470 Mr. H. Wilde on Helium and its place in the 



and Runge and Paschen be accepted, i. e. that the new gas is 

 a mixture, the density of helium proper will be further 

 increased. 



The low specific gravity of the cleveite gas, and its occlusive 

 affinity for the platinum electrodes of the vacuum-tubes during 

 the transmission of the electric discharge, as first noticed by 

 Lockyer*^ indicate that helium proper is the typical element 

 H3 at the head of the uneven series H3n, homologous in 

 position and analogous in properties with hydrogen, and that 

 the lighter constituent of reputed helium is the typical element 

 H2 at the head of the positive even series H2n. Further re- 

 search, however, is necessary before the characteristic lines 

 belonging to each of the constituents of reputed helium can 

 be determined with certainty, and the complete separation of 

 the other new gases obtained from minerals, by the distillation 

 method, is the problem which awaits solution. 



|I have shown in former papers that the characteristic 

 spectral lines of the alkaline metals in the series Hn ? and 

 their homologues of position in the series I13n containing 

 thallium and its analogues, indium and gallium, advance 

 towards the more refrangible end of the spectrum in the in- 

 verse order of their atomic weights J. The same relation is 

 also observable in the spectra of the alkaline-earth metals, 

 and in other well-defined series of elements. M. Lecoq de 

 Boisbaudran had previously formulated the same relation 

 towards the least refrangible end of the spectrum, in the 

 direct order of the atomic weights§. 



Profs. Runge and Paschen, in their recent communication 

 to the Berlin Academy ||, have pointed out that, while the 

 spectra of each vertical series of chemically related elements 

 like Li, Na, K, Rb, Cs, shift towards the least refrangible 

 side of the spectrum with increasing atomic weight, the 

 spectra of elements in homologous positions in each hori- 

 zontal series like Na, Mg ; K, Oa; Cu, Zn ; Rb, Sr ; Ag, Cd, 

 shift the opposite way, so that the spectrum of the element 

 of greater atomic w r eight is, as a whole, situated on the more 

 refrangible side. An examination of the spectra of the 

 different elements will show that this generalization holds 

 fairly good for the first and second series, and is also obser- 

 vable, but in a much less degree, in members of the third 

 series. So great, however, is the difference of displacement 



* Proa Roy. Soc., June 13, 1895, p. 195. 

 t Note read before the Society, October 15, 1895. 

 j Manchester Memoirs, 1878, 1887, 1895 ; Proc. Roy. Soc. 1893. 

 § Comptes Rendus, vol. lxix. 1869. 



|| Proceedings of the Berlin Academy, July 11, 1895, p. 757 j Phil. 

 Mag. September 1895 ; ' Nature,' Sept. 2(3. 



