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carbons are set burning, tlie distance is also nincli larger than the 

 normal one, when the arc hisses, the reverse takes place ; the lines 

 grow fainter, and the distance grows smaller, in case of ver>' decided 

 hissing they can even become entirely invisible. Also in ditferent 

 places of the arc the distance is ditferent, for the negative carbon 

 the distance is much larger — about twice as large as a rule — 

 than for the positive carbon. 



Between the two absorption maxima lies also a region of absorption, 

 which on the whole is of only little greater intensity than the 

 maxima of absorption. Now and then it makes the impression that 

 there are still more feeble maxima of absorption in this region ; I 

 have, however, not been able to ascertain this with perfect cer(aint3\ 



I could artificially modify the aspect of the absorption lines very 

 considerably by blowing a little soda mist into the arc by means 

 of a tube placed pai-allel to the positive carbon, which lies horizontal ; 

 in order not to disturb the equilibrium of the ai'c the blown in air 

 current had to be only very weak, while the quantity of sodium 

 could be modified by varying the concentration of the sodium 

 solution. It then appeared that always when soda was blown into 

 the arc the distance of the components of the two Z)-lines increased, 

 these becoming vaguer at the same time. The greater the quantity 

 was of the soda that was blown in, tiie further the components 

 were split up, and the less sharp they became. This splitting up 



o 



could even reach an amount of about 0,3 A. LT., in which case they 

 were, however, very \ague. The splitting was always perfectly 

 symmetrical with respect to the oi-iginal double absorption line. The 

 maxima of absorption were — for so far as [)erceptible — of equal 

 intensity, the sharpness of the two components also seeming pretty 

 well equal. I have not undertaken further quantitative measurements 

 about this, since it would have been impossible to determine the 

 quantity of sodium in the arc, even when the velocity of supply 

 Avas known; at most this quantiiy could be very roughly estimated; 

 nor was the phenomenon perfectly constant. Similar phenomena, 

 still less constant, however, were observed when carbons were used 

 which were soaked in a diluted solution of soda. 



When the Wood tube is placed in the way of the rays of light 

 so that it follows the spectrometer, which cuts a small portion out 

 of the spectrum, so e.g. between object glass and eye-glass of the 

 reading glass, the absorption lines of the sodium vapour appear also 

 to l)e double, in which the distance of the components increases as 

 the temperature rises. At the same time, however, distinct phenomena 

 of anomalous dispersion are perceptible, as soon as the heating of 



