URINARY CALCULI. 561 



When the solution of this acid is concentrated it deposites crys- 

 tals of oxalate of urea, and then of pure oxalic acid. Hence at 

 the time that Dr Wollaston made his experiments (1797), it was 

 impossible for him to have drawn any other conclusion than that 

 the acid constituent of the calculus was oxalic. 



Calculi occur, which from their appearance have been called 

 hemp-seed calculi. They are alwsfys small, pale-coloured, and 

 remarkably smooth on the surface. Dr Wollaston examined 

 them, and found them also to consist of oxalate of lime. It would 

 be an object of some consequence to ascertain by a chemical an- 

 alysis whether the acid which they contain be oxalic or oxaluric. 

 These hemp-seed calculi must be very rare. I have only seen 

 one specimen in all the numerous collections of calculi which I 

 have examined. 



Dr Marcet met with three different specimens of mulberry 

 calculi, passed per urethram by three different persons, and hav- 

 ing each a distinct crystalline texture. The shape of the crys- 

 tal was a very flat octahedron.* 



6. Urate of soda calculi. Dr Wollaston first showed that the 

 chalk stones which form in the joints of gouty patients consist 

 chiefly of urate of soda. Berzelius informs us that M. Lind- 

 bergson analyzed a urinary calculus which he found composed 

 of urate of soda and carbonate of magnesia. I met with a par- 

 cel of very small calculi in the collection of Dr George Mon- 

 teath, which were coated on the surface with urate of soda. They 

 were obtained from a man of 60 years of age, who laboured un- 

 der a diseased prostate gland. These calculi were 40 in num- 

 ber, about the size of a pea, some cylindrical and others approach- 

 ing to the cubic shape. Within they were yellow, but the ex- 

 ternal surface was white. The yellow portion was uric acid. 

 From the white surface I extracted uric acid and soda. 



There was in the same collection another calculus extracted 

 after death. It consisted of a mulberry nucleus, covered by a 

 pretty thick coat of uric acid. The surface was white, and had 

 exactly the appearance of the surface of the small calculi just 

 mentioned'. Hence I considered it probable that it consisted al- 

 so of urate of soda, but I had not an opportunity of examining it. 



9. Cystic oxide calculi. This rare calculus, the substance con- 

 stituting which has already been described under the name of 



* Marcet's Essay, p. 78. 



N n 



