URINARY CALCULI. 559 



This calculus is rare, only about a dozen of such occur in the 

 Hunterian collection. 



4. Ammonia-phosphate of magnesia calculi. Strictly speaking, 

 this does not constitute a peculiar species, as the double phosphate 

 always contains a considerable mixture of phosphate of lime ; at 

 least, if any exist composed of the double phosphate of magnesia 

 and ammonia alone, I have never happened to see them. These 

 calculi are yellowish-white, and have usually a tuberculated sur- 

 face. It is not uncommon to meet with calculi containing crys- 

 tals of the double phosphate. When these crystals are exposed 

 to the action of the blowpipe ammonia is disengaged, and biphos- 

 phate of magnesia remains, which undergoes an imperfect fusion. 



5. Fusible calculi. These calculi are composed of a mixture 

 of double phosphate and phosphate of lime. They are more 

 abundant than any other species, except the uric acid calculi. 

 They are whiter and more friable than any other species, Some- 

 times they resemble a mass of chalk, and leave a white dust upon 

 the fingers. They easily separate into laminae, the interstices of 

 which are often studded with crystals of the double phosphate. 

 But this laminated structure is not always observable. They 

 often acquire a very large size, sometimes nearly filling the whole 

 cavity of the bladder. When these calculi are urged by the 

 blowpipe they readily melt into a vitreous globule ; in conse- 

 quence of the mutual action of the biphosphate of magnesia and 

 phosphate of lime on each other. 



When this calculus is pulverized and treated with acetic 

 acid, the ammonia-phosphate of magnesia is dissolved, and the 

 phosphate of lime remains nearly pure ; muriatic acid being 

 poured upon this residue, dissolves the phosphate of lime, and 

 usually leaves a quantity of uric acid, which not uncommonly 

 constitutes the nucleus of the calculus. The proportions of these 

 constituents, and with them the appearance of the calculus, varies 

 very much. When the calculus is large the outermost crust not 

 unfrequently contains a greater proportion of ammonia-phosphate 

 of magnesia than the internal parts. It is not uncommon to find 

 a nucleus of uric acid or oxalate of lime covered by a crust of 

 phosphate of lime, and that again by a crust of fusible calculus. 



6. Carbonate of lime calculi. These calculi are common in 

 the inferior animals, but very rare in man. It has been already 

 stated that human calculi containing carbonate of lime were first 



