1879.J on Spectroscopic Investigation. 209 



and so much expanded as to be almost in contact. The width of the 

 dark lines decreased as the ca3sium evaporated, but they remained 

 quite distinct for a long time. A similar effect was jiroduced by the 

 rubidium mixture, only it was necessary to have the tube very much 

 hotter, in order to get enough of violet light to see the reversal of 

 the rubidium lines. In this case the two lines were so much expanded 

 as to form one broad dark band, which gradually resolved itself 

 into two as the rubidium evaporated. The reversal of these lines 

 of ca3sium and rubidium seems to take place almost or quite as 

 readily as that of the D lines by sodium, and the vapours of those 

 metals must be extremely opaque to the light of the refrangibility 

 absorbed, for the absorption was conspicuous when only very minute 

 quantities of the metals were present. The red, yellow, and green 

 parts of the spectrum were carefully searched for absorption lines, 

 but none due to caesium or rubidium could be detected in any case. 

 It is perhaps worthy of remark that the liberation of such extremely 

 electro-positive elements as caesium and rubidium from their chlorides 

 by sodium and by lithium, though it is probably only partial, is a proof, 

 if proof were wanting, that so-called chemical affinity only takes a 

 part in determining the grouping of the elements in such mixtures ; 

 and it is probable that the equilibrium arrived at in any such case is a 

 dynamical or mobile equilibrium, continually varying with change of 

 temperature. 



It is difficult to prevent the oxidation of magnesium in the iron 

 tubes, and tubes wider than half an inch did not give satisfactory results. 

 With half-inch tubes, the lines in the green were clearly and sharply 

 reversed, also some dark lines, not measured, were seen in the blue. 

 The sharpness of these lines depended on the regulation of the 

 hydrogen current, by which the upper stratum of vapour could be 

 cooled at will. 



(1) The absorption spectrum of magnesium consists of 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 5,210. These 

 lines are the first and the last to be seen, and were first taken for the 

 extreme lines of the h group. 



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

 to measure exactly, but very near the jilace of the brightest blue 

 line of magnesium. This line was not always visible, indeed rarely 

 when magnesium alone was placed in the tube. It was better seen 

 when a small quantity of potassium or sodium was added. The 

 measure of the less refrangible edge of this band gave a wave-length 

 of very nearly 4,615. 



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

 than the h group. This is best seen when jwtassium and magnesium 

 are introduced into the tube, but it may also be seen with sodium 

 and magnesium. The less refrangible edge of this band is sharply 



