Chapter XIX: Part 19

Ramsden (_Phil. Trans._ vol. xix. p. 419) suggested the division of
the small speculum of a Cassegrain telescope and the production of
double image by micrometric rotation of the semispecula in the plane
passing through their axis. Brewster (_Ency. Brit._ 8th ed. vol. xiv.
p. 749) proposed a plan on a like principle, by dividing the plane
mirror of a Newtonian telescope. Again, in an ocular heliometer by
Steinheil double image is similarly produced by a divided prism of
total reflection placed in parallel rays. But practically these last
three methods are failures. In the last the field is full of false
light, and it is not possible to give sufficiently minute and steady
separation to the images; and there are of necessity a collimator, two
prisms of total reflection, and a small telescope through which the
rays must pass; consequently there is great loss of light.

_Micrometers Depending on Double Refraction._--To the Abbe Rochon
(_Jour. de phys._ liii., 1801, pp. 169-198) is due the happy idea of
applying the two images formed by double refraction to the
construction of a micrometer. He fell upon a most ingenious plan of
doubling the amount of double refraction of a prism by using two
prisms of rock-crystal, so cut out of the solid as to give each the
same quantity of double refraction, and yet to double the quantity in
the effect produced. The combination so formed is known as Rochon's
prism. Such a prism he placed between the object-glass and eye-piece
of a telescope. The separation of the images increases as the prism is
approached to the object-glass, and diminishes as it is approached
towards the eye-piece.

D. F. J. Arago (_Comptes rendus_, xxiv., 1847, pp. 400-402) found that
in Rochon's micrometer, when the prism was approached close to the
eye-piece for the measurement of very small angles, the smallest
imperfections in the crystal or its surfaces were inconveniently
magnified. He therefore selected for any particular measurement such a
Rochon prism as when fixed between the eye and the eye-piece (i.e.
where a sunshade is usually placed) would, combined with the normal
eye-piece employed, bring the images about to be measured nearly in
contact. He then altered the magnifying power by sliding the field
lens of the eye-piece (which was fitted with a slipping tube for the
purpose) along the eye-tube, till the images were brought into
contact. By a scale attached to the sliding tube the magnifying power
of the eye-piece was deduced, and this combined with the angle of the
prism employed gave the angle measured. If p" is the refracting angle
of the prism, and n the magnifying power of the eye-piece, then p"/n
will be the distance observed. Arago made many measures of the
diameters of the planets with such a micrometer.

Dollond (_Phil. Trans._, 1821, pp. 101-103) describes a double-image
micrometer of his own invention, in which a sphere of rock-crystal is
substituted for the eye-lens of an ordinary eye-piece. In this
instrument (figs. 18, 19) a is the sphere, placed in half-holes on the
axis bb, so that when its principal axis is parallel to the axis of
the telescope it gives only one image of the object. In a direction
perpendicular to that axis it must be so placed that when it is moved
by rotation of the axis bb the separation of the images shall be
parallel to that motion. The angle of rotation is measured on the
graduated circle C. The angle between the objects measured is = r sin
2[theta], where r is a constant to be determined for each magnifying
power employed,[21] and [theta] the angle through which the sphere has
been turned from zero (i.e. from coincidence of its principal axis
with that of the telescope). The maximum separation is consequently at
45 deg. from zero. The measures can be made on both sides of zero for
eliminating index error. There are considerable difficulties of
construction, but these have been successfully overcome by Dollond;
and in the hands of Dawes (_Mem. R.A.S._ xxxv. p. 144 seq.) such
instruments have done valuable service. They are liable to the
objection that their employment is limited to the measurement of very
small angles, viz. 13" or 14" when the magnifying power is 100, and
varying inversely as the power. Yet the beautiful images which these
micrometers give permit the measurement of very difficult objects as a
check on measures with the parallel-wire micrometer.

On the theory of the heliometer and its use consult Bessel,
_Astronomische Untersuchungen_, vol. i.; Hansen, _Ausfuhrliche Methode
mit dem Fraunhoferschen Heliometer anzustellen_ (Gotha, 1827);
Chauvenet, _Spherical and Practical Astronomy_, vol. ii. (Philadelphia
and London, 1876); Seeliger, _Theorie des Heliometers_ (Leipzig,
1877); Lindsay and Gill, _Dunecht Publications_, vol. ii. (Dunecht,
for private circulation, 1877); Gill, _Mem. R.A.S._ vol. xlvi. pp.
1-172, and references mentioned in the text. (D. Gi.)

FOOTNOTES:

[1] The circles by Reichenbach, then almost exclusively used in
Germany, were read by verniers only.

[2] The diameter of Venus was measured with one of these heliometers
at the observatory of Breslau by Brandes in 1820 (_Berlin Jahrbuch_,
1824, p. 164).

[3] The 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
of 15/1000th of an inch when an angle of 1 deg. is measured. For 2
deg. the difference would amount to nearly 1/10th of an inch. Bessel
confined his measures to distances considerably less than 1 deg.

[4] In 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.

[5] This 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 deg., the images remaining as sharp and perfect as when the
smallest angles are measured.

