154 ESSAY IX. 



any nitrous acid ; for, on the one hand, paper tinged with 

 lacmus scarce shows any signs of its presence ; and, on 

 the other hand, the colour is destroyed by the addition 

 of any acid, without being ever afterwards recoverable, 

 either by alkalies or any other means known to me. If 

 the solution be quickly evaporated upon a heated furnace, 

 it at last swells into innumerable bubbles. This foam grows 

 redder and redder, and, after it is quite dry, it appears 

 dark red. This dark mass communicates its colour to a 

 much larger quantity of water than before, and dissolves 

 very readily in all acids, even in such as have no action 

 on the calculus ; but they entirely destroy the colour, and 

 that the more quickly the stronger they are. Even alum, 

 with its small excess of acid, has this effect. Caustic 

 alkalies dissolve the colouring matter, and destroy it, but 

 more slowly. 



The nitrous acid has a peculiar effect upon all phlogistic 

 substances ; and as the colour of bodies is especially de- 

 pendent on phlogiston, it appears why no other acid has the 

 power of educing such a colour from the calculus. But in 

 order to obtain it, a proportional quantity of acid is requisite. 

 The diluted acid of nitre ought therefore to be made use of, 

 in order not so easily to transgress the proper limits ; for if 

 too much be used, it will not produce the proper effect ; but, 

 in proportion to its superabundance, goes farther, and 

 destroys more or less, or the whole of it. If it be poured 

 undiluted upon powdered calculus, it is converted in a few 

 moments, and without any assistance of heat, into a mere 

 foam. 



The acid of calculus is the more easily separated from 

 the aquafortis by evaporation, as the latter is rendered more 

 volatile by the inflammable particles of the former. Alkali 

 added to them both united does not produce any precipita- 



