200 NUTRITION OF FARM ANIMALS 



care and skill exercised in the choice of the check animal. The as- 

 sumed identity of composition of the two animals cannot in the nature 

 of things be proved and is very unlikely to be absolute. In a short 

 experiment, therefore, the error thus possibly introduced may be rela- 

 tively large. Its importance diminishes the greater the increase 

 made over the original weight, i.e., the longer the period covered 

 by the experiment. Furthermore, an experiment by this method 

 can be divided into periods only by the use of additional check 

 animals, involving additional assumptions as to identity of compo- 

 sition at different times, while even these subdivisions, for the reason 

 just stated, must be fairly long. Finally, the method is labori- 

 ous, especially with the larger animals. The different parts of the 

 carcass must be separated, the weight of each part accurately deter- 

 mined, avoiding mechanical losses and making due allowance for 

 evaporation of water. A correct sample of each part must be 

 taken promptly and at once so treated as to preclude any changes 

 previous to its analysis. The task of analyzing the carcass of a 

 hog or sheep, and still more that of a steer, with the degree of 

 accuracy required in -a scientific investigation is not one to be un- 

 dertaken lightly. 



285. The balance experiment. The comparative slaughter 

 test attempts to determine the weights of the several ingredients 

 contained in the body at two different times. The balance 

 experiment, on the contrary, consists of a comparison of in- 

 come and outgo and does not attempt to determine the original 

 stock in the body. If I know that I have a balance of $50 in 

 bank at the beginning of the month and $150 at the end, it is 

 clear that I have gained $100 in the meantime. This is the 

 principle of the comparative slaughter test. On the other hand, 

 if I know that my deposits during the month were $500 and my 

 drafts $400, I am equally sure that I have gained $100, even 

 if I do not know whether my balance at the beginning was $50 

 or $500. This is the principle of the balance experiment. If, 

 for example, a steer digests 750 grams of protein out of his daily 

 ration and if the amount of nitrogenous products excreted in 

 24 hours shows that he has katabolized 500 grams of protein, it 

 is evident that his original stock of protein, whatever its amount 

 may have been, has been increased by 250 grams. By compari- 

 sons based on the same general principle, although more com- 

 plicated as to details, the increase or decrease of the body's 

 stock of fat, glycogen, ash and water or of chemical energy may 



