Editing Helium (section)

===Scientific discoveries===
The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a [[wavelength]] of 587.49 nanometers in the [[Emission spectrum|spectrum]] of the [[chromosphere]] of the [[Sun]]. The line was detected by French astronomer [[Pierre Janssen|Jules Janssen]] during [[Solar eclipse of August 18, 1868|a total solar eclipse]] in [[Guntur]], India.<ref name="frnch">{{Cite journal|title = French astronomers in India during the 17th&nbsp;– 19th centuries |journal = Journal of the British Astronomical Association|volume =101|issue = 2|pages = 95–100|bibcode = 1991JBAA..101...95K|author = Kochhar, R. K.|date=1991}}</ref><ref name="nbb" /> This line was initially assumed to be [[sodium]]. On October 20 of the same year, English astronomer [[Norman Lockyer]] observed a yellow line in the solar spectrum, which he named the D<sub>3</sub> because it was near the known D<sub>1</sub> and D<sub>2</sub> [[Fraunhofer line]]s of sodium.<ref name="Lockyer 1868">{{cite journal |last1=Lockyer |first1=J. N. |title=Notice of an observation of the spectrum of a solar prominence |journal= Proceedings of the Royal Society of London |volume=17 |date=October 1868 |pages=91–92 |url= https://babel.hathitrust.org/cgi/pt?id=hvd.32044106279359;view=1up;seq=109 |jstor=112357 |access-date=3 June 2018 |bibcode=1868RSPS...17...91L |doi=10.1098/rspl.1868.0011|s2cid=163097539 }}</ref><ref name="enc">{{Cite book|title= The Encyclopedia of the Chemical Elements |pages =256–268 |first = Clifford A. |last=Hampel |location=New York |isbn = 978-0-442-15598-8 |date = 1968 |publisher =Van Nostrand Reinhold}}</ref> He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer named the element with the Greek word for the Sun, ἥλιος (''[[helios]]'').<ref>{{OEtymD|helium}}</ref><ref>{{Cite journal |last=Thomson |first=William |date=August 3, 1871 |volume=4 |pages=261–278 [268] |doi=10.1038/004261a0 |title=Inaugural Address of Sir William Thomson |journal=Nature |url=https://books.google.com/books?id=IogCAAAAIAAJ&pg=PA268 |quote=Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium |bibcode=1871Natur...4..261. |issue=92 |pmc=2070380 |access-date=February 22, 2016 |archive-url=https://web.archive.org/web/20161202011154/https://books.google.com/books?id=IogCAAAAIAAJ&pg=PA268 |archive-date=December 2, 2016 |url-status=live }}</ref> It is sometimes said that English chemist [[Edward Frankland]] was also involved in the naming, but this is unlikely as he doubted the existence of this new element. The ending "-ium" is unusual, as it normally applies only to metallic elements; probably Lockyer, being an astronomer, was unaware of the chemical conventions.<ref name=jensen>{{Cite journal |last=Jensen |first=William B. |date=2004 |volume=81 |issue=7 |page=944 |doi=10.1021/ed081p944 |title=Why Helium ends in "-ium" |journal=Journal of Chemical Education|bibcode=2004JChEd..81..944J }}</ref>
[[File:Helium spectrum.jpg|left|thumb|Spectral lines of helium|alt=Picture of visible spectrum with superimposed sharp yellow and blue and violet lines]]
