418 CHEMISTRY IN AGRICULTURE AND PHYSIOLOGY. 
decomposes, liberating the carbon as carbonic acid, and the 
hydrogen as water. 
Fat contains 80 per cent, of carbon. Sugar, starch, and 
fat, are composed of carbon, hydrogen, and oxygen. The 
most decisive experiments and observations have proved, 
that whenever oxygen unites with carbon to form carbonic 
acid, or with hydrogen to form water, the act of combination 
cannot take place without the disengagement of a definite 
amount of heat . 
If we suppose the carbon of the food to be converted into 
carbonic acid within the body, it must give out exactly as 
much heat as if it had been burnt in the air or in oxygen. 
This is the source of animal heat. (Liebig.) 
The whole history of hybernating animals, and the well- 
established fact of the periodical accumulation in various 
animals of fat, which at other periods entirely disappears, 
prove that the oxygen in the respiratory process consumes 
all those substances which are capable of entering into com¬ 
bination with it. Besides carbon, and a small portion of 
sulphur, the animal body contains, as a combustible element, 
with which oxygen can combine only hydrogen. 
A fat man or a dormouse will live longer without a re¬ 
newal of food than a lean one of either species. In cold 
weather animals require the largest proportion of carbon¬ 
aceous food to act as fuel to keep them warm. Our bodies 
may be likened to little steam-engines, or a blacksmith’s” 
fire, our lungs operating as a pair of bellows, our mouth as a 
chimney, and our food as the coal; our bellows are always 
going, or we soon die : if we do not eat, what would be the 
consequence, would the bellows cease work ? No, they would 
go on working till every particle of animal matter, fuel, fat, 
&c., was consumed. This shows that if we take no food to 
supply the waste of the body, we must, like the fire, go out; 
on the other hand, if the blacksmith overloads his fire, and 
does not blow his bellows enough, his flame goes out, and 
we, if we are always eating, and never blowing our bel¬ 
lows enough, we are also liable to go out. The life of all 
animals, as we said before, depends on the action of oxygen 
and the supply of food. The constant tendency of this 
action of oxygen upon the animal system is to destroy it, and 
unless the vital force is supported by food, it soon destroys 
the body. The supply of food forms an antagonistic force, 
which is able to resist oxygen as long as the supply is kept 
up, but oxygen never ceases in its pursuit of its victim till it 
has ultimately consumed it. 
It is agreed among physiologists that the tissues are formed 
