1122 



THE LIGHT FACTOR. II. QUANTUM YIELD 



CHAP. 29 



tion in light can be eliminated by subtracting from the total gas exchange 

 in the two combined beams the gas exchange in one beam alone. (There 

 seems to be no difference between this method and calculation of 7 from the 

 difference in yield at two light intensities in light of the same color.) The 

 results of these measurements are shown in Table 29. V. The 7 values 



Table 29.V 



POLAROGRAPHIC DETERMINATION OF QUANTUM YiELDS OF Chlorella IN ReD 



AND Red plus Blue Light 

 (after INIoore, and Duggar 1949) 



Initial beam 



Added beam 



Abs. 



% 



Abs., 



% 



1/7 



6500 

 6500 

 4350 

 6500 

 6500 

 6500 



1958 

 783 

 1288 

 1610 

 1392 

 1188 



76 

 46 

 91 

 49 

 74 

 52 



0.074 



0.12 



0.09 



0.10 



0.10 



0.11 



4358 



1575 



100 



0.11 



9.1 



" Intensities in erg/cm.^ sec. 



for the superimposed beam (0.08 to 0.11) are not significantly different 

 from those for the single beam (0.074 to 0.12), a result that can be inter- 

 preted as indicating two things: approximate identity of quantum yields 

 in blue and in red light (despite the absorption of light by carotenoids in 

 the first-named region; cf. chapter 30), and approximate linearity of the 

 Hght curve up to the total intensity of the two combined beams. 



Several objections can be made (and have been made by Warburg) against the 

 polarographic quantum yield determinations: 



1. The determination of the number of absorbed photons was not satisfactory. 

 A thermopile was placed immediately behind the reaction cell ; the energy flux falling 

 onto the thermopile with and without the algae in the reaction vessel (illuminated by 

 parallel light) was multiplied by the ratio vessel area:thermopile area and the difference 

 between the two products was assumed to be the absorbed flux. This assumption im- 

 plies that scattering out of the beam intercepted by the thermopile is compensated by 

 scattering into this beam, and thus neglects large angle scattering. The error caused by 

 this could lead to too high a value for absorbed light energy, and hence to too low a 

 value for the quantum yield. 



2. The suspension was not stirred. (Stirring during measurements is impossible 

 by the nature of the method; stirring between measurements was attempted, but found 

 not to influence the results and was therefore abandoned). The algae did not settle 

 during a run; and stirring is obviously of less importance when oxygen determination is 

 made in the body of the liquid than when it is carried out in the gas phase above it. On 

 the other hand, carbon dioxide exhaustion conceivably could occur in the immediate 

 neighborhood of the cells, and cau.se a diminution of the quantum yield. However, this 

 danger should not be serious when measurements are made at or below the compensation 

 point. 



