348 



PHYSIOLOGICAL CHEMISTRY. 



urates of ammonium, calcium, magnesium, potassium and sodium. 

 The ammonium urate may occur in neutral, alkaline or acid urine, 

 whereas the other forms of urates are confined to the sediments of 

 acid urines. Sodium urate occurs in sediments more abundantly 

 than the other urates. The urates of calcium, magnesium and po- 

 tassium are amorphous in character, whereas the urate of ammon- 

 ium is crystalline. Sodium urate may be either amorphous 

 or crystalline. When crystalline it forms groups of fan-shaped 

 clusters or colorless, prismatic needles (Fig. 102, below). Am- 

 monium urate is ordinarily present in the sediment in the burr-like 

 form of the "thorn-apple" crystal, i. e., yellow or reddish-brown 

 spheres, covered with sharp spicules or prisms (Plate VI, oppo- 

 site). The urates are all soluble in hydrochloric acid or acetic 



FIG. 102. 



ACID SODIUM URATE. 



acid and their acid solutions yield crystals of uric acid upon stand- 

 ing. They also respond to the murexide test. The clinical sig- 

 nificance of urate sediments is very similar to that of uric acid. A 

 considerable sediment of amorphous urates does not necessarily 

 indicate a high uric acid content, but ordinarily signifies a concen- 

 trated urine having a very strong acidity. 



Cystine. Cystine is one of the rarer of the crystalline urinary 

 sediments. It has been claimed that it occurs more often in the 

 urine of men than of women. Cystine crystallizes in the form of 

 thin, colorless, hexagonal plates (Fig. 32, p. 73, and Fig. 103, 

 p. 349) which -are insoluble in water, alcohol and acetic acid and 



