rock crystal: see quartz.

liquid crystal, liquid whose component particles, atoms or molecules, tend to arrange themselves with a degree of order far exceeding that found in ordinary liquids and approaching that of solid crystals. As a result, liquid crystals have many of the optical properties of solid crystals. Moreover, since the order is not as firmly fixed as that of a solid crystal, it can be easily modified with corresponding changes in the optical properties; typically, a small electrical impulse darkens the crystal so that it is clearly visible against the lighter background of neutral crystals. An array of seven lozenges, each of which can be darkened by a separate impulse, can yield any digit. Thus, liquid crystal displays (LCDs) have become the most common means of producing visual readouts on such devices as digital clocks and electronic calculators. Some liquid crystals vary the color of the light that they reflect as their temperature changes. Since the colors reflected at any given temperature are quite specific, temperature can be measured by this means to an accuracy of 0.1°C;.

crystal, a solid body bounded by natural plane faces that are the external expression of a regular internal arrangement of constituent atoms, molecules, or ions. The formation of a crystal by a substance passing from a gas or liquid to a solid state, or going out of solution (by precipitation or evaporation), is called crystallization.

Classification of Crystals

The particles in a crystal occupy positions with definite geometrical relationships to each other. The positions form a kind of scaffolding, called a crystalline lattice; the atomic occupancies of lattice positions are determined by the chemical composition of the substance. A crystalline substance is uniquely defined by the combination of its chemistry and the structural arrangement of its atoms. In all crystals of any specific substance the angles between corresponding faces are constant (Steno's Law, or the First Law of Crystallography, published by the Danish geologist Nicolaus Steno in 1669). Crystalline substances are grouped, according to the type of symmetry they display, into 32 classes. These in turn are grouped into seven systems on the basis of the relationships of their axes, i.e., imaginary straight lines passing through the ideal centers of the crystals.

Crystals may be symmetrical with relation to planes, axes, and centers of symmetry. Planes of symmetry divide crystals into equal parts (mirror images) that correspond point for point, angle for angle, and face for face. Axes of symmetry are imaginary lines about which the crystal may be considered to rotate, assuming, after passing through a rotation of 60°, 90°, 120°, or 180°, the identical position in space that it originally had. Centers of symmetry are points from which imaginary straight lines may be drawn to intersect identical points equidistant from the center on opposite sides.

The crystalline systems are cubic, or isometric (three equal axes, intersecting at right angles); hexagonal (three equal axes, intersecting at 60° angles in a horizontal plane, and a fourth, longer or shorter, axis, perpendicular to the plane of the other three); tetragonal (two equal, horizontal axes at right angles and one axis longer or shorter than the other two and perpendicular to their plane); orthorhombic (three unequal axes intersecting at right angles); monoclinic (three unequal axes, two intersecting at right angles and the third at an oblique angle to the plane of the other two); trigonal, or rhombohedral (three equal axes intersecting at oblique angles); and triclinic (three unequal axes intersecting at oblique angles). In all systems in which the axes are unequal there is a definite axial ratio for each crystal substance.

Physical Properties of Crystals

Crystals differ in physical properties, i.e., in hardness, cleavage, optical properties, heat conductivity, and electrical conductivity. These properties are important since they sometimes determine the use to which the crystals are put in industry. For example, crystalline substances that have special electrical properties are much used in communications equipment. These include quartz and Rochelle salt, which supply voltage on the application of mechanical force (see piezoelectric effect), and germanium, silicon, galena, and silicon carbide, which carry current unequally in different crystallographic directions, as semiconductor rectifiers.

See solid-state physics.

Crystal Palace, building designed by Sir Joseph Paxton and erected in Hyde Park, London, for the Great Exhibition in 1851. In 1854 it was removed to Sydenham, where, until its damage by fire in 1936, it housed a museum of sculpture, pictures, and architecture and was used for concerts. In 1941 its demolition was completed because it served as a guide to enemy bombing planes. The building was constructed of iron, glass, and laminated wood. One of the most significant examples of 19th-century, proto-modern architecture, it was widely imitated in Europe and America.

Crystal Lake, city (1990 pop. 24,512), McHenry co., NE Ill., in a farming area; inc. 1874. Electrical components, tools, plastics, and machinery are manufactured.

