 8 to 13 tubular green petals containing honey; and 5 to 10 free carpels. There are several forms, the best beingmaximus. The Christmas rose is extensively grown in many market gardens to provide white flowers forced in gentle heat about Christmas time for decorations, emblems, &c.
H. orientalis, the Lenten rose, has given rise to several fine hybrids withH. niger, some of the best forms being clear in colour and distinctly spotted.H. foetidus, stinking hellebore, is a native of England, where likeH. viridis, it is confined chiefly to limestone districts; it is common in France and the south of Europe. Its leaves have 7- to 11-toothed divisions, and the flowers are in panicles, numerous, cup-shaped and drooping, with many bracts, and green sepals tinged with purple, alternating with the five petals.
H. viridis, or green hellebore proper, is probably indigenous in some of the southern and eastern counties of England, and occurs also in central and southern Europe. It has bright yellowish-green flowers, 2 to 4 on a stem, with large leaf-like bracts. O. Brunfels and H. Bock (16th century) regarded the plant as the black hellebore of the Greeks.
H. lividus, holly-leaved hellebore, found in the Balearic Islands, and in Corsica and Sardinia, is remarkable for the handsomeness of its foliage. White hellebore isVeratrum album(seeVeratrum), a liliaceous plant.
Hellebores may be grown in any ordinary light garden mould, but thrive best in a soil of about equal parts of turfy loam and well-rotted manure, with half a part each of fibrous peat and coarse sand, and in moist but thoroughly-drained situations, more especially where, as at the margins of shrubberies, the plants can receive partial shade in summer. For propagation cuttings of the rhizome may be taken in August, and placed in pans of light soil, with a bottom heat of 60° to 70° Fahr.; hellebores can also be grown from seed, which must be sown as soon as ripe, since it quickly loses its vitality. The seedlings usually blossom in their third year. The exclusion of frost favours the production of flowers; but the plants, if forced, must be gradually inured to a warm atmosphere, and a free supply of air must be afforded, without which they are apt to become much affected by greenfly. For potting,H. nigerand its varieties, andH. orientalis,atrorubensandolympicushave been found well suited. After lifting, preferably in September, the plants should receive plenty of light, with abundance of water, and once a week liquid manure, not over-strong. The flowers are improved in delicacy of hue, and are brought well up among the leaves, by preventing access of light except to the upper part of the plants. Of the numerous species of hellebore now grown, the deep-purple-floweredH. colchicusis one of the handsomest; by crossing withH. guttatusand other species several valuable garden forms have been produced, having variously coloured spreading or bell-shaped flowers, spotted with crimson, red or purple.
The rhizome ofH. nigeroccurs in commerce in irregular and nodular pieces, from about 1 to 3 in. in length, white and of a horny texture within. Cut transversely it presents internally a circle of 8 to 12 cuneiform ligneous bundles, surrounded by a thick bark. It emits a faint odour when cut or broken, and has a bitter and slightly acrid taste. The drug is sometimes adulterated with the rhizome of baneberry,Actaea spicata, which, however, may be recognized by the distinctly cruciate appearance of the central portion of the attached roots whencut across, and by its decoction giving the chemical reactions for tannin.1The rhizome is darker in colour in proportion to its degree of dryness, age and richness in oil. A specimen dried by Schroff lost in eleven days 65% of water.