ill the Museum of St, Bartholomew's Hospital, 415 



sist essentially of urate of ammonia. That the ammonia in 

 these calculi is combined with uric acid may be shown in the 

 following manner. Let a small quantity of boiling water be 

 poured over a few grains of the calculus placed in a small 

 paper filter ; the solution will on cooling deposit a copious 

 flocculent white precipitate of urate of ammonia, which from 

 its appearance alone^ may be easily distinguished from the 

 scanty crystalline precipitate which takes place when uric acid 

 calculi are similarly treated: should too much water have 

 been added, it will be necessary to evaporate the solution a 

 little before precipitation of the urate of ammonia will take 

 place. Free uric acid is very frequently present in these 

 calculi, and may be observed in the form of minute crystals 

 mixed with the amorphous precipitate of urate of ammonia, 

 or adhering to the sides of the vessel ; when however the 

 triple phosphate is present this test is of no value, for the 

 mere affusion of hot water over a mixture of pure uric acid 

 and phosphate of ammonia and magnesia will give rise to the 

 formation of urate of ammonia, and thus vitiate the result. 

 This fact appears to add considerable weight to the opinion 

 of Dr. Prout, that uric acid is seldom or ever deposited in a 

 free state together with the phosphates. There is another 

 distinctive character by which these calculi may always be 

 recognised, viz. their decrepitating on the application of heat: 

 in no well-marked specimen has this property been wanting, 

 and I consider it as perfectly characteristic of the species. 

 To what this peculiarity is owing it is not easy to determine, 

 for pure urate of ammonia does not decrepitate when heated. 

 It has been generally referred to the small quantity of oxalate 

 of lime contained in these calculi ; but this is hardly probable, 

 as oxalate of lime calculi undergo combustion silently, and 

 the same property is possessed by those specimens in which 

 the phosphates form the predominating admixture. It may 

 possibly arise from the sudden extrication of ammonia, and its 

 degree of force depend upon the compactness of the body, for 

 in those calculi which from the predominance of the earthy 

 phosphates are porous and friable, this property is considerably 

 impaired or altogether lost. As far as my observation has 

 gone the urate of ammonia in these calculi is never in a state 

 of purity, all of them containing variable quantities of oxalate 

 of lime, the phosphates, uric acid, and in some few instances 

 urate of lime. 



The quantity of earthy matter however in the compact 

 varieties is very small, seldom exceeding a few parts per cent. 

 15 grs. of a specimen which was rather disposed to crumble, 



