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The first successful attempt to liquefy hydrogen was made by two Polish investigators, Wroblewski and Olszewski, in 1884. They cooled the gas to -183°, and allowed it to expand from 100 atm. pressure, obtaining evidence of liquefaction, but getting no liquid in bulk. The latter was first obtained by Dewar in 1895, at the Royal Institution in London. By compressing hydrogen to 200 atm., cooling it to -200°, and expanding it through a valve, he obtained colourless liquid hydrogen, readily boiling off. Olszewski in 1895 found that the critical temperature of hydrogen is about -234° (the accurate value is -239.9°; the critical pressure is 12.8 atm.), and that the slight heating effect produced by expansion through a valve at the ordinary temperature (Joule-Kelvin effect) changes, on cooling to - 80.5° at 113 atm., into a cooling effect. This inversion point makes it necessary in the liquefaction of hydrogen first to cool the gas strongly before expansion.
Liquid hydrogen is a colourless, transparent liquid, with the very small density of 0-07105 at -252.8° and 745.52 mm. It boils at -252.78°. By rapidly evaporating the liquid under reduced pressure in a tube immersed in liquid hydrogen in a double Dewar vessel,
 | Fig: Preparation of solid hydrogen |
its temperature is reduced to -259°, when it freezes to a colourless, transparent solid or a white, snow-like mass. At the temperature of liquid hydrogen all other gases except helium and neon are frozen to solids which at the extreme cold show practically no vapour pressure.
If a Geissler tube containing air is attached to a bulb containing charcoal, and the latter dipped into liquid hydrogen, the vacuum in the Geissler tube becomes so intense that no electrical discharge will pass even with a powerful coil
 | Fig: Hydrogen low temperature Experiment to show the very low temperature of liquid hydrogen. |
If liquid hydrogen is poured into an ordinary test-tube, a white coating of ice at once covers the outside. From this, drops of liquid air are seen to fall.
Liquid hydrogen may be prepared in the modification of Travers' apparatus devised by Nernst
 | Fig: Liquid hydrogen Preparation of liquid hydrogen. |
Compressed hydrogen enters through the copper coil, A, and passes through an extension, A', of the coil immersed in liquid air in a large Dewar vessel. The cooled gas then passes through an extension of the coil, A", composed of two coils in parallel inside a small Dewar tube completely enclosed in a brass vessel, B. At the end of this coil is an expansion valve, V operated from outside. In the tube A'' the previously cooled gas is liquefied by the cold expanded gas from the valve sweeping over the coil, and liquid hydrogen collects in the inner Dewar vessel. The cold hydrogen gas passes out through a copper coil, C, wound in contact with the coil A, and takes heat from the incoming hydrogen in the latter. The liquid air boiling in the outer Dewar vessel gives off cold air, which passes out through a copper coil, D, wound between the two coils A and C, and also takes up heat from the incoming hydrogen. The brass vessel, B, is in two parts, screwed together, to permit of the inner Dewar tube being inserted. 300-400 c.c. of liquid hydrogen are obtained per hour, with a gas velocity of 2-3 c.c. per second, and the use of about 300 c.c. of liquid air.
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