214 THE FEEDING OF ANIMALS 



measuring actual energy production. The older forms of 

 respiration apparatuses simply allowed an estimation of 

 the carbon dioxid and water given off by the animal. 

 How much of the water was formed by the oxidation of 

 the hydrogen of the food and how much was simply 

 evaporated from the store taken in as water, it was 

 impossible to know by direct determination. This could 

 only be calculated. The carbon dioxid was, on the other 

 hand, a direct and accurate measure of the combustion 

 of carbon. Later devices, as, for instance, the one used 

 by Zuntz, allow a direct determination, not only of the 

 products of combustion, but of the oxygen absorbed by 

 breathing. This method of work seemed to have advan- 

 tages, as one measurement not only checks the other, but 

 makes it possible to ascertain the actual oxygen consump- 

 tion during any given period of the experiment, as, for 

 instance, when the animal is at rest, when masticating 

 food, or when performing a given amount of external 

 work. In this way, Zuntz made his masterly demon- 

 strations of the differences in oxygen use with different 

 foods during the period of mastication. 



300. Respiration calorimeter. None of the older 

 apparatuses, whether allowing the determination of 

 oxygen consumption or not, measured the heat radia- 

 tion from the animal body, or, in other words, the amount 

 of energy actually evolved from internal combustion. 

 Professors Atwater and Rosa first devised a respiration 

 apparatus which was at the same time a calorimeter. 

 The quantity of heat radiated from a man or other 

 animal confined in this calorimeter is absorbed by a 

 known volume of water and is thus determined. This 

 is a great advance towards certainty, because direct 

 measurements of the energy production of a ration are 



