12 NEW HAMPSHIRE EXPERIMENT STATION [Bull. 242 



around the rod, standing up in the center of the crucible. This makes 

 it possible for the oxygen-rich air to come in contact with all of the 

 sample. More satisfactory and consistent results can be obtained by 

 regulating the rate of circulation in the closed system of the oxy- 

 calorimeter with an external resistance governing the speed of the 

 motor. 



Feces were not analyzed in this research. The feces of both hu- 

 mans and animals were, however, studied in the original development 

 of the apparatus 1 and require no different preparation for combus- 

 tion from that described here for foods. 



CALCULATION OF RESULTS 



The volume of oxygen (reduced to 0° C, dry, and 760 mm. pres- 

 sure) consumed in the combustion of one gram of an air-dry food is 

 calculated by the formula: 



LxKxFxM 



V= 



W 



in which V is the reduced volume of oxygen per gram of air-dry 

 matter, expressed in cubic centimeters; W is the air-dry weight of 

 the substance in grams; L is the change in level of the spirometer 

 bell, expressed in millimeters; K is the apparent volume of the bell 

 in cubic centimeters per millimeter of its length (in our particular 

 apparatus 20.80 c. c.) ; F is the constant correction factors 1.0112 

 which was obtained by the standardization tests with cane sugar 

 (see page 9); and M is the factor for reduction of the apparent 

 volume to standard conditions of 0° C, dry, and 760 mm. pressiire. 

 This last factor, M, is based upon the prevailing barometric pressure 

 and the average temperature of the spirometer. The value of M is 

 found by reference to the standard tables published by Carpenter, 3 

 the air in the apparatus being considered completely saturated. 



The analysis of a chicken sandwich may be used as a : typical 

 illustration of the method of calculation. When 2.55 grams of the 

 air-dry material were burned, it was found that L, the change in 

 level of the spirometer bell, was 149 mm. Since the average tempera- 

 ture of the spirometer was 25.8° C. and the barometric pressure 

 was 762 mm., the value of M is 0.886. The formula then becomes 



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

 912; ibid., Journ. Biol. Chem., 1925, 66, p. 783. 



(2) In the initial work with the oxy-calorimeter, corrections were made 

 for the 5 c. c. of oxygen required for the ignition of the iron wire and for 

 the slight rise in temperature of the spirometer. These corrections are 

 eliminated by the use of the factor F. 



(3) Carpenter, T. M., Carnegie Inst. Wash. Pub. No. 303A, 1924, tables 7 

 and 8, pp. 39 to 70. The barometric pressure in millimeters, as recorded 

 across the top of these tables, represents the barometric reading corrected 

 for brass scale reading only and not for tension of aqueous vapor. The cor- 

 rection for tension of aqueous vapor is taken care of in the logarithms given 

 in the body of the tables. 