[6] Bessel 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.

[7] The tube, being of wood, was probably liable to warp and twist in
a very uncertain way.

[8] We have been unable to find any published drawing showing how the
segments are fitted in their cells.

[9] We have been unable to ascertain the reasons which led Bessel to
choose _ivory_ planes for the end-bearings of his screws. He actually
introduced them in the Konigsberg heliometer in 1840, and they were
renewed in 1848 and 1850.

[10] A 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.

[11] This 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.

[12] _Description de l'observatoire central de Pulkowa_, p. 208.

[13] Steinheil 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.

[14] Manuel Johnson, M.A., Radcliffe observer, _Astronomical
Observations made at the Radcliffe Observatory, Oxford, in the Year
1850_, Introduction, p. iii.

[15] The 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.

[16] For a detailed description of this instrument see _Dunecht
Publications_, vol. ii.

[17] _Mem. Royal Astronomical Society_, xlvi., 1-172.

[18] The 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.

[19] For description of the earliest form see _Cambridge Phil.
Trans._ vol. ii., and _Greenwich Observations_ (1840).

[20] Dawes (_Monthly Notices_, January 1858, and _Mem. 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 aperture _exactly equal_ to 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 the _exact_ required
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.

[21] Dollond provides for changing the power by sliding the lens d
nearer to or farther from a.

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 Re'-Harmakhis, falcon-headed, and Etom, 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 Etom. The sun-god Re' (see EGYPT: _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 Re'. 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" Re', 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's _Egypt_. (F. Ll. G.)

HELIOSTAT (from Gr. [Greek: helios], the sun, [Greek: statos], 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.

FIG. 3.--Silbermann's Heliostat.]

Many forms of heliostats have been devised, the earliest having been
described by Wilhelm Jacob s' Gravesande in the 3rd edition of his
_Physices 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's _Cours de
physique_, is shown in fig. 3. The mirror mm is attached to the
framework _pafe_, the members of which are parallel to the incident
and reflected rays SO, OR, and the diagonal pf is perpendicular to the
mirror. The framework is attached to two independent circular arcs Cs
and rr' having their centres at O and provided with clamps D and A on
the axis F of the instrument. The arc Cs is 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 Cs until a
needle indicates true time on the hour dial B. The arc rr' 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, or TURNSOLE, _Heliotropium_ (Gr. [Greek: heliotropion], 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 tribe _Heliotropieae_ of 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, as _H. 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 than _H. peruvianum_. The species commonly grown in Russian gardens is _H. 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 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.

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.

_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.

_H. orientalis_, the Lenten rose, has given rise to several fine hybrids with _H. niger_, some of the best forms being clear in colour and distinctly spotted. _H. foetidus_, stinking hellebore, is a native of England, where like _H. 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 is _Veratrum album_ (see VERATRUM), 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 deg. to 70 deg. 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. niger_ and its varieties, and _H. orientalis_, _atrorubens_ and _olympicus_ have 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-flowered _H. colchicus_ is one of the handsomest; by crossing with _H. guttatus_ and 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 of _H. niger_ occurs 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 when cut across, and by its decoction giving the chemical reactions for tannin.[1] The 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.

_H. niger_, _orientalis_, _viridis_, _foetidus_, and several other
species of hellebore contain the glucosides _helleborin_, C36H42O6,
and _helleborein_, C23H20O15, the former yielding glucose and
_helleboresin_, C30H38O4, and the latter glucose and a violet-coloured
substance _helleboretin_, C14H20O3. Helleborin is most abundant in _H.
viridis_. A third and volatile principle is probably present in _H.
foetidus_. Both helleborin and helleborein act poisonously on animals,
but their decomposition-products helleboresin and helleboretin seem to
be devoid of any injurious qualities. Helleborin produces excitement
and restlessness, followed by paralysis of the lower extremities or
whole body, quickened respiration, swelling and injection of the
mucous membranes, dilatation of the pupil, and, as with helleborein,
salivation, vomiting and diarrhoea. Helleborein exercises on the heart
an action similar to that of digitalis, but more powerful, accompanied
by at first quickened and then slow and laboured respiration; it
irritates the conjunctiva, and acts as a sternutatory, but less
violently than veratrine. Pliny states that horses, oxen and swine are
killed by eating "black hellebore"; and Christison (_On Poisons_, p.
876, 11th ed., 1845) writes: "I have known severe griping produced by
merely tasting the fresh root in January." Poisonous doses of
hellebore occasion in man singing in the ears, vertigo, stupor,
thirst, with a feeling of suffocation, swelling of the tongue and
fauces, emesis and catharsis, slowing of the pulse, and finally
collapse and death from cardiac paralysis. Inspection after death
reveals much inflammation of the stomach and intestines, more
especially the rectum. The drug has been observed to exercise a
cumulative action. Its extract was an ingredient in Bacher's pills, an
empirical remedy once in great repute in France. In British medicine
the rhizome was formerly official. _H. foetidus_ was in past times
much extolled as an anthelmintic, and is recommended by Bisset (_Med.
Ess._, pp. 169 and 195, 1766) as the best vermifuge for children; J.
Cook, however, remarks of it (_Oxford Mag._, March 1769, p. 99):
"Where it killed not the patient, it would certainly kill the worms;
but the worst of it is, it will sometimes kill both." This plant, of
old termed by farriers ox-heel, setter-wort and setter-grass, as well
as _H. viridis_ (Fr. _Herbe a seton_), is employed in veterinary
surgery, to which also the use of _H. niger_ is now chiefly confined
in Britain.

