352 



Profs. Liveing and Dewar 



[May 2, 



rubidium are reversed in our tubes, and both these violet lines are seen 

 when the spark of an induction-coil, without jar, is passed between 

 beads of rubidium chloride fused on platinum wire, though only one of 

 them appears when a Leyden jar is used. 



We have not repeated these observations on caesium and rubidium 

 in iron tubes, because the light emitted from the hot iron does not 

 extend so far into the violet as the rubidium lines, and the amount of 

 caesium at our command at present is very small. 



We have extended our observations on the absorption of magnesium 

 and of mixtures of magnesium with potassium and sodium, using iron 

 tubes placed vertically in a small furnace fed with Welsh coal, as 

 described in our former communication. 



The result of several observations, when commercial magnesium 

 (i.e., magnesium with only a small percentage of sodium in it) was 

 used, is that the absorption produced by magnesium consists 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 5,210. 

 These hues 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, but very near the place 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 was added. The measure of the 

 less refrangible edge of this band then gave a wave length of very 

 nearly 4,615. It was also seen when sodium and magnesium were 

 used together, but it was more difficult to get a measure of it in tljis 

 case, as the sodium obscures the blue part of the spectrum. A measure 

 taken when sodium was used gave a wave length 4,580, but this can 

 only be regarded as an approximation. 



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

 than the b group. This is best seen when potassium as well as 

 magnesium is introduced into the tube, but it may also be seen with 

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

 sharply defined, and has a wave length about 5,140, and it fades away 

 towards the blue. 



These absorptions are all seen both when potassium and when sodium 

 are used along with magnesium, and may be fairly ascribed to mag- 

 nesium, or to magnesium together with hydrogen. 



But besides these, other absorptions are seen which appear to be due 

 to mixed vapours. 



* On one occasion a dark fine corresponding to b* was also seen. 



