134 THE SECRETIONS: 



19. Peroxide of iron. The amount of iron contained in the 

 urine is frequently very minute, and can only be detected in 

 the ash, which must be dissolved in hydrochloric acid. Upon 

 the addition of ferrocyanide of potassium to the acid solution, a 

 deep blue colour, or a very slight precipitate of prussian blue 

 is produced : while the addition of hydrosulphate of ammonia 

 or infusion of galls effects a dark colouring. 



QUANTITATIVE ANALYSIS OF THE URINE. 



Method of separating all the proximate constituents. 



An exact quantitative determination of all the constituents of 

 the urine is a task beyond the powers of animal chemistry in 

 its present condition. Our ignorance of the proximate consti- 

 tuents of the extractive matter, and our inability to separate 

 them, are alone sufficient to preclude the hope of a perfect an- 

 alysis : we must therefore content ourselves with pursuing the 

 same course as we have already done with the blood, and must 

 rest satisfied with effecting the separation (as accurately as we 

 can) of those constituents which at present we regard as the 

 most important, and which present no peculiar chemical diffi- 

 culty ; whilst others, as for instance the various extractive mat- 

 ters, must be associated in groups. 



Even this abbreviated and comparatively simple method does 

 not yield absolute estimates, only a few of the constituents of 

 the urine, as, for instance, the fire-proof salts, yielding quanti- 

 tative results with analytical exactness : the determination of 

 the organic constituents, of the urea, uric acid, ammonia-com- 

 pounds, and extractive matters is more or less insecure and fluc- 

 tuating, and we must regard a quantitative analysis of urine as 

 giving us certainly an idea of its probable constitution, but not 

 by any means of its actual composition. 



I shall now explain a method of analysing the urine, by which 

 the principal constituents may be isolated and determined. 



It is impossible to estimate the various constituents of the 

 urine from a single portion ; different portions of the same urine 

 must be used for the determination of the various constituents, 

 as will be presently shown. 