In 1881, Italian physicist [[Luigi Palmieri]] detected helium on Earth for the first time through its D<sub>3</sub> spectral line, when he analyzed a material that had been [[Sublimation (phase transition)|sublimated]] during a recent eruption of [[Mount Vesuvius]].<ref name="Palmieri 1881">{{cite journal |last1=Palmieri |first1=Luigi |title=La riga dell'Helium apparsa in una recente sublimazione vesuviana |trans-title=The line of helium appeared in a recently sublimated material [from Mt.] Vesuvius. |journal=Rendiconto dell'Accademia delle Scienze Fisiche e Matematiche (Naples, Italy) |volume=20 |date=1881 |page=223 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.hnl7mr;view=1up;seq=251 |access-date=1 May 2017 |quote= ''Raccolsi alcun tempo fa una sostanza amorfa di consistenza butirracea e di colore giallo sbiadato sublimata sull'orlo di una fumarola prossima alla bocca di eruzione. Saggiata questa sublimazione allo spettroscopio, ho ravvisato le righe del sodio e del potassio ed una lineare ben distinta che corrisponde esattamente alla D<sub>3</sub> che è quella dell'Helium. Do per ora il semplice annunzio del fatto, proponendomi di ritornare sopra questo argomento, dopo di aver sottoposta la sublimazione ad una analisi chimica.'' (I collected some time ago an amorphous substance having a buttery consistency and a faded yellow color which had sublimated on the rim of a fumarole near the mouth of the eruption. Having analyzed this sublimated substance with a spectroscope, I recognized the lines of sodium and potassium and a very distinct linear line which corresponds exactly to D<sub>3</sub>, which is that of helium. For the present, I'm making a mere announcement of the fact, proposing to return to this subject after having subjected the sublimate to a chemical analysis.) |archive-url= https://web.archive.org/web/20180901111504/https://babel.hathitrust.org/cgi/pt?id=hvd.hnl7mr;view=1up;seq=251 |archive-date=1 September 2018 |url-status=live }}</ref>

[[File:William Ramsay working.jpg|thumb|upright|Sir [[William Ramsay]], the discoverer of terrestrial helium]]
[[File:Clevite sample (35321726345).jpg|thumb|upright|The cleveite sample from which Ramsay first purified helium<ref name="Kirk">{{cite web| last1=Kirk|first1=Wendy L.| title= Cleveite [not Clevite] and helium| url=https://blogs.ucl.ac.uk/museums/2013/01/11/cleveite-and-helium-not-clevite/| website=Museums & Collections Blog| publisher=[[University College London]]| access-date=18 August 2017|archive-url= https://web.archive.org/web/20181018054313/http://blogs.ucl.ac.uk/museums/2013/01/11/cleveite-and-helium-not-clevite/| archive-date=18 October 2018|url-status= live}}</ref>]]
On March 26, 1895, Scottish chemist [[William Ramsay|Sir William Ramsay]] isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10% [[rare-earth elements]]) with mineral [[acid]]s. Ramsay was looking for [[argon]] but, after separating [[nitrogen]] and [[oxygen]] from the gas, liberated by [[sulfuric acid]], he noticed a bright yellow line that matched the D<sub>3</sub> line observed in the spectrum of the Sun.<ref name="enc" /><ref>{{Cite journal|title = On a Gas Showing the Spectrum of Helium, the Reputed Cause of D<sub>3</sub>, One of the Lines in the Coronal Spectrum. Preliminary Note| last = Ramsay | first= William|author-link = William Ramsay| journal = Proceedings of the Royal Society of London|volume = 58|issue = 347–352|pages = 65–67| date = 1895|doi = 10.1098/rspl.1895.0006| bibcode = 1895RSPS...58...65R| s2cid = 129872109| url = https://zenodo.org/record/1432083|doi-access = free}}</ref><ref>{{Cite journal| title = Helium, a Gaseous Constituent of Certain Minerals. Part I|last = Ramsay | first= William|journal = Proceedings of the Royal Society of London|volume = 58| issue = 347–352|pages = 81–89|date = 1895 |doi = 10.1098/rspl.1895.0010| bibcode = 1895RSPS...58...80R|doi-access = free}}</ref><ref>{{Cite journal |title = Helium, a Gaseous Constituent of Certain Minerals. Part II – Density|last = Ramsay | first= William| journal = Proceedings of the Royal Society of London|volume = 59|issue = 1|pages = 325–330|date = 1895 |doi = 10.1098/rspl.1895.0097|bibcode = 1895RSPS...59..325R|s2cid = 96589261}}</ref> These samples were identified as helium by Lockyer and British physicist [[William Crookes]].