208 Professor Dewar [June 6, 



place of the less refrangible of the ceesium blue lines. During this 

 time no dark line could be observed in the red ; but as the tempera- 

 ture rose, a broad absorption band appeared in the red, with its centre 

 about midway between B and C, ill defined at the edges, and though 

 plainly visible not very dark. The lines in the violet had now 

 become so broad as to touch each other and form one dark band. On 

 cooling, the absorption band in the red became gradually lighter 

 without becoming defined, and was finally overpowered by the chan- 

 nelled spectrum of sodium in that region. The double dark line in 

 the violet became sharply defined again as the temperature fell. 

 There are two blue lines in the spectrum of rubidium taken with an 

 induction-coil very near the two blue lines of caesium ; but they are 

 comparatively feeble, and the two dark lines in the blue observed 

 in the places of the characteristic blue lines of ceesium must have 

 been due to a small quantity of caesium chloride in the sample of 

 rubidium chloride. These blue lines were not, however, visible 

 when some of the rubidium chloride was held in the flame of a 

 Bunsen's burner, nor when a spark was taken from a solution of the 

 chloride ; but the more refrangible of them (Csa) was visible in the 

 spark of an induction-coil, without a Leyden jar, taken between beads 

 of the rubidium chloride fused on platinum wires. 



When a tube containing ceesium chloride and sodium was observed, 

 in the same way, the two dark lines in the blue were seen very soon 

 after the heating began, and the more refrangible of them broadened 

 out very sensibly as the temperature increased. The usual channelled 

 spectrum of sodium was seen in the green, and an additional channelling 

 appeared in the yellow, which may be due to csesium or to the mixture 

 of the two metals. Indeed the CfBsium chloride was not free from 

 rubidium, and the dark lines of rubidium were distinctly seen in 

 the violet. Metallic lithium acts on the chlorides of caesium and 

 rubidium, giving the same results as sodium. 



It is remarkable that these absorption lines of caesium coincide 

 with the blue lines of caesium as seen in the flame, not with the 

 green line which that metal shows when heated in an electric spark of 

 high density. It is to be observed, however, that when sparks from an 

 induction-coil without a jar are taken between beads of caesium chlo- 

 ride fused on platinum wires, a spectrum similar to the flame spec- 

 trum is seen, and it is only when a Leyden jar is used that the 

 spectrum is reduced to a green line. In like manner both the violet 

 lines of rubidium are reversed, 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. 



Mixtures of carbonate of caesium with carbon, and of carbonate of 

 rubidium with carbon, prepared by charring the tartrates, heated in 

 narrow porcelain tubes, placed vertically in a furnace, gave sharp 

 results. A small quantity of the caesium mixture, introduced into a 

 tube at a bright red heat, showed instantly the two blue lines reversed 



