ssB Dr. Wollaston on Cystic Oxide , 
from numerous experiments for ascertaining the presence of 
phosphoric acid, I gave the preference to one in which nitrate 
of mercury was employed, on account of the facility of ex- 
tracting the acid from the phosphate of mercury, by heat 
alone. But since the whole of the phosphoric acid is not pre- 
cipitated by nitrate of mercury, sulphate of magnesia will not 
be formed on the addition of sulphuric acid, and the magnesia 
cannot be obtained separate by the same process. 
It may have been in consequence of this over-sight, that a 
mistake on that subject has occurred in the succeeding volume 
of the Transactions. 
A calculus is there described, which had been taken by Mr. 
Thomas from the bladder of a dog, and a series of experi- 
ments are related, from which it was inferred to consist of 
super-phosphate of lime, and phosphate of ammonia. But 
from the appearance of this calculus (which was exhibited to 
the Society at the time when the paper was read) I was much 
inclined to think, that the nature of it was mistaken, and upon 
full consideration of the experiments, they did not appear to 
me conclusive. 
I therefore obtained a portion of the calculus, and by the 
following process, the earth contained in it was proved to 
consist almost wholly of magnesia. 
It was dissolved, with the exception of a very small resi- 
duum, by distilled vinegar. 
The whole of the phosphoric acid was then precipitated by 
acetate of lead, added to excess. 
The liquor was then poured off, and sulphuric acid was 
added, w hich precipitated the excess of lead, and at the same 
time formed sulphate of magnesia in solution. 
