PHOSPHATES. 169 



this will precipitate 0.00354 gramme of chlorin, equal to 

 0.00584 gramme of sodium chlorid. Calculate the per- 

 centage of chlorin by weight in the urine. The change of 

 color from white to red can be more plainly seen by yellow 

 light (gaslight) than by daylight. There are present in 

 the urine some other substances which are precipitated by 

 silver nitrate like the chlorin. To make approximate cor- 

 rection for these, 1 cubic centimeter may be subtracted 

 from the number used. 



366. A more accurate result can be obtained if the organic 

 matter is first destroyed. To 10 cubic centimeters of urine in a 

 thin porcelain or platinum dish add about 1 gramme of sodium 

 nitrate and 2 grammes of potassium nitrate, both free from 

 chlorids. Evaporate to dryness and carefully heat to fusion. 

 Cool, dissolve in water, slightly acidify with nitric acid, then make 

 exactly neutral with sodium carbonate and titrate with silver 

 nitrate, calculating the amount of chlorin as in the preceding 

 experiment. 



PHOSPHATES. 



The phosphoric acid of the urine is united with two 

 classes of bases: the alkalies, sodium and potassium, 

 and the alkaline earths, calcium and magnesium. The 

 compounds are called, respectively, "alkaline" and "earthy" 

 phosphates. The alkaline phosphates are soluble in water. 

 The earthy phosphates are insoluble in water or alkalies, 

 but are dissolved by acids. They consequently appear in 

 the urine in the insoluble form whenever it becomes alka- 

 line, either by fermentation or by the addition of reagents. 

 They may also be precipitated by boiling. The amorphous 

 white precipitate thus obtained is often mistaken for albu- 

 min. It can be distinguished by being easily soluble in 

 acids, which is not the cr.se with albumin. "When ammonia 

 is present, as in fermentation, the magnesium forms an 



