345 
we obtain: taking (a) and (c) ... A = 1,73 
... (b) and (c) . . . 1,49 
In two papers, due to L. Lorenz and K. Prytz x ) further data 
respecting the dispersive power of some vapours (such as CH 4 0, 
c 2 h 6 o, c 2 h 5 i, c,h 6 etc.) can be found. This evidence, however, 
resting at it does on the results in every case of a single pair of 
experiments (indices are given for X — 5,893 and 6,708 only) is 
not of a conclusive character; it was thought best not to include 
it here. 
A beautiful series of experiments dealing with the refractivities 
of several elements in the gaseous state has been published recently 
by Messrs C. Cuthbertson and E. P. Metcalfe 1 2 ). The research wâs 
designed primarily to obtain a single value of the index v for each 
substance but in the course of the work it was found that the dis¬ 
persion would be considerable. To quote the words of the authors: 
"'it was only with the object of satisfying ourselves that the refrac- 
"tivity measured was not affected to an abnormal degree by the 
" existence of an absorption band for a wave-length near to that 
"by which the observations were made that readings were taken 
"with different wave-lengths’’. The following table shows (in round 
numbers) the per cent variation of the refractivity v — I, for the 
total range of wave-lengths employed, observed in the experiments 
of Cuthbertson and Metcalfe: 
Gaseous Cadmium: about 4°/ 0 Gaseous Selenium: about 2°/ 0 
. . . Zinc: . . . 7°/ 0 . . . Tellurium: . . . 10°/ 0 
. . . Arsenic: . . . 2°/ 0 • • • Mercury 3 * * ): . . . 3°/ 0 
Considering the degree of accuracy attained in the measurements 
we see from this that zinc and tellurium are the only elements 
for which it may be safe to attempt calculations similar to that pre¬ 
viously recorded. 
1 ) Annalen d. Phys. u. Ch., Bd. 11 ., p. 70; ibidem, p. 104. 1880. 
2 ) Philosophical Trans. Roy. Soc. of L. (A). Vol. 207. p. 185. 1907. 
Cf. two previous papers: Phil. Trans. R. S. L. (A). Vol. 204, p. 323. 1905 and 
Proc. R. S. L. (A). Vol. 76., p. 426. 1905. 
3 ) According- to a courteous private Communication from Mr. Cuthbertson, 
of date March 16, 1907, the value of 10 6 (v Ci)i — 1) for X = 6,562 in the case of 
Mercury, is 1835 (instead of 1799, as stated in the paper). 
