STERILIZATION OF WATER WITH ULTRAVIOLET RADIATION 165 



energy changes involved in the vibrational changes may be compara- 

 tively small and do not enter into the picture; hence, E v0 is very small. 

 Therefore, for some one electron change from its lowest energy state 

 n = to some higher energy state n = 1, the frequency of the absorbed 

 energy is 



v = v e + B ± 2Br + Cr 2 



where v e is due to an electronic change, B and C are constants for the 

 band, and r takes successive values of 0, 1, 2, 3, etc. Thus each line in 

 such a band corresponds to the same electronic shift and to the same 

 vibrational shift. If this relation is plotted for various values of r a 

 parabola of two branches is obtained. The theoretical band for a dia- 

 tomic molecule has the appearance of possessing a sharp edge at the 

 long-wavelength end and gradually decreasing in intensity towards the 

 short-wavelength end. Somewhat similar properties are exhibited by 

 the experimental absorption bands previously discussed. 



Instead of computing the theoretical energy values and from them the 

 spectral frequencies, the problem usually is the converse, viz., obtaining 

 the energy values from the experimentally determined spectrum and 

 trying to arrive at their theoretical significance through an equation 

 similar to that developed above. 



The observed facts, however, have far outstripped our understanding 

 of the mechanism of absorption. Although only a superficial elementary 

 analysis of electronic band spectra has been given, it should be sufficient 

 to enable the reader to appreciate the difficulties and the direction in 

 which progress in this field is now being made. 



Sterilization of Water with Ultraviolet Radiation 



In order to appreciate the limitations set by water, either as a solvent 

 or for immersion, on the lethal action of ultraviolet radiation, it is 

 necessary to examine the transmission factors of water for ultraviolet 

 spectral radiation. Information on the absorption characteristics of 

 water in the ultraviolet is meager. 



o 



In general it is known that an absorption band exists at 6000 A with 



o 



probably two weaker bands at 6500 and 5200 A. These give to water 

 the predominant blue color when viewed through thick layers. 



In the ultraviolet the absorption has been traced from 3000 A to its 



o 



opaque limit just below 1800 A. These results are summarized in 

 Fig. IV-20. This curve shows why water becomes opaque near 1800 A. 



o 



Since midsummer sunlight is limited at 2950 A by the opacity of the 

 atmosphere, it becomes apparent that sunlight cannot exert a lethal 

 action on bacteria immersed in water at any great depth. To sterilize 



