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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Physical crystallography before X-rays | 1/5 | https://en.wikipedia.org/wiki/Physical_crystallography_before_X-rays | reference | science, encyclopedia | 2026-05-05T16:17:31.939542+00:00 | kb-cron |
Physical crystallography before X-rays describes how physical crystallography developed as a science up to the discovery of X-rays by Wilhelm Conrad Röntgen in 1895. In the period before X-rays, crystallography can be divided into three broad areas: geometrical crystallography culminating in the discovery of the 230 space groups in 1891–1894, chemical crystallography and physical crystallography. Physical crystallography is concerned with the physical properties of crystals, such as their optical, electrical, magnetic, thermal, and mechanical properties.
The interaction between crystals and electromagnetic radiation is covered in the following sections: double refraction, rotary polarization, conical refraction, absorption and pleochroism, luminescence, reflection from opaque materials, and infrared optics. The effect of electricity and magnetism on crystals is covered in: electrical conduction, magnetic properties, and dielectric properties. The effect of temperature change on crystals is covered in: thermal expansion, thermal conduction, thermoelectricity, and pyroelectricity. The effect of mechanical force on crystals is covered in: elasticity, photoelasticity, and piezoelectricity. The study of crystals in the time before X-rays was focused more on their geometry and mathematical analysis than their physical properties. Unlike geometrical crystallography, the history of physical crystallography has no central story, but is a collection of developments in different areas.
== Symmetry == During the 19th century crystallography was progressively transformed into an empirical and mathematical science by the adoption of symmetry concepts. In 1832 Franz Ernst Neumann used symmetry considerations when studying double refraction. Woldemar Voigt, who was a student of Neumann, in 1885 formalized Neumann's principle as "if a crystal is invariant with respect to certain symmetry operations, any of its physical properties must also be invariant with respect to the same symmetry operations". Neumann's principle is sometimes referred to as the Neumann–Minnigerode–Curie principle based on later work by Bernhard Minnigerode (another student of Neumann) and Pierre Curie. Curie's principle "the symmetries of the causes are to be found in the effects" is a generalization of Neumann's principle. At the end of the 19th century Voigt introduced tensor calculus to model the physical properties of anisotropic crystals.
== Interaction with electromagnetic radiation ==
=== Double refraction ===
Double refraction occurs when a ray of light incident upon a birefringent material, is split by polarization into two rays taking slightly different paths. The double refraction and rhomboidal cleavage of crystals of calcite, or Iceland spar, were first recorded in 1669 by Rasmus Bartholin In 1690 Christiaan Huygens analyzed double refraction in his book Traité de la lumière. Huygens reasoned that the cleavage rhombohedron resulted from the stacking of spherical particles and that the peculiarities of the transmission of light can be traced to the particular asymmetry of the crystal. In 1810 Étienne-Louis Malus determined that natural light, too, when reflected through a certain angle, behaves like one of the rays exiting a double-refracting crystal. Malus called this phenomenon polarization. In 1812 Jean-Baptiste Biot defined optically positive and negative crystals for the first time. In 1819 David Brewster found that all crystals could be classified as isotropic, uniaxial or biaxial. Augustin-Jean Fresnel was a significant researcher in the whole field of crystal optics, and published a detailed paper on double refraction in 1827 in which he described the phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarization. Crystal optics was an active research area during the 19th century and comprehensive accounts of the field were published by Lazarus Fletcher (1891), Theodor Liebisch (1891) and Friedrich Pockels (1906).
=== Rotary polarization ===