solid-state physics, study of the properties of bulk matter rather than those of the individual particles that compose it. Solid-state physics is concerned with the properties exhibited by atoms and molecules because of their association and regular, periodic arrangement in crystals. The descriptive side of the study of solids is crystallography. From a practical point of view, searches are made for new characteristics and behavior of various materials. The most spectacular discovery resulting from these searches has been the transistor. From a theoretical point of view, attempts are made to predict and explain the nature of aggregates of atoms in terms of the basic laws of the quantum theory and the well-understood properties of individual atoms. An important concern of solid-state physics is the mechanical and thermal behavior of solids; specific areas of study include the allowed vibration modes of crystals (see phonon), the transmission of vibrational energy (thermal conductivity), the amount of energy that must be absorbed to produce a given change in temperature (specific heat), and phase transitions such as the melting points of crystals. Although the crystalline, mechanical, thermal, and optical properties of solids are of great interest, it is the electrical properties that most clearly demarcate the various types of materials and which exhibit the greatest diversity of behavior. The single most important electrical characteristic of a solid is its electrical conductivity (the ease with which electric currents flow through it). See conduction. Metals are highly conductive solids that offer little resistance to electric currents. Most solid nonmetals, on the other hand, are insulators (solids whose conductivity is nearly zero); they offer virtually infinite resistance to electric currents. A third class of solids possesses electrical conductivity that is neither very high nor very low; these solids are called semiconductors. A principal triumph of quantum mechanics in solid-state physics is the explanation of these extreme variations of electrical conductivity in terms of the atomic structure of the three types of solids.