






aes} . ‘ Py. wet Lt Sty tbe FO 
, sae i 2 A r ans t--i-- 7 = i ae Ml 
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EFFECT OF -NITROGENOUS~FERTILIZERS. 205 
In previous reports the distinction between “ heats of com- 
bustion ” and “ fuel values ” has not been explained as clearly 
as seems desirable. As the terms are here used the heat of 
combustion of a given food material is the total energy of that 
material as determined by burning a given amount of it with 
oxygen in the bomb calorimeter; while the fuel value of the 
same material is the energy which can become available to the 
body when the material is eaten. That is to say, the fuel 
value is the total heat of combustion of the food less that of the 
corresponding feces and urine. 
In the experiments here reported the heats of combustion 
of the food and of the feces were determined by the bomb calor- 
imeter, and that of the urine was estimated; the difference be- 
tween the first and the sum of the last two was taken as the 
measure of the energy of the food digested, i. ¢., the fuel value 
of the food eaten.* | 
The nitrogenous matter of the digested food.is not com- 
_ pletely oxidized in the body, but a portion is eliminated in the 
urine as urea and kindred compounds. The potential energy 
of these compounds is not available to the body, and is de- 
ducted from the energy of the material digested to give the 
amount of energy available from any given material. The 
amount of energy in the compounds in urine is roughly calcu- 
lated in the manner described on page 178 of the Annual Re- 
port of this Station for 1896, in the discussion of digestion ex- 
periments with men. The assumptions made there give re- | 
sults that are probably rather too low. Late research seems to 
indicate that a larger factor should be assigned to the fuel value 
of the nitrogenous matter of the urine. 
_ General conclusions from these experiments will hardly 
be permissible until more data are available. One point, how- 
ever, is brought out very clearly. Among the feeding stuffs 
tested those rich in protein, such as the legumes, are more di- 
gestible than those with little protein, such as corn fodder, 
oat fodder, millet, and the like. 
Table 40, which follows, gives a summary of the results of 
the digestion experiments thus far made with sheep by the 
Station. These experiments are arranged, according to the 
character of the feeding stuffs used, under the headings: Mill- 
ing products (with hay), cured fodders and hays, and green 
fodders and grasses. 

* See Report of the Station for 1897, pages 155, 156. 
