Willem Hendrik Keesom (21.06.1876-24.03.1956) was a Dutch physicist and a member of the Royal Netherlands Academy of Arts and Sciences (1924). He was born on Texel. He graduated from the University of Amsterdam (1900). In 1900-09 he worked at the University of Leiden, in 1909 — 23 at the University of Utrecht (professor from 1918), and in 1923—45 he was professor at the University of Leiden and director (from 1926) of the cryogenic laboratory. His research concerned low-temperature physics and the chemistry of isotopes. He carried out measurements of the boiling point of liquid helium, of its vapour pressure, permittivity, surface tension, thermal conductivity, heat of fusion, compressibility and viscosity. He determined the boiling point of helium as 4.126 K. He obtained (1926) helium in the solid state. In 1928, together with M. Wolfke, he discovered at a temperature of 2.19 K a phase transition of the second kind in liquid helium, which showed that liquid helium has two states: helium in the state above the transition temperature is called helium I, and below the transition point — helium II. With K. Clusius he detected (1932) anomalies in the temperature dependence of the specific heat capacity of liquid helium and established the existence of a point at which the heat capacity jumps (the λ-point). In 1931—32, by pumping off the vapour of liquid helium, he reached a temperature of 0.71 K. He constructed (1928) an entropy diagram of liquid helium. He studied the equation of state of liquid helium and was the first (1933) to obtain the isotherms of the ρ—p diagram. Together with his daughter he was the first to detect (1935) the unusually high heat transfer through narrow heat conduits filled with helium II (the anomalously high thermal conductivity of helium II). In 1938 he obtained an X-ray photograph of a single crystal of solid helium at a temperature of 1.45 K and a pressure of 37 atm. He proved experimentally that isotopes can be enriched by the method of fractional distillation.