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 carbamate, 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 Urixary 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, Ztsrhr. f. Biol., Miinchen, 1892, Bd. xi. S. 387-39", where other references 
will be found. 
- Ztschr.f. physiol. C'hem., Strassburg, 1891, Bd. xv. S. 161. 
3 Most analyses bear out this statement, but those of Bunge, given on p. ">73. show an 
excess of lime. 
