1888.] Investigations on the Spectrum of Magnesium. 241 



IV. u Investigations on the Spectrum of Magnesium. No. II." 

 By G. D. LiVEiNG, M.A., F.R.S., Professor of Chemistry, 

 and J. Dewar, M.A., F.R.S., Jacksonian Professor, Univer- 

 sity of Cambridge. Received May 16, 1888. 



Since our last communication on this subject, we have made many 

 additional observations on the spectrum of magnesium under various 

 circumstances, and have arrived at some new results. Speaking 

 generally, we find that differences of temperature, such as we gefc in 

 the flame of burning magnesium, in the arc, and in the spark, 

 produce less differences in the spectrum than we had before attri- 

 buted to them. For instance, the lines which previously we had 

 observed only in the spark discbarge, we have since found to be deve- 

 loped in the arc also, provided the discharge occur between electrodes 

 of magnesium.* In making these experiments we used thick electrodes 

 of magnesium, and brought them together inside a glass globe about 

 6 inches in diameter, fitted with a plate of quartz in front and filled 

 from time to time with various gases. The arc was an instantaneous 

 flash which could not be repeated more than twice without rendering 

 the sides of the vessel opaque with a complete coating of magnesium. 

 It was therefore analogous to an explosion of magnesium vapour. 

 The strong blue line \4481, two pairs about \3895, 3893, and X3855, 

 3848, the strong pair about A.2935, 2927, and the two weaker lines of 

 the quadruple group, namely, \2789'9 and 2797, all come out in the 

 arc given by a Siemens' dynamo between magnesium electrodes in 

 air, in nitrogen, and in hydrogen. We have observed most of them 

 also when the arc is taken in carbonic acid, in ammonia, in steam, 

 in hydrochloric acid, in chlorine, and in oxygen. The relative 

 intensities of these lines, as compared with one another and with the 

 other lines of the spectrum, vary considerably under different cir- 

 cumstances, of which temperature is doubtless one of the most 

 important ; but none of the spark lines seem to be absent from the 

 arc, and even the blue line X4481, so characteristic of the spark, 

 which we never found in the electric arc taken between carbon poles 

 in a crucible of magnesia even on addition of magnesium, is some- 

 times quite as strongly shown in the arc between magnesium 

 electrodes. There are still several lines of the arc which we have 

 never observed in the spark, such as the series of triplets of wave- 

 length less than 2770, but their presence may be dependent more on 

 the large quantity of incandescent matter in the arc than upon its 

 relative temperature. The observations, however, render doubtful 



* Compare the appearance of the lines of hydrogen in the arc discharge, ' Koy. 

 Soc. Proc.,' toI. 30, p. 157 ; and vol. 35, p. 75. 



