274 SCIENCE PROGRESS. 



by inserting first a jar and then an air-break into the circuit, 

 the effect is to increase the brilliancy and the breadth of all 

 the lines, the brilliancy and breadth being greatest when the 

 longest air-break is used. 



(2) Contrariwise, when we are dealing with a known 

 compound gas ; at the lowest tension we may get the 

 complete spectrum of the compound without any trace of 

 its constituents, and we may then, by increasing the tension, 

 gradually bring in the lines of the constituents, until, when 

 complete dissociation is finally reached, the spectrum of the 

 compound itself disappears. 



Working on these lines the spectrum of the spark at 

 atmospheric pressure passing through the gas or gases, 

 distilled from broggerite, has been studied with reference 

 to the special lines C (hydrogen), D 3 , 667, and 447. 



The first result is that all the lines do not vary equally 

 as they should do if we were dealing with a simple gas. 



The second result is that at the lowest tension 667 is 

 relatively more brilliant than the other lines ; on increasing 

 the tension C and D 3 considerably increase their brilliancy, 

 667 relatively and absolutely becoming more feeble, while 

 447, seen easily as a narrow line at low tension, is almost 

 broadened out into invisibility as the tension is increased 

 in some of the tubes, or is greatly brightened as well as 

 broadened in others (Fig. 12). 



Fig. 12. — Diagram showing changes in intensities of lines brought about by varying the 

 tension of the spark, i. Without air-break. 2. With air-break. 



The above observations were made with a battery of 

 five Grove cells ; the reduction of cells from 5 to 2 made 

 no difference in the phenomena except in reducing their 

 brilliancy. 



Reasoning from the above observations it seems evident 

 that the effect of the higher tension is to break up a com- 

 pound or compounds, of which C, D 3 , and 447 represent 

 constituent elements ; while, at the same time, it would 



