Part 44

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 should be 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," the _tricoccum_, and a somewhat taller plant, the _helioscopium_; the former, it has been supposed, is _Croton tinctorium_, and the latter the [Greek: heliotropion mikron] of Dioscorides or _Heliotropium 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 term _herba verrucaria_ applied 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 sol_ and _Actinosphaerium eichhornii_ (fig. 2), known as sun animalcules to the older microscopists, float freely in stagnant or slow-flowing waters, and _Myriophrys_ is able by an investment of long flagelliform cilia to swim freely. The majority, however, lurk among confervae or the light debris 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 genera _Zoochlorella_ and _Sphaerocystis_, both probably--the latter certainly--vegetative stages of a Chlamydomonad (FLAGELLATA, q.v.) and of symbiotic significance.

[Illustration: FIG. 1.--Heliozoa. 1. _Actinophrys sol_, Ehrb. a, food-particle lying in a large food-vacuole; b, deep-lying finely granular protoplasm; c, axial filament of a pseudopodium extended inwards to the nucleus; d, the central nucleus; e, contractile vacuole; f, superficial much vacuolated protoplasm. 2. _Clathrulina elegans_, Cienk. 3. _Heterophrys marina_, H. and L. a, nucleus; b, clearer protoplasm surrounding the nucleus; c, the peculiar felted envelope. 4. _Raphidiophrys pallida_, F. E. Schultze. a, food-particle; b, contractile vacuole; c, the nucleus; d, central granule in which all the axis-filaments of the pseudopodia meet. The tangentially disposed spicules are seen arranged in masses on the surface. 5. _Acanthocystis turfacea_, Carter. a, probably the central nucleus; b, clear protoplasm around the nucleus; c, more superficial protoplasm with vacuoles and chlorophyll corpuscles; d, coarser siliceous spicules; e, finer forked siliceous spicules; f, finely granular layer of protoplasm. The long pseudopodia reaching beyond the spicules are not lettered. 6. Bi-flagellate "flagellula" of _Acanthocystis aculeata_. a, nucleus. 7. Id. of _Clathrulina elegans_. a, nucleus; b, granules. 8. _Astrodisculus ruber_, Greeff. a, red-coloured central sphere (? nucleus); b, peripheral homogeneous envelope.]

The Heliozoa can move by rolling over on their extended pseudopods; _Acanthocystis ludibunda_ traversing 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 species _Actinosphaerium 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.

[Illustration: FIG. 2.--Heliozoa. 1. _Actinosphaerium eichhornii_, Ehr.; a, nuclei; b, deeper protoplasm with smaller vacuoles and numerous nuclei; c, contractile vacuoles; d, peripheral protoplasm with larger vacuoles. 2. A portion of the same specimen more highly magnified and seen in optical section. a, Nuclei; b, deeper protoplasm (so-called endosarc); d, peripheral protoplasm (so-called ectosarc); e, pseudopodia showing the granular protoplasm streaming over the stiff axial filament: f, food-particle in a good-vacuole. 3, 4. Nuclei of _Actinosphaerium_ in the resting condition. 5-13. Successive stages in the division of a nucleus of _Actinosphaerium_, showing fibrillation, and in 7 and 8 formation of an equatorial plate of chromatin substance (after Hertwig). 14. Cyst-phase of _Actinosphaerium eichhornii_, showing the protoplasm divided into twelve chlamydospores, each of which has a siliceous coat; a, nucleus of the spore; g, gelatinous wall of the cyst; h, siliceous coat of the spore.]

_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 in _Acanthocystis_; 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 in _Actinophrys_; 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 about 1/20th 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 of _Actinophrys_--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-nuclear _Actinosphaerium_ leave 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.

S 1. Spicules netted or free in the protoplasm. Heterophrys Arch. (fig. 1 (3)), Raphidiophrys Arch. (fig. 1 (4)), Pinacodocystis, Hertw. and Less.

S 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. S 1 attached by a stalk. Clathrulina Cienk. (fig. 1 (2, 7)), Hedriocystis, Hertw. and Less.

S 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 "Resume of Recent Contributions to the Knowledge of Freshwater Rhizopods," _ibid._ xvi., xvii. (1876-1877). See also R. Hertwig and Lesser, "Uber Rhizopoda und denselben nahestehenden Organismen," in _Archiv fur mikroscopische Anatomie_, x. (1874), p. 35; R. Schaudinn, "Heliozoa" in _Tierreich_ (1896); E. Penard, _Les Heliozoaires d'eau douce_ (1904); the two last named contain full bibliographies. (M. Ha.)

HELIUM (from Gr. [Greek: helios], 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 refused to 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 D3 line of the solar chromosphere, observed in 1868 by Sir J. Norman Lockyer, and ascribed by him to a hypothetical element _helium_. 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 see RADIOACTIVITY.) 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 deg. 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 deg., 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:

Red 7066 Red 6678 Yellow 5876 Green 4922 Blue 4472 Violet 4026

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 the behaviour of a constant volume gas thermometer filled with helium. For the pressure coefficient per degree, between 0 deg. and 100 deg. 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 about 1/10 deg. at the temperature of liquid oxygen, and about 1/5 deg. 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 deg. abs. (-268.7 deg. C.), the density of the liquid 0.154, the critical temperature 5 deg. abs., and the critical pressure 2.3 atmospheres (_Communications from the Physical Laboratory at Leiden_, No. 108; see also LIQUID 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). (R. J. S.)

HELIX (Gr. [Greek: helix], 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. _Holle_, Dutch _hel_), 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 Hebrew _Sheol_, and in the New Testament the Greek [Greek: hades], Hades, and [Greek: geenna], Hebrew _Gehenna_ (see ESCHATOLOGY).

HELLANICUS of Lesbos, Greek logographer, flourished during the latter half of the 5th century B.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; the _Carneonikae_, a list of the victors in the Carnean games (the chief Spartan musical festival), including notices of literary events; an _Atthis_, 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 with little 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; _Troica_ and _Persica_, 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 Muller, _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" in _Jahrbucher fur klass. Philologie_ (Supplementband, xxvii. 455 sqq.) (1902), which contains new edition and arrangement of fragments; C. F. Lehmann-Haupt, "Hellanikos, Herodot, Thukydides," in _Klio_ vi. 127 sqq. (1906); J. B. Bury, _Ancient Greek Historians_ (1909), pp. 27 sqq.

HELLEBORE (Gr. [Greek: helleboros]: mod. Gr. also [Greek: skaphe]: Ger. _Nieswurz_, _Christwurz_; Fr. _hellebore_, and in the district of Avranche, _herbe enragee_), 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 Noel_ or _rose d'hiver_), is found in southern and central Europe, and with other species was cultivated in the time of Gerard (see _Herball_, 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 being _maximus_. 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.