METHODS OF INVESTIGATION. 69 
the proteid metabolism is now almost exclusively expressed in 
terms of either nitrogen or protein. 
The Gain or Loss of Fat.—As the balance between income 
and outgo of nitrogen serves to measure the gain or loss of protein 
by the schematic body, so the balance between income and outgo 
of carbon furnishes the means for estimating the gain or loss of fat. 
The income of carbon is, of course, the carbon of the food. 
The outgo of carbon consists of— 
First, the carbon of the undigested food contained in the feces. 
Second, the carbon of the products of metabolism contained 
in feces, urine, and perspiration. 
Third, the carbon of the gaseous excreta, including the carbon 
dioxide given off by the lungs and skin and the carbon dioxide and 
hydrocarbons resulting from fermentations in the digestive tract. 
RESPIRATION APPARATUS.—The carbon of the visible excreta, 
is readily determined by the ordinary analytical methods. The 
determination of the carbon of the gaseous excreta requires the use 
of a special apparatus, commonly called a respiration apparatus. 
In early experiments upon respiration the animal was simply 
placed in a known confined volume of air which was analyzed before 
and after the experiment. By this method, however, the oxygen 
of the air is progressively diminished, while the respiratory products 
accumulate, both of which conditions are liable to disturb the 
normal respiratory exchange, although Kaufmann,* who has re- 
cently reverted to this primitive method, claims to have secured 
accurate results in rather short experiments. 
The obvious desirability of renewing the oxygen and removing 
the products of respiration soon led to the construction of more 
complicated forms of apparatus of which three principal types 
may be distinguished. 
The Regnault Apparatus.—The oldest of these is the Regnault ¢ 
or closed circuit respiration apparatus. In this type of apparatus 
the subject breathes in a confined volume of air, the carbon dioxide 
being removed by suitable absorbents and weighed, while the oxy- 
gen consumed is replaced from a receiver containing pure oxygen, 
the amount admitted to the apparatus being measured. These 
* Archives de Physiol., 1896, p. 329. ; 
+Regnault & Reiset, Ann. de Chim. et de Physique, 3d series, 26, 299. 
