Page:Proceedings of the Royal Society of London Vol 60.djvu/234

214 density of 1.871 ; and the residue diffused in 266.6″, which corresponds to a density of 2.187. In each of these experiments about half the helium passed through the porous ping.

The denser portion of this gas was again diffused five times, lighter portions being removed. This corresponds to a residue of 30 c.c. from 400 c.c. of the original gas. The rate of diffusion of this sample compared with that of hydrogen was almost identical with the last, namely 208″ to 143″, and corresponds to a density of 2.133. The gas is therefore not increased in density by this process.

The lighter gas was submitted to a similar fractionation, and the ratio of its diffusion-rate to that of hydrogen was 246.75″ to 181.0″, as a mean of several closely concordant experiments. This corresponds to a density of 1.874. We have accordingly:—

Not content with this, we pushed fractionation still further; the helium was divided into seven portions (by fractionation) and then submitted to methodical fractional diffusion, in which the heavier portions were transferred to the “denser” side, and the lighter portions to the “lighter” side. This process was repeated four times, and the end portions were each divided into two; the lighter portion of the “lighter” was collected separately, and its rate determined. It took 258.5″ to diffuse, compared with 189.5″ for an equal volume of hydrogen; its density calculated from these rates was 1.876. It is clear, therefore, that the limit has been reached in purifying the lighter portion by diffusion.

It should have been mentioned that the portion of 2.133 density as well as that of 1*874 density had been sparked with oxygen in presence of potash, and in a vacuum tube showed mere traces of hydrogen, every other gas being absent. The spectrum of hydrogen is still visible, even when 0*01 per cent, of that gas is present.

At various times during the attempt to separate helium, the spectrum has been carefully examined. very first portions of the lightest gas gave an identical spectrum, seen with a hand-spectroscope, with the very last portions of the heaviest gas. Professor Ames, of the Johns Hopkins University, has however kindly undertaken to photograph the spectra using a dispersion-grating; so that if any difference can be detected, it will ere long be made known.

Lord Rayleigh was so kind as to measure the refractivity of these extreme portions of the fractionated gas. His process has been described in the ‘Proceedings,’ vol. 59, p. 202. For the sample of helium sent him in July, 1895, he found the number 0.146. The lighter portion of the fractionated gas of density 1.876 had a refrac-