506 Infra-red and Ultra-violet Absorption of Sulphur Dioxide,. 



dioxide. The apparatus employed consisted of a quartz- 

 ended glass cell of special construction connected by glass 

 tubing to a reservoir containing the dry gas and to a 

 mercury manometer. As the light source, the condensed 

 spark between nickel electrodes was used and the absorptive 

 power was determined by photography with the help of a 

 quartz spectrograph and a Hilger rotating sector photo- 

 meter. 



As is well known, the broad absorption band exhibited by 

 sulphur dioxide in the liquid and solution phases is resolved: 

 into a number of small band groups which, at any rate in 

 the less refrangible ultra-violet region, are symmetrically 

 distributed around a central band group of maximum 

 absorption. By varying the pressure and the thickness of 

 the absorption layer of the gas, the temperature being kept 

 constant, I have been able to trace out the whole of the 

 complex absorption and to determine the molecular extinction 

 coefficient for each component band group. 



In the first place, it may be noted that series of constant 

 frequency differences occur between the centres of the band 

 groups, and, although with the relatively small dispersive 

 power of the spectrograph it was found to be a matter of some 

 difficulty exactly to determine the centre of any one band 

 group, yet the averaging of these frequency differences has 

 enabled me to arrive at their real values with considerable 

 accuracy. 



In the second place, the whole region of absorption can 

 be divided into three portions, namely, a less refrangible^ 

 portion in which the average difference between the wave- 

 numbers of the centres of the consecutive bands is 22*4, 

 an extreme ultra-violet region where the average wave- 

 number difference is about 35, and an intermediate region 

 where combinations of these differences occur together with, 

 a third difference of about 12 *. 



In the following tables are given the wave-lengths and 

 wave-numbers of the centres of the small band groups 

 together with the differences. In Table III. the numbers 

 (1), (2), (3) after the differences refer to the values 22*4, 

 35, and 12 respectively. The molecular extinction coeffi- 

 cients are given at the more important points. 



* For the sake of convenience wave-numbers (reciprocals of wave- 

 lengths) are used, expressed in four figures; that is to say, the number 

 of waves in 1 mm. 



