Editing 1866 (section)

=== July–December ===
[[File:Georg Friedrich Bernhard Riemann.jpeg|thumb|right|110px|[[Bernhard Riemann]]]]
* [[July 20]] – [[Bernhard Riemann]], German mathematician (b. [[1826]])
* [[July 25]] – [[Floride Calhoun]], Second Lady of the United States (b. 1792)
* [[July 29]] – [[Madame Clicquot Ponsardin]], French champagne producer  (b. [[1777]]) 
* [[August 1]] – John Ross, long-serving principal chief of the [[Cherokee Nation]], of natural causes, in Washington D. C. (born 1790 in Cherokee Nation East).
* [[August 6]] – [[Christian Eric Fahlcrantz]], Swedish writer (b. [[1790]])
* [[August 20]] – [[Maria De Mattias]], Italian Catholic saint (b. [[1805]])
* [[August 29]] – [[Tokugawa Iemochi]], 14th ''[[shōgun]]'' of the [[Tokugawa shogunate]] of Japan (b. [[1846]])
* [[September 4]] – [[Theresa Pulszky]], European author (b. [[1819]])
* [[September 30]] – [[Per Gustaf Svinhufvud af Qvalstad]], Swedo-Finnish treasurer of [[Tavastia (historical province)|Tavastia province]], manor host, and paternal grandfather of President [[P. E. Svinhufvud]] (b. 1804)<ref>[https://suomenpresidentit.fi/svinhufvud/?lang=en PEHR EVIND SVINHUFVUD (1861–1944), 1ST REGENT OF FINLAND (1918), 3RD PRESIDENT OF FINLAND (1931–1937)]</ref>
* [[October 13]] – [[Celadon Leeds Daboll]], American merchant and inventor (b. [[1818]])
* [[October 18]] – [[Manuel Bulnes]], Chilean general and politician, [[President of Chile]] (b. [[1799]])
* [[November 11]] – [[Agustín Jerónimo de Iturbide y Huarte]], Prince Imperial of Mexico (b. [[1807]])
* [[November 14]] – King [[Miguel I of Portugal]] (b. [[1802]])
* [[November 26]] – [[Jean-Jacques Willmar]], Luxembourg politician (b. [[1792]])
* [[December 1]] – [[George Everest]], Welsh geodesist (b. [[1790]])
* [[December 21]] – [[William J. Fetterman]], United States Army officer (b. [[1835]]?)<ref>{{cite journal |url=https://archive.org/details/annalsofwyom63141991wyom/page/42 |title=Price of Arrogance: The Short and Controversial Life of William Judd Fetterman |last=McDermott |first=John D. |journal=Annals of Wyoming by Wyoming State Historical Society |volume=63 |issue=2 |pages=42–53 |date=Spring 1991}}</ref>
* December 21 – [[Mercedes Marín del Solar]], Chilean poet, reform educator (b. [[1804]])
* [[December 25]] – [[Hayrullah Efendi]], Ottoman physician, historian, and official (b. [[1818]])<ref>{{TDV Encyclopedia of Islam|volume=17 |pages=67–75 |last=Akün |first=Ömer Faruk |title=Hayrullah Efendi |url=https://islamansiklopedisi.org.tr/hayrullah-efendi}}</ref>