490 



Prof. J. Dewar and A. Scott. 



[Dec. 11, 



III. " Further Experiments on the Vapour Densities of Potas- 

 sium and Sodium." By Professor James Dewar, M.A., 

 F.R.S., and Alexander Scott, B.A. Received November 

 27, 1879. 



In our former communication ''On the Vapour Densities of Potas- 

 sium and Sodium," we pointed out the chief obstacles which lay in the 

 way of an exact determination of these constants. Having overcome 

 the chief manipulative difficulties in connexion with the method we 

 described, there still remained the problem for solution as to how far 

 the use of iron bottles in our experiments might affect the results. 

 If the iron retained the metals or allowed their vapours to diffuse 

 with rapidity through it, a considerable error might be produced 

 without its being easily detected. We noted an action of this kind, 

 remarking that "both metals seemed to have a tendency to behave 

 in the same manner as zinc, viz., to form an alloy with the iron, 

 because the vapour evolved after a few minutes was absorbed com- 

 pletely, in spite of all precautions to prevent oxidation." The ten- 

 dency of iron and the alkali metals to form alloys, we thought, would 

 be diminished, so as not to interfere with the value of the results, " by 

 raising the temperature as much as possible, so as to induce rapid 

 volatilisation," and the fair accordance of the series of experiments 

 formerly given led us to suppose that this difficulty had been success- 

 fully obviated. Recent spectroscopic observations, by Professors 

 Liveing and Dewar, on the luminous absorption of the vapours of 

 sodium under different conditions in iron and platinum vessels — which 

 will be presented shortly to the Society — having led to the inference 

 that mere rapidity of volatilisation, of such small weights of the metals 

 as can be conveniently employed for the measurement of the volumes 

 of the vapours, in presence of a relatively enormous mass of iron, would 

 not eliminate the absorptive action, new experiments were made in a 

 platinum apparatus of similar shape and size to the iron one, and in 

 the same manner as formerly described. Magnesia was used instead 

 of sand to pack the platinum vessel in the crucibles employed as a 

 protection against the diffusion of furnace gases, and the volatilisation 

 of the alkali metals took place in nitrogen. Each set of experiments 

 was made at different times, and the results are given in the order 

 of observation, the bottle being freed from the vapours after each 

 experiment by a strong current of hydrogen. The molecular weights 

 .are calculated for 22'34 cub. centims. at 0° C. and 760 mm. pressure. 



The following tables contain the record of the results obtained : — 



