634 THE CHEM1STR Y OF THE URINE. 



moting a retention of chlorides ; a fact for which we have no sufficient 

 explanation. 1 



Upon ordinary diet, about from 6 to 10 grms. HC1 is excreted per 

 diem by a healthy adult. 



To demonstrate the presence of chlorides, the urine is diluted, made 

 acid with nitric acid, and mixed with nitrate of silver solution ; a white 

 curdy precipitate of silver chloride falls, which, if filtered off, is found 

 to be soluble in ammonia. 



To estimate the hydrochloric acid, a known quantity of silver salt is added, 

 together with nitric acid, to a measured amount of urine, taking care that the 

 silver is in excess. The precipitate is filtered off, and the excess of silver 

 titrated in the nitrate with a standard solution of ammonium thiocyanate, 

 ferrous sulphate being used as an indicator. Knowing the amount of silver 

 added, and that left in the nitrate, the difference indicates that combined as a 

 chloride, from which the hydrochloric acid can be calculated (Volhard's 

 method). 



Carbonic acid is found even in acid urines ; some 50 c.c. being 

 present per litre. 2 In acid urines the greater part is not in firm chemical 

 combination, as it is driven out of solution by the passage of a stream 

 of air. But when the urine contains abundant fixed bases, and especially 

 when it is actually alkaline from these, considerable quantities of car- 

 bonates may be present, the urine of herbivora being exceptionally rich 

 in these. 



Nitric and nitrous acids may be present in normal urine in the 

 form of salts, but in quite unimportant quantity. 



The nitrates are derived, not from metabolism, but directly from the 

 food ; the nitrites are not present when the urine is first passed, but 

 appear to arise always from the nitrates, as an effect of the reducing 

 action of micro-organisms. 



Silicic and hydrofluoric acids may appear in traces, simply as 

 an effect of the presence of their salts in various foodstuffs. 



Sodium and potassium. Of sodium about 5 grms. per diem is 

 excreted upon a mixed diet, and of potassium about half this quantity. 

 The proportion of the latter metal is increased when the dietary is more 

 exclusively composed of flesh, and it is raised during starvation, and in 

 febrile conditions. 3 We have already referred to the interesting fact 

 that the ingestion of large quantities of potassium salts may lead to 

 increased elimination of sodium from the body, and it is this driving out 

 of the latter essential constituent of the body-fluids which makes the 

 consumption of common salt with the food a necessity in all cases 

 where the diet is rich in potassium. 4 



Calcium and magnesium. In human urine, about - 2 to 0'4 grms. 

 of lime (CaO) is excreted per diem. 



Of the lime salts present in the food, only a small proportion is 

 excreted by the urine. Much of the lime remains in an insoluble form, 

 and is not absorbed at all, while of that which does enter the circulation 



1 Of. Salkowski and Leube, " Lehre vom Earn," 1882, S. 174, 464, 465 ; see also Kast, 

 Ztschr. f. pliysiol. Chem., Strassburg, 1888, Bd. xii. S. 271. 



2 Wurster and Schmidt, Centralbl. f. Physiol., Leipzig u. Wien, 1887, Bd. i. S. 421. 



3 Of. Salkowski, rirchota'sArchiv, 1871, Bd. liii. S. 209 ; Munk, Berl. Uin, JFcJinschr., 

 1887, S. 432. 



4 Bunge, "Lehrbuch der physiol. Chem." 



