370 



Profs. Liveing and Dewar 



[Feb. 27, 



mistake in Thalen's observation, which ascribed it to caesium. "We 

 have never detected this line with caesium, which, on the other hand, 

 seems always to give the characteristic blue lines, both in the spark 

 and in the flame, as well as to give the same lines reversed when its 

 vapour is used as an absorbent. 



When metallic indium was introduced into the crucible, both the 

 lines with wave-lengths 4,101 and 4,509 were at once seen strongly 

 reversed, and so continued for some time. ISTo other absorption line 

 of indium could be detected. 



It is apparent that the expansion of lines, so often observed when 

 fresh materials are introduced, must be ascribed to increase in the 

 density of the vapours, not to any increase of temperature. Moreover, 

 the length of tube which reaches a very high temperature in the expe- 

 riments above described is very short in the lime crucibles, and still 

 shorter in the carbon crucibles, so that the reversing layer is also short 

 in many cases. We are, therefore, directing our attention to the means 

 of heating up a longer length of the tubes, either by introducing oxy- 

 hydrogen jets, or additional electric arcs one above another ; and also 

 to the introduction of reducing gas (hydrogen or carbonic oxide) to 

 counteract the oxidising action of the air which is drawn in through 

 the lateral openings. 



The curious behaviour of the lines of different spectra with regard 

 to reversal has induced us to compare the bright lines of the chromo- 

 sphere of the sun, as observed by Young, with those that are reversed 

 in our crucibles. It is well known that some of the principal lines of 

 metals giving comparatively simple spectra, such as lithium, aluminium, 

 strontium, and potassium, are not represented amongst the dark lines 

 of Fraunhofer, while other lines of those metals are seen : and an 

 examination of the bright chromospheric lines shows that special rays 

 highly characteristic of bodies which appear from other rays to be 

 present in the chromosphere are absent, or are less frequent in their 

 occurrence than others. 



In the following tables the relation between our observations on 

 reversals and Young's on the chromospheric lines is shown. 



Lines in Frequency 

 wave-lengths. 1 m chromo- 

 ° sphere. 



Behaviour. 

 Reversal in our 

 tubes. 



Eemarks. 



Sodium.. 6,160 1 1 



6.154 J j U 

 D I 50 



5,687 11 9 

 5,681 J ! ^ 



5.155 11 „ 

 5,152 J | 

 4,983 lj 

 4,982 J ! u 



Expanded. 



. Principal ray. 



Difficultly reversed. 

 Tery diffused. 



J) 5) 



