258 Dr. Venables on 



of my paper in this Journal, favoured me with a communica- 

 tion upon this interesting subject, and, at his request, I had the 

 honour of furnishing him with some specimens of silex passed 

 by my patient. In comparing them with those discovered by 

 himself, he observes that ' they bear some resemblance (though 

 they are more minute and are of an amber tinge) to those 

 which I have mentioned as coming under my own view *.' 



Very lately I had another opportunity of detecting very 

 small siliceous particles in a crystallized form in some gravelly 

 fragments passed by a patient of Mr. Green's, who happened 

 to be visiting at this place. Having accidentally met with this 

 gentleman, he was mentioning the circumstances of his case, 

 particularly his frequently voiding quantities of gravelly matter. 

 Having obtained a specimen recently passed, I found them to 

 consist of lithic acid with volatile and fixed alkali and lime. The 

 volatile alkali was evolved by heating with caustic potassa, and be- 

 came sensible by the pungent smell. The presence of fixed alkali 

 was proved by fusing a particle with a small quantity of very 

 finely divided silex. Exposed on charcoal to the flame of the 

 blow-pipe, the mass fused into a globule, the transparency of 

 which was different in different instances. The gravel was of a 

 pale cream colour, and seemed like so many fragments or scales 

 of the outer covering of a small nucleus, having both a con- 

 cave and convex surface. Among these I found a small 

 nucleus of an irregularly rounded shape, and of about the bulk 

 of a snipe shot. It was insoluble in muriatic acid, but soluble 

 with effervescence in the nitric. 



Among the fragments which were of a cream -colour, I ob- 

 served several which were of a much whiter appearance, and 

 about the bulk of mustard-seed shot, irregular in shape. From 

 their appearance I took them to be the mixed or fusible phos- 

 phates, but upon urging a particle with the blow-pipe, I was 

 surprised to find that it underwent little or no observable 

 change, except that during the ignition it assumed a very bright 

 or brilliant incandescence. After ignition it had a strongly 

 alkaline reaction, and when moistened with distilled water it 

 slaked and fell to powder like caustic lime, which an additional 

 quantity of the water dissolved. On subjecting this solution 

 * On the Tendency to Calwlua Disorders. Phil, Trans, 1830. 



