1880.] 



On the Spectra of Magnesium and Lithium. 



93 



great complications, and until this difficulty is overcome, experiments 

 at higher temperatures must remain uncertain. All the experiments 

 show that for an approach to a knowledge of temperatures beyond 

 the range of our actual thermometric scale, the law given in 1873 is a 

 sufficiently correct reproduction of the facts, considering the limited 

 data at our disposal. 



The intensity of the radiation of the positive pole of the Siemens' 

 arc, as compared with the same surface heated with a large oxy- 

 hydrogen blowpipe, was determined by employing a hollow negative 

 carbon which allowed the intensely heated surface to radiate directly 

 on to the pile, as shown in fig. 3. A large number of observations have 

 been made by this method at different times, and with slight modifica- 

 tions in the order of the experiments, leading to the average result 

 that' the intensity of the total radiation of the positive pole of the 

 Siemens' arc is ten times that of the same substance at the tempera- 

 ture of the oxyhydrogen flame. If we take an average result of nine 

 to one, then we may infer that the temperature of the limiting posi- 

 tive pole is about 6,000° C, seeing that the mean temperature of the 

 oxhydrogen may be taken as 2,000° C. The mean value of the total 

 radiation of the Siemens' arc was determined by observing the rate of 

 flow of the water through the annular vessel, represented in fig. 1. 

 together with the mean increment of temperature. This gave on the 

 average 34,000 gram-units per minute, or a little more than three 

 horse-power. 



IV. " On the Spectra of Magnesium and Lithium." By G. D. 

 LlVEiNG, M.A., F.R.S., Professor of Chemistry, and J. 

 Dewar, M.A., F.R.S., Jacksoman Professor, University of 

 Cambridge. Received January 8, 1880. 



In a former communication ("Proc. Roy. Soc," vol. xxvii, p. 350) 

 we described our observations on the absorption spectrum of magne- 

 sium, and of magnesium with potassium and sodium, as seen in iron 

 tubes in an atmosphere of hydrogen. These absorptions consisted of — 



" (1.) Two sharp lines in the green, of which one, which is broader 

 than the other, and appears to broaden as the temperature increases, 

 coincides in position with the least refrangible of the b group, while 

 the other is less refrangible and has a wave-length very nearly 5210. 

 These lines are the first and the last to be seen and very constant, and 

 we at first took them for the extreme lines of the b group. 



" (2.) A dark line in the blue, always more or less broad, difficult to 

 measure exactly. 



" (3.) A third line or band, in the green, rather more refrangible 