<ref>{{Cite journal|title = On the new gas obtained from uraninite. Preliminary note, part II|author = Lockyer, J. Norman|author-link = Norman Lockyer| journal = Proceedings of the Royal Society of London|volume = 58|issue = 347–352| pages = 67–70|date = 1895|doi = 10.1098/rspl.1895.0008|doi-access = free}}</ref><ref>See:
* {{cite journal| last= Crookes| first= William | year= 1895 | url= https://books.google.com/books?id=YCLOAAAAMAAJ&pg=PA151 | title= The spectrum of the gas from clèveite | journal= The Chemical News and Journal of Physical Science| volume= 71 | number= 1844 | page= 151}}
*  {{cite journal| last= Crookes| first= William | year= 1895 | url= https://books.google.com/books?id=lSLOAAAAMAAJ&pg=PA87 | title= The spectrum of helium | journal= The Chemical News and Journal of Physical Science | volume= 72 | number= 1865 | pages=87–89}}</ref> It was independently isolated from cleveite in the same year by chemists [[Per Teodor Cleve]] and [[Abraham Langlet]] in [[Uppsala]], Sweden, who collected enough of the gas to accurately determine its [[atomic weight]].<ref>See:
*  {{cite journal |last1=Clève |first1=P.T. |title=Sur la présence de l'hélium dans le clévéite |journal=Comptes rendus hebdomadaires des séances de l'Académie des sciences |date=1895 |volume=120 |page=834 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015035451122&view=1up&seq=844&skin=2021 |trans-title=On the presence of helium in cleveite |language=French}}
*  English translation:  {{cite journal |last1=Clève |first1=P.T. |title=On the presence of helium in clèveite |journal=The Chemical News and Journal of Physical Science |date=1895 |volume=71 |issue=1849 |page=212 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044048675458&view=1up&seq=272&skin=2021}}
*  {{cite journal |last1=Thorpe |first1=T. E. |title=Terrestrial helium? |journal=Nature |date=1895 |volume=51 |issue=1329 |page=586 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.31210011061270&view=1up&seq=632&skin=2021}}
*  {{cite journal |last1=Clève |title=Sur la densité de l'hélium |journal=Comptes rendus hebdomadaires des séances de l'Académie des sciences |date=1895 |volume=120 |page=1212 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015035451122&view=1up&seq=1226&skin=2021 |trans-title=On the density of helium |language=French}}</ref><ref>{{Cite journal|title = Das Atomgewicht des Heliums|trans-title = The atomic weight of helium|author = Langlet, N. A.|journal = Zeitschrift für Anorganische Chemie|volume = 10|issue = 1| pages = 289–292|date = 1895|url=https://books.google.com/books?id=sHcWAAAAIAAJ&pg=PA289|doi =10.1002/zaac.18950100130|language= de}}</ref><ref name="nbb" /><ref>{{cite book |last1=Weaver |first1=E.R. |title=Circular of the Bureau of Standards No. 81: Bibliography of Scientific Literature Relating to Helium |date=1919 |page=6 |publisher=U.S. Government Printing Office |location=Washington, D.C., USA |url=https://nvlpubs.nist.gov/nistpubs/Legacy/circ/nbscircular81.pdf}}</ref> Helium was also isolated by American geochemist [[William Francis Hillebrand]] prior to Ramsay's discovery, when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to [[nitrogen]].