CHARACTERISTICS OF URINARY EXCRETIVES. 635 



a considerable fraction is re-excreted into the lower bowel. 1 The 

 administration of dilute mineral acids, which decomposes to some extent 

 the insoluble phosphates of the food, increases the urinary lime salts, 

 and, conversely, when sodium phosphate is taken in large quantities, the 

 lime may almost disappear from the urine. 



Very interesting is the observation of G. Hoppe-Seyler, 2 who found 

 that the excretion of lime salts by the kidneys is much greater during 

 conditions of rest than during exercise, a fact which doubtless depends, 

 in part at least, upon the effect of exercise on the excretion into the 

 bowel. 



As a general rule, the urine contains about twice as much magnesia 

 as lime. 3 Most food-stuffs, other than milk and eggs, contain more 

 magnesium than calcium salts. The phosphates of the former are also 

 more soluble, and as both bases are largely present as phosphates in the 

 food, it is to be understood that more magnesium will be absorbed and 

 excreted. When, as during starvation, the ingestion of magnesium salts 

 ceases, the lime is found to be in the greater proportion. 



The presence of calcium in urine is easily demonstrated, and its amount 

 determined by acidifying the fluid with acetic acid, and adding ammonium 

 oxalate, when all the lime separates as the insoluble crystalline calcium oxalate, 

 which may be filtered off and weighed as calcium carbonate, into which it 

 is converted on gentle ignition. In the filtrate from this, the magnesium is 

 precipitated as triple phosphate upon the addition of ammonium chloride, 

 ammonia, and, if necessary, of some extraneous alkaline phosphate. 



Iron. The urine contains, as a rule, a very minute quantity of iron, 

 and frequently no detectable trace. It has been found increased in 

 diseases, such as pernicious anaemia, but never rises to more than a few 

 milligrammes in the twenty-four hours. It is a remarkable fact that 

 this metal, if present at all, is, to a" large extent, precipitated in association 

 with the pigmented crystals of uric acid, which separate when the urine 

 is acidified with hydrochloric acid. It may be detected in the ash of 

 large quantities of the urine, by taking a solution of this to dryness with 

 a little nitric acid, dissolving the residue in water, and testing with 

 potassium sulphocyanide, which gives with ferric salts a blood red 

 coloration. 



GENERAL CHARACTERISTICS OF THE URINARY EXCRETIVES. 



It might perhaps be expected that the waste products of meta- 

 bolism, on leaving the body, would in general represent the simplest 

 compounds of physiological chemistry, and would stand farthest of all 

 removed from the complexity of the tissue proteids. That this is not 

 entirely the case, however, will have been clear from the facts set forth 

 in previous sections ; it is, indeed, striking to observe how many of the 

 organic excretives arise by synthetic processes from simpler precursors 

 in the body. 



There is one form of chemical change which takes an important and 



1 Voit, Ztsehr. f. Biol., Miinchen, 1892, Bd. xi. S. 387-397, where other references 

 will be found. 



2 Ztsehr. f. physiol. Chem., Strassburg, 1891, Bd. xv. S. 161. 



3 Most analyses bear out this statement, but those of Bunge, given on p. 573, show an 

 excess of lime. 



