Liquefaction of Helium

The failure of many attempts to liquefy helium suggested that its critical temperature must be very low, and from the behaviour of the gas in a vacuum tube connected with a charcoal absorption tube Dewar concluded that its boiling-point was not higher than 6° absolute.

The actual liquefaction of helium was preceded by a study of the isothermals down to the temperature of liquid hydrogen, from which it appeared probable that at low temperatures the value of K for the Joule-Thomson effect would change sign, and that at the temperature of solid hydrogen the expansion of helium from a considerable pressure would cause a lowering of temperature sufficient to make a self-intensive cooling process successful. That this would be so had been predicted by Dewar.

Onnes prepared his helium from monazite sand and purified it by Dewar's method of fractionation from cold charcoal. The gas was circulated through an apparatus in which it was cooled first to the temperature of liquid air, then to 15° absolute by means of liquid hydrogen boiling under 60 mm. pressure, and finally passed through a specially constructed liquefier of the Hampson type. Thus 60 c.c. of liquid was obtained in 3 hours from 300 litres of gas.

Liquid helium is a colourless, very mobile liquid. Its density is 0.154, and it is thus much lighter than any other known solid or liquid: it is stated to have a point of maximum density at about 2° absolute. The ratio of the densities of liquid helium and its vapour at the boiling-point is 11:1. Calculating from the density by the formula of Lorentz and Lorenz, the refractive index is found to be 1.03. By an ingenious device it was made possible to have a clear view of the liquid that collected in the vacuum- jacketed tube of the liquefier, and it was observed that the meniscus was sharply visible and straight: this indicates that the surface tension is very small. The boiling-point is about 4.3° absolute, the critical temperature is 5.25° absolute, and the critical pressure is 2.26 atmospheres. The calculated critical volume is 0.00271.

The following table shows the vapour pressure of liquid helium at temperatures below its critical temperature: -

Absolute temperature	4.28	4.97	5.10	5.15	5.22	5.25
Vapour pressure, mm.	767	1329	1520	1569	1668	1718

Solid helium has not yet been obtained: efforts to produce it by evaporation of the liquid under reduced pressure have failed, although a temperature in the neighbourhood of 2.5° absolute has been reached. It seems probable that the triple-point pressure of helium is below 10 mm. of mercury.

Liquid helium has been used as a refrigerating agent in the prosecution of low temperature research at Leyden.