THE ENERGETICS OF PHOTOSYNTHESIS 47 



The increase in the amount of gas is therefore 



Ml 

 /ou \ y / 



Furthermore 



As the manometer does not contain mercury but another fluid with a normal 

 pressure of P^ we find that 



(1 G ^ + 1 fo: \ 

 K /^^ ^^^ 



It follows from equation 7 that the amount of gas produced is proportional 

 to the change of pressure and is independent of the initial pressure P. The 

 expression between parentheses is a constant, called vessel constant A', so that 



.V = hK (8) 



Depending on the nature of the gas fOo or CO-j) produced, we distinguish 

 between vessel constants A'o„ and A'^o,; the absorption coefficients of these 

 gases are aQ„ = 0.038 and a^^y,, = 1.182 at 10° C. We therefore obtain the 

 equations 



xo„ = /zA'(>2 



The vessel constants are always positive and are expressed in mm-. The 

 sign of h is positive when gas is produced and negative when the gas is ab- 

 sorbed. The values for v can thus be either positive or negative. 



In measuring either respiration or photosynthesis we have to consider two 

 gases — O2 and CO., — instead of only one. The pressure change h is equal to 

 the sum of the changes in the partial pressures of both gases, so that 



h = ho, + //CO, (9) 



The A- values of both gases are 



xo, = ho,Ko, (10) 



>^co2 = /'cojA'coj (11) 



Further, we know that in photosynthesis 



7 = ^^^ (1) 



xo. 



From the equations 1, 9, 10 and 1 1 we can derive 



•^02 = h ^ , ^f. (1-^) 



AcOa I 7-^02 



, Ko^KcOi 



and xco-, = « 



Aco, 



+ Ao, (13) 



