280 BIOLOGICAL EFFECTS OF RADIATION 



to immerse the thermopile in a water-tight box provided with a (quartz) 

 window and submerged in the thermostat directly behind the reaction 

 vessel. 



Calculations. — The total number of moles reacting is determined 

 by chemical or physical measurements. The number of moles reacting 

 is then multiplied by the Avogadro number 6.06 X 10^^ in order to 

 determine the number of molecules reacting. The number of ergs 

 absorbed by the reacting mixture is determined with the calibrated 

 thermopile and the number of seconds is recorded during which the 

 radiation is absorbed. Next, the number of ergs in one quantum is 

 calculated and this quantity is divided into the total number of ergs 

 absorbed. The quotient gives the number of quanta absorbed. In 

 calculating the number of ergs in one quantum the wave-length of light 

 is expressed in centimeters and divided into the velocity of light 

 (3 X 10^° cm.) in order to obtain the frequency of light. This frequency 

 of light is then multiplied by Planck's constant h (6.55 X 10"^^ erg-sec). 

 Having determined the number of quanta absorbed, and the number of 

 molecules reacting, the quantum yield $ is calculated. It is the number 

 of molecules reacting divided by the number of quanta absorbed. A 

 typical example illustrating the calculation of the quantum yield is 

 given below. 



Wave-length = 4360A. 



Bromine reacting = 0.0054 X 10~'' mole 



Absorbed-light intensity = 12.92 X 10^ ergs/sec. 



Time of exposure = 895 sec. 



^ , , . u I J 12.92 X 10'^ X 895 X 4.36 X 10-^ 

 iotal quanta absorbed = fi g;g; v 10-" v '^ v inio — ^ 



2.57 X lO's 

 Molecules reacting = 0.0054 X 10-^ X 6.06 X lO^^ = 3.27 X lO^^ 

 _ molecules reacting _ 3.27 X 10^* _ „ 

 quanta absorbed 2.57 X 10^^ 

 With the best technique now available quantum yields in the neighbor- 

 hood of unity can be determined with a precision of about 1 per cent. 

 The absolute values of the radiation standard are probably known with 

 an accuracy no greater than two or three per cent and in general an 

 accuracy within 0.05 is about all that can be expected at present in the 

 determination of $. Usually the accuracy of the chemical analysis 

 constitutes the limiting factor. Sometimes the determination of the 

 quantum yield is a very difficult matter and although it is always desir- 

 able, it is oftentimes unnecessary for a particular purpose and the time 

 is not worth the expenditure of effort which it entails. The importance 

 of determining the value of the quantum yield lies in the fact that deduc- 

 tions may often be made concerning the primary and secondary processes. 

 If the quantum yield is unity, the chances are that the primary process 



