HEMICYCLE (Gr. [Greek: hêmi-], half, and [Greek: kyklos], circle), a semicircular recess of considerable size which formed one of the most conspicuous features in the Roman Thermae, where it was always covered with a hemispherical vault. A small example exists in Pompeii, in the street of tombs, with a seat round inside, where those who came to pay their respects to the departed could rest. An immense hemicycle was designed by Bramante for the Vatican, where it constitutes a fine architectural effect at the end of the great court.
HEMIMERUS, an Orthopterous or Dermapterous insect, the sole representative of the family _Hemimeridae_, which has affinities with both the _Forficulidae_ (earwigs) and the _Blattidae_ (cockroaches). Only two species have been discovered, both from West Africa. The better known of these (_H. hanseni_) lives upon a large rat-like rodent (_Cricetomys gambianus_) feeding perhaps upon its external parasites, perhaps upon scurf and other dermal products. Like many epizoic or parasitic insects, _Hemimerus_ is wingless, eyeless and has relatively short and strong legs. Correlated also with its mode of life is the curious fact that it is viviparous, the young being born in an advanced stage of growth.
HEMIMORPHITE, a mineral consisting of hydrous zinc silicate, H2Zn2SiO5, of importance as an ore of the metal, of which it contains 54.4%. It is interesting crystallographically by reason of the hemimorphic development of its orthorhombic crystals; these are prismatic in habit and are differently terminated at the two ends. In the figure, the faces at the upper end of the crystal are the basal plane k and the domes o, p, l, m, whilst at the lower end there are only the four faces of the pyramid P. Connected with this polarity of the crystals is their pyroelectric character--when a crystal is subjected to changes of temperature it becomes positively electrified at one end and negatively at the opposite end. There are perfect cleavages parallel to the prism faces (d in the figure). Crystals are usually colourless, sometimes yellowish or greenish, and transparent; they have vitreous lustre. The hardness is 5, and the specific gravity 3.45. The mineral also occurs as stalactitic or botryoidal masses with a fibrous structure, or in a massive, cellular or granular condition intermixed with calamine and clay. It is decomposed by hydrochloric acid with gelatinization; this property affords a ready means of distinguishing hemimorphite from calamine (zinc carbonate), these two minerals being, when not crystallized, very like each other in appearance. The water contained in hemimorphite is expelled only at a red heat, and the mineral must therefore be considered as a basic metasilicate, (ZnOH)2SiO3.