METHODS OF INVESTIGATION. 249 
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, a description 
of which would occupy too much space here, 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. In 
test experiments the apparatus has given extremely accurate re- 
sults. 
An especial advantage of this apparatus is that it is practicable 
to make it of large size, and also to continue the experiments for an 
indefinite length of time. The original apparatus was of a size 
sufficient for experiments on man, while all previous forms were 
restricted to experiments on small animals. Recently a modified 
Atwater-Rosa apparatus has been completed under the writer’s 
direction at the Pennsylvania Experiment Station, with the co- 
operation of the Bureau of Animal Industry of the United States 
Department of Agriculture, of a size sufficient for investigations 
with cattle, and still larger ones are in process of construction. 
ComputaTIon oF Heat Propuction.—The respiration-calorim- 
eter, in its more perfected forms, is a complicated and costly appara- 
tus both in construction and use, and, moreover, is a rather recent 
development. It was natural, therefore, that attempts should be 
made to determine the heat production indirectly by computations 
based on the kind and amount of matter oxidized in the body. 
We may conveniently distinguish three distinct although. closely 
related methods of attacking the problem, all of which assume as a 
fundamental postulate that the oxidation of a given substance in 
the body liberates the same amount of energy as does its oxidation 
outside the organism. In the next chapter we shall examine into 
the correctness of this postulate; for the present we are con- 
cerned simply with the methods of computation based on it. 
Computation from Gaseous Exchange.—From a knowledge of the 
ultimate composition and heat of combustion of a substance it is 
easy to compute the amount of heat which will be produced by the 
oxidation of an amount of it sufficient to yield a unit of carbon 
dioxide or to consume a unit of oxygen. Conversely, then, we can 
compute from the carbon dioxide evolved or the oxygen consumed 
in a given time the corresponding amount of energy liberated. 
