408 Royal Society : — 



series ; 2nd, those in which the associated elements varied in each 

 series. The following salts were mapped : — 



PbF 2 , PbCl 2 , PbBr 2 , Pbl 2 ; Sr F 2 , Sr Cl 2 , Sr Br 2 , Srl 2 ; 

 BaF a , BaCl 2 , Ba Br 2 , Ba I a ; Mg F 2 , Mg Cl 2 , Mg Br 2 , 

 Mg I 2 ; Na F, Na CI, Na Br, JSTa I. 



The conditions of the experiments are described. The same alu- 

 minium cups, described in the first paper, were used ; and the poles 

 were arranged in such a manner that they could at will be sur- 

 rounded with any gas or vapour. Hydrogen was used in some of 

 these experiments ; it was purified in the usual manner by drying 

 and freeing from traces of sulphuretted hydrogen; it was then 

 passed over clean-cut pieces of sodium, and admitted to the poles. 

 An induction-spark from 5 one-pint Grove cells was used, the 

 circuit being ivithout the Leyden jar. 



The lead compounds behaved (in air) as follows : — 



The fluoride gave the eleven longest lines of the metal ; but four 

 were very faint. 



The chloride gave nine lines * one of these was very short. 



The bromide gave six lines ; but one was a mere dot on the pole. 



The iodide gave four lines distinctly and two as dots, one of which 

 was scarcely visible. 



It is pointed out that the decrease in length and number of 

 lines follows the increase in the atomic weight of the non-metallic 

 element, the lines dying out in the order of their length. 



Barium was next experimented on, the same series of salts being 

 used. A marked departure from the results obtained in the case 

 of the lead compounds was observed, especially in the case of the 

 fluoride, its spectrum being much the simplest ; in fact it consisted 

 of only 4 lines. Strontium behaved like barium ; and so did mag- 

 nesium fluoride. This anomalous behaviour was found to be most 

 probably due to the exceedingly refractory nature of these fluorides, 

 all of them being quite infusible, and non-volatile in any spark that 

 was used. 



Sodic fluoride, sodic chloride, sodic bromide, and sodic iodide 

 exhibited a behaviour exactly the reverse of that of lead ; i. e. the 

 iodide showed most of the metallic spectrum. 



The difference between flame-spectra and those produced by a 

 weak electric discharge are then discussed. Beads of the chlorides 

 &c. were heated in a Bunsen gas-flame. Bal 2 gave a " structure " 

 spectrum (since proved to be due to the oxide) and the line at 

 wave-length 5534-5, by very far the longest metallic line of ba- 

 rium. The bromide behaved like the iodide ; and so did the chlo- 

 ride, except that its spectrum was more brilliant. Baric fluoride 

 gave scarcely a trace of a spectrum, the oxide structure being 

 scarcely visible, and 5534*5 very faint indeed. The strontium 

 salts follow those of barium — 4607*5, the longest strontium line, 

 appearing in conjunction with an oxide spectrum. The strontic 

 fluoride, however, refused to give any spectrum whatever. These 