<ref>Hillebrand (1890) [https://babel.hathitrust.org/cgi/pt?id=uc1.b2968310&view=1up&seq=511 "On the occurrence of nitrogen in uraninite and on the composition of uraninite in general,"] ''Bulletin of the U.S. Geological Survey'', no. 78, pp. 43–79.</ref> His letter of congratulations to Ramsay offers an interesting case of discovery, and near-discovery, in science.<ref>{{Cite book|last=Munday|first=Pat|author-link=Pat Munday|date=1999|title=Biographical entry for W.F. Hillebrand (1853–1925), geochemist and U.S. Bureau of Standards administrator in American National Biography|editor=John A. Garraty|editor2=Mark C. Carnes|volume=10–11|publisher=Oxford University Press|pages= 808–9; 227–8|title-link=American National Biography}}</ref>

In 1907, [[Ernest Rutherford]] and [[Thomas Royds]] demonstrated that [[alpha particle]]s are helium [[atomic nucleus|nuclei]] by allowing the particles to penetrate the thin glass wall of an [[evacuated tube]], then creating a discharge in the tube, to study the spectrum of the new gas inside.<ref>{{Cite journal| doi = 10.1080/14786440808636511| title = XXIV.Spectrum of the radium emanation| journal = Philosophical Magazine| series = series 6| volume = 16| issue = 92| pages = 313–317| year = 1908| last1 = Rutherford| first1 = E.| last2 = Royds| first2 = T.| url = https://babel.hathitrust.org/cgi/pt?id=umn.31951000614205r;view=1up;seq=349}}</ref> In 1908, helium was first liquefied by Dutch physicist [[Heike Kamerlingh Onnes]] by cooling the gas to less than {{convert|5|K|C F}}.<ref>Onnes, H. Kamerlingh (1908) [https://web.archive.org/web/20180809111624/https://babel.hathitrust.org/cgi/pt?id=uva.x002433831;view=1up;seq=309 "The liquefaction of helium,"] ''Communications from the Physical Laboratory at the University of Leiden'', '''9''' (108) : 1–23.</ref><ref>{{Cite journal|title = Little cup of Helium, big Science |author = van Delft, Dirk |journal = Physics Today |url = http://www-lorentz.leidenuniv.nl/history/cold/VanDelftHKO_PT.pdf |pages = 36–42 |date = 2008 |access-date = 2008-07-20|archive-url = https://web.archive.org/web/20080625064354/http://www-lorentz.leidenuniv.nl/history/cold/VanDelftHKO_PT.pdf |archive-date = June 25, 2008|url-status=dead|bibcode = 2008PhT....61c..36V|volume = 61|doi = 10.1063/1.2897948|issue = 3}}</ref> He tried to solidify it by further reducing the temperature but failed, because helium does not solidify at atmospheric pressure. Onnes' student [[Willem Hendrik Keesom]] was eventually able to solidify 1&nbsp;cm<sup>3</sup> of helium in 1926 by applying additional external pressure.<ref>See:
* Preliminary notice: Keesom, W. H. (17 July 1926) Letters to the Editor: "Solidification of helium," ''Nature'', '''118''' : 81.
* Preliminary notice: Keesom, W. H. (1926) [https://archive.org/stream/ComptesRendusAcademieDesSciences0183/ComptesRendusAcadmieDesSciences-Tome183-Juillet-dcembre1926#page/n25/mode/2up "L'hélium solidifié,"] {{Webarchive|url=https://web.archive.org/web/20161022075647/https://archive.org/stream/ComptesRendusAcademieDesSciences0183/ComptesRendusAcadmieDesSciences-Tome183-Juillet-dcembre1926#page/n25/mode/2up |date=2016-10-22 }} ''Comptes rendus'' ... , '''183''' : 26.
