Some apology is needed for a third paper, by the same authors, on the refraction and dispersion of neon. In 1909* we published determinations made with 300 c.c. of gas purified by H. E. Watson for Sir William Ramsay. The index for the green mercury line (X 5462-23) was found to be 1 -00006716. For the dispersion the cadmium red (X 6438) and blue (X 4800) lines were also used, and the results were found to be expressed by the formulawhere n is the frequency of the light = V/X. Only 200 bands could be counted, and the figures for the dispersive power were not believed to be trustw orthy to less than 5 per cent.In 1910f we remeasured the dispersion with greater accuracy, assuming the previous value of the refraction for X 5462-23. The number of bands counted was 440 and the dispersion was observed at seven points in the visible spectrum, between XX 6708-4800.The dispersion was best represented by _ I =

I.On the Refraction and Dispersion of the , Halogen , S t e a m, Oxides of Nitrogen B y Cl iv e C u t h b e r t s o n ,Fellow o f University , London and M a u d e Cu t h b e r t so n .

By the kindness of Sir William Ramsay and Mr. H. E. Watson we have had an opportunity of measuring the refractive index of neon with a large quantity of the gas (300 c. c.) which had been carefully purified by Mr. Watson. Subject to small corrections which he has not yet completed, the weight of 1 litre of the gas was 0·9005 gramme at 0° C. and 760 mm. pressure in London. From 11 experiments which hardly varied by 1 part in 500 we find the refractive index at 0° C. and 760 mm. for the green mercury line (λ = 5461) to be 1·00006716 or, doubling for comparison with diatomic gases, 1·0001343.

M a u d e Cuthbektson.

The refractive index of heavy water, 100% pure, was found by Lewis and Luten, and Luten to be less than that of normal water by 0·00456 at 20°C for λ5893, and this result has been approximately confirmed by the work of Washburn, Smith, and Frandsen, Tilton and Taylor (J. K.), and Taylor (H. S.) and Selwood. Luten, and Tilton and Taylor also measured the dispersion of D 2 O, and found that the curve was slightly flatter than that of normal water. Lutten gives the figures shown in Table I. So large a diminution of the normal index is surprising, for, even if we assume that the D atom is identical with the H atom in refractive effect, and allow for the increase in the molecular volume of the D 2 O molecule above that of H 2 O, the change is greater than would be expected.