THE RESPIRATION CALORIMETER. 1045 



INTERPRETATION Or RESULTS. 



A fccdiiio- experiment conducted with the aid of the respiration 

 calorimeter is not fundamentally different from one made according 

 to simpler and more familiar methods. In ])oth cases we attempt to 

 compare the results obtained, either from two or more rations under 

 identical conditions or from identical rations under differing but con- 

 trolled conditions. The difference lies in the extent to which we are 

 able to control the conditions and in the accuracy and minuteness with 

 which it is possible to compare the rations and their results. 



The simplest and most obvious form of feeding experiment is that 

 in which the amounts of feed consumed are noted and their effects 

 measured by the increase in the live or dressed weight of the animal 

 or by the weight of milk or wool produced. This method, when skill- 

 fully carried out with a considerable number of animals and under the 

 conditions of actual practice, is particularly adapted, and indeed may 

 be said to be indispensable, to the study of the economic aspects of 

 stock feeding. 



But while this is true, it is also the fact that no considera])le or pro- 

 found knowledge of the principles of feeding can be gained by means 

 of experiments of this class. The factors entering into the prol)lem 

 are too complex. Chennstry has shown that each one of the feeding 

 stuffs consumed consists of a great variety of substances- -useful, indif- 

 ferent, and even injurious — mingled in the most diverse and varying 

 proportions, wdiile physiological investigation has demonstrated not 

 only the considerable and irregular fluctuation of live weight from day 

 to day, but especially that a given increase or decrease may be of very 

 varying significance according as it consists of proteid tissue, fat, min- 

 eral matter, or simply water. The result of a live-weight experiment, 

 therefore, may be the resultant of any one of many possible combina- 

 tions of these factors, and no safe conclusion as to its actual cause is 

 usually possible. The history of this class of experiments amply cor- 

 roborates this conclusion. Great accumulations of experimental data 

 have been made, but relatively few general conclusions have issued 

 from them. 



The earliest step in advance was the attempt to separate the factor 

 "food" into its elements. Of these attempts the one which has 

 secured general acceptance is the familiar one of Henneberg which 

 groups the chemical ingredients of feeding stuffs into "protein," 

 "carbohydrates," "fat," and "ash," subdividing the carbohydratCvS 

 into "crude fiber" and "nitrogen-free extract," and distinguishing 

 further between the digestible and the indigestible portions of each 

 group. A great mass of investigation along these lines in the labora- 

 tory and digestion stall has materially enlarged our knowledge of 

 foedino- stuffs, althou<>-h much still remains to be done. It is now a 

 comparatively easy matter, by the familiar methods of the digestion 



