32 Feeds and Feeding. 



gives only the digestibility of that nutrient. In using this method 

 it has been found that the digestibility varies with the length of 

 time the solution is allowed to act, the temperature maintained, 

 etc. It is therefore only fairly accurate, but is useful as a quick 

 method of determining the relative digestibility of the substances 

 under comparison, rather than an absolute one for general guid- 

 ance. G. Kiihn has shown that the results may be reached by 

 the use of pepsin solutions only. * 



53. Water extracts. — It has been found that the total quantity 

 of solid matter which can be extracted from a fodder by the use 

 of boiling water measures, with considerable accuracy, the quan- 

 tity of digestible nitrogen-free extract which it contains. Some 

 of the dissolved matter, however, is protein and ash, so that this 

 method is not directly accurate, but a guide rather, for practical 



purposes. 



n. Respiration Studies. 



54. The respiration apparatus. — A "respiration apparatus" 

 is a device for measuring the products of respiration of animals 

 confined therein. Its leading feature is an air-tight chamber in 

 which the animal experimented on is confined, with an inlet for 

 supplying fresh air and an outlet for drawing off the air with its 

 respiration products. The air entering the chamber as well as 

 the gases drawn from it are measured and analyzed, to determine 

 the products given off by the subject on trial. The most com- 

 plete and satisfactory forms of the respiration apparatus are those 

 constructed according to the plan of Pettenkofer of the Physio- 

 logical Institute at Miinich, the first of which was built through 

 the munificence of the King of Bavaria. There was recently 

 constructed at the Wesleyan University, Connecticut, by the 

 co-operation of the Storrs (Connecticut) Experiment Station 

 and the United States Department of Agriculture, Washington, 

 a very complete respiration apparatus especially designed for 

 the study of human nutrition. ' 



In conducting experiments, animals or men, or both, are placed 



in the respiration chamber. In some cases labor is performed; 



» Landw. Vers. Sta., 44. pp. 188-256. 



» For a popular description of this apparatus and some of the results 

 obtained, see Century Magazine, June, 1897. 



