URINE. 139 



7. Determination of the fixed salts. 



The determination of these constituents is of much import- 

 ance; they are composed of potash, soda, lime, magnesia, sul- 

 phuric acid, phosphoric acid, and hydrochloric acid. The de- 

 termination of the silicic acid would also be interesting, if a 

 series of analyses in reference to this point were instituted. Iron, 

 manganese, and fluoride of calcium exist in too minute quan- 

 tities to be successfully determined, or indeed always detected. 

 The bases and acids which were first named, viz. the potash, &c., 

 may be determined in the following manner. Three or four 

 ounces of urine are evaporated, and the residue incinerated. As 

 the carbonaceous matter does not readily burn off, in conse- 

 quence of being entangled with the melting salts, it is expedient 

 to add some nitric acid to the urine, and to place a cover on the 

 crucible. A white melted ash is soon obtained, the weight of 

 which must be determined. A certain proportion of this ash, 

 if the whole quantity is sufficiently large, may be weighed, and 

 used for the determination of the chlorine, or a separate quantity 

 of urine may be evaporated and incinerated for this purpose. 



For the determination of the other constituents a known 

 quantity of the salts is dissolved in water, to which a little nitric 

 acid has been added ; this solution (A) is filtered; what remains 

 on the filter is silicic acid, mixed perhaps with a little carbon. 

 It must be washed, burnt with the filter, and its weight esti- 

 mated. The solution (A) and the water with which the con- 

 tents of the filter were washed, are mixed together, and a slight 

 excess of free ammonia added. The mixture is then warmed. 

 By this means the earthy phosphates are precipitated, and, as in 

 healthy urine the phosphoric acid is in excess as compared with 

 the earths, the latter are completely thrown down. They are 

 quickly washed, dried, exposed to a strong heat, and weighed. 

 In order to determine the quantity of lime in the earthy phos- 

 phates, they must be dissolved in very diluted nitric acid, and 

 the free acid saturated with ammonia; the lime may then be 

 precipitated by oxalate of ammonia. The filtered fluid will yield 

 the magnesian salt, by precipitation with free ammonia. 



The ammoniacal solution from which the earthy phosphates 

 have been precipitated must be mixed with the water used 



