94 



Profs. Liveing and Dewar. 



[Jan. 15, 



than the b group The less refrangible edge of this band 



is sharply denned, and has a wave-length about 5140, and it fades away 

 towards the blue. 



"These absorptions are all seen both when potassium and when 

 sodium are used along with magnesium, and may be fairly ascribed to 

 magnesium, or to magnesium together with hydrogen. 



" (4). When sodium and magnesium are used together, a dark line 

 with ill-defined edges is seen in the green, with a wave-length about 

 .5300. 



" (5.) When potassium and magnesium are used together, a pair of 

 dark lines are seen in the red. The less refrangible of these sometimes 

 broadens into a band with ill-defined edges, and has a mean wave- 

 length of about 6580. The other is always a fine sharp line with a 

 wave-length about 6475- 



" (6.) On one occasion, with a mixture of potassium and magnesium, 

 -another dark line was seen in the blue, with a wave-length nearly 

 4820." 



We then described the emission spectrum seen when sparks are 

 taken between electrodes of magnesium with a small induction coil 

 without a Leyden jar as consisting of lines with the wave-lengths 

 4481, 4590, 4570, 4797, 4930, 4945, 4957, 4969, 4980, 4990, 5000, the 

 well-known b group, and 5528. 



As the absorption lines above described correspond to no known 

 emission lines of magnesium, we could only ascribe their origin to the 

 mixtures employed as distinct from the separate elements, and we were 

 led to investigate the conditions under which corresponding emission 

 lines could be produced. 



Shortly afterwards (" Proc. Hoy. Sod," vol. xxvii, p. 494) we de- 

 scribed the emission spectrum of sparks from an induction coil taken 

 between magnesium points in an atmosphere of hydrogen, as follows : — 

 " A bright line regularly appeared with a wave-length about 5210. 

 . ■ This line does not usually extend across the whole interval 

 between the electrodes, and is sometimes seen only at the negative 

 electrode. Its presence seems to depend on the temperature, as it is not 

 seen continuously when a large Leyden jar is employed, until the pres- 

 sure of the hydrogen, and its resistance, is very much reduced. When 

 well-dried nitrogen or carbonic oxide is substituted for hydrogen, this 

 line disappears entirely; but if any hydrogen or traces of moisture 

 be present, it comes out when the pressure is much reduced. In such 

 cases the hydrogen lines C and F are always visible as well. Some- 

 times several fine lines appear on the more refrangible side of this line 

 between it and the b group, which give it the appearance of being a 

 narrow band shaded on that side." " In addition to the above-men- 

 tioned line, we observed that there is also produced a series of fine 

 lines, commencing close to the most refrangible line of the b group, and 



