18 PEODUCTION OF HEAT. 



pounds we have mentioned arise. Now it is a fact of the utmost signifi- 

 cance that the compounds thus originating from the direct artificial "burn- 

 ing of matters proper for food are the very same that are dismissed from 

 the animal system in which food has been submitted to the air introduced 

 by respiration. They are such substances as carbonic acid, water, am- 

 monia, sulphates and phosphates. 



It may impress these truths more deeply upon us to learn that the 

 facts at which we have thus arrived may also be recognized m the 

 changes of destruction presented by the vegetable kingdom. The leaves 

 of trees, after they have fallen in autumn, quickly decay, and even the 

 heart- wood itself has a limit beyond which it does not last. Sooner or 

 later every part of a plant is destroyed by the atmospheric air. Such 

 limits of duration in animal structures are short. A very brief time, per- 

 haps only a few hours, is all that is wanted for putrefaction to set in, and 

 the entire mass, undergoing dissolution, is lost in the surrounding air. 



This final disappearance of all organized structures is brought about 

 by the action of that energetic element, oxygen. If by any contrivance 

 its influence is prevented and its presence avoided, these changes do not 

 take place. Putrefaction and decay are slow combustions, true burnings 

 taking time. There equally arise from the fallen leaf and from the de- 

 caying body carbonic acid, water, and ammonia, the self-same substances 

 dismissed from the economy during the continuance of life. 



Processes of combustion and processes of decay are therefore both due 

 to the action of atmospheric oxygen on the changing substance. They 

 differ chiefly from one another in the relative rapidity with which they 

 are accomplished. 



The facts thus set forth wan-ant the following statements. The mat- 

 ters which a man receives as food are combustible bodies ; those dismissed 

 Production of from his system have been burned. To that, as to any other 

 animal heat. g^^ burning, oxygen gas is absolutely requisite. There is, 

 therefore, a plain conclusion before us, which, in its far-reaching conse- 

 quences, covers the whole science of physiology, and betrays to us the 

 function which every animal discharges, viz., that oxidation is incessant- 

 ly going on in the interior of the system through the agency of atmos- 

 pheric air introduced by the process of breathing. 



An animal, in this point of view, is an oxidizing machine, into the in- 

 terior of which atmospheric air is constantly introduced. The active con- 

 stituent, oxygen, satisfies its chemical affinities at the expense of those 

 parts of the system which are wasting away. And as the act of breath- 

 ing, that is, the introduction of this gas, takes place day and night, wak- 

 ing and sleeping, so too must the production of burned bodies ; a part 

 escaping by the lungs, a part by the skin, a part in the urine. To com- 

 pensate the loss which ensues s nearly 1000 pounds weight of combustible 



