CONCRETIONS. 435 



but does not fuse before the blowpipe, and then acts in the 

 manner described in (e) ; and if a small portion of the fresh 

 specimen dissolved in boiling water affords the ordinary evi- 

 dence of uric acid when treated with nitric acid and ammonia. 

 If the assay slightly fuses and runs together, and is then par- 

 tially soluble in water, urate of soda or potash is mixed with the 

 urate of lime ; the solution has a strong alkaline reaction, and 

 effervesces on the addition of an acid. 



i. It is urate of magnesia (which occurs very rarely in 

 calculi, and then only with uric acid) if it readily burns white 

 before the blowpipe, but does not fuse ; and if the residue is 

 insoluble in water, but soluble without (or with very slight) 

 effervescence in dilute sulphuric acid; and if caustic potash 

 throws down a precipitate from this solution. 



k. It contains silica (a rare constituent) if, after prolonged 

 exposure to heat, and digestion of the residue in hot hydro- 

 chloric acid, an insoluble residue remains, which becomes white 

 before the blowpipe, and fuses into a clear bead when mixed 

 with carbonate of potash or soda. 



3. The specimen may be partially consumed on exposure 

 to heat, while the residue undergoes no further change under 

 the action of the blowpipe. Concretions of this sort are by 

 no means rare ; they consist of a mixture of the compounds of 

 the 1st and 2d classes. Indeed, calculi, composed merely of one 

 of the substances already enumerated, are very rare, for although 

 in one class of calculi uric acid may be the preponderating 

 constituent, in another oxalate of lime, and in a third the 

 earthy phosphates, we almost always find associated with 

 these substances a certain amount of other matters; for in- 

 stance, uric acid and the urates are of frequent occurrence in 

 calculi chiefly composed of oxalate of lime or of the earthy 

 phosphates. 



Intestinal concretions consist for the most part of earthy 

 phosphates with a little fat, extractive matters, and vegetable 

 fibre ; biliary concretions, of cholesterin mixed with a little 

 bile-pigment and biliary resin, or of bile-pigment and biliary 

 resin with a little cholesterin. Other classes of concretions 

 (with the exception of arthritic concretions, which consist for 

 the most part of urate of soda) are composed of earthy phos- 



