10 NEW HAMPSHIRE EXPERIMENT STATION [Bull. 242 



Nitrogen determinations. During the combustion of nitrogenous 

 substances there is a liberation of pure nitrogen. This nitrogen ac- 

 cumulates in the closed system and obviously affects the apparent 

 decrease in volume of the oxygen in the spirometer, since each cubic 

 centimeter of nitrogen liberated takes the place of a cubic centimeter 

 of oxygen which would otherwise pass from the spirometer into the 

 air current. Thus the apparent contraction in volume due to the 

 absorption of oxygen is too low and a correction is necessary. This 

 correction is arrived at by determining the nitrogen in the substance 

 to be burned. It is usually desirable to know the protein content of 

 a food as well as the caloric value. Hence total nitrogen determina- 

 tions by the Kjeldahl method have been included in our assessment of 

 food values. These determinations not only indicate the amount of 

 protein in the food but make it possible to correct the oxygen measure- 

 ment obtained with the oxy-calorimeter. Pure nitrogenous sub- 

 stances, such as urea, hippuric acid and uric acid, were burned in the 

 oxy-calorimeter, and the accuracy of the correction for nitrogen was 

 thoroughly established. 



Adaptability and accuracy of the apparatus. The adaptability 

 of the oxy-calorimeter for studying problems in nutrition, especially 

 with humans, is evidenced by the fact that the combustion chamber 

 can be applied to almost all of the respiration apparatus used at the 

 present day for determining basal metabolism. Tests made at the 

 Nutrition Laboratory with practically all of the various types of 

 respiration apparatus show that the combustion chamber is as well 

 adapted to any of them as to the particular type used in our experi- 

 ments. 



With the ordinary food mixtures eaten by man and animals, and 

 particularly with excreta, the difficulty of securing a truly character- 

 istic sample is so great that the accuracy of the oxy-calorimeter is 

 far inside of any possible limit of accuracy which could be expected 

 in the sampling. Indeed, the oxy-calorimeter has been recommended 

 for determining the energy value of industrial fuels, coals, and fuel 

 oil,i which for economic reasons require extremely close determina- 

 tions. 



PREPARATION OF SAMPLE FOR COMBUSTION 



Practically all of the foods eaten by man are too moist to burn 

 readily without previous preparation. All foods except candies, dry 

 cereals, crackers and the like must, therefore, be dried until they con- 

 tain not more than 20 per cent and usually nearer 10 per cent of 

 moisture. For this purpose the fresh food is accurately weighed in 



(1) Benedict, F. G., and E. L. Fox, Indus, and Eng. Chem., 1925, 17, p. 

 912. 



