208 URINARY SEDIMENTS. 



amined with high powers. The so-called "brick-dust" 

 sediment is a mixture of the sodium and potassium urates. 

 Sodium urate is also found in fan-shaped clusters or irregu- 

 lar groups of fine crystals, and sometimes in granules. 

 (Plate II, 8.) Ammonium urate makes up the " thorn- 

 apple" crystals: brown, spherical masses covered with 

 curved spicules. (Plate II, 9.) The urates can be differ- 

 entiated from other sediments by being soluble on gently 

 warming the liquid, as well as in alkalies. The urates, as 

 well as the free acid, give the murexid test (Experiment 

 354). Uric acid is especially important when found as a 

 sediment, from its tendency to form calculi. The same is 

 true, to a less extent, of the urates. 



CALCIUM OXALATE. 



This salt is most frequent in acid urine. It may exist 

 in two forms: the crystalline, or "envelope shaped/' and 

 the "dumb-bell shaped." Its appearance under the micro- 

 scope affords the best method of identification. (Plate II, 

 10.) The crystalline form consists of octahedral crystals. 

 They are never large, often being smaller than a red blood- 

 corpuscle. When sufficiently magnified, they have some- 

 what the appearance of the back of a square envelope, the 

 crossed lines being formed by the angles of the crystal. 

 In the shape of the crystals they resemble some forms of 

 triple phosphate, from which they can be distinguished by 

 their insolubility in acetic acid and by their smaller size. 

 The amorphous form of calcium oxalate is disk shaped, 

 with a contraction on opposite sides, so that it somewhat 

 resembles a dumb-bell. Calcium carbonate has much the 

 same form, but dissolves in acids with effervescence. 

 Calcium oxalate is insoluble in acetic, but soluble in hydro- 