* Keesom, W. H. (1926) "Solid Helium," ''Communications from the Physical Laboratory at the University of Leiden'', '''17''' (184) .</ref><ref>{{Cite news| title = Coldest Cold| publisher = Time Inc.| date = 1929-06-10| url = http://www.time.com/time/magazine/article/0,9171,751945,00.html| access-date = 2008-07-27| archive-url = https://web.archive.org/web/20081206015739/http://www.time.com/time/magazine/article/0,9171,751945,00.html| archive-date = 2008-12-06| url-status = dead}}</ref>

In 1913, [[Niels Bohr]] published his "trilogy"<ref name = Hoyer>{{cite book|first = Ulrich|last = Hoyer|chapter = Constitution of Atoms and Molecules|pages = 103–316 (esp. pp. 116–122)|title = Niels Bohr – Collected Works: Volume 2 – Work on Atomic Physics (1912–1917)|chapter-url = https://books.google.com/books?id=zGczmJjSO6kC&pg=PA117|editor-first = Ulrich|editor-last = Hoyer|publisher = [[North Holland Publishing Company]]|location = Amsterdam|year = 1981|isbn = 978-0720418002}}</ref><ref>{{cite book|last = Kennedy|first = P. J.|year = 1985|chapter = A Short Biography|editor1-last = French|editor1-first = A. P.|editor2-last = Kennedy|editor2-first = P. J.|title = Niels Bohr: A Centenary Volume|pages = 3–15|publisher = [[Harvard University Press]]|isbn = 978-0-674-62415-3|chapter-url-access = registration|chapter-url = https://archive.org/details/nielsbohrcentena00bohr}}</ref> on atomic structure that included a reconsideration of the [[Pickering–Fowler series]] as central evidence in support of his [[Bohr model|model of the atom]].<ref>{{cite journal|last = Bohr|first = N.|author-link = Niels Bohr|year = 1913|title = On the constitution of atoms and molecules, part I|journal = [[Philosophical Magazine]]|volume = 26|issue = 151|pages = 1–25|doi = 10.1080/14786441308634955|url = http://web.ihep.su/dbserv/compas/src/bohr13/eng.pdf|access-date = 2017-12-27|archive-url = https://web.archive.org/web/20190404184145/http://web.ihep.su/dbserv/compas/src/bohr13/eng.pdf|archive-date = 2019-04-04|url-status = live|bibcode = 1913PMag...26....1B}}<br />{{cite journal|last = Bohr|first = N.|author-link = Niels Bohr|year = 1913|title = On the constitution of atoms and molecules, part II: Systems Containing Only a Single Nucleus|journal = [[Philosophical Magazine]]|volume = 26|issue = 153|pages = 476–502|url = http://web.ihep.su/dbserv/compas/src/bohr13b/eng.pdf|doi = 10.1080/14786441308634993|access-date = 2017-12-27|archive-url = https://web.archive.org/web/20171215041355/http://web.ihep.su/dbserv/compas/src/bohr13b/eng.pdf|archive-date = 2017-12-15|url-status = live|bibcode = 1913PMag...26..476B}}<br />{{cite journal|last = Bohr|first = N.|author-link = Niels Bohr|year = 1913|title = On the constitution of atoms and molecules, part III: Systems containing several nuclei|journal = [[Philosophical Magazine]]|volume = 26|issue = 155|pages = 857–875|doi = 10.1080/14786441308635031|url = https://zenodo.org/record/1430922|bibcode = 1913PMag...26..857B}}</ref><ref name = Robotti>{{cite journal|title = The Spectrum of ζ Puppis and the Historical Evolution of Empirical Data|first = Nadia|last = Robotti|author-link=Nadia Robotti|journal = [[Historical Studies in the Physical Sciences]]|volume = 14|issue = 1|year = 1983|pages = 123–145|doi = 10.2307/27757527|jstor = 27757527}}</ref> This series is named for [[Edward Charles Pickering]], who in 1896 published observations of previously unknown lines in the spectrum of the star [[Zeta Puppis|ζ Puppis]]<ref>{{cite journal|last = Pickering|first = E. C.