Leiden University at the university. The learning and reputation of Jacobus Gronovius, Herman Boerhaave, Tiberius Hemsterhuis and David Ruhnken, among others, enabled Leiden to maintain its reputation for excellence down to the end of the 18th century. At the end of the nineteenth century, Leiden University again became one of Europe's leading universities. At the world’s first university low-temperature laboratory, professor Heike Kamerlingh Onnes achieved temperatures of only one degree above absolute zero of −273 degrees Celsius. In 1908 he was also the first to succeed in liquifying helium and can be credited with the discovery of the superconductivity in metals. The
Johannes Kuenen experiments with x-rays with the physiologist Edward Waymouth Reid. In 1907 he was appointed professor of physics at Leiden University. Heike Kamerlingh Onnes and Kuenen led the Kamerlingh Onnes physics Laboratory. On the basis of his scientific work he was elected in 1911 as a member of the Royal Netherlands Academy of Arts and Sciences (KNAW), and became a member of the Hollandsche Maatschappij der Wetenschappen in 1915. He discovered retrograde condensation and published his findings in 1892 in the Ph.D. thesis with the title "Metingen betreffende het oppervlak van Van der Waals voor mengsels van koolzuur en chloormethyl". (Measurements