Nov, 1931] FOODS IN A COLLEGE COMMUNITY 11 



pinge on the surface of the glass chimney without danger, a visible de- 

 posit of carbon is undesirable, and especially toward the end of the com- 

 bustion the canying of bits or flakes of unbumed carbon out of the 

 chamber by the air current must be avoided. The spirometer, S, should 

 be carefully counterpoised, and one should occasionally wipe the inside 

 and the outside of the bell with a lightly oiled rag. to minimize the ad- 

 herence of water to the metal. 



The essential features of this modification of the spirometer type 

 of oxy-calorimeter are, therefore, the simplification of the combustion 

 chamber, particularly the base and the metallic tube below it, the sub- 

 vstitution of a metallic soda-lime container for the glass container, and 

 the provision for external cooling of this container. With these modifi- 

 cations, if care is exercised in keeping the initial and the final tempera- 

 tures of the closed system the same, extraordinarily uniform results can 

 be obtained in the various combustions, and the precision claimed for 

 the original model can not only be secured but with less difficulty. 



With the spirometer type of oxy-calorimeter provided with the cool- 

 ing device for accurate temperature control, as described in the preced- 

 ing pages, the calculation of the result of a combustion may be expressed 

 bj' the formula: 



_ (LXlvXM)— 5 

 ^(0°C., dry; 760 mm.) w 



in which V is the volume of oxygen consumed in the combustion of one 

 gram of an air-diy food, this volume being expressed in cubic centi- 

 meters and reduced to 0°C. (dry) and 760 mm. pressure; 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 (usually not 

 far from 21 c.c.) ; M is the factor for reduction of the apparent volume 

 to standard conditions of temperature and pressure. This factor M is 

 based upon the prevailing barometric pressure and the average temper- 

 ature of the spirometer and is found by referring to the standard tables 

 published by Carpenter,^ the air in the apparatus being considered com- 

 pletely saturated. The minus correction of 5 represents the cubic centi- 

 meters of oxygen consumed by the ignition of the iron wire. This re- 

 duced volume of oxygen consumed per gram of air-dry matter must be 

 con-ected for the amount of nitrogen liberated per gram of air-dr>' sub- 

 stance during the combustion. Since the percentage of nitrogen in the 

 food sample under consideration is determined by the Kjeldahl method 

 and since it is known that one milligram of nitrogen occupies 0.8 c.c. 

 under standard conditions of temperature and pressure, one can readily 

 compute the total volume of nitrogen liberated in the combustion of 

 one gram of the air-dry substance. The reduced volume of oxygen per 

 gram of air-dry material should be increased by this volume of nitrogen 

 liberated, and this i-esult, converted from cubic centimeters to liters, 



^Carpenter, T. M., Carnegie Inst. Wash. Pub. No. 303A, 1924, tables 7 and 

 8, pp. 39 to 70. 



