May, 1929] FOODS IN A COLLEGE COMMUNITY 9 



A weighed amount of the food (approximately two grams), 

 which has previously been brought to an air-dry condition (See 

 page 11), is placed loosely in a small nickel crucible, and the crucible 

 is placed inside the combustion chamber, supported by three small 

 metal prongs in the base of the chamber. A fine iron wire is at- 

 tached to two insulated, vertical posts either side of the crucible, 

 and the central part of this wire rests upon the food sample. The 

 glass lamp chimney is placed over the crucible, the apparatus is 

 filled with oxygen, the motor is started, and the position and tempera- 

 ture of the spirometer bell are recorded. A current of electricity is 

 passed through the iron wire, which is immediately raised to in- 

 candescence, and the food is ignited. Since the air in the combustion 

 chamber is highly enriched with oxygen, i the combustion takes place 

 rapidly and completely. At the end of the combustion, the lamp 

 chimney is cooled with a damp cloth, and the final readings of the 

 position and temperature of the spirometer bell are taken. 



Standardization tests. The initial work with the oxy-calorimeter 

 was controlled by direct combustions with a standardized bomb calori- 

 meter. Pure organic substances, which burn readily, such as cane 

 sugar, benzoic acid, and salicylic acid, were selected for this purpose. 

 The volume of oxygen required in the combustion of a gram of each 

 of these substances was determined with the oxy-calorimeter, and 

 the actual amount of heat liberated per gram was found with the 

 bomb calorimeter. Thus the caloric value of oxygen (that is, the 

 calories per liter of oxygen required to burn the substance) was di- 

 rectly established and found to agree with theory. In addition to 

 these initial tests, the accuracy of the instrument was frequently con- 

 trolled during its use in this particular research by burning in it a 

 substance of known chemical composition, such as pure sugar. From 

 the chemical equation C-^Jl^.,0-^^-\-12 Og = 12 COj-j-ll H^O it can 

 be computed that each gram of cane sugar requires in its 

 combustion 785.5 c. c. of oxygen at standard conditions of tempera- 

 ture and pressure. In a series of combustions of cane sugar 

 made with the oxy-calorimeter at the beginning of this research it 

 was found that the apparatus recorded on the average a consumption 

 of 776.8 c. c. of oxygen per gram of cane sugar or 98.88 per cent of 

 theory.- The results of other combustions of cane sugar made during 

 the progress of the research, corrected by this amount, have shown 

 astonishingly close agreement with the theoretical value. An addi- 

 tive correction of 1.12 per cent was, therefore, made in all the values 

 for heats of combustion obtained with this apparatus. 



(1) The oxygen which we have used in this apparatus is that commonly 

 furnished in cylinders for ordinary acetylene welding- or cutting of steel by 

 modern methods. 



(2; See note 2, page 12. 



