A copious flow is the result. But it is just this contrast between the capacity of removing urea possessed by living cells, and the passage of urea in solution from one side to the other of a membrane, which justifies the retention of the expression “vital.” Mechanical conditions are those which we can imitate in a model; vital conditions, those which at present we are unable to reproduce. =Nitrogenous Waste.=—Meat, fish, eggs, milk, vegetable-albumins, are the sources of nitrogen. The kidney is the organ which eliminates it from the body. Since all nitrogenous food which is digested is eventually reduced to simple, soluble compounds which appear in the urine (the quantity thrown off in perspiration is so small as to be negligible), the proportion which the nitrogen of the urine bears to the nitrogen in the food is a measure of the efficiency of digestion. A certain quantity of the nitrogen eliminated is in the form of uric acid, creatinin, and other compounds of a like order; but these less oxidized substances, though always present in some degree, are not, in Man and other mammals, the normal end-products of nitrogenous metabolism. Urea is the final and simplest product. It is therefore sufficient to estimate the quantity of urea excreted, and to compare the nitrogen which it contains with the nitrogen ingested in the form of “animal food.” About nine-tenths of the nitrogen ingested should be accounted for by urea. When alimentation is excessive or digestion imperfect, the proportion is less than this; some nitrogenous food is not absorbed; some that is absorbed is imperfectly oxidized. =Urea= is characteristically an animal product. Inorganic chemistry deals with stable, organic chemistry with unstable, compounds. Not that there is any boundary between inorganic 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 inorganic chemistry has to do with such substances as carbonates, 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 CO₂; ammonia, the formula NH₃. Urea is a combination of the two compounds. It is carbonic acid in which one (divalent) atom of oxygen is replaced by two (monovalent) atoms of ammonia. It is ammonia in which two (monovalent) atoms of hydrogen are replaced by one (divalent) atom of carbonic acid. Carbonic Urea Ammonia anhydride NH₂ / CO₂ CO NH₃ \ NH₂