ORGANIC NITROGEN. ITS NATURE. 49 
complicated than the original proteid of the food. Thus, in the 
bodies of plants and animals alike the nitrogen reaches a condition 
allied to proteid. But, while proteids may serve as food for animals 
and for the great class of colorless plants (fungi) they are quite out 
of the reach of the green plants, which are the great food producers 
of nature. Our next problem, then, must be to learn how these 
proteids are reduced to their original condition of nitrate. 
Part of the proteid thus built up into the body of the plant or the 
animal remains there until the animal or plant dies, and at death it 
is still a proteid and as complex as ever. In this form it may be- 
come incorporated into the soil when the animal or plant dies, or it 
may become eaten as food and pass through the body of another 
animal. But much of it will eventually reach the soil while still 
in the form of proteid. 
A second portion of the proteid is used up in the animal's 
body to furnish energy and heat; it is metabolized, as we say. 
When it is thus used its complex chemical molecule is broken to 
pieces, and it is reduced to much simpler compounds. But it is not 
decomposed sufficiently to bring the nitrogen back within the reach 
of plant life. The carbon in this proteid is in part removed from it 
and combined with oxygen, to be exhaled as CO 2 . The molecule 
falls to pieces and various simpler by-products arise; but in the 
animal's body, practically all of it eventually assumes the form of 
urea (CON 2 H 4 ). Though this urea is a nitrogen molecule far 
simpler than proteid, still it is not simple enough for a plant food. 
Urea, or a closely allied compound, is the form in which nearly all of 
the nitrogenous material resulting from proteid metabolism in the 
animal body is excreted. Urea thus represents one stage in the 
destruction of proteid compounds, and to this stage the proteids are 
brought as the result of the metabolism in the life processes of 
animals. In some animals this urea is secreted as urine by the 
kidneys, but in others (birds) it is mixed with the feces; in all cases 
it contains the nitrogen which is no longer of any use to the animal 
world. It is estimated that some 38,0x30 tons of urea are excreted 
daily by the human race. To this quantity must be added the far 
greater amount excreted by other animals, for all animals, large 
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