THE URINE. 433 



rough surface, are white or pale yellow, and have a crystalline fracture. They 

 are not very hard ; they burn up almost entirely on a platinum foil, burning 

 with a bluish flame. They give the above-mentioned reactions for cystin. 



Xanthin calculi are very rarely found. They are also of primary formation. 

 They vary from the size of a pea to that of a hen's egg. They are whitish, 

 yellowish brown, or cinnamon-brown in color, medium hard, with amorphous 

 fracture, and on rubbing appear like wax. They burn up completely when 

 heated on a platinum foil. They give the xanthin reaction with nitric acid 

 and alkali, but this must not be mistaken for the murexid test. 



Urostealith calculi have only been observed a few times. In the moist state 

 they are soft and elastic at the temperature of the body, but in the dry state 

 they are brittle, with an amorphous fracture and waxy appearance. They 

 burn with an illuminating flame when heated on a platinum foil, and generate 

 an odor similar to resin or shellac. Such a calculus, investigated by KRUKEN- 

 BERG, consisted of paraflfine derived from a parafline bougie used as a sound 

 on the patient. Perhaps the urostealith calculi observed in other cases had a 

 similar origin, although the substances of which they consisted have not been 

 closely studied. 



Fibrin calculi sometimes occur. They consist of more or less changed 

 fibrin coagulum. On burning they develop an odor of burnt horn. 



The chemical investigation of urinary calculi is of great practi- 

 cal importance. To make such an examination actually instructive 

 it is necessary to investigate separately the different layers which 

 constitute the calculus. For this purpose saw the calculus, which 

 has been wrapped in paper, with a fine saw so that the nucleus is 

 sawed through and accessible. Then peel off the different layers, 

 or, if the stone is to be kept, scrape off enough of the powder from 

 each layer for examination. This powder is then tested by heating 

 on a platinum foil. It must not be forgotten that a calculus is 

 never entirely burnt up, and also that it is never so free from 

 organic matter that on heating it does not carbonize. Do not, 

 therefore, lay too great stress on a very insignificant unburnt residue 

 or on a very small amount of organic matter, but consider the cal- 

 culus in the former case as completely burnt and in the latter as 

 not burnt. 



When the powder is in great part burnt up but a significant 

 quantity of unburnt residue remains, then the powder in question 

 contains as a rule urates mixed with inorganic bodies. In such 

 cases remove the urate with boiling water, and then test the filtrate 

 for uric acid and the expected bases. The residue is then tested 

 according to the following schema of HELLER, which is well adapted 

 to the investigation of urinary calculi. In regard to more detailed 

 examination the reader is referred to special works on the subject. 



