ANIMAL CALORIMETRY 29 



The various sources of error clue to faulty technique have been 

 gradually eliminated, and the resultant calorimeters that bear 

 the names of Atwater, Rosa, and Benedict and that of Williams 

 produce results that are sufficient to convince even the most 

 sceptical of honest observers that the oxidation of assimilated 

 foodstuffs in the living body produces the same evolution of 

 energy as they would if burned in the bomb calorimeter, provided 

 the end-products are identical. 



The direct method is not of such general use as the indirect. 

 Study of the papers from the Carnegie Institute of Washington 

 or of those from Cornell University makes clear the complexity 

 of the machine and the intricacy of its manipulation. The cost, 

 except for the smallest Williams' boxes, is prohibitive. The 

 apparatus can be much simplified if the direct estimation of the 

 energy-changes is omitted and the observer confines himself to 

 measuring the respiratory gases and the urinary output. 



(b) Indirect. As we have seen, the basis of this method also 

 was laid by Crawford. It depends upon the following established 

 facts : 



(I) The quantity of energy liberated depends on the chemical 

 composition of the food used. 



(II) The quantity of oxygen absorbed depends also on the 

 chemical composition of the food used ; therefore, 



(III) There must be some relation between the energy evolved 

 and the quantity of oxygen absorbed. 



(IV) The three proximate principles of food differ markedly 

 in chemical composition, (a) Proteins contain nitrogen, which 

 is eliminated almost entirely in the urine, (b) Carbohydrates 

 and fats differ widely in their proportion of O to C. 



(V) If a determination were made of the amount of heat and 

 the amount of C and O 2 which corresponds to each gram of 

 urinary nitrogen, one could, from the nitrogen excreted, calculate 

 the heat liberated from the protein of the diet. (1 gram of 

 urinary nitrogen=26-51 cals.) Having deducted the protein O 2 

 from the total O 2 absorbed and the protein CO 2 from the total 

 CO 2 eliminated, one arrives at the figures corresponding to the 

 non-protein O 2 and CO 2 respectively. 



(VI) Now, as we have said, carbohydrates differ from fats in 

 their respective contents of C and O 2 . Carbohydrates have the 

 general formula *(CH 2 O), while fats contain less O 2 compared 

 with their content of oxidisable matter, e.g. a?(C 9 H 12 O). There- 

 fore, when carbohydrates alone are used, the ratio of the volume 



