1904 in science
- Johannes Franz Hartmann discovers the interstellar medium.
- Edward Walter Maunder plots the first sunspot "butterfly diagram".
- Notable asteroid 522 Helga is discovered by Max Wolf in Heidelberg.
- December 3 – The sixth moon of Jupiter, later called Himalia, is discovered at Lick Observatory.
- Van der Grinten projection proposed.
- Henri Poincaré discovers the Poincaré homology sphere, leading him to formulate the Poincaré conjecture.
- Helge von Koch describes the "Koch snowflake", one of the earliest fractal curves described.
- Charles Spearman develops his rank correlation coefficient.
- Ernst Zermelo formulates the axiom of choice to formalize his proof of the well-ordering theorem.
- Epinephrine first artificially synthesized by Friedrich Stolz.
- Antoni Leśniowski presents to a meeting of the Warsaw Medical Society a surgical specimen of an inflammatory tumour of the terminal ileum with a fistula to the ascending colon, consistent with what will later become known as Crohn's disease.
- Vacuum tube invented by John Ambrose Fleming.
- James H. Jeans's The Dynamical Theory of Gases is published in Cambridge.
- J. J. Thomson proposes the plum pudding model for the atom.
- Hantaro Nagaoka develops the Saturnian model for the atom.
- July 4 – Piero Ginori Conti demonstrates the use of geothermal power to generate electricity, at Larderello in Italy.
- July 23 – A continuous track tractor is patented by David Roberts of Richard Hornsby & Sons of Grantham in England.
- November 24 – A continuous track tractor is demonstrated by the Holt Manufacturing Company in the United States.
- The first diesel engined submarine, the Z, is built in France.
- The Heckelphone variety of oboe is invented by Wilhelm Heckel and his sons.
- The sleeve valve is invented by Charles Yale Knight.
- The turbine-powered Bliss-Leavitt torpedo, designed by Frank McDowell Leavitt and manufactured by the E. W. Bliss Company of Brooklyn, is put into service by the United States Navy.
- Lucien Bull produces the first successful chronophotography (of insect flight), working in France.
- Rue Franklin Apartments, Paris, are completed by Auguste Perret and his brother Gustave, an early example of an exposed reinforced concrete frame building.
- Nobel Prizes
- January 21 – Edris Rice-Wray Carson (died 1990), American-born physician, pioneer in family planning.
- January 26 – Ancel Keys (died 2004) American nutritionist.
- March 13 – René Dumont (died 2001), French agronomist.
- March 20 – B. F. Skinner (died 1990), American behavioral psychologist.
- April 11 – Arthur Mourant (died 1994), Jersiais hematologist.
- April 22 – J. Robert Oppenheimer (died 1967), American physicist.
- June 3 – Charles R. Drew (died 1950), African American physician, pioneer in blood transfusion
- July 5 – Ernst Mayr (died 2005), German-born evolutionary biologist.
- August 5 – Kenneth V. Thimann (died 1997), English-American plant physiologist and microbiologist known for his studies of plant hormones.
- August 17 – Cornelis Simon Meijer (died 1974), Dutch mathematician.
- August 28 – Secondo Campini (died 1980), Italian jet pioneer.
- November 11 – J. H. C. Whitehead (died 1960), British mathematician.
- March 7 – Ferdinand André Fouqué (born 1828), French geologist, petrologist and volcanologist.
- May 10 – Henry Morton Stanley (born 1841), Welsh-born explorer and journalist.
- September 24 – Niels Ryberg Finsen (born 1860), Icelandic/Faroese/Danish physician and scientist, recipient of the Nobel Prize in Physiology or Medicine.
- October 7 – Isabella Bird (born 1831), British explorer, writer, photographer and naturalist.
- October 21 – Isabelle Eberhardt (born 1877), Swiss-Algerian explorer.
- Asimov, Isaac. Asimov's Biographical Encyclopedia of Science and Technology (2nd ed.).
- "Sur une courbe continue sans tangente, obtenue par une construction géométrique élémentaire".
- Addison, Paul S. (1997). Fractals and Chaos: An Illustrated Course. Bristol: Institute of Physics. p. 19. ISBN 0-7503-0400-6.
- Crilly, Tony (2007). 50 Mathematical Ideas you really need to know. London: Quercus. p. 145. ISBN 978-1-84724-008-8.
- Zermelo, Ernst (1904). "Beweis, dass jede Menge wohlgeordnet werden kann" (reprint). Mathematische Annalen. 59 (4): 514–16. doi:10.1007/BF01445300.
- Reported by him in Pamiętnik Towarzystwa Lekarskiego Warszawskiego. Bartnik, W. (December 2003). "Inflammatory bowel disease – Polish contribution". Journal of Physiology and Pharmacology. Kraków: Polish Physiological Society. 54 (S3): 205–210. PMID 15075474. Retrieved 2008-03-28.[permanent dead link]
- British Patent No. 16,345. Robinson, Peter. Lincoln's Excavators: The Ruston years 1875–1930. Nynehead: Roundoak. ISBN 1-871565-42-1.
- Newpower, Anthony (2006). Iron Men And Tin Fish: The Race to Build a Better Torpedo During World War II. Westport, Conn.: Greenwood Publishing Group. p. 18. ISBN 0-275-99032-X.
- Reported by him in "Motional mechanism of the insect wing", Comptes rendus de l'Académie des sciences 138:590–592 (29 February); "Application of the electric spark to the chrono-photography of rapid motions", Comptes rendus 138:155–157 (21 March); "Chronophotography of rapid motions", Bulletin de la Société Philomathiclue (Paris) (June 11); Synthesis in chronophotography, Bulletin de la Société Philomathiclue (November 12).
- "Rue Franklin Apartments". GreatBuildings. Retrieved 2012-05-29.
- Rothschild, Walter (1904-05-20). "Microgoura, n. gen". Bulletin of the British Ornithologists' Club. 14 (CVII): 77–78. Retrieved 2015-09-29.