URINARY CALCULI. 441 



grain of urinary gravel in the centre of several calculi of uric 

 acid, and the central portions are often darker coloured than 

 the peripheral. The uric acid in these calculi is never pure, 

 but is always mixed with colouring matter the uroerythrin 

 (which always accompanies uric acid), frequently with alkaline 

 urates, and occasionally with small quantities of earthy phos- 

 phates. A very minute quantity of fat and of extractive 

 matter occurs in this as well as in most other sorts of urinary 

 calculi. 



In order to obtain an accurate knowledge of the composition 

 of a calculus, it must be sawed through the centre, and the 

 different strata submitted to distinct analyses if they present 

 any variation in their physical characters : it does not often 

 happen that a calculus which consists externally of uric acid 

 is composed in its interior of earthy phosphates, oxalate of 

 lime or cystin, but on the other hand uric acid often forms 

 a nucleus to calculi formed of other constituents. In order to 

 determine whether fixed alkaline urates, or earthy phosphates 

 are contained in a calculus, a portion must be incinerated and 

 the residue analysed. If they are present they may be deter- 

 mined by the rules given in 2, 0, b, c, f, g, h, and 3. 



A portion of each lamina, or if the calculus is uniform 

 throughout, some of the dust separated in the operation of 

 sawing, is reduced to an impalpable powder ; a weighed quan- 

 tity is placed in a small porcelain basin, and after being 

 warmed for some time on the water-bath, is placed under the 

 exsiccator in order to remove every trace of moisture. A known 

 portion of the dried powder is placed in a small glass flask, and 

 repeatedly extracted with ether, whereby the fat is removed ; 

 and the residue is boiled with alcohol of specific gravity '850, 

 which takes up some extractive matter. The powder is then 

 boiled with distilled water till nothing further can be removed 

 by that menstruum. On evaporating the watery solution in a 

 small porcelain capsule we obtain the urates as a residue. If 

 it is requisite that this part of the analysis should be carried 

 further, we dry the residue and weigh it; we then heat it in a 

 little water and add hydrochloric acid; the uric acid sepa- 

 rates, and the bases combine with the hydrochloric acid. The 

 uric acid is collected on a filter, washed with water, dried, and 

 weighed; the hydrochloric- acid solution is evaporated, and 



