130 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 

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

 in cold, but readily soluble in warm water. HCl and acetic aci<l 

 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 aflfections 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 CsR:i^iO:i'N&. 



Neutral sodic urate ... CgH-jN^OaNao. 



Acid ammonium urate .... C5H3N4O3' NH4). 



Acid potassic urate .... C5H3N4O3K. 



Wlien 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 ui-ate 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 piu-ple. 

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

 border is fairly well defined. 



(h.) 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. 



((.*.) 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. 



{fL) Examine the deposit microscopically. The urates are 

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



