1879.] on Spectroscopic Investigation. 207 



l)ct\vccu the red and green light is an enormous extension of the D 

 absorption line, while a still broader dark space intervenes between 

 the green and the blue light. The dark lino in the green (wave- 

 length about 5,510) now becomes more sharply defined. This lino 

 appears to have been observed by Roscoe and Schuster, and regarded 

 by them as coinciding with the double sodium line next in strength 

 to the D lines, but it is considerably more refrangible than that double 

 line. In the next stage, the channelled-space spectrum comes out in 

 tlie dark space between the green and blue, and finally in the red. 

 Gradually the light extends, the channels disappear, the D line 

 absorption narrows, but still the dark lino in the green is plainly 

 discernible. Lastly, there is only D lines absorption. 



The method of observation described may be used to observe 

 emission as well as absorption spectra ; for if the closed end of the 

 tube be placed against the bars of the furnace so as to be relatively 

 cooler than the middle of the tube, the light emitted by the vapours 

 in the hottest part is more intense than that emitted by the bottom of 

 the tube. This succeeds admirably with sodium. 



The volatility of rubidium and caesium rendered it advisable to try 

 the effects first in glass tubes. For this purpose a piece of combustion 

 tubing had one end drawn out and the end turned up sharjjly, and 

 sealed oft' (like an ill-made combustion tube of the usual form), so as 

 to produce an ai^proximately plane face at the end of the tube ; a 

 small bulb was then blown at about an inch from the end, and the 

 tube drawn out at about an inch from the bulb on the other side, so 

 as to form a long narrower tube. Some dry rubidium or caesium 

 chloride was introduced into the bulb, and a fragment of fresh cut 

 sodium, and the narrow j^art of the tube turned up, so as to allow 

 the tube and bulb to be seen through in the direction of the axis 

 of the tube. The shape of tube is given in Plate I. The open 

 end was then attached to a Sj)rengel pump, and the air exhausted ; 

 the sodium was then melted, and afterwards either dry hydrogen or 

 dry nitrogen admitted, and the end of the tube sealed off at nearly 

 the atmospheric pressure. It is necessary to have this pressure of 

 gas inside the tube, otherwise the metal distilled so fast on heating 

 that the ends were speedily obscured by condensed drops of metal. 

 Through those tubes placed lengthways in front of a spectroscope, a 

 lime light was viewed. On warming the bulb of a tube in which 

 rubidium chloride had been sealed up with sodium, the D lines were 

 of course very soon seen ; and very soon there aj^peared two dark 

 lines near the extremity of the violet light, which, on measurement, 

 were found to be identical in position with the well-known violet 

 lines of rubidium. Next appeared faintly the channelled sjjectrum 

 of sodium in the green, and then a dark line in the blue, very sharp 

 and decided, in the place of the more refrangible of the charac- 

 teristic lines of caesium in the flame spectrum. As the temperature 

 rose, these dark lines, especially those in the violet, became sensibly 

 broader ; and then another fine dark line appeared in the blue, in tho 



