EXCRETION 211 



and organic chemistry. They are merely terms which it is 

 convenient to use to indicate the groups of atoms which 

 occupy the chemist's attention at the time. Nor is stability 

 an attribute of certain groups, instability an attribute of others. 

 Stability is relative, not absolute. But admitting these terms 

 as convenient indications of degree, it may be said that in- 

 organic chemistry has to do with such substances as carbon- 

 ates, nitrates, ammonia ; organic chemistry, with compounds in 

 which carbon is not satisfied with oxygen, as it is in carbonic 

 acid ; nitrogen not satisfied with oxygen, as in nitric acid, or 

 with hydrogen, as in ammonia. Carbonic acid (anhye)drid 

 has the formula C0 2 ; ammonia, the formula NH 3 . Urea is a 

 combination of the two compounds. It is carbonic acid in 

 which one (divalent) atom of oxygen is replaced by two (mono- 

 valent) atoms of ammonia. It is ammonia in which two 

 (monovalent) atoms of hydrogen are replaced by one (divalent) 

 atom of carbonic acid. 



Carbonic anhydride Urea Ammonia 



C0 2 CO NH 3 



NH 2 



Urea is an amide carbonic diamide. It very readily takes 

 water into its molecule, changing into carbonate of ammonia. 

 N 2 H 4 CO + 2H 2 = (NH 4 ) 2 C0 3 . This change is rapidly brought 

 about by the influence of bacteria in urine exposed to the air. 



In thinking of the transformations which proteid substances 

 undergo in the system, it is legitimate to regard their nitrogen 

 as from the first united with hydrogen in the form of ammonia. 

 Not that the grouping is so simple as this. An albumin is not 

 an amide. But in the dance of atoms of its great molecule as 

 it progresses through the system forming part of the blood, 

 taken up by the cells as floating protein, incorporated in the 

 protoplasm of the cells, shaken into smaller aggregates in the 

 muscles nitrogen and hydrogen are partners. They leave the 

 body hand in hand. Gusts of oxygen atoms enter through the 

 lungs ; use blood-corpuscles as carriages ; dismounting, they 

 traverse lymph, forcing their way into the interior of the cells ; 

 they join in the dance. With their strong arms they detach 

 carbon atoms and hydrogen atoms from the huge albumin chain. 



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