of Radium in the Periodic System, 



477 



of certain analogies with the spectra of the alkalies, the line- 

 pair of the primary series, the line-pair of the first and that 

 of the secondary series. In the line-pair of the first secondary 

 series there occurs by the side of the line of greater wave- 

 length a feebler line on the side of greater wave-lengths, 

 which Runge and Paschen term a satellite. Measured on 

 the scale of frequency, the two lines of each of the three pairs 

 are the same distance apart for any one element, provided 

 that in the case of the pair of the first secondary series the 

 satellite be taken instead of one of the lines. This distance 

 varies, on the other hand, from one element to another, in- 

 creasing in a perfectly regular manner with the atomic 

 weight, as will be seen below. In a magnetic field these 

 lines, as has been shown by Runge and Paschen, split 

 up in various ways into components, but in such a manner 

 that, referred to the scale of frequency, the resolution 

 of each line of any one element is precisely the same 

 as that of the corresponding line of each of the other 

 elements. 



Now we have found that this also holds for radium, so that 

 Ra is to be classed along with Mg, Ca, Sr, and Ba in a group 

 of chemically allied elements — a conclusion which is sup- 

 ported by the chemical behaviour of radium, in so far as this 

 is known. 



The following table exhibits the correspondence between 

 the lines : — 





■ 

 Mg. 



Ca. 



Sr. 



Ba. 



Ra. 



Primary Series •; 



2803 

 2796 



3969 

 3934 



4216 



4078 



4934 

 4554 



4682 

 3815 



1st secondary Series < 



2798 

 2791 



3181 

 3179 

 3159 



3475 

 3465 



3381 



4166 

 4131 



3892 



4436 

 4341 

 3650 



2nd „ „ | 



2937 

 2929 



3737 

 3706 



4306 

 4162 



4900 

 4525 



5S14 

 4533 



The resolution 

 easily observable 



of the radium lines in a magnetic field is 

 with the strongest lines. It is only the 



resolution of the satellite and of the yellow line 5814 that we 



have as yet been unable to effect. 



For any one element the distances apart of the two lines of 



each pair are, as already remarked above, equal if measured 



