430 PRINCIPLES OF ANIMAL NUTRITION. 
can give only mixed results varying with the quantities com- 
pared. 
It seems tolerably clear, then, that the whole subject of the net 
availability of foods and nutrients needs reinvestigation by more 
rigorous methods and with due regard to the amounts of the food 
materials compared and to the thermal environment of the animals 
experimented upon. 
Discussion of Results. 
For the reasons just stated, any strict quantitative discussion 
of the above results seems impossible. At the same time, certain 
general conclusions may be at least tentatively deduced from them 
which, even though to a considerable extent speculative, may at 
least serve provisionally as a connecting thread between the known 
facts. 
Influence of Amount of Food on Availability.—In the fore- 
going paragraphs it has been tacitly assumed that the amount of 
food eaten has no influence on its availability, or, to state it in 
another way, that the expenditure of energy in digestion and assimi- 
lation is proportional to the quantity of food. To express the same 
thing in mathematical terms, we have assumed, in constructing the 
diagram on p. 410, that the net available energy is a linear function 
of the metabolizable energy. 
While it seems highly probable that such is the case the only ex- 
periments bearing specifically upon this question of which the writer 
is aware are those upon timothy hay just cited. An examination 
of the graphic representation of the results strongly supports the 
hypothesis that the net availability of the food is independent of 
its amount, but the evidence of so few experiments must naturally 
be accepted with some reserve. The other recorded results, as 
computed above, apart from the possible source of uncertainty 
pointed out on p. 429, show such considerable variations in indi- 
vidual cases that it scarcely seems possible to reach any definite 
conclusions from them regarding the influence of quantity of food. 
As will appear in the next chapter, the extensive respiration exper- 
iments made in recent years at the Méckern Experiment Station by 
G. Kithn and O. Kellner upon fattening cattle indicate that the 
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