EXAMINATION OF URINARY DEPOSITS. 175 



When urinary deposit is examined with the microscope, it 

 will be found either crystalline, amorphous, or organized. 

 When, as is frequently the case, the deposit consists of a 

 mixture of different forms, each of them in succession should 

 be examined, until the nature of the whole deposit is clearly 

 understood. 



CRYSTALLINE DEPOSITS will probably be either uric acid, 

 pMfesphate of lime and magnesia (from which the triple phos- 

 phate is formed), oxalate of lime, or perhaps cystine. 



Triple Phosphate. This salt (called also the double phos- 

 phate of ammonia and magnesia) is formed by supersaturating 

 with ammonia. . Phosphate of lime is also precipitated by the 

 same means, but may be distinguished by the microscope. 

 The crystals of the triple phosphate are stellate or triangular 

 prisms, as seen in Fig. 54. They disappear on the addition 

 of acetic acid. 



Uric (or Lithic) Acid. This salt, like the earthy phos- 

 phates, exists in a small quantity in healthy urine, but as the 

 proportion varies considerably in many forms of disease, its 

 determination when in abnormal quantity affords much assis- 

 tance in diagnosis. 



It is insoluble in alcohol, and nearly so in dilute hydro- 

 chloric and sulphuric acid ; but it combines with the alkalies, 

 forming salts, which are insoluble or very sparingly soluble in 

 water. 



The action of nitric acid upon uric acid is characteristic. It 

 will gradually dissolve it, carbonic acid and nitrogen being 

 given off with effervescence, leaving behind a mixture of 

 alloxan (C 8 N 2 H 4 O 10 ), alloxantine (C 4 H 3 N 5 O s ), and other 

 compounds. This may be evaporated nearly to dryness, when 

 a red residue will be left, which, when cold, should be moist- 

 ened with ammonia, which will develope a beautiful purple 

 color, owing to the formation of murexide (C 13 N 5 H 8 ). 



