vii KATABOLIC CONSTITUENTS OF UKINE 379 



last figures of the specific gravity by the constant coefficient '2'33 

 (Haeser). A litre of urine of normal mean specific gravity, e.g., 

 contains solids to an approximate amount of 20 x 2'33 = 46'60 

 grms. ; and 1J litre of this urine secreted on an average in the 

 24 hours contains 20 x 23 x 31'5 = 69'90 grms. solid substances. 



The acidity of the total urine in man in the 24 hours is, under 

 normal conditions, about equivalent to that of 2 grms. oxalic acid, 

 with which it is usually compared. It is largely due to the 

 presence of acid sodium phosphate (Liebig). But the degree of 

 acidity naturally varies at different times of the day. During 

 gastric secretion, owing to the formation of hydrochloric acid and 

 reabsorption of the liberated sodium in the blood, the alkalinity 

 of the blood increases and the acidity of the urine therefore 

 diminishes (01. Bernard, Bence-Jones, Gley), increasing again 

 when gastric digestion is over. The nature of the diet has more 

 influence on the reaction of the urine. With vegetable food, 

 which contains an excess of alkali, the acidity of the urine 

 diminishes, so that it may exhibit a neutral or amphoteric, some- 

 times even an alkaline reaction, when, e.g., only potatoes, which are 

 very rich in potassium salts, are eaten. On a flesh diet, on the 

 contrary, in which the earthy bases predominate, the urine is 

 always distinctly acid. 



For the same reasons the urine of herbivora is normally 

 alkaline and that of carnivora acid. But in fasting, when both 

 carnivora and herbivora consume their own tissues, the urine of 

 the latter also becomes acid (01. Bernard). 



The acidity of human urine sometimes increases for a certain 

 time after micturition, owing to a fermentation which gives rise 

 to the development of new acid substances (Scherer). This causes 

 a precipitation of acid urates which makes the whole of the urine 

 cloudy, and slowly forms a sediment consisting principally of 

 urates, plus oxalates, mucus and desquamated epithelial cells from 

 the urinary passages. When present in large amount, these 

 urates may be precipitated, as the urine cools, independent of 

 any fermentation. In this case the acidity of the urine is not 

 increased, but is diminished, owing to the precipitation of the acid 

 urates (Voit and Fr. Hofmann). 



As was said above in discussing the reaction of the blood (see 

 Vol. I. p. 94), recent work in chemical physiology has greatly 

 modified our notions as to the reaction of urine and of all the 

 tissue fluids in general. Thus, according to the results of 

 Auerbach and Friedenthal (1903) either with physico-chemical 

 methods, or by simply using a suitable indicator (e.g. phenol- 

 phthalein), human urine always has a neutral or weakly acid 

 reaction, even when an alkaline reaction is indicated by litmus. 

 In fact, neither litmus paper nor methyl orange can be used as 

 indicators in presence of carbonic acid. 