In the early days of medicine two kinds of hellebore were recognized,
the white or _Veratrum album_ (see VERATRUM), and the black, including
the various species of _Helleborus_. The former, according to
Codronchius (_Comm.... de elleb._, 1610), Castellus (_De helleb.
epist._, 1622), and others, is the drug usually signified in the
writings of Hippocrates. Among the hellebores indigenous to Greece and
Asia Minor, _H. orientalis_, the rhizome of which differs from that of
_H. niger_ and of _H. viridis_ in the bark being readily separable
from the woody axis, is the species found by Schroff to answer best to
the descriptions given by the ancients of black hellebore, the [Greek:
helleboros melas] of Dioscorides. The rhizome of this plant, if
identical, as would appear, with that obtained by Tournefort at Prusa
in Asia Minor (_Rel. d'un voy. du Levant_, ii. 189, 1718), must be a
remedy of no small toxic properties. According to an early tradition,
black hellebore administered by the soothsayer and physician Melampus
(whence its name _Melampodium_), was the means of curing the madness
of the daughters of Proetus, king of Argos. The drug was used by the
ancients in paralysis, gout and other diseases, more particularly in
insanity, a fact frequently alluded to by classical writers, e.g.
Horace (_Sat._ ii. 3. 80-83, _Ep. ad Pis._ 300). Various superstitions
were in olden times connected with the cutting of black hellebore. The
best is said by Pliny (_Nat. hist._ xxv. 21) to grow on Mt Helicon. Of
the three Anticyras that in Phocis was the most famed for its
hellebore, which, being there used combined with "sesamoides," was,
according to Pliny, taken with more safety than elsewhere.

The British Pharmaceutical Conference has recommended the preparation which it terms _the tinctura veratri viridis_, as the best form in which to administer this drug. It may be given in doses of 5-15 minims. The tincture is prepared from the dried rhizome and rootlets of green hellebore, containing the alkaloids jervine, veratrine and veratroidine. It is recommended as a cardiac and nervous sedative in cerebral haemorrhage and puerperal eclampsia. Black hellebore is a purgative and uterine stimulant.

FOOTNOTE:

[1] For the microscopical characters and for figures of transverse
sections of the rhizome, see Lanessan, _Hist. des drogues_, i. 6
(1878).

HELLENISM (from Gr. [Greek: hellenizein], to imitate the Greeks, who were known as [Greek: Hellenes], after [Greek: Hellen], the son of Deucalion). The term "Hellenism" is ambiguous. It may be used to denote ancient Greek culture in all its phases, and even those elements in modern civilization which are Greek in origin or in spirit; but, while Matthew Arnold made the term popular in the latter connexion as the antithesis of "Hebraism," the German historian J. G. Droysen introduced the fashion (1836) of using it to describe particularly the latter phases of Greek culture from the conquests of Alexander to the end of the ancient world, when those over whom this culture extended were largely not Greek in blood, i.e. _Hellenes_, but peoples who had adopted the Greek speech and way of life, _Hellenistai_. Greek culture had, however, both in "Hellenic" and "Hellenistic" times, a common essence, just as light is light whether in the original luminous body or in a reflection, and to describe this by the term Hellenism seems most natural. But whilst using the term in the larger sense, this article, in deference to the associations which have come to be specially connected with it, will devote its principal attention to Hellenism as it appeared in the world after the Macedonian conquests. But it will be first necessary to indicate briefly what Hellenism in itself implied.

No verbal formula can really enclose the life of a people or an age, but we can best understand the significance of the old Greek cities and the life they developed, when, looking at the history of mankind as a whole, we see the part played by reason, active and critical, in breaking down the barriers by which custom hinders movement, in guiding movement to definite ends, in dissipating groundless beliefs and leading onwards to fresh scientific conquests--when we see this and then take note that among the ancient Greeks such an activity of reason began in an entirely novel degree and that its activity in Europe ever since is due to their impulsion. When Hellenism came to stand in the world for something concrete and organic, it was, of course, no mere abstract principle, but embodied in a language, a literature, an artistic tradition. In the earliest existing monument of the Hellenic genius, the Homeric poems, one may already observe that regulative sense of form and proportion, which shaped the later achievements of the race in the intellectual and artistic spheres. It was not till the great colonizing epoch of the 8th and 7th centuries B.C., when the name "Hellene" came into use as the antithesis of "barbarian," that the Greek race came to be conscious of itself as a peculiar people; it was yet some three centuries more before Hellenism stood fully declared in art and literature, in politics and in thought. There was now a new thing in the world, and to see how the world was affected by it is our immediate concern.