725 



111 the ligiire I have also indicated the mean amoujit of the width 

 of the region of tlie arc absorption by means of short dotted lines. 

 Fnrther the cnr\ e indicated bv p denotes the vaponr tension of the 

 sodium vapour for the telnperature in question. The scale of the 

 ordinates on the right hand side of the tigure refers to this curve. 

 For the determination of this cur\ e I made use of Kroner's ') research 

 on the \'apour tension of alkalimetals ; for this it was necessary to 

 extrapolate the values found by Kroner, for which piirpose I used 

 Gratz's ') formula, which is formed from Dupre-Hertz's •') formnla 

 based on that of Clapeyron by assuming the validity of Van der 

 Waals's law for the vaponr instead of that of Boyle-Gay Ia'ssac. 

 Gratz's formula 



p .e^'T — fcT-o* ^^ ^ 



containing four constants, 1 had to assume four points of the vapour 

 tension curve. I took three points for them, which had been directly 

 determined for sodium by Kroner, viz. 7'= 693, /j = 2.00 ; 7'= 733, 

 p = 4.20; T= 773, p=z S.Q4: (temperatures in absolute scale, pres- 

 sures in m.m. of mercury). I derived the fourth point, availijig myself 

 of Ramsay and Young's rule, from Kroner's determinations for pot- 

 assium and found for it 7':= 589, p=z0.11. I found from this for 

 the constants using Brigg's logarithms, « = 28.877, /c»// 1- = J 64.88, 

 711 = 48.748, n ^= 18143. By the aid of these constants I calculated 

 the values of /; given in the tigure. 



One might be inclined to ascribe the observed phenomena to ano- 

 malous dispersion ; the observed dark lines would then be no absorp- 

 tion lines proper, but ''dispersion lines". If this were actually the 

 case, the light of the considered wa\e length would ordy have 

 changed its direction, without haxing undergone absorption. As to 

 the absorption lines in the light ai-c, taking the comparatively small 

 value of the anomalous dispei-sion at the densities in question into 

 consideration, the point of intersectioji of the rays of light coming 

 from the arc with the plane of the spectrometer slit could never be 

 far distant from the crater iiUcige. When this image is moved over 

 the plane of the slit we should therefore have to see light lines in 

 some positions instead of dark ones. As I have nexer observed 

 anything like this, not when 1 placed the arc lamp in other positions 



1) A. Kroner. Ann. der Phys. 40. p. 438. 1913. 



2) Gratz. Zeitschr. f. Math. u. Pliys. 49 p. -289. 1903. 

 ") Hertz Wied. Ann. 17. p, 177. 1882. 



DupRÉ. Theorie mécanique de la chaleur. p. 69. Paris 1869. 



