in the Structure of Calculi. 24 1 
sand, and is frequently mixed with variable proportions of 
phosphate of lime. In the latter it makes its appearance after 
the urine has remained undisturbed for some hours in an open 
vessel, generally in the form of a fine pellicle, or of crystal- 
line laminae, which when collected and dried bear some re- 
semblance to boracic acid. 
Its putting on this form is accounted for, from its being held 
in solution in the first instance by carbonic acid, and as this 
flies oft, the triple salt makes its appearance. If a portion of 
the urine be preserved in a phial closely stopped, the carbonic 
acid cannot escape, and consequently no phosphate is observed 
to separate. There is also a quantity of phosphoric acid pre- 
sent, which keeps another portion of the ammoniaco-magne- 
sian phosphate, and also some lime (in the state of super 
phosphate of lime) in solution. 
It is therefore obvious, that whenever the urine is deprived 
of a portion of the acid which is natural to it, the deposition 
of the triple phosphate, and phosphate of lime, more readily 
takes place : this is effected by the exhibition of the alkalies. 
It may therefore be asserted, that although alkaline medi- 
cines often tend to diminish the quantity of uric acid, and thus 
to prevent the addition of that substance in its pure state, to a 
calculus in the bladder ; they favour the deposition of the 
phosphates. 
It cannot be doubted that the alkalies reach the bladder, 
since in cases where large doses of sub-carbonate of potash 
have been exhibited, I have seen evident traces of it in the 
urine. 
Where the phosphates only are voided, it has been pro- 
mdcccviii, I i 
