Liquefaction of Argon

No difficulty is met with in the production of both liquid and solid argon, as the boiling-point and melting-point lie between the boiling-point of freshly prepared liquid air and that of liquid air which has been kept.

Liquid argon is colourless and transparent and boils at -186.1° C.; at the boiling-point the density is 1.4046: when cooled by external means or by its own evaporation, it solidifies to a white mass which crystallises in the regular system, and melts at –187.9° C., less than 3° below the boiling-point.

The critical constants of argon are as follows: -

Critical temperature = -122.44°; Critical pressure = 47.996 atmos; Critical density = 0.509 gm./cc;

The triple point of argon is -189.3°. The vapour pressure of solid argon is 411 mm. At -190.9°: by addition of oxygen it is raised to 420 mm. at that temperature and then remains constant, in spite of continued addition of oxygen, as long as any solid argon remains. We are here dealing with a saturated solution of solid argon in liquid oxygen; it contains 92.7 per cent, of argon by.volume, and the vapour in equilibrium with it contains 97.8 per cent, of argon. The vapour pressures of liquid argon from the critical temperature to the triple point and of solid argon from the triple point to -206° C. have been determined, and from these data it has been calculated that the molecular heat of vaporisation of the liquid is 12.92 calories at -125.49°, 24.01 calories at –140.80°, and 35.00 calories at -183.06°.

The equation for the rectilinear diameter for argon has been found to be Dt = 0.20956 – 0.0026235t (temperature in degrees C.). The coefficient of t, i.e. the slope of the diameter, is greater than for any other gas except xenon. The density of liquid argon varies from 1.4233 at 84° abs. To 1.3845 at 90° abs.

In conclusion, it may be of interest to note that it has been recorded in one case that argon exhibited "phosphorescent effects" during an experiment in which it was repeatedly passed over heated metallic calcium and copper oxide respectively. The phosphorescence is stated to have become more intense as the gas became purer. No attempt seems to have been made to explain this phenomenon; but there is at least a superficial resemblance to the observations recorded under Neon, and the matter appears to be worth further investigation.