L. Bell — Ultra-violet Spectrum of Cadmium. 431 



course only rough approximations, but they are sufficient to 

 raise some rather interesting questions. 



The great resolving power of the apparatus used in these ex- 

 periments resulted in the discovery of several new double lines, 

 notably the one at w. 1. 2264, of which the components are 

 quite sharp and of about equal intensity. A negative in the 

 second spectrum showed lines w. 1. 2979\8 and w. 1. 2880*2 to 

 be also doubles, each of them having a faint component on the 

 side of greater wave-length. And this discovery brings out a 

 rather striking analogy. The group having wave-lengths 2979, 

 2880 and 2836 and the prominent group of wave-lengths 3611, 

 3609, 3466, 3465 and 3402 are in relative distances, character 

 and intensity, almost precisely alike. In each the first two 

 lines are double and in each case the fainter component has the 

 greater wave-length. 



Now turn to the spectrum of zinc. The most prominent 

 group in the ultra-violet is a triplet of wave-lengths, 3344, 

 3301, 3281. And further in the ultra-violet is another group, 

 of wave lengths 2800, 2770, 2754, closely resembling the former 

 in arrangement. If now these be compared with the cadmium 

 groups before mentioned, the close analogy is at once apparent. 

 Each zinc group has an analogue in the cadmium spectrum, 

 somewhat expanded and of greater wave-length, but very sim- 

 ilar in grouping and general character. A photograph of the 

 former zinc group was taken in the second spectrum which dis- 

 closed the fact that, as in the corresponding cadmium group, 

 the two first lines were double, each with a faint component 

 on the side of greater wave-length. Ciamician has pointed out 

 a similarity in the visible spectra, and as the spectra are more 

 thoroughly examined the analogv becomes all the more strik- 

 ing. Plot to scale these zinc lines: 6360, 4924, 4911,4809, 

 4722, 4679, 3344, 3301, 3281, 2800, 2770, 2754, 2557, 2501, 

 and the following cadmium lines, 6438, 5338, 5379, 5086, 4800, 

 4678, 3609, 3465, 3402, 2979, 2880, 2836, 2748, 2572, and the 

 extraordinary resemblance between the spectra is evident. So 

 striking is it that it can hardly have escaped notice, but the 

 discovery of the double zinc lines noted above so emphasizes 

 it that it is worthy of more than passing attention. The re- 

 semblance extends not only to the grouping of the lines but to 

 those individual peculiarities that every spectroscopist knows to 

 be almost as characteristic of certain lines as their wave-lengths. 

 In both spectra it is analogous lines that are longest in the 

 spark, that are reversed in the sun, and that are most persistent 

 in dilute solutions. Certain it is, that zinc and cadmium must 

 have some great similarity in atomic structure, and the extent 

 and meaning of this is a subject by no means unworthy of 

 investigation. 



