Ultra-violet Absorption System of Sulphur Dioxide. 513 



Owing to the great dispersion used, Miss Lowafcer was 

 able to measure the wave-lengths or! the component absorp- 

 tion lines with great accuracy, and it should at once be 

 possible to arrive at far more accurate values of the basis 

 constants from her numbers now that the central wave- 

 number of the less refrangible band is known. From a 

 knowledge of these it should also be possible to calculate 

 the wave-number of every single component line in the 

 absorption band group, and thus put the theory brought 

 forward by one of us in previous papers * to a far more 

 rigid test than has yet been possible with the compounds 

 already studied. Owing to the very great accuracy of 

 Miss Lo water's measurements the wave-numbers in the 

 following pages will be expressed in five figures, and, 

 moreover, note must be made of the fact that her values 

 were obtained in air and were not reduced to vacuum. 

 This correction is necessary before any calculations from 

 them can be made. 



It was not possible to apply the general theory to sulphur 

 dioxide until an accurate determination of the central 

 wave-number of the ultra-violet band group had been made. 

 As stated in the preceding paper, it has now been found 

 that this central line lies at \ = 2961 or l/\ = 33780, but it 

 must be remembered that the accuracy of this measurement 

 is far below that obtained by Miss Lowater. There are 

 three lines of maximum intensity observed by her near this 

 position, any one of which might be the true central line. 

 Their wave-numbers in air are 33770*3, 33767*2, 33761*2, 

 and the question arises as to which one of these should be 

 selected. The whole essence of the Bjerrum conception lies 

 in the fact that the component absorption lines in any one 

 band group lie symmetrically distributed about the central 

 line, and it is a matter of simple calculation to prove that 

 the greatest symmetry is obtained when the wave-number 

 -33761*2 is taken as centre. This number when reduced to 

 vacuum becomes 33751*6, and this may be taken as the true 

 value for the centre of the less refrangible ultra-violet 

 absorption band of sulphur dioxide. 



The relationships found by Garrett may now be dealt 

 with, since the central wave-number is known and the basis 

 constants calculated. The wave-number of the most impor- 

 tant absorption band in the short wave infra-red region of 

 sulphur dioxide is given by 33751-6/25 = 135006, corre- 

 sponding to a wave-length of 7*407 /j.. Again, the least 



* Phil. Mag. xxvii. p. 632 (1914); xxix. p. 223, xxx. p. 510 (1915). 

 Phil. Mag. S. 6. Vol. 31. No. 186. June 1916. 2 M 



