1881.] Investigations on the Spectrum of Magnesium. 181) 



III. "Investigations on the Spectrum of Magnesium." By 

 G. D. Liveing, M.A., F.R.S., Professor of Chemistry, and 

 J. Dewar, M.A., F.R.S., Jacksouian Professor, University 

 of Cambridge. Received April 28, 1881. 



[Plate 1.] 



Since our last communication on this subject (" Proc. Roy. Soc," 

 vol. 30, p. 93) several authors — Ciamician, Cornu and Fievez — have 

 published observations on the spectrum of magnesium, to some of 

 which allusion is made in the sequel, but these observations by no 

 means exhaust the subject. Our own observations, carried on for a 

 considerable time, have extended to new regions and a variety of cir- 

 cumstances, and the summary of them which we now present to the 

 Society will, we hope, help to bring out the connexion between some 

 of the variations in the spectrum of this element and the conditions 

 under which it is observed, and throw additional light on the ques- 

 tion of the emissive power for radiations of short wave-length of 

 substances at the relatively low temperature of flame to which we 

 alluded in our paper on the spectrum of water ( t; Proc. Roy. Soc," 

 vol. 30, p. 580). 



We begin with an account of these observations. 



Spectrum of the Flame of Burning Magnesium. 

 When magnesium wire or ribbon is burnt in air, we see the three 

 lines of the b group, the blue line about wave-length 4570, first noticed 

 by us in the spark spectrum (" Proc. Roy. Soc," vol. 27, p. 350) ; 

 and photographs show, besides, the well-known triplet in the ultra- 

 violet between the solar lines K and L sharply defined, and the line 

 for which Cornu has found the wave-length 2850 very much expanded 

 and strongly reversed. These lines are all common to the flame, arc, 

 and spark spectra ; and the last of them (2850) seems to be by far the 

 strongest line both in the flame and arc, and is one of the strongest in 

 the spark. But, in addition to these lines, the photographs of the 

 flame show a very strong, somewhat diffuse, triplet, generally re- 

 sembling the other magnesium triplets in the relative position of its 

 components, close to the solar line M ; and a group of bands below it 

 extending beyond the triplet near L. These bands have, for the most 

 part, each one sharply defined edge, but fade away on the other side ; 

 but the diffuse edges are not all turned towards the same side of the 

 spectrum. The positions of the sharp edges of these bands, and of the 

 strong triplet near M, are shown in the figure, No. 1. It is remarkable 

 that the triplets near P and S are absent from the flame spectrum, 

 and that the strong triplet near M is not represented at all either 

 in the arc or spark. The hydrogen-magnesium series of lines, begin- 

 ning at a wave-length about 5210, are also seen sometimes, as already 