|author-link = Edward Charles Pickering|journal = [[Harvard College Observatory Circular]]|volume = 12|title = Stars having peculiar spectra. New variable stars in Crux and Cygnus|pages = 1–2|year = 1896|bibcode = 1896HarCi..12....1P}} Also published as: {{cite journal|title = Stars having peculiar spectra. New variable stars in Crux and Cygnus|last1 = Pickering|first1 = E. C.|author-link = Edward Charles Pickering|last2 = Fleming|first2 = W. P.|author-link2 = Williamina Fleming|journal = [[Astrophysical Journal]]|volume = 4|pages = 369–370|year = 1896|doi = 10.1086/140291|bibcode = 1896ApJ.....4..369P|doi-access = free}}</ref> (these are now known to occur with [[Wolf-Rayet star|Wolf–Rayet]] and other hot stars).<ref>{{cite journal|title = The relation between the Wolf–Rayet stars and the planetary nebulae|first = W. H.|last = Wright|journal = [[Astrophysical Journal]]|volume = 40|pages = 466–472|year = 1914|doi = 10.1086/142138|bibcode = 1914ApJ....40..466W|doi-access = free}}</ref> Pickering attributed the observation (lines at 4551, 5411, and 10123&nbsp;[[Ångström|Å]]) to a new form of hydrogen with half-integer transition levels.<ref>{{cite journal|title = Stars having peculiar spectra. New variable Stars in Crux and Cygnus|first = E. C.|last = Pickering|author-link = Edward Charles Pickering|year = 1897|journal = [[Astronomische Nachrichten]]|volume = 142|issue = 6|pages = 87–90|doi = 10.1002/asna.18971420605|bibcode = 1896AN....142...87P|url = https://zenodo.org/record/1424755|access-date = 2019-08-24|archive-url = https://web.archive.org/web/20190824143848/https://zenodo.org/record/1424755/files/article.pdf|archive-date = 2019-08-24|url-status = live}}</ref><ref>{{cite journal|title = The spectrum of zeta Puppis|last = Pickering|first = E. C.|author-link = Edward Charles Pickering|year = 1897|journal = [[Astrophysical Journal]]|volume = 5|pages = 92–94|doi = 10.1086/140312|bibcode = 1897ApJ.....5...92P|doi-access = free}}</ref> In 1912, [[Alfred Fowler]]<ref>{{cite book|title = The Methodology of Scientific Research Programmes|first = Imre|last = Lakatos|author-link = Imre Lakatos|publisher = [[Cambridge University Press]]|year = 1980|isbn = 9780521280310|editor1-first = John|editor1-last = Worrall|editor2-first = Gregory|editor2-last = Currie|chapter-url = https://books.google.com/books?id=RRniFBI8Gi4C&pg=PA62|chapter = Bohr: A Research Programme Progressing on Inconsistent Foundations|pages = 55–68}}</ref> managed to produce similar lines from a hydrogen-helium mixture, and supported Pickering's conclusion as to their origin.<ref>{{cite journal|title = Observations of the Principal and other Series of Lines in the Spectrum of Hydrogen|first = A.|last = Fowler|author-link = Alfred Fowler|journal = [[Monthly Notices of the Royal Astronomical Society]]|volume = 73|issue = 2|year = 1912|pages = 62–63|doi = 10.1093/mnras/73.2.62|bibcode = 1912MNRAS..73...62F|doi-access = free}}</ref> Bohr's model does not allow for half-integer transitions (nor does quantum mechanics) and Bohr concluded that Pickering and Fowler were wrong, and instead assigned these spectral lines to ionised helium, He<sup>+</sup>.<ref>{{cite journal|title = The Spectra of Helium and Hydrogen|first = N.