DETERMINATION OF ENERGY STORED BY PLANT 233 



tion, I, produces a deflection D of the Radiometer, the 

 transmitted radiation, T, producing a smaller deflection d. 



Coefficient of transmission = =- . . • • (3) 



„ absorption =1—— . . . (4) 



Comparison of the independent determinations of the 



coefficient of transmission -i- and =- given in the equations 



(1) and (3) affords means of testing the reliability of either 

 method. Experimental results will be presently given which 

 show the great reliability of each of the two methods. 



Determination of Energy stored by the Plant 



The exact value of this is obtained from the number of 

 bubbles given out by the Bubbler (with its known constant) 

 whilst the plant is exposed to light for three minutes, during 

 which the energy absorbed by the plant is being measured 

 by the Calorimeter. We calculate the weight of this volume 

 of oxygen and multiply it by the carbohydrate-factor to 

 obtain the quantity of carbohydrate that had been formed 

 during exposure to light. The energy stored is converted 

 into heat-units by multiplying the quantity of carbohydrate 

 by its heat of combustion in calories per gramme. 



In the determinations of the stored energy made by 

 previous observers there has been much uncertainty in the 

 values of the carbohydrate-factor and of the heat of com- 

 bustion of the product. In the present determination there 

 is, however, no such uncertainty. Specimens of plants were 

 taken for experiment from the pond exposed to the light of 

 the northern sky, which gave a very constant carbohydrate- 

 factor, the value of which indicates (p. 218) that glucose and 

 starch are formed in these specimens in about equal pro- 

 portion. It would therefore be better to take for the heat 



