5X6 Prof. E. C. 0. Baly on 



it does not evidence itself with any definiteness in benzene 

 derivatives. It is most probable that this band and the re- 

 mainder of less intensity are due to multiples of the basis 

 constants which happen to fall near together, with the result 

 that their effectiveness as absorbers is enhanced. Thus 

 10x10-125 gives \= 9*88, and 2S x 37 = 9-65 /*, and the 

 mean of these is 9*76 yit. 



Added to this, the other two bases give bands in the imme- 

 diate neighbourhood which tend still further to increase the 

 intensity. All the remaining bands can be accounted for at 

 once in the same way by the fact that multiples of two of 

 the bases happen to be near together, and it is interesting 

 to note that when such multiples do not happen to be very 

 close, a weak and very broad band is shown. 



It is evident from the above that no further absorption 

 bands of any importance can be exhibited by benzene of 

 shorter wave-length than 1 //<, except at some multiple of the 

 least common multiple of any two of the basis constants. 

 If, therefore, the approximate position of the ultra-violet 

 band group of benzene be known, it should now be possible 

 to calculate every single absorption line in such group on 

 Bjerrum's principle, for all the necessary data are at hand. 

 It is peculiarly simple in this case because from previous 

 knowledge it is clear that the central line of the ultra-violet 

 band group is 10 X 405 (the lowest common multiple of 4'0 

 and 10*125). The wave-numbers therefore of the absorption 

 lines constituting the group can be calculated from 



4050 + ^x3*7, 4050 + nx 4-0, 4050 + nx7'6, and 

 4050 + 72x10-125. 



From the wave-numbers obtained in this way the wave- 

 lengths can be calculated, but in order to compare them 

 with observed measurements they must be corrected for the 

 refractive index of air. 



I have calculated the wave-lengths of the absorption lines 

 of benzene from the above formulae, and there are about 

 600 between A, = 2761 and A.=2226 which are the limits 

 experimentally observed by Hartley. Hartley measured the 

 wave-lengths of about 300 lines in the band, and the agree- 

 ment between the calculated and observed values is very 

 good. In Table I. are shown the results for the first few 

 lines on both sides of the central line together w r ith the 

 values observed by Hartley* and by Grebe |. 



* Phil. Trans, cviii. A. p. 475 (1908). 

 t Zeit. wiss. Phot. ix. p. 130 (1911). 



