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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Physical crystallography before X-rays | 5/5 | https://en.wikipedia.org/wiki/Physical_crystallography_before_X-rays | reference | science, encyclopedia | 2026-05-05T16:17:31.939542+00:00 | kb-cron |
Pyroelectricity is the generation of a temporary voltage in a crystal when subjected to a temperature change. The appearance of electrostatic charges upon a change of temperature has been observed since ancient times, in particular with tourmaline and was described, among others, by Steno, Linnaeus, Aepinus and René Just Haüy. Aepinus published an account of his observations in 1756. Haüy made detailed investigations of pyroelectricity; he detected pyroelectricity in calamine and showed that electricity in tourmaline was strongest at the poles of the crystal and became imperceptible at the middle. Haüy published a book on electricity and magnetism in 1787. Haüy later showed that hemihedral crystals are electrified by temperature change while holohedral (symmetric) crystals are not. Research into pyroelectricity became more quantitative in the 19th century. In 1824 David Brewster gave the effect the name it has today. In 1840 Gabriel Delafosse, Haüy's student, theorized that only molecules which are not symmetrical can be polarized electrically. Both William Thomson in 1878 and Woldemar Voigt in 1897 helped develop a theory for the processes behind pyroelectricity. A detailed history of pyroelectricity has been written by Sidney Lang; shorter histories have also been published.
== Effect of mechanical force ==
=== Elasticity ===
Some minerals, for example mica, are highly elastic, springing back to their original shape after being bent. Others, for example talc, may be readily bent but do not return to their original form when released. The initial theory of the elasticity of solid bodies were developed in the 1820s. Augustin-Louis Cauchy and Siméon Denis Poisson published theories of the mutual action of a regular arrangement of particles for a non-cubic body in 1823 and 1829 respectively. In 1827 Claude-Louis Navier published a theory for an isotropic body. Also during the 1820s Friedrich Mohs introduced his eponymous scale of hardness. In 1834 Franz Ernst Neumann published a paper on the elasticity of homohedral crystals. In 1828 Cauchy generalised the problem and showed that 36 independent constants were required to describe elasticity in crystals. George Green (1837) introduced the limitation that the force between any two elements of a crystal, however small, must lie along the line joining their centres. This reduced the number of constants from 36 to 21. William Thomson (1857) showed that Green's assumption was unnecessary and that the thermodynamic requirements of a reversible process require only 21 constants, without any special assumptions. In 1874 Woldemar Voigt measured the elasticity of rock salt and G. Baumgarten measured the elasticity of calcite. In 1887 Wilhelm Röntgen and J. Schneider measured the cubic compressibility of sodium and potassium chlorides. In 1877 Lambros Koromilas measured the elasticity of gypsum and mica by twisting mineral bars.; in 1881 H. Klang carried out similar experiments with fluorites. In the period 1874–1888 Voigt was the leading researcher on the elasticity of crystals. Voigt showed that the number of elasticity constants reduces as more symmetry is introduced into the crystal. For a triclinic crystal, which is the most general case, 21 elasticity constants are required. For a monoclinic crystal there are 13 elasticity constants, for a rhombic crystal 9, for a hexagonal crystal 7, for a tetragonal crystal 6, and finally for a cubic crystal there are only 3. A summary of developments in the field was published by W. A. Wooster.
=== Photoelasticity === Photoelasticity describes changes in the optical properties of a material under mechanical deformation. The photoelastic phenomenon in transparent, non-crystalline materials (gels and glasses) was first discovered by David Brewster in 1815. Brewster then detected the effect in crystals and showed that uniaxial crystals could be made biaxial. In 1822 Augustin-Jean Fresnel experimentally confirmed that the photoelastic effect was a stress-induced birefringence. Franz Ernst Neumann investigated double refraction in stressed transparent bodies. In 1841 Neumann published his elastic equations, which describe, in differential form, the changes which polarized light experiences when travelling through a stressed body. The Neumann equations are the basis of all subsequent photoelasticity research. The photoelastic effect was analyzed by Friedrich Pockels, who also discovered the Pockels electro-optic effect (the production of birefringence of light on the application of an electric field). In 1889–1890 Pockels produced a phenomenological theory for both of these effects for all crystal classes.
=== Piezoelectricity ===
In 1880 Pierre and Jacques Curie discovered piezoelectricity (an electric charge that accumulates in response to applied mechanical stress) in certain crystals, including quartz, tourmaline, cane sugar and sodium chlorate. The Curies, however, did not predict the converse piezoelectric effect (the internal generation of a mechanical strain resulting from an applied electric field). The converse effect was deduced by Gabriel Lippmann in 1881. The Curies immediately confirmed the existence of the effect, and went on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals. In 1890 Woldemar Voigt published a phenomenological theory of the piezoelectric effect based on the symmetry of crystals without centrosymmetry.
== Research community ==
Before the 20th century crystallography was not a well-established academic discipline. There were no academic positions specifically in crystallography. Workers in the field normally carried out their crystallographic research as an ancillary to other employment(s), or had independent means. The leading workers in the field of physical crystallography were employed as follows:
Professors Mathematics or science: Airy, Arago, E. Becquerel, Biot, Curie, Drude, Hamilton, Linnaeus, Mitscherlich, Pasteur, Pockels, Plücker, Stokes, Tyndall, Thomson, Voigt Mineralogy: Groth, Haüy, Liebisch, Mohs, Neumann, Sénarmont Other employment: Bartholinus (physician), Brewster (editor), Fresnel (engineer), Hooke (municipal official), Malus (military officer) Independently wealthy: Herschel, Huygens In the nineteenth century there were informal schools of physical crystallography researchers in France (Arago, E. Becquerel, Biot, Fresnel, Haüy, Sénarmont), Germany (Drude, Groth, Liebisch, Mitscherlich, Mohs, Neumann, Pockels, Voigt) and the British Isles (Airy, Brewster, Hamilton, Stokes, Thomson). Until the founding of Zeitschrift für Krystallographie und Mineralogie by Paul Groth in 1877 there was no lead journal for the publication of crystallographic papers. The majority of crystallographic research was published in the journals of national scientific societies, or in mineralogical journals. The inauguration of Groth's journal marked the emergence of crystallography as a mature science independent of geology.
== See also == History of crystallography before X-rays Chemical crystallography before X-rays Geometrical crystallography before X-rays Timeline of crystallography
== Citations ==
== Works cited ==