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Physical Properties of Neon

The density of neon has been determined by Watson using a sample of the gas prepared by method (4) above. He found 0.9002 gm. ± 0.0003 gm. for the weight of the normal litre, which corresponds to a density of 20.16 (O2 = 32). Ramsay and Travers had previously found the value 19.94 (O2 = 32), but the later determination is undoubtedly more accurate. Leduc gives 0.899 gm. as the weight of a normal litre.

From Watson's value for the density, and Burt's value given below for the pressure compressibility coefficient, it follows from the method of limiting densities that the molecular weight of neon is 20.20.

The compressibility coefficient of neon at 0° between 0 and 1 atmosphere is, according to Burt, -0.00105; that is, neon, like hydrogen, is an ultra-perfect gas. Leduc's value is -0.00046 at 17°.

The refractivity of neon was given by Ramsay and Travers as 0.2345 times that of air for white light, whence the refractive index is =1.0000687.2 This figure agrees fairly well with that found by C. and M. Cuthbertson, namely, 1.00006716 at 0° and 760 mm. for the green mercury line (λ = 5461).

The dispersion at N.T.P. is approximately twice that of helium, the relationship between wave-length and refractive index being given by

where C×10-27 = 5.18652 and n02×10-27 = 38916.2.

The absorption coefficient of neon in water is 0.0114 at 0°, and increases gradually up to 0.0317 at 50°. The solubility curve appears to have a minimum at 0°.

The viscosity of neon at 0° C. is 2.981×10-4 in absolute units. The dielectric cohesion of pure neon at 17° is 5.6 (hydrogen = 205), which is considerably less than that of helium, whereas one would expect it to be between that of helium and argon. The value of the dielectric cohesion is very much raised even by traces of foreign gases; e.g. l/20th per cent, of impurity raises it to 6.8, and the determination of this constant therefore affords a good criterion of the purity of the gas.

When an electrical discharge is passed through neon under reduced pressure a brilliant orange-pink light is emitted. The spectrum is characterised by lines in the orange and red regions and, like that of helium, argon, krypton, and xenon, is materially altered by inclusion of a Leyden jar and spark-gap, the strongest of the ordinary lines being only just visible, while many new lines appear in the more refrangible region. The chief lines are enumerated below: -


An investigation of three series of lines in the neon spectrum has been made by Rossi.

The Zeeman effect in neon is very complicated.

A curious property of neon has been observed by Collie. When the gas is shaken in a tube with mercury a red glow is observed which is equally bright at atmospheric pressure or at 120-200 mm. By unequal heating and other methods tubes were obtained which would glow at certain parts but not at others. Moisture inhibits the phenomenon.

No explanation appears to have been advanced: possibly it may be connected with the glow seen in an exhausted receiver containing neon moving near an electrostatic field. It is stated that neon is very resistant to absorption by the electrodes of a discharge tube.

The minimum spark potential in neon is 200 volts, with a spark-gap of 2.8 mm.

The earlier investigators had not sufficient neon at their disposal to permit the determination of the ratio of the specific heats; but from the fact that mixtures of neon with argon were already known to have γ = 1.66, it was concluded that the value of γ for neon was that required by theory for a monatomic gas. Subsequently accurate determinations of this constant have been made for pure neon according to the method of Kundt and Warburg: the figures obtained are: -

TemperatureHalf Wave-Length.Density
Air19°27.92 14.48

The value thus found for the ratio Cp/Cv for neon is 1.642.

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