As already noted, ammonia is evidently a normal precursor of urea, being changed to the latter in part in the muscles and other tissues generally and in part during its passage through the liver. In accordance with this view we find that the elimination of nitrogen as ammonia may be notably increased at the expense of urea: (1) in structural diseases of the liver; (2) after injecting mineral acids which combine with ammonia in the body, forming stable ammonium salts; (3) in cases of a pathological excess of acids in metabolism, such as often occurs in diabetes and in fevers. All of these are, of course, abnormal conditions. Normally, about 2 to 6 per cent of the total nitrogen eliminated is in the form of ammonium salts, the amount depending largely upon the relation between the amounts of acid-forming and of base-forming elements in the food, which will be discussed in connection with the study of the ash constituents of food and of mineral metabolism (Chapter X).