262 Dr. Venables on 



detected the silex in the carbonate of lime fragment only, from 

 the minute portion, and no silex being discoverable in the 

 general mass, I might have been tempted to suppose that 

 the carbonate of lime was an accidental impurity, not legiti- 

 mately referable to a urinary source, although it is not easy to 

 credit or even suspect that the whole of the intermixed frag- 

 ments of carbonate of lime were mere accidental contami- 

 nations. 



But the detection of silex imbedded in lithic salts leaves no 

 doubt of its urinary connexion ; and this circumstance renders 

 the urinary origin of the crystallized specimen no longer equi 

 vocal. What the peculiar circumstances are which determine 

 the mode of appearance, it is difficult, nay impossible, in the 

 present state of our knowledge, to determine. The more the 

 earthy diathesis seems to prevail, the greater the tendency in 

 the silex to separate in the crystallized form ; and, indeed, so 

 far as the few facts ascertained upon this subject will warrant 

 an inference, the deposition of silex seems connected with an 

 earthy diathesis. Fourcroy and Vauquelin found the silex in 

 phosphate of lime ; Dr. Yelloly found it in oxalate of lime ; 

 Professor Wurzer found it in a calculus consisting principally 

 of lithic acid with a small proportion of phosphate of lime. 

 Wurzer's calculus yielded 1 per cent, on analysis. Its com- 

 position was as follows : 



Phosphate of lime 17 .33 



Lithic acid 75 .34 



Animal matter 6 .33 



Silex 1 .00 



100.00 



Alemani gives the chemical composition of a urinary cal- 

 culus, containing not only silica in large proportion, but also 

 phosphate of iron. It was as under :- 



Magnesia 51.00 



Silica 20.00 



Phosphate of iron 21 .84 



Carbonate of magnesia 4 . 00 



Loss 3.16 



100.00 



However, the composition of this concretion is so extra- 



