URINARY CALCULI. 437 



f wells with innumerable aerial bubbles, and forms 

 a froth which is at firft ruddy, and when more 

 entirely evaporated, becomes black. This black 

 matter tinges a great deal more water than the 

 weak folution, and is foluble even by thofc acids 

 which act not upon the calculus, and always the 

 Ilronger the acid infufcd, the fooncr does theco- 

 lour difappcar. Even alum, in which there is but 

 a final 1 proportion of acid, dcilroys the colourof 

 this froth. 



Nitrous acid acls in a fmgular manner on in- 

 flammable matters; and as inflammable matter is 

 the principle of colour, hence it is eailly undcr- 

 ilood wliy none but the nitrous acid extracts the 

 colour from the calculus. A due proportion of 

 the acid is t however, rcquilitc to render the co- 

 lour permanent. Diluted nitrous acid fhould 

 therefore be employed to avoid the inconveni- 

 ence of an excels; for an excels docs not produce 

 too flrong a red, but dellroys the colour by the 

 abforption of all the phlogillon, Strong nitrous 

 acid, mixed with calculous matter, is, after a 

 fliort interval, converted, without the application 

 of heat, into froth. 



The acid of the urinary calculi is calily fepa- 

 raleil from the nitrous acid by evaporation; the 

 nitrous acid being rendered more volatile by 

 combination with phlogitlon. Alkaline iults 

 do not feparatc thefe acids; for it almolt always 

 happens in the cafe of two acids being mixed, 

 E c 3 that 



