the Rotation-Energy of Molecules. 75 



the measurements of the water-vapour band at 6fi made by 

 Paschen * at various temperatures. 



The constancy in the last column can be considered as 

 quite satisfactory. 



In accordance with what has been said above, it must be 

 held probable that Bjerrum's view as to the influence of 

 rotation on the ultra-red absorption is right in the main. 

 From this follows that, if the rotation- frequencies are dis- 

 tributed in accordance with Maxw-elPs law of distribution, 

 the absorption-bands cannot be resolved into more than the 

 said maxima, and these must be continuous. This is, how- 

 ever, far from being the case. By his remarkable investi- 

 gations on the long-wave spectrum Rubens f has shown that 

 the absorption of water-vapour by no means forms a con- 

 tinuous area here, but on the contrary a series of well- 

 defined bands ; and by an indirect method I have shown that, 

 e. a. the absorption-bands of carbon monoxide and nitrous 

 oxide (see fig. 1) are not continuous. Finally, I have also 

 succeeded in directly measuring a whole series of separate 

 maxima in the water-vapour band at 6*3//, and the hydro- 

 chloric acid band at 3*5yu, J. 



Thus fig. 2, curve (fr), shows an energy-curve for rays from 

 a Nernst lamp which have passed through 2 m. of undried 

 -air. The curve (a) is an energy-curve measured by Paschen §, 

 given by rays which have been transmitted through 7 cm. of 

 water-vapour at 100 degrees. As may be seen, some of the 

 separate maxima appear even in this curve. According to 

 Bjerrum every separate maximum that occurs in fig. 2 ought 

 to correspond to an absorption maximum in the long-wave 

 spectrum. If the positions of these latter maxima are 

 calculated, then the values inserted in Table II. cols. 1 and 2 

 tire obtained. The first series is calculated from the right- 

 hand part of the band, the other from the left-hand part. 

 The brackets round the values 69 and 64, 50 and 48 denote 

 that these absorption-bands so merge into one another (see 

 fig. 2) that, in the case of direct measurement according to 

 Rubens' method, they must presumably present themselves 

 as one single absorption-band. As is to be noticed, the two 

 series are, on the whole, in agreement, and they also corre- 

 spond well with the values found by Rubens (col. 3). The 

 values between 10 and 20//, inserted in the same column will 

 he referred to below. 



* F. Paschen, Wied. Ann. liii. p. 334 (1894). 



t H. Rubens, Bed. Akad. Ber. 1913, p. 513. 



J E. v. Bahr, Verh. d. D.phjs. Ges. xv. pp. 710,731 & 1150 (1913). 



5 F. Paschen, Wied. Ann. Hi. p. 215 (1894). 



