Editing 1873 in science (section)

==Physics==
* February 20 – English electrical engineer [[Willoughby Smith]] publishes his discovery of the [[photoconductivity]] of the element [[selenium]]<ref>{{cite journal|doi=10.1038/007303e0|title=Effect of Light on Selenium During the Passage of an Electric Current|year=1873|journal=[[Nature (journal)|Nature]]|volume=7|issue=173|page=303|bibcode = 1873Natur...7R.303. |doi-access=free}}</ref>
* June 14 – [[Johannes Diderik van der Waals]] defends his thesis, ''[http://www.scs.uiuc.edu/~mainzv/exhibit/vanderwaals.htm Over de Continuiteit van den Gas en Vloeistoftoestand]'' (On the continuity of the gaseous and liquid state) at [[Leiden University]]; in this, he introduces the concepts of molecular volume and molecular attraction; gives a semi-quantitative description of the phenomena of [[condensation]] and critical temperatures; and derives the [[van der Waals equation]]<ref>{{cite journal|authorlink=James Clerk Maxwell|last=Clerk-Maxwell|first=J.|year=1874|title=Van der Waals on the Continuity of the Gaseous and Liquid States|journal=Nature|volume=10|pages=477–480|doi=10.1038/010477a0|bibcode=1874Natur..10..477C|issue=259|url=https://zenodo.org/record/1429215/files/article.pdf}}</ref>
* September 22 – [[James Clerk Maxwell]] delivers a discourse on [[molecule]]s to the [[British Association for the Advancement of Science]] meeting in Bradford<ref>{{cite journal|first=J.|last=Clerk-Maxwell|title=Molecules|journal=Nature|doi=10.1038/008437a0|volume=8|issue=204|date=25 September 1873|pages=437–41|bibcode=1873Natur...8..437.|doi-access=free}} Also digitised at [http://www.victorianweb.org/science/maxwell/molecules.html ''The Victorian Web'']. [https://web.archive.org/web/20120510161223/http://www.victorianweb.org/science/maxwell/molecules.html Archived] 2012-02-23.</ref>
* December – [[J. Willard Gibbs]] describes the principle of [[Gibbs free energy]]<ref>{{cite journal|first=J. W.|last=Gibbs|title=A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces|journal=Transactions of the Connecticut Academy of Arts and Sciences|volume=2|year=1873|pages=382–404}}</ref>
* James Clerk Maxwell's ''[[A Treatise on Electricity and Magnetism]]'' first presents the [[partial differential equations]] known as ''[[Maxwell's equations]]'' which form the foundation of [[classical electrodynamics]], [[optics]] and [[electric circuit]]s
* [[Frederick Guthrie (scientist)|Frederick Guthrie]] is the first to report observing [[thermionic emission]]<ref>{{cite book|url=https://books.google.com/books?id=PrbOIoMnxnwC&pg=PA196|title=The emission of positive ions by hot metals|first=Owen|last=Richardson|year=1921|accessdate=2009-07-11|isbn=978-1-929148-10-3}}</ref>