216 Prof. W. N. Hartley. On the Spectrum of Cyanogen 



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 supposi- 

 tion 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 refrac- 

 tivities in no known relation to their densities, as, for example, 

 hydrogen O5, oxygen nearly 1. But the refractivities of the dif- 

 ferent portions of helium are proportional to their densities ; a 

 statement which is true of any one gas, inasmuch as refractivifcy 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. Tt is therefore 

 permissible to seek for an explanation of its remarkable properties in 

 framing any hypothesis which admits of being put to the test. 



"On the Spectrum of Cyanogen as produced and modified 

 by Spark Discharges." By W. N. HARTLEY, F.R.S., Royal 

 College of Science, Dublin. Received July 13, 1896. 



The Production of Cyanogen in the Electric Arc. The very careful 

 and numerous experiments of Liveing and Devvar* 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. 

 They 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. 152162, 494509 : vol. 34, pp. 123130 and 

 pp. 418429. 



f " Ueber die Spectren der Elemente. Z welter Abschnitt. Ueber die im galva- 

 trischen Liclitbogen auftretenden Bandenspectren der Kolile." ' Abh. K. Preuss. 

 Ak. Wiss.,' 1889, p. 9. 



