Art. XLVIII.—Note on the Vapour-density of Mercury.

By Douglas Hector.
Communicated by Professor Easterfield.

[Read before the Wellington Philosophical Society, 12th March, 1901.]

It is well known that the vapour-densities of many elements diminish at high temperatures owing to dissociation of the gaseous molecules. Thus the researches of Victor Meyer^* and of Crafts and Meier^† have shown that the vapour-density of iodine is constant between the temperatures of 250°-700°, but that above the latter temperature dissociation commences, and is complete at temperatures above 1,400° (at 152 mm. pressure).

Very few systematic experiments appear to have been made in the opposite direction—i.e., with the object of determining to what extent, if any, an association of the gaseous molecules occurs when the temperature is reduced.

Professor Easterfield has suggested to me that a certain interest would attach to determinations of the vapour-density of the elements made at the lowest possible temperature. I have therefore chosen quicksilver for the first series of experiments.

V. Meyer^‡ has shown that mercury is monatomic at temperatures between 440° and 1,565°. More recently H. Brereton Baker has determined the vapour-density of the carefully purified metal in absolutely dry nitrogen at 440°, but no evidence of association could be detected. Vapour-density determinations at lower temperatures than 440° do not appear to have been attempted.

By employing a slightly changed form of the beautiful method described by V. Meyer,^§ in which the substance under investigation is gasified in an atmosphere of hydrogen at the ordinary pressure, we have been able to determine the vapour-density of mercury at 236°—i.e., 121° below its boiling-point. Even at this low temperature, however, the molecule of mercury remains monatomic. The actual vapour-density found was 112; calculated for monatomic mercury, 100. The experiment is being extended to other elements.