−259.14 °C (−434.45 °F, 14.01 K)
−252.87 °C (−423.17 °F, 20.28 K) 
Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.
To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be cooled to 20.28 K (−423.17 °F/−252.87°C) while still pressurized. One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of hydrogen storage. As in any gas, storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. Once liquefied it can be maintained as a liquid in pressurized and thermally insulated containers.
Liquid hydrogen consists of 99.79% parahydrogen, 0.21% orthohydrogen.
1756 - The first documented public demonstration of artificial refrigeration by William Cullen, Gaspard Monge liquefied the first gas producing liquid sulfur dioxide in 1784. Michael Faraday liquefied ammonia to cause cooling, Oliver Evans designed the first closed circuit refrigeration machine in 1805, Jacob Perkins patented the first refrigerating machine in 1834 and John Gorrie patented his mechanical refrigeration machine in 1851 in the US to make ice to cool the air, Siemens introduced the Regenerative cooling concept in 1857, Carl von Linde patented equipment to liquefy air using tile Joule Thomson expansion process and regenerative cooling in 1876, in 1885 Zygmunt Florenty Wróblewski published hydrogen's critical temperature as 33 K; critical pressure, 13.3 atmospheres; and boiling point, 23 K.
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