264 Prof. E. Wiedemann on the 



must also be opaque ; i. e. a film which contains per square 

 centimetre 



2 p; xl i °" 4 =4xlO- 8 gr.Na. 

 5 x 10 3 & 



In the sodium flame examined by us there is present in a 

 layer of 1 square centimetre 6*9 x 10 -7 gr., that is about 

 twenty times as much as would be necessary to produce such 

 opacity with the platinum. 



If, therefore, KirchhofF's law is to hold good here the 

 flame must be absolutely opaque for the yellow rays. In fact 

 such a flame shows reversal ; i. e. in the centre of each of the 

 yellow sodium lines a dark line appears when a ray of white 

 light traverses it, nevertheless the absorption is by no means 

 so great as one would have expected according to KirchhofF's 

 law, since the dark line is confined to the centre. Hence 

 it would seem that in the sodium flame luminescence 

 phenomena appear together with with the usual luminous 

 phenomena. In fact, highly complicated chemical processes 

 occur in such a flame ; further researches will show this more 

 clearly. 



Direct Comparison of the Coefficients of Total and True 

 Emission of Platinum and Sodium. 



31. In continuation of the foregoing, an experimental 

 arrangement was employed, which will find frequent applica- 

 tion in later investigations. 



In one and the same flame sodium was distributed and a 

 platinum wire was heated to luminosity, and the brightness 

 of each was compared with the amyl-acetate lamp. 



The apparatus shown in fig. 3 (PI. III.) was employed. In 

 the interior of an Ebert's* burner, B, a thin platinum-wire 0'26 

 millim. thick was fastened at a ; it had thus the same thickness 

 as the wire previously investigated, which was heated by the 

 current; at its upper end it was attached to a small hook 

 which was suspended to one arm of a lever, movable about 

 the horizontal axis e, which was capable of being adjusted 

 as to height. The lever was weighted on the other side by 

 the weight /in order to keep the wire stretched when hot. 



Then water only was scattered in the flame, which was 

 colourless, and the brightness of the platinum wire was 

 measured ; then by a slight movement of the support the wire 

 was put out of the field of view, the flame was fed with 

 sodium solution, and its brightness measured again. These 

 measurements showed that the ratio of brightness of the pla- 



* Wied. Ann. xxxii. p. 345 (1887). 



