Liquefaction of Neon
Neon can be liquefied under the ordinary atmospheric pressure at the temperature of liquid hydrogen boiling under ordinary conditions.
Its boilingpoint has not been exactly determined, but it certainly lies above that of hydrogen (20.4° abs. on the helium scale), and is probably in the neighbourhood of 25° abs. Its vapour pressure is 12.8 mm. At 20.4° abs., and 2.4 mm. At 15.65° abs.
Its triplepoint pressure is 35 cm. of mercury, and its critical pressure is 29 atmospheres; but the critical temperature has not been determined. There is, however, a connection between the critical temperatures and the viscosities of the inert gases. The variation of η with T is expressed by Sutherland's equation: 
where η and η_{0} are the viscosities at temperatures T° and T_{0}° abs., and C is a constant for each gas.
When the values of η at the critical temperature are calculated for the other inert gases, it is found that the relationship η_{k}^{2} = 3.93×10^{10} at. wt., holds good. By interpolation the value of η_{k} and hence, by Sutherland's equation, of T_{k} for neon may be found to be 61.1° absolute. This value agrees with that suggested by Travers, Senter, and Jaquerod (loc. cit.), viz. 60° abs. According to Onnes and Crommelin, the critical temperature of neon is a few degrees below 55° absolute.
The surface tension, density, and refractive index of liquid neon have not yet been measured; but Rudorf calculates that its refractive index should be about 1.24. 


