70 On the Order of Reversibility of the Lithium Lines. [Mar. 8, 



irregularity in the width of the lines, the effect resembles that of an 

 outburst of hydrogen in the solar atmosphere. The elements of the 

 water are, as we must suppose, separated in the arc, but from the 

 explosive character of the effect they are not uniformly distributed in 

 the arc. The arc being horizontal and the image of it projected on 

 to the slit of the spectroscope, it was really a very small section of the 

 arc which was under observation, and this renders the variation in 

 the width of the lines the more remarkable. 



Ill . " Note on the Order of Reversibility of the Lithium Lines." 

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

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

 sity of Cambridge. Received March 1, 1883. 



In our communications on the reversal of the lines of metallic 

 vapours, we have several times noticed (" Proc. Roy. Soc," vol. 28, 

 pp. 357, 369, 473) the reversal of the lithium lines, and concluded 

 that the blue line is more easily reversed than the orange line. This, 

 however, does not appear to be really the case. When much lithium 

 is introduced into the arc, a second blue line is developed close to but 

 slightly more refrangible than the well-known blue line. This second 

 blue line produces with the other the appearance of a reversal, which 

 deceived us until we became aware of the existence of the second line. 

 The blue line (wave-length 4604) is really reversed without difficulty 

 when sufficient lithium is present, but under these circumstances the 

 orange line is also reversed. The latter line is also the one which 

 first (of the two) shows reversal, and also the one which is more per- 

 sistently reversed. Hence we place the lines in order of reversibility as 

 follows : red, orange, blue, green, violet. 



