NET AVAILABLE ENERGY—MAINTENANCE. 439 
energy any increase in the consumption of water (taking this as the 
typical case) will increase the metabolism by an amount sufficient 
to warm the water to the body temperature. Above the critical 
amount of food the excess of heat arising from the digestive work 
will, we may reasonably suppose, be applied to the warming of the 
additional water consumed, and only when this is insufficient will 
an increased metabolism be required to make up the deficit. In 
case of farm animals, however, it would appear that the waste heat 
even on a maintenance ration is ordinarily sufficient, and more 
than sufficient, to supply all the energy needed for warming the 
ingesta. 
The Time Element.—One important factor in modifying the 
results of the demand for heat, particularly with relation to the 
water consumption, is what we may call the timeelement. Hitherto 
it has been tacitly assumed that all the factors making up the 
demand for heat act at a uniform rate. Asa matter of fact this is at 
best only partially true. Ordinarily a farm animal is watered but 
once or twice per day and then consumes a relatively large amount 
in a few minutes. A sudden demand for heat is thus set up, since 
this water must be raised to body temperature within a compara- 
tively short time. It is quite conceivable, therefore, that the demand 
for heat may temporarily exceed the supply, requiring the deficit 
to be made up by an increased metabolism, while if the same water 
consumption were distributed uniformly over the twelve or twenty- 
four hours no such effect would be produced. Such a temporary 
increase in the heat production, however, cannot be made up for 
later when the heat production is in excess, but is a permanent loss. 
Once converted into heat, the energy of food or tissue has, so to 
speak, escaped from the grasp of the organism, which appears to 
have no power to reconvert it into any other form of energy. We 
may plausibly suppose that these considerations constitute a partial 
explanation of the advantages observed in practice from the warm- 
ing of drinking-water and the installation of self-watering devices 
in the stable. 
What is true in regard to the consumption of water is of course 
equally true of other forms of the demand for heat. The time ele- 
ment is an important factor. Thus an exposure of an hour or two 
in a cold yard or to a cold rain may cause an increased metabolism 
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