278 SCIENCE PROGRESS. 



suggested to us the idea that the two pairs of series belonged 

 to different elements. One of the two pairs being by far 

 the stronger, we assume that the stronger one of the two 

 remaining series belongs to the same element as the stronger 

 pair. We thus get two spectra consisting of three series 

 each, two series ending at the same place, and the third 

 leaping over the first two in large bounds and ending in the 

 more refrangible part of the spectrum. This third series we 

 suppose to be analogous to the so-called principal series in 

 the spectra of the alkalis, which show the same features. 

 It is not impossible, one may even say not unlikely, that 

 there are principal series in the spectra of the other elements. 

 But so far they have not been shown to exist. 



" Each of our two spectra now shows a close analogy to 

 the spectra of the alkalis. 



"We therefore believe the gas in cleveite to consist of 

 two, and not more than two, constituents." 



To the one containing the line D 3 , which I discovered 

 in 1868, the name helium remains ; the other for the present 

 we may call " gas X V 



The chief lines of these two constituents are as follows, 

 according to Runge and Paschen, the wave-lengths being 

 abridged to five figures. 



i 



Hi I ol 



I II] L 



8 

 If ' 



Fig. 14. — Runge and Paschen's results suggesting that cleveite gives off 

 two gases, each with three series of lines. 



1 In the many comparisons I had to make, I soon found the incon- 

 venience of not having a name for the gas which gave 667, 501 and other 

 lines. When, therefore, Professors Runge and Paschen, who had endorsed 



