METHODS OF INVESTIGATION. 243 
small. On a mixed diet containing any considerable proportion of 
vegetable matter, and particularly in the case of herbivorous ani- 
mals consuming large amounts of coarse fodders, only an actual 
determination of the heat of combustion can be depended upon. 
Since the feces of these animals contain a larger proportion of the 
indigestible lignin, etc., than does their food, the heat of combustion 
of the feces is correspondingly higher, but its actual value must 
obviously depend to a considerable degree on the character of the 
food. 
Combustible Gases.—Since it is impracticable to collect sepa- 
rately the combustible intestinal gases, we must of necessity com- 
pute the amount of potential energy carried off in these substances. 
This computation has been based on the amount of carbon con- 
tained in these gases, determined in the manner indicated on p. 72, 
upon the assumption that only methane (CH,) was present. It 
has been shown that this gas exists in considerable amounts in 
the digestive tract of herbivora, and it is probable that the above 
assumption is substantially accurate, although a small amount of 
hydrogen has been found by some observers. In experiments by 
Fries,* at the Pennsylvania Experiment Station, in which both 
the carbon and hydrogen of the combustible gases excreted by a 
steer consuming chiefly timothy hay were determined, the follow- 
ing ratios of hydrogen to carbon were obtained: 
Period A. 
First day, 1:2.900 
Second day, 1:2.916 
Period B. 
First day, 1:2.978 
Second day, 1:2.947 
Period C. 
ms First day, 1:2.899! 
Second day, 1:2.951 
Period D. 
First day, 1:3.051 
Second day, 1:3.096 
Average, 1:2.967 
Computed for CH,, 1:2.976 
* Proc. Soc. Prom. Ag. Sci., 1902. 
