290 BIOLOGICAL EFFECTS OF RADIATION 



as well, were contained in large reservoirs and mixed in a cylindrical 

 quartz cell, the pressure being determined with a glass gauge. Light 

 from a mercury- vapor lamp was passed through filters to make it approxi- 

 mately monochromatic, then passed through the reaction chamber and 

 on to a thermopile. 



The concentration of chlorine was determined by application of Beer's 

 law. Previous calibration and determination of the absorption coefficient 

 permitted a direct reading of the concentration of chlorine from the 

 galvanometer deflection. The same galvanometer deflection was used 

 also in determining the energy absorbed, and thus it was possible to 

 determine quantum yields directly from the galvanometer deflections 

 alone. This method for determining quantum yields is quite superior to 

 some of the methods used previously. There is no pressure change 

 during the reaction, and manometer measurements cannot be used for 

 following the course of this reaction. In one method water is added, and 

 the hydrogen chloride and the chlorine are absorbed. Both from pres- 

 sure changes and from the chemical titrations the course of the reaction 

 can then be followed. In other methods the gas mixture is frozen out 

 in an attached U-tube, and the pressure of the remaining hydrogen is 

 determined. These methods are less accurate and slower than the 

 direct determination in the cell by means of Beer's law. 



The primary process is the photodissociation of chlorine molecules 

 into chlorine atoms. At the shorter wave-lengths one of the atoms is in 

 an excited state. The chlorine atoms combine wdth hydrogen molecules, 

 forming hydrochloric acid and releasing hydrogen. A series of reactions 

 leading to a long chain is set up, as indicated by the following equations : 



1. CI2 + hv = CI + CI 



2. CI + H2 = HCl + H 



3. H + CI2 = HCl + CI, etc. 



4. H + HCl = H2 + CI 



5. CI + CI - CI2 



The retarding effect of hydrochloric acid and the influence of the 

 concentration of the hydrogen and chlorine, together with the fact that 

 the reaction rate depends upon the intensity of the light absorbed, can 

 all be expressed quantitatively by an empirical formula (44). 



The maximum quantum yield obtained was about 100,000 molecules 

 per quantum absorbed. The chains are continued until stopped by 

 some chemical reaction, as for example when a chlorine atom combines 

 with another chlorine atom to form a chlorine molecule. 



It has long been known that oxygen exerts a strong inhibiting effect 

 on this reaction. It apparently stops the reaction chains. It was found 

 that in oxygen-rich mixtures the quantum efficiency is independent of 

 the intensity of the absorbed light and varies inversely as the partial 

 pressure of the oxygen (40). 



