268 RADIATION BIOLOGY 



for red light. Past researches have stressed the importance of determin- 

 ing the maximum yield, and it is unlikely that optimum conditions can 

 be maintained long enough to grow many grams of new plant material. 

 But more attention could well be paid to the direct weighing of the prod- 

 ucts of photosynthesis. 



Possibly, between the micro scale of the laboratory and the macro scale 

 of the "yield per acre" of agricultural crops, there may be a fruitful area 

 for experimental studies of photosynthetic efficiencies. The growth of 

 gram quantities of algae in tanks with suitable actinometers should be 

 attractive (Geoghegan, 1951). 



Oxygen is paramagnetic, whereas most other common gases are slightly 

 diamagnetic. This property has been utilized by PauHng et al. (1946) 

 for determining the oxygen concentration in a gas. A convenient and 

 practical instrument has been developed by Beckman in which gas is 

 passed through a chamber containing a light, dumbbell-shaped test body 

 which is suspended by a fine quartz fiber in a strong, nonhomogeneous 

 permanent magnetic field. A mirror attached to the fiber deflects a beam 

 of light onto photocells. Than an electronic circuit is affected in such a 

 way as to give a voltmeter reading that is directly proportional to the 

 oxygen concentration of the gas passing through the magnetic field. This 

 instrument has been found very satisfactory for measuring the oxygen 

 changes in a circulating-gas system that involves photosynthesis or respi- 

 ration. Its accuracy can be about the same as that of a Warburg manom- 

 eter, and the reading is specific for oxygen. 



The concentration of carbon dioxide in a gas stream can be determined 

 by infrared absorption. Carbon dioxide has a distinctive absorption 

 band at 4.3 m in the infrared, which was used by McAlister (1937) in 

 studies of photosynthesis. Evans (1951) has used this method for deter- 

 mining the quantum yield in photosynthesis.* This infrared absorption 

 method of analysis works very well for low concentrations of a few 

 hundredths or tenths of a per cent, but it calls for a good deal of care in 

 operating above 1 per cent carbon dioxide, which is the interesting range 

 for investigations of photosynthesis. This infrared method is valuable 

 because it is specific for carbon dioxide in a mixture with oxygen and 

 nitrogen. The water vapor can be removed with absorbents. 



The electrical conductance of a solution has been used as a measure of 

 the carbon dioxide present under certain conditions. This method has 

 been described and perfected by Wolf et al. (1952). 



The mass spectrometer offers a method for the simultaneous determi- 

 nation of oxygen and carbon dioxide. It has been developed and used 

 successfully by Brown et al. (1952). It can be used equally well in tracer 

 experiments with isotopes. 



* Added in proof: An infrared gas analyzer by Grubb Parsons gives good results 

 as reported by Yuan (1954, 1955). 



