URINAUY CALCULI. 443 



uric acid, or oxalate or phosphate of lime. Urate of ammonia 

 acts before the blow-pipe in just the same manner as uric acid, 

 and their reactions with nitric acid are also similar : they may, 

 however, be readily distinguished by the comparative solubility 

 of the former in boiling water, and by the evolution of am- 

 monia that takes place on triturating it with caustic potash. 

 In a careful examination of a calculus of urate of ammonia, the 

 first point is to ascertain if other constituents are present, which 

 is usually found to be the case. If there is a residue left after 

 heating it before the blow-pipe, that residue may consist of 

 earthy phosphates, or earthy or alkaline carbonates : the alka- 

 line carbonates correspond with the alkaline urates, the carbo- 

 nate of lime with the oxalate of lime. In this case the calculus 

 must be reduced to a very fine powder and dried ; a weighed 

 portion must then be freed from fat and extractive matter by 

 ether and alcohol, and afterwards repeatedly boiled in small 

 quantities of distilled water, till the water is no longer affected. 

 When the calculus is finely powdered the urate of ammonia 

 dissolves with tolerable facility in boiling water. The earthy 

 phosphates and the oxalate of lime must be extracted with 

 dilute hydrochloric acid, 1 precipitated by ammonia, dried, 

 weighed, and exposed to a high temperature. If we again dis- 

 solve this residue in hydrochloric acid, and throw down the 

 earthy phosphates with ammonia, chloride of calcium remains 

 in solution, arising from the oxalate of lime. Whatever remains 

 unacted on by dilute hydrochloric acid is uric acid. 



The urates of soda and potash, as well as the urate of lime 

 are (as I have already mentioned) often found in calculi of 

 uric acid ; they likewise occur in calculi of urate of ammonia. 

 All these urates are soluble in boiling water ; their mode of 

 separation has been already described. If urate of magnesia 

 should also be present (which is probably seldom the case), a 

 different method of separation must be adopted. Hydrochloric 

 acid is added to the evaporated aqueous solution in order to 

 precipitate the uric acid; the acid solution, after filtration, is 

 evaporated on the water-bath, and we then obtain a residue of 



1 [Unless the hydrochloric acid is tolerably strong, it will not dissolve the oxalate 

 of lime.] 