|last = Bohr|author-link = Niels Bohr|journal = [[Nature (journal)|Nature]]|volume = 92|issue = 2295|year = 1913|pages = 231–232|doi = 10.1038/092231d0|bibcode = 1913Natur..92..231B|s2cid = 11988018|url = https://zenodo.org/record/1429570}}</ref> Fowler was initially skeptical<ref>{{cite journal|title = The Spectra of Helium and Hydrogen|first = A.|last = Fowler|author-link = Alfred Fowler|journal = [[Nature (journal)|Nature]]|volume = 92|issue = 2291|year = 1913|pages = 95–96|doi = 10.1038/092095b0|bibcode = 1913Natur..92...95F|s2cid = 3972599|url = https://zenodo.org/record/1429568}}</ref> but was ultimately convinced<ref>{{cite journal|title = Reply to: The Spectra of Helium and Hydrogen|first = A.|last = Fowler|author-link = Alfred Fowler|journal = [[Nature (journal)|Nature]]|volume = 92|issue = 2295|year = 1913|pages = 232–233|doi=10.1038/092232a0|bibcode = 1913Natur..92..232F|s2cid = 3981817|url = https://zenodo.org/record/1429568}}</ref> that Bohr was correct,<ref name = Hoyer /> and by 1915 "spectroscopists had transferred [the Pickering–Fowler series] definitively [from hydrogen] to helium."<ref name = Robotti /><ref>{{cite journal|title = The Spectra of Hydrogen and Helium|first = N.|last = Bohr|author-link = Niels Bohr|journal = [[Nature (journal)|Nature]]|volume = 95|issue = 6–7|pages = 6–7|year = 1915|doi = 10.1038/095006a0|bibcode = 1915Natur..95....6B|s2cid = 3947572|url = https://zenodo.org/record/1429597}}</ref> Bohr's theoretical work on the Pickering series had demonstrated the need for "a re-examination of problems that seemed already to have been solved within classical theories" and provided important confirmation for his atomic theory.<ref name = Robotti />

In 1938, Russian physicist [[Pyotr Leonidovich Kapitsa]] discovered that [[helium-4]] has almost no [[viscosity]] at temperatures near [[absolute zero]], a phenomenon now called [[superfluidity]].<ref>{{Cite journal|title = Viscosity of Liquid Helium below the λ-Point |author = Kapitza, P. |author-link = Pyotr Leonidovich Kapitsa |journal =Nature|volume = 141|issue = 3558 |page = 74 |doi = 10.1038/141074a0 |date = 1938|bibcode = 1938Natur.141...74K |s2cid = 3997900 |doi-access = free }}</ref> This phenomenon is related to [[Bose–Einstein condensation]]. In 1972, the same phenomenon was observed in [[helium-3]], but at temperatures much closer to absolute zero, by American physicists [[Douglas D. Osheroff]], [[David M. Lee]], and [[Robert Coleman Richardson|Robert C. Richardson]]. The phenomenon in helium-3 is thought to be related to pairing of helium-3 [[fermion]]s to make [[boson]]s, in analogy to [[Cooper pairs]] of electrons producing [[superconductivity]].<ref>{{Cite journal|title = Evidence for a New Phase of Solid He<sup>3</sup> |author = Osheroff, D. D. |author2 = Richardson, R. C. |author3 = Lee, D. M. |journal = Phys. Rev. Lett. |volume = 28 |issue = 14 |pages = 885–888 |doi = 10.1103/PhysRevLett.28.885 |date = 1972 |bibcode=1972PhRvL..28..885O|s2cid = 89609083 |doi-access = free }}</ref>

In 1961, Vignos and Fairbank reported the existence of a different phase of solid helium-4, designated the gamma-phase. It exists for a narrow range of pressure between 1.45 and 1.78 K.<ref>{{Cite journal |last1=Vignos |first1=James H. |last2=Fairbank |first2=Henry A. |date=1961-03-15 |title=<nowiki>New Solid Phase in ${\mathrm{He}}^{4}$</nowiki> |url=https://link.aps.org/doi/10.1103/PhysRevLett.6.265 |journal=Physical Review Letters |volume=6 |issue=6 |pages=265–267 |doi=10.1103/PhysRevLett.6.265}}</ref>