Quantitative Determination of Heat of Combustion. 1 37 



decimetre per hour the error in the increase in dry weight obtained 

 by the half leaf method in an experiment lasting 5 hours would be 

 as much as 100%, whereas the error in the results obtained by 

 measuring the carbon dioxide absorption would amount to no more 

 than 2%. They therefore reject Sachs' method as quite untrust- 

 worthy. 



As the dry weight method, if it could be made sufficiently 

 accurate, would have its uses we agree with Thoday " that it should 

 not lightly be abandoned." Thoday makes some useful suggestions 

 in regard to decreasing the inaccuracy of the method, but he does 

 not furnish data which enable one to determine the degree of 

 accuracy obtainable when all suggested precautions are taken. It 

 appears to us that the only way of finding this is to make a number 

 of such estimations and determine the probable error of the mean 

 result. 



C. The Quantitative Determination of the 

 Heat of Combustion of the Products of Assimilation. 



Although the measurement of heats of combustion offers no 

 particular difficulties, very few such measurements have been made in 

 plant physiology. Brown and Escombe assume that the heat of 

 combustion of the products of assimilation is the same as that of 

 glucose, but this assumption is not justified by the values obtained by 

 experiment for the heat of combustion of one gram of material pro- 

 duced in assimilation. It will be seen however from the heats of 

 combustionof various substances recorded in the accompanying table, 

 that measurements of the actual heats of combustion of the products 

 of assimilation mijjht afford helpful information as to the relative 

 proportion of the different products. 



Table XXX IX. 

 Heats of Combustion in Gram-Calories of Various Substances. 



Substance ' "> Heat of Combustion per gram. 



Ethyl Alcohol 7-18x10^ 



Glucose 3-76x10' 



Sucrose 3-99x10' 



Dextrin 4-i x 10' 



Starch 4-1 x 10' 



Cellulose 4-2 x 10' 



Leucin 6-5 x 10' 



Vitellin 5-7 x 10' 



Linseed Oil 9-47x10' 



Olive Oil 9-51x10' 



