NUTRITIVE VALUE OF FEEDING STUFFS 47 



ing the bowels. The less digestible matter in a feed, the lower is, 

 therefore, its value to animals. 



Second, there are certain losses through fermentations in the 

 paunch and intestines which result in the evolution of incompletely 

 oxidized gases that escape from the alimentary tract (p. 30). 



Third, incompletely oxidized protein substances are excreted as 

 urea, and the fuel value which they represent is, therefore, of no 

 value to animals. The total energy less that lost through these 

 three sources furnishes the available energy, or so-called fuel value 

 of the feed. This may be determined by means of the respiration 

 apparatus, or its improved form, the respiration calorimeter. 



The Respiration Calorimeter. The Pettenkofer respiration ap- 

 paratus was greatly improved by Atwater and Rosa by making the 

 respiration chamber into a calorimeter. The original apparatus 

 built at Wesleyan University, Connecticut, has been. further modi- 

 fied by Armsby and associates at the Pennsylvania station, where 

 an apparatus was built in 1898 by the Pennsylvania station, in 

 cooperation with the U. S. Department of Agriculture. This appa- 

 ratus is sufficiently large to allow of investigations with mature 

 cattle, and it is possible to continue the experiments for a con- 

 siderable length of time, if desired (Fig. 9). 



" The apparatus consists of a Pettenkofer respiration apparatus pro- 

 vided with special devices for the accurate measurement, sampling, and 

 analysis of the air-current. A current of cold water is led through copper- 

 absorbing pipes near the top of the respiration chamber and takes up the 

 heat given off by the subject. The volume of water used being measured, 

 and its temperature when entering and leaving being taken at frequent 

 intervals, the amount of heat brought out in the water-current is readily 

 calculated. To this is added the latent heat of the water-vapor brought out 

 in the ventilating air-current. By means of ingenious electrical devices, 

 . . . the temperature of the interior of the apparatus is kept constant, and 

 any loss of heat by radiation through the walls or in the air-current is 

 prevented." 2 



Trials with this apparatus have been conducted since about 

 1901, and the results have greatly enlarged our knowledge of nutri- 

 tive processes and the value of different feeding stuffs. The con- 

 duct of such trials involves an immense amount of chemical work 

 and calculations; during the actual experiments alone the services 

 of at least seven men are required, exclusive of the assistants in 

 charge of the feeding and collection of excreta. 3 



2 Armsby, " Principles of Animal Nutrition," p. 248. 



3 For a description of the Pennsylvania respiration calorimeter, see 

 U. S. Department of Agriculture Year Book, 1910, pp. 307 to 318; Ex- 

 periment Station Record, vol. 15, p. 1037. 



