THE ELIMINATION OF METABOLIC PRODUCTS. 591 



supply of phosphoric acid. In increased diuresis the amount of urea 

 and of common salt in the urine increases considerably, but the amount 

 of phosphoric acid present remains remarkably constant. 



The reaction of the urine depends, as we have already said, upon the 

 nature of the food. The urine of herbivora is neutral or alkaline, while 

 that of the carnivora is acid as a rule. The direct connection which 

 this has with the food may be indicated theoretically by compar- 

 ing the ash of plants with that of animals. That it is not any par- 

 ticular difference in the metabolism taking place in different classes of 

 animals which causes the different reaction of the urine, may be shown 

 by feeding vegetables to the carnivora. The urine then becomes neutral 

 or alkaline. Conversely, herbivora may be forced to become carnivora 

 by starvation. The animal is then obliged to live upon its own tissue, 

 and the urine then has an acid reaction. Alkaline urine, especially in 

 herbivora, is usually turbid on account of the precipitation of alka- 

 line earth salts. The urine of a normal man with a mixed diet shows an 

 acid reaction. The acid reaction is caused by the fact that during meta- 

 bolism acid products are formed by the combustion of neutral substances 

 such as albumin and lecithin for example. The sulphur contained in 

 albumin is largely converted into sulphuric acid, the phosphorus of leci- 

 thin and of the nucleic acids is oxidized to phosphoric acid. Further- 

 more, organic acids, as, for example, hippuric acid, uric acid, oxalic acid, 

 and aromatic oxyacids, are also formed. The organism, moreover, pos- 

 sesses ways and means for keeping the acidity within certain limits. 

 For one thing, the acid formed may be neutralized by means of alkali 

 carbonate; and if there is not enough of this present, then the ammonia 

 which is set free by the decomposition of proteins comes into play. 



It is perhaps well here to make a few general observations concerning 

 the conception of acidity. An acid may be defined from two stand- 

 points. 1 The chemist understands by an acid a substance whose hydro- 

 gen atom, or atoms, may be replaced by metals. When the metal enters 

 the molecule the acid character is neutralized. Thus the acidity of a 

 solution may be estimated by measuring the amount of alkali which is 

 necessary to replace all of the acid hydrogen. Our discussion of the acidity 

 of the urine was from this point of view. The physico-chemist, on the 

 other hand, defines an acid as a chemical compound which when dis- 

 solved in water is dissociated, yielding positively charged hydrogen atoms 

 (H + ). According to the degree of dissociation, we characterize an acid 

 as strong or weak. A weak acid, for example, is one which at a given 

 concentration is less dissociated than a strong acid. The difference 

 between these two points of view may be perhaps best illustrated by an 



1 Cf. R. Hoeber: Hofmeister's Beitrage, 3, 525 (1903). 



