THE NITROGENOUS EXCRETION. URINE. 357 



the feces. The analysis of the urine gives us then in ordinary 

 cases a fairly accurate measure of the alkali metals taken in 

 with our food and drink; in normal condition there is no ac- 

 cumulation of alkali salts in the body. 



CALCIUM AND MAGNESIUM COMPOUNDS. 



The full significance of these salts in the urine we can not 

 explain, since without complete analyses of the feces we do 

 not know the relation of the excreted to the ingested alkali- 

 earths. Our natural drinking waters contain usually quite 

 appreciable amounts of these salts, with those of calcium in 

 excess as a rule. In Lake Michigan water, for example, we 

 have about 125 milligrams per liter of these salts as carbonates, 

 but in our common animal and vegetable foods we consume 

 daily much greater quantities than we could get from water. 

 The ash of wheat contains about 12 per cent of magnesia and 

 3 per cent of lime, while in the ash of muscle we have over 3 

 per cent of magnesia and between 2 and 3 per cent of lime. 

 500 grams of lean meat would furnish us then with over 150 

 milligrams of magnesia and with something less than that 

 amount of lime. 



But only fractions of these compounds find their way into 

 the urine. In the original foods they exist, in part at least, 

 in insoluble forms. While some of these substances may be 

 dissolved in the stomach the conditions are reversed in the 

 intestines, and insoluble phosphates, carbonates and sulphates 

 are lost with the feces. There has been much discussion as to 

 the exact nature of the calcium and magnesium salts excreted. 

 In a measure the discussion is fruitless, as we must certainly 

 admit the free exchange of ions in solution. Under ordinary 

 conditions the acid ions of the urine appear to be in slight 

 excess of the metals, which prevents precipitation of insoluble 

 phosphates, for example. Temperature plays a very impor- 

 tant part in the problem of the stability of the calcium and 

 magnesium compounds in the urine, and the problem is further 

 complicated by the presence of uric acid, the peculiar behavior 

 of which will be touched upon below. 



