in the Structure of Calculi. 239 
1. Uric acid, either in a pure state, or with a very small 
proportion of the phosphates. 
2. The phosphates, either pure, or with a small proportion 
of uric acid. 
The first species, which generally appears in the form of 
minute crystalline grains, of a reddish brown colour, or of an 
impalpable brown powder, is either entirely soluble in pure 
alkaline solutions, not emitting an ammoniacal odour, in which 
case it consists of pure uric acid : or it does emit an ammo- 
niacal odour, and is not entirely soluble, in which case it con- 
tains the triple phosphate of ammonia and magnesia. 
When this substance is observed in the urine, the alkalies 
are recommended. They are exhibited either in a pure state, 
or as carbonates, and in each instance the uric sediment gene- 
rally diminishes rapidly, and during the continued use of alka- 
line medicines, occasionally disappears altogether. 
It however frequently happens that the matter voided, is 
not diminished in quantity by the use of alkalies, but that its 
form and composition are altered, and that it assumes the ap- 
pearance of a gray powder, and is composed of uric acid with 
variable portions of the ammoniaco-magnesian phosphate. 
From these facts therefore, it cannot be doubted that the 
internal exhibition of alkalies, often prevents the formation of 
uric acid, and hence must likewise prevent the increase of a 
calculus in the bladder, as far at least as uric acid is concerned ; 
but it has also been supposed that the alkalies are capable of 
acting upon the stone itself, and even of effecting its complete 
solution. It is true that if we immerse a calculus, composed 
of uric acid, in a dilute solution of caustic alkali, that it will 
be slowly acted upon, and after some time entirely dissolved. 
