778 Prof. H. Rubens and Mr. H. Hollnagel on 



D is the percentage transmission. Reflexion at the surfaces 

 of the substances investigated is not taken into consideration. 



Attention is called to the marked transmission of the 

 quartz, paraffin, and ebonite in comparatively thick layers. 



The water measurements were made on a soap film, which 

 contained 10 per cent of glycerine and 1 per cent, of sodium 

 oleate. The thickness of this film was maintained constant 

 by means of an apparatus used in earlier researches *. The 

 determination of this thickness was also carried out in a 

 manner similar to that then utilized. The results obtained 

 are to be considered as approximate since the thickness 

 varied between 2*2 and 3 /u. 



In measuring the absorption of water vapour, a brass 

 cylinder 40 cm. long and 9 cm. in diameter, electrically 

 heated to 150° C. and open at both ends, was placed in the 

 optical path. Through a small tube set in the side of this 

 cylinder, water vapour from a flask of water, maintained at 

 the boiling-point under atmospheric pressure, was led in. 

 Within the absorption tube the vapour was overheated, hence 

 no condensation set in until the vapour reached a considerable 

 height above the open ends. There small clouds appeared. 

 While the Reststrahlen of rock-salt and sylvine are almost 

 completely absorbed by such a layer of vapour, a considerable 

 percentage of the Reststrahlen of potassium bromide and 

 potassium iodide is transmitted. This indicates that the 

 absorption of water vapour has, to be sure, a considerable 

 value in the region of 80 to 100 /jl f , though not so great as 

 in the domain between 50 and 70 yu-. The same holds good 

 for the liquid state ; a consideration of the extinction co- 

 efficients (g) shows this. For the Reststrahlen of rock-salt 

 <7 = 0*68 X was obtained in an earlier research, while for 

 that of potassium bromide we find here § 



^=lognat.TT4^=0-66, 



which proves that for the greater wave-length there is a 

 lesser weakening referred to the same optical path. 



As is well known, Drude || calculated the region of 



* H. Rubens and E. Ladenburg-, Verh. der Dt. Phys. Ges. xi. p. 16 

 (1909). 



t That the Reststrahlen of KBr are absorbed by the water vapour 

 is to be expected, since the Bunsen flame emits these same rays to a 

 marked extent ; by far the greater portion of these rays are, however, 

 given out by the Welsbach mantle. 



X H. Rubens and E. Ladenburg, Le Radivm, vi. p. 108 (1909). 



§ A. marked reflexion on the surfaces of the water-film is not to be 

 expected because of the small absorption and thickness of the layer. 



|| P. Drude, Physik des Aethers, p. 533. 



