1866 in science

Astronomy

• May – William Huggins studies the emission spectrum of a nova and discovers that it is surrounded by a cloud of hydrogen.
• June 4 – Pluto reaches its only aphelion between 1618 and 2113.
• Giovanni Schiaparelli realizes that meteor streams occur when the Earth passes through the orbit of a comet that has left debris along its path.

  Biology

• Gregor Mendel publishes his laws of inheritance.
• Ernst Haeckel challenges the plant/animal division of life, observing that single celled organisms, the protists, do not fit into either category.
• Élie Metchnikoff describes the early separation of "polecells" in parthenogenetic Diptera.
• Robert John Lechmere Guppy discovers the guppy in Trinidad.
• Frederick Smith first discovers Formica candida in the Bournemouth district of England, describing it as Formica gagates.
• Nikolai Kaufman publishes his Moscow Flora.

  Chemistry

• Dynamite invented by Alfred Nobel.
• August von Hofmann proposes the now standard system of hydrocarbon nomenclature and invents the Hofmann voltameter.
• Emil Erlenmeyer proposes that naphthalene has a structure of two fused benzene rings.

  Earth sciences

• January 26 – Volcanic eruption in the Santorini caldera begins.

  Mathematics

• The second smallest pair of amicable numbers is discovered by teenager B. Nicolò I. Paganini.

  Medicine

• July – Elizabeth Garrett Anderson opens the St Mary's Dispensary in London where women can seek medical advice from exclusively female practitioners.
• Max Schultze discovers two sorts of 'receptors' in the retina.
• Dr John Langdon Down publishes his theory that different types of mental condition can be classified by ethnic characteristics, notably "Mongolism", the genetic developmental disability now known as Down syndrome.
• Invention of a clinical thermometer by Thomas Clifford Allbutt.
• A cholera epidemic in London causes over 5,000 deaths.
• Patrick Manson starts a school of tropical medicine in Hong Kong.

  Paleontology

• American paleontologist Joseph Leidy describes the new genus and species Laelaps aquilunguis, demonstrating that theropod dinosaurs walked on their hind limbs rather than on all fours as in earlier reconstructions.

  Physics

• James Clerk Maxwell formulates the Maxwell–Boltzmann distribution in the kinetic theory of gases.

  Technology

• January 12 – Royal Aeronautical Society is formed as 'The Aeronautical Society of Great Britain' in London, the world's oldest such society.
• July 27 – The successfully completes laying the transatlantic telegraph cable between Valentia Island, Ireland and Heart's Content, Newfoundland, permanently restoring a communications link.
• August 23 – Ralph H. Twedell patents the hydraulic riveter in the United Kingdom.

  Awards

• Copley Medal: Julius Plücker
• Wollaston Medal for Geology: Charles Lyell

  Births

• February 1 – Agda Meyerson, Swedish nurse and healthcare profession activist
• February 8 – Moses Gomberg, Russian-born chemist
• February 14 – Victor Despeignes, French pioneer of radiation oncology
• February 26 – Herbert Henry Dow, Canadian-born chemist
• April 17 – Ernest Starling, English physiologist
• July 13 – Emily Winifred Dickson, Irish-born gynaecologist
• September 13 – Arthur Pollen, English inventor
• September 21 – H. G. Wells, English scientific populariser
• September 25 – Thomas Hunt Morgan, American biologist, Nobel laureate in Physiology
• October 8 – Reginald Fessenden, Canadian pioneer of radio broadcasting
• November 30 – Robert Broom, Scottish-born paleontologist
• December 7 – Maude Delap, Irish marine biologist

  Deaths

• March 6 – William Whewell, English scientist, philosopher and historian of science
• March 14 – Alexander Morison, Scottish physician and psychiatrist
• April 4 – William Dick, Scottish veterinarian
• April 5 – Thomas Hodgkin, English physician
• July 20 – Bernhard Riemann, German-born mathematician
• October 18 – Philipp Franz von Siebold, German physician, botanist and traveler in Japan
• December 1 – George Everest, British surveyor and geographer