Combined nitrogen, it has been well said, is the common capital of the living kingdom. In an environment like that of the earth's surface, with an atmosphere of which free nitrogen constitutes four fifths of the gases, the element can nevertheless be taken up by most plants in the form only of ammonia, nitrites or nitrates. To animals even these compounds have appeared to be useless; and the only source of nitrogen to this class of living organisms, which includes man, has seemed to be the protein which has been built up by the agency of the plant cells. It has not been easy to dispose of the possibility that gaseous or elementary nitrogen may play a part in the processes of animal metabolism. The careful experiments of Oppenheimer1 and Krogh2 have at length demonstrated conclusively, however, that no nitrogen is eliminated in the free state from the
THE SIGNIFICANCE OF NITRATES IN HUMAN URINE. JAMA. 1916;LXVII(2):122–123. doi:10.1001/jama.1916.02590020038013
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