1866 in science

• May – William Huggins studies the emission spectrum of a nova and discovers that it is surrounded by a cloud of hydrogen.{{cite ODNB|url=http://www.oxforddnb.com/view/article/34039|title=Huggins, Sir William (1824–1910)|first=Barbara J.|last=Becker|year=2004|doi=10.1093/ref:odnb/34039|accessdate=2011-03-04}}
• June 4 – Pluto (not known at this time) 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.

• Gregor Mendel publishes his laws of inheritance.{{cite journal|last=Mendel|first=J. G.|year=1866|title=Versuche über Pflanzenhybriden|journal=Verhandlungen des naturforschenden Vereines in Brünn|volume=IV|pages=3–47 (Abhandlungen)}} For the English translation, see {{cite journal|last1=Druery|first1=C. T.|first2=William|last2=Bateson|title=Experiments in plant hybridization|journal=Journal of the Royal Horticultural Society|volume=26|pages=1–32|year=1901|url=http://www.esp.org/foundations/genetics/classical/gm-65.pdf|accessdate=2009-10-09}}
• 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" (progenital cells) in parthenogenetic Diptera.{{cite journal|first=Alexander|last=Petrunkevitch|title=Russia's Contribution to Science|journal=Transactions of the Connecticut Academy of Arts and Sciences|volume=23|year=1920|page=239}}
• Robert John Lechmere Guppy discovers the guppy (fish) 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.Great Soviet Encyclopedia (1979).

• Dynamite invented by Alfred Nobel.{{cite book|last1=Schück|first1=H.|last2=Sohlman|first2=R.|year=1929|title=The Life of Alfred Nobel|location=London|publisher=Heinemann}}
• August von Hofmann proposes the now standard system of hydrocarbon nomenclature and invents the Hofmann voltameter.{{cite book|last=von Hofmann|first=A. W.|title=Introduction to Modern Chemistry: Experimental and Theoretic; Embodying Twelve Lectures Delivered in the Royal College of Chemistry, London|publisher=Walton and Maberly|location=London|year=1866|url=https://books.google.com/books?id=KT8JAAAAIAAJ}}
• Emil Erlenmeyer proposes that naphthalene has a structure of two fused benzene rings.{{cite journal|title=Studien über die s. g. aromatischen Säuren|first=Emil|last=Erlenmeyer|journal=Annalen der Chemie und Pharmacie|volume=137|issue=3|pages=327–359|year=1866|doi=10.1002/jlac.18661370309|url=https://zenodo.org/record/1427251}}

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

• The second smallest pair of amicable numbers (1184, 1210) is discovered by teenager B. Nicolò I. Paganini.

• February 21 – Lucy Hobbs Taylor becomes the world's first woman to receive a doctorate from a dental college (Ohio College of Dental Surgery).{{cite journal|jstor=44443642|title=The First Woman Dentist Lucy Hobbs Taylor, D.D.S. (1833–1910)|last=Edwards|first=Ralph W.|journal=Bulletin of the History of Medicine|year=1951|volume=25|issue=3|pages=277–283|pmid=14848611}}
• 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.Zur Anatomie und Physiologie der 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.{{cite journal|first=J. Langdon H.|last=Down|title=Observations on the Ethnic Classification of Idiots|journal=Clinical Lectures and Reports by the Medical and Surgical Staff of the London Hospital|volume=3|pages=259–62|year=1866}}
• Invention of a clinical thermometer by Thomas Clifford Allbutt.
• A cholera epidemic in London causes over 5,000 deaths.{{cite book|chapter=1866|title=The People's Chronology|editor=Everett, Jason M.|publisher=Thomson Gale|year=2006}}
• Patrick Manson starts a school of tropical medicine in Hong Kong.{{citation needed|date=March 2011}}

• 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.{{cite book|last=Holtz|first=Thomas R.|authorlink=Thomas R. Holtz Jr.|year=2004|chapter=Tyrannosauroidea|editor= Weishampel, David B.|editor2=Dodson, Peter|editor3=Osmólska, Halszka|title=The Dinosauria|edition=2nd|publisher=University of California Press|location=Berkeley|pages=111–136|isbn=0-520-24209-2}}

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

• 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 {{SS|Great Eastern}} successfully completes laying the transatlantic telegraph cable between Valentia Island, Ireland and Heart's Content, Newfoundland, permanently restoring a communications link.{{cite book|title=Penguin Pocket On This Day|publisher=Penguin Reference Library|isbn=0-14-102715-0|year=2006}}
• August 23 – Ralph H. Twedell patents the hydraulic riveter in the United Kingdom.{{cite book|first=Ian|last=McNeill|title=Hydraulic Power|location=London|publisher=Longman|year=1972|isbn=0-582-12797-1}}

• Copley Medal: Julius Plücker{{cite web |title=Copley Medal {{!}} British scientific award |url=https://www.britannica.com/science/Copley-Medal |website=Encyclopædia Britannica |accessdate=23 July 2020 }}
• Wollaston Medal for Geology: Charles Lyell

• February 1 – Agda Meyerson (died 1924), Swedish nurse and healthcare profession activist
• February 8 – Moses Gomberg (died 1947), Russian-born chemist
• February 14 – Victor Despeignes (died 1937), French pioneer of radiation oncology
• February 26 – Herbert Henry Dow (died 1930), Canadian-born chemist
• April 17 – Ernest Starling (died 1927), English physiologist
• July 13 – Emily Winifred Dickson (died 1944), Irish-born gynaecologist
• July 25 – Frederick Blackman (died 1947), English plant physiologist
• September 13 – Arthur Pollen (died 1937), English inventor
• September 21 – H. G. Wells (died 1946), English scientific populariser
• September 25 – Thomas Hunt Morgan (died 1945), American biologist, Nobel laureate in Physiology
• October 8 – Reginald Fessenden (died 1932), Canadian pioneer of radio broadcasting
• November 11 – Martha Annie Whiteley (died 1956), English chemist and mathematician
• November 30 – Robert Broom (died 1951), Scottish-born paleontologist
• December 7 – Maude Delap (died 1953), Irish marine biologist

• March 6 – William Whewell (born 1794), English scientist, philosopher and historian of science
• March 14 – Alexander Morison (born 1779), Scottish physician and psychiatrist
• April 4 – William Dick (born 1793), Scottish veterinarian
• April 5 – Thomas Hodgkin (born 1798), English physician
• July 20 – Bernhard Riemann (born 1826), German-born mathematician
• September 16 – François Mêlier (born 1798), French physician {{Cite web|language=fr |url=https://cths.fr/an/savant.php?id=570# |title=François Mêlier |author=Jules Bergeron, Alexandre Wauthier |website=cths.fr |publisher= Comité des travaux historiques et scientifiques |access-date=2020-10-27}}
• October 18 – Philipp Franz von Siebold (born 1796), German physician, botanist and traveler in Japan
• December 1 – George Everest (born 1790), British surveyor and geographer

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Category:1860s in science

Category:19th century in science