NATUEE OF MATTEES EESTOEED. 17 



Such being the nature and properties of the things received, we may 

 now examine in the same general manner those which are Properties of 

 dismissed from the system. Here, at the very outset, we en- substances 



. -,. -, dismissed bv 



counter the important fact that they are oxidized or burned the system." 

 bodies. 



1st. As respects the urine and its constituents. Its liquid part, wa- 

 ter, is an oxide of hydrogen, of which, though the greater portion may 

 not have been produced in the economy, yet a certain quantity unques- 

 tionably has. In it, too, are to be found sulphuric acid, which is an ox- 

 ide of sulphur ; phosphoric acid, which is an oxide of phosphorus ; and its 

 leading solid constituent, urea, is the representative of bodies which arise 

 when processes of oxidation have been going on. 



2d. The expired and transpired matters present similar burned com- 

 pounds. At the head of these products stand carbonic acid gas, which is 

 an oxide of carbon, and water, which, as we have already said, is an ox- 

 ide of hydrogen. We here omit any consideration of the nature or con- 

 stitution of the fecal matter, because much of it has never been properly 

 in the interior of the system, though it has passed through the intestine. 



The general result at which we arrive is, then, that the food consists of 

 combustible matter, and that the substances dismissed from the economy 

 are oxidized bodies. A burning must, therefore, have been go- c om k ust j on 

 ing on, and this could only have been accomplished by the air occurs in the 

 introduced by breathing acting upon the substance of the body oc y ' 

 itself and its contents, and,* to repair the waste which must have ensued, 

 a due weight of food has been required. Since this, in its turn, as a 

 part of the living mechanism, is destined to undergo the like destructive 

 action, we may present the entire series of facts under consideration cor- 

 rectly by regarding them as arising remotely from the action of the air 

 upon the food. 



With this statement before us, we next inquire what ensues when sub- 

 stances appropriate for food are exposed in artificial experiments at a cer- 

 tain temperature to the action of atmospheric air. 



A piece of flesh, or even of any vegetable body, consisting of carbon, 

 hydrogen, oxygen, and nitrogen, submitted to those condi- Results of arti- 

 tions, undergoes combustion. Its carbon, by uniting with ox- fi . cial combus- 



\ J & tion the same 



ygen, produces carbonic acid, its hydrogen for the most part as that in the 

 water, but a residue thereof, combining with the nitrogen, may bodjr - 

 give rise to the production of ammonia. If there be any sulphur and 

 phosphorus present, they also burn, and salts of sulphuric and phosphoric 

 acids are the result. 



Such is what occurs outside of the body in a common case of artificial 

 combustion where atmospheric air has access. The constituents of which 

 the food is composed thus satisfy their chemical affinities, and the com- 



B 



