354 



EXCRETION. 



Fit:. 115. 



a slight darkening of the color, owing to the action of the acid upon 

 the organic coloring matter of the urine. If the mixture, however, 

 be allowed to stand for some time, the urates of soda, potassa, &c v 

 will be decomposed, and pure uric acid, which is very insoluble, 

 will be deposited in a crystalline form upon the sides and bottom 

 of the glass vessel. The crystals of uric acid have most frequently 

 the form of transparent rhomboidal plates, or oval laminaB with 

 pointed extremities. They are usually tinged of a yellowish hue 

 by the coloring matter of the urine which is united with them 

 at the time of their deposit. They are frequently arranged in 

 radiated clusters, or small spheroidal masses, so as to present the 



appearance of minute calcu- 

 lous concretions. (Fig. 115.) 

 The crystals vary very much 

 in size and regularity, ac- 

 cording to the time occupied 

 in their formation. 



If a free alkali, such as 

 potassa or soda, be added to 

 the urine so as to neutralize 

 its acid reaction, it becomes 

 immediately turbid from a 

 deposit of the earthy phos- 

 phates, which are insoluble 

 in alkaline fluids. 



The addition of nitrate of 

 baryta, chloride of barium 



or subacetate of lead to healthy urine, produces a dense precipi- 

 tate, owing to the presence of the alkaline sulphates. 



Nitrate of silver produces a precipitate with the chlorides of 

 sodium and potassium. 



Subacetate of lead and nitrate of silver precipitate also the or- 

 ganic substances, mucus and coloring 'matter, present in the urine. 

 All the above reactions, it will be seen, are owing to the presence 

 of the natural ingredients of the urine, and do not, therefore, indi- 

 cate any abnormal condition of the excretion. 



Beside the properties mentioned above, the urine has several 

 others which are of some importance, and which have not been 

 usually noticed in previous descriptions. It contains, among other 

 ingredients, certain organic substances which have the power of 

 interfering with the mutual reaction of starch and iodine, and even 



URIC ACID; deposited from urine. 



