260 Dr. Venables on 



upon this subject I may observe that it is not always present 

 in the urine of the human subject, nor is its appearance constant 

 even when occasionally discoverable. I have repeatedly analysed 

 the urine of the same individual, and have at times readily 

 found distinct and satisfactory traces of silex ; while at others, 

 after the most careful and minute investigation, I have been 

 unable to discover the slightest indication of the presence of 

 this substance. 



Being anxious to ascertain whether the silex was confined 

 to this one fragment, or whether it existed in any proportion 

 in the remainder, a quantity, amounting to a grain in weight*, 

 was taken promiscuously from the mass, and being introduced 

 into a test tube, and tolerably strong nitric acid added, heat 

 was applied. It dissolved with effervescence. Distilled water 

 was now added, and the tube, fixed in its stand, was placed 

 under a glass jar and left at rest for forty- eight hours. There 

 was no deposition whatever, which would have been the case 

 if there had been any intermixture of silex. Ammonia being 

 added till neutralization was nearly effected, oxalate of am- 

 monia precipitated a considerable proportion of oxalate of 

 lime. The presence of lithic acid was proved by exposing a 

 portion to the action of caustic potassa in excess aided by heat, 

 and then pouring off the clear solution. Acetic acid being 

 added in excess, the precipitate was washed and collected on 

 a capsule. The solution of this precipitate in nitric acid being 

 evaporated to dryness, and then acted on by ammonia, proved 

 the presence of lithic acid by the formation of purpurate of 

 ammonia. 



The presence of carbonic acid in the white limy-looking 

 particles was proved as follows : A very small test tube being 

 filled with and inverted over mercury, a particle or two of the 

 calcareous carbonate was introduced, and immediately rose to 

 the top of the mercury. A small quantity of moderately di- 

 luted hydrochloric acid was introduced, by means of another 

 test tube, into that containing the carbonate. The diluted acid 

 immediately rose to the surface of the mercury in the tube, 

 and, acting on the carbonate, dissolved it with considerable 

 effervescence ; the mercury at the same time descending in 

 * The entire specimen with which I was furnished did not amount to two grains. 



