Mr. Venables on Siliceous Gravel. 239 



whole of the red colour was removed, and the mass became 

 nearly colourless. They resembled the very fine crystals which 

 we frequently see in sand or gravel upon the sea-side. On 

 taking a small crystal, and pressing it with my nail, and draw- 

 ing it over a piece of window glass, it scratched it like common 

 flint. Mixed with soda, and heated with the blowpipe upon 

 charcoal, they melted slowly, and with effervescence, into a 

 vitreous globule. In very small quantity, they gave a perfectly 

 transparent globule of an amber or orange colour ; in larger 

 quantity, the vitrification was less perfect, and the globule 

 more opaque. These characters perfectly agree with those of 

 silex. 



I have since had several opportunities of attending to the 

 particulars of this case ; and I have satisfied myself, beyond 

 the possibility of doubt, that this siliceous sand is passed with 

 the urine. On the first examination, I thought it possible that 

 the sand might have been in the cup, and the urine have been 

 voided upon it ; but, upon attending to all the circumstances, 

 it is impossible to admit of such a solution. In the first place, 

 the red colour was owing to a thin coating of the alkaline 

 lithates, ammonia, and lime. Thus, by treating them with 



menstruum. Hence, no mistake could arise from the hasty exposure to 

 the action of liq. potassae, to which the sand was subjected. Nothing 

 but the lithic acid coating was dissolved. This was satisfactorily proved 

 by the addition of distilled vinegar, the separating and washing the 

 precipitate, and re-dissolving it in nitric acid, which was attended with the 

 usual appearances. On evaporation to dryness, the carmine red colour 

 afforded sufficient proof of the presence of lithic acid, which was still 

 further confirmed by the re-agency of the vapour of ammonia. 



However, having treated a small quantity of the sand with nitric, 

 muriatic acid, and caustic potass in succession, and washing after ex- 

 posure to each re-agency, th$. residue being pulverised in an agate 

 mortar, and reduced to a state.of extreme comminution, was digested in 

 potass and dissolved. The solution being supersaturated with muriatic 

 acid, the silica was precipitated, and reduced by evaporation to a gela- 

 tinous mass. It was then evaporated to dryness, and was tried with 

 warm distilled water by means of " Wollaston's Fountain of Compres- 

 sion," till the washings ceased to give a precipitate with nitrate of silver ; 

 the residue, being mixed with soda, fused into a transparent vitreous 

 amber or orange- coloured globule. Quartz, treated in the same way, 

 afforded precisely a similar result. The quantity of sand that could be 

 obtained from all the specimens of urine was not sufficient to institute 

 experiments upon a more extended scale without sacrificing the whole 

 product, which I was unwilling to do, as I wished to preserve a small 

 portion as a record of the most important feature in this interesting case. 