Transparent variety of the silica mineral quartz that is valued for its clarity and total lack of colour or flaws. Rock crystal formerly was used extensively as a gemstone, but it has been replaced by glass and plastic; rhinestones originally were quartz pebbles found in the Rhine River. The optical properties of rock crystal led to its use in lenses and prisms; its piezoelectric properties (see piezoelectricity) are used to control the oscillation of electrical circuits.

Learn more about rock crystal with a free trial on Britannica.com.

officially International Movement of the Red Cross and Red Crescent formerly International Red Cross

Humanitarian agency with national affiliates worldwide. Established for the care of victims of battle, it now aids in the general prevention and relief of human suffering. It arose out of the work of Jean-Henri Dunant, who proposed the formation of voluntary relief societies in all countries, the first of which came into being in 1864. The name Red Crescent, adopted in 1906 at the insistence of the Ottoman Empire, is used in Muslim countries. In peacetime, the Red Cross aids victims of natural disasters, maintains blood banks, and provides supplementary health care services. In wartime, it serves as an intermediary between belligerents and visits prisoner-of-war camps to provide relief supplies, deliver mail, and transmit information between prisoners and their relatives. Its operating principles are humanity, impartiality, and neutrality. Its headquarters are in Geneva. Individual national organizations run community programs and coordinate natural-disaster relief efforts. The American Red Cross was founded by Clara Barton in 1881 and first chartered by Congress in 1900; it runs the world's largest blood-donor service. In 1901 Dunant received the first Nobel Prize for Peace; the Red Cross itself received the prize in 1917, 1944, and 1963.

Learn more about Red Cross with a free trial on Britannica.com.

Optoelectronic device used in displays for watches, calculators, notebook computers, and other electronic devices. Current passed through specific portions of the liquid crystal solution causes the crystals to align, blocking the passage of light. Doing so in a controlled and organized manner produces visual images on the display screen. The advantage of LCDs is that they are much lighter and consume less power than other display technologies (e.g., cathode-ray tubes). These characteristics make them an ideal choice for flat-panel displays, as in portable laptop and notebook computers.

Learn more about liquid crystal display (LCD) with a free trial on Britannica.com.

Substance that flows like a liquid but maintains some of the ordered structure characteristic of a crystal. Some organic substances do not melt directly when heated but instead turn from a crystalline solid to a liquid crystalline state. When heated further, a true liquid is formed. Liquid crystals have unique properties. The structures are easily affected by changes in mechanical stress, electromagnetic fields, temperature, and chemical environment. Seealso liquid crystal display.

Learn more about liquid crystal with a free trial on Britannica.com.

Any solid material whose atoms are arranged in a definite pattern and whose surface regularity reflects its internal symmetry. Each of a crystal's millions of individual structural units (unit cells) contains all the substance's atoms, molecules, or ions in the same proportions as in its chemical formula (see formula weight). The cells are repeated in all directions to form a geometric pattern, manifested by the number and orientation of external planes (crystal faces). Crystals are classified into seven crystallographic systems based on their symmetry: isometric, trigonal, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. Crystals are generally formed when a liquid solidifies, a vapour becomes supersaturated (see saturation), or a liquid solution can no longer retain dissolved material, which is then precipitated. Metals, alloys, minerals, and semiconductors are all crystalline, at least microscopically. (A noncrystalline solid is called amorphous.) Under special conditions, a single crystal can grow to a substantial size; examples include gemstones and some artificial crystals. Few crystals are perfect; defects affect the material's electrical behaviour and may weaken or strengthen it. Seealso liquid crystal.

Learn more about crystal with a free trial on Britannica.com.

Giant glass-and-iron exhibition hall in Hyde Park, London, that housed the Great Exhibition of 1851. It was taken down and rebuilt (1852–54) at Sydenham Hill, where it survived until its destruction by fire in 1936. Designed by the greenhouse builder Sir Joseph Paxton (1801–1865), it was a remarkable assembly of prefabricated parts. Its intricate network of slender iron rods sustaining walls of clear glass established an architectural standard for later international exhibitions, likewise housed in glass conservatories.

Learn more about Crystal Palace with a free trial on Britannica.com.