XXIV.] URINARY DEPOSITS, ETC. I5I 



(B.) Tf mcomlncifilJe, or if it leaves much ashj it may consist of 

 urates with a fixed base (Na, Mg, Ca), oxalate, carbonate, or 

 phosphate of lime, or triple phosphate. 



5. A. Combustibl^.— Of tliis group, uric acid and urate of 

 ammonium give the nnirt'xide test. 



(i.) Uric Acid is by far the most common form, and constitutes 

 five-sixths of all renal concretions. Concretions the size of a 

 split-pea, or smaller, may be discharged as (/ravel. "When retained 

 in the bladder, they are usually spheroidal, elliptical, and some- 

 what flattened; are tolerably hard; the surface may be smooth 

 or studded with fine tubercules; the colour may be yellowish, 

 reddish, reddish brown, or very nearly white. When cut and 

 polished, they usually exhibit a concentric arrangement of layers. 

 Not unfrequently a uric acid calculus is covered with a layer of 

 phosphates, and some calculi consist of alternate layers of uric 

 acid and oxalate of lime. Its chemical relatio7is : nearly insoluble 

 in boiling water; soluble in KHO, from which acetic acid preci- 

 pitates uric acid crystals (microscopic) ; gives the murexide test 

 (Lesson XX. 3). 



(ii.) Urate of Ammonium Calculi are very rare, and occur 

 chiefly in the kidneys of children ; they form small irregular, soft, 

 fawn-coloured masses, easily sohible in hot water. 



(iii.) If the calculus is combustible and gives no murexide test, 

 it may consist of xanthin, which is very rare, and of no practical 

 importance. 



(iv.) Cystin is very rare, has a smooth surface, dull yellow 

 colour, which becomes greenish on exposure to the air; and a 

 glistening fracture with a peculiar soapy feeling to the fingers ; 

 soft, and can be scratched with the nail. It occurs sometimes in 

 several members of the same family. It is soluble in ammonia 

 and after evaporation it forms regular microscopic hexagonal 

 plates (fig. 78). 



The other calculi of this group are very rare. 



6. (A.) Group. — Apply the Murexide Test. 



It is / Treat the original powder with \ No odour = Uric acid. 

 obtained \ potash. J Odour of NH3 == Ammonium uraU, 



The residue is not coloured, but becomes yellowish-red \^ = Xanthin. 



on adding caustic potash ..... j 

 The residue is not coloured either by KHO or NH4HO ; ) 



the original substance is soluble in ammonia, and ^ '^Cystin, 



on evaporation yields hexagonal crystals . . ) 

 On heating, it gives an odoui' of burned feathers ; the "j 



substance is soluble in KHO, and is precipitated V-'Proteid, 



therefrom by excess of HNO3 • • . > j 



