406 CHEMISTEY. 



heated apartments in which some manufactures are carried on. There 

 are other objections to Liebig's classification, but we will not dwell on 

 them. The division is probably in the main correct, and yet there are 

 some facts that seem to show that plastic food is sometimes used for 

 the production of heat, and that fuel -food is sometimes used for build- 

 ing. If so, it is an exception to a general rule, and indicates that the 

 chemistiy of life is not bound by such strict lines as is inorganic chem- 

 istry. 



575. Amount of Animal Heat. At first thought it seems 

 strange that so much of the food, ordinarily four fifths, 

 should be expended as fuel in producing heat, because we 

 are in the habit of thinking of food as doing good only in 

 building up and repairing the system. This is the com- 

 mon popular view of nourishment, and it is only the in- 

 vestigations of the chemical physiologist that make us 

 realize of what importance heat is in the maintenance of 

 living action. It is calculated that the amount of heat 

 produced in the body of an adult man in one year would 

 suffice to raise from twenty to twenty-five thousand pounds 

 of water from the freezing to the boiling point. The heat, 

 then, required to run the animal machine, as we may ex- 

 press it, is very great, and therefore there must be a large 

 provision of fuel. 



576. Uses of Pat in the System. There is one form of 

 heat-food fatty matter which is used quite extensively 

 in building up the structures of the system, though in the 

 wear and tear of these structures it may eventually serve 

 as fuel. But it is Liebig's idea that the fat which is so 

 largely present in many tissues is not really a part of them, 

 but is contained in them very much as water is contained 

 in the interstices of a sponge. The combination, however, 

 is certainly more intimate than this; perhaps it may be 

 considered like that of phosphate of lime with gelatin in 

 bone. And in the case of the brain there seerns to be even 

 a chemical combination of fatty matter with phosphoric 



