point to a similar conclusion; although no great alteration in density has been produced, yet there is a sign that a kind of separation is being effected electrically. There is also in favour of the supposition the unlikelihood that two or more gases, so like one another as the constituents of helium, should exist with densities so near each other; and the probability that some separation should have been detected by aid of the spectroscope.
Lastly, the refractivities of both gases, if there be two, appear to be equally abnormal; now, different gases have different refractivities in no known relation to their densities, as, for example, hydrogen 05, oxygen nearly 1. But the refractivities of the different portions of helium are proportional to their densities; a statement which is true of any one gas, inasmuch as refractivity is directly proportional to pressure, i.e., mass in unit volume. The refractivity of. helium, also, is so small that it totally differs in this respect, as, indeed, it does in most of its physical properties from every other gas, and it is moreover a monatomic gas. It is therefore permissible to seek for an explanation of its remarkable properties in framing any hypothesis which admits of being put to the test.
The Production of Cyanogen in the Electric Arc.—The very,careful and numerous experiments of Liveing and Dewar* have very generally been accepted as affording evidence sufficient to establish the existence of an emission spectrum of cyanogen as distinct from that of carbon in the electric arc. Kayser and Runge,"f though at first disinclined to accept such a conclusion, obtained additional evidence by experimenting with the arc in air, and in carbon dioxide. I hey found that the ordinary carbon spectrum and that of cyanogen appeared with rapidity alternately in the arc in air, though there could be no difference in temperature sufficient to account for the production of two different carbon spectra. With the poles immersed in carbon dioxide no such changes were seen, the carbon spectrum alone being visible, which evidence led them to concur in the views of Liveing and Dewar. The chief evidence of the existence of a cyanogen spectrum rests on the fact that this substance is actually synthesised in the arc when nitrogen is present, and because without
- ‘ Roy. Soc. Proc.,’ vol. 30, pp. 152—162, 494—509 : vol. 34, pp. 123—130 and
pp. 418—429.
t “ Ueber die Spectren der Elemente. Zweiter Absehnitt. Leber die im galvanischen Liclitbogen auftretenden Bandenspectren der Kohle.” ‘ Abh. K. Preuss. Ak. Wiss.,’ 1889, p. 9.
