13 PRACTICAL PHYSIOLOGY. [XX. 



covering it with another watch-glass or small beaker. Examine the threads 

 microscopically for the characteristic crystals of uric acid, which are soluble 

 in KHO. A similar reaction may be done on a microscopic slide. 



4. Uric Acid Salts (Urates, " Lithates "). Uric acid forms salts 

 (chiefly acid), with various bases, which are soluble with difficulty 

 in cold, but readily soluble in warm water. HC1 and acetic acid 

 decompose urates,. and then the uric acid crystallises. 



Urates form one of the commonest and least important deposits in urine. 

 There is usually a copious precipitate, varying in colour from a light pink or 

 brick-red to purple. They occur in catarrhal affections of the intestinal canal, 

 after a debauch, in various diseases of the liver, in rheumatic and feverish 

 conditions. They frequently occur as the " milky " deposit in the urine of 

 children. Urates constitute the " lateritious " deposit or "critical" deposit 

 of the older writers. Urates frequently occur even in health, especially when 

 the skin is very active (in summer), or after severe muscular exercise ; when 

 much water is given off by the skin and a small quantity by the kidneys. 

 The following are the formulae of the more common urates : 



Acid sodic urate C^H 8 N 4 O s Na. 



Neutral sodic urate .... C 5 HoN 4 3 Na. 2 . 



Acid ammonium urate .... C 5 H 3 N 4 3 (NH 4 ). 



Acid potassic urate .... C 5 H 3 N 4 3 K. 



When the urine is passed it is quite clear, but on standing for 

 a time it becomes turbid, and a copious reddish-yellow some- 

 times like pea-soup or purplish precipitate occurs, because urates 

 are more soluble in warm water than in cold; and when there 

 is only a small quantity of water to hold the urates in solution, 

 on the urine cooling they are precipitated. Their occurrence is 

 favoured by an acid reaction, a concentrated condition of the urine, 

 and a low temperature. 



The urates deposited in urine consist chiefly of sodic urate mixed 

 with a small amount of ammonium urate. 



5. Tests for " Urates " or " Lithates " in urine. 



(a.} Observe the naked-eye characters. The deposit is usually 

 copious = yellowish-pink, reddish, or even shading into purple. 

 The deposit moves freely on moving the vessel, and its upper 

 border is fairly well denned. 



(b.) Place some in a test-tube. Heat gently the upper stratum. 

 It becomes clear, and on heating the whole mass of fluid, it also 

 becomes clear, as the urates are dissolved by the warm liquid. 



(c.) Place some of the deposit on a glass slide, add a drop of 

 hydrochloric acid, and uric acid is deposited in one or more of 

 its many crystalline forms. Examine the crystals microscopically. 



(d.) Examine the deposit microscopically. The urates are 

 usually " amorphous," but the urate of soda may occur in the form 



