388 Fourcroy’s Examination of 
each other and exhibited no difference. We mutt how-. 
ever add to this fact, which forms one of the new points of 
the differtation of Dr, Pearfon, that of the difcovery of his 
ouric oxyd in arthritic concretions. But it may be readily 
perceived, that this difference between our author and the 
chemifts whom he feems to combat no way affe€ts the in- 
timate nature of the real calculous fubftance, and it is only 
on the latter that 1t is of importance to fix the opinions of 
philofophers. 
_ The queftion between us is now reduced to this fimple 
point: Ought the properties of the matter which conftitutes 
the urinary calculus to induce us to confider it as a peculiar 
acid, or only as an animal oxyd? 
It is fufficiently agreed, that every fubftance more or lefs 
foluble in water, thatturns fome blue colours red, that, above. 
all, is fufceptible of uniting ftrongly and readily with alkalies, 
and of forming with them cryftallifable compounds, in fuch 
a manner that their peculiar attractive power for each of 
thefe falifiable bafes, as well as their affinities, compared to 
that of other well known acids, may be eftimated, ought to 
be ranked among the number of acids. Thus the metals 
called tung /ien, molybdena, and chrome, faturated with oxy- 
gen by complete combuition, have naturally aflumed a place 
in the clafs of and next to the moft powerful and longeft 
known acids. ..It is thus in particular that the matter con- 
tained in the gall-nut which blackens iron; that which co- 
fours it blue, and which is furnifhed by the decompofition of 
animal fubftances, have received the character of acid bo- 
dies, and the names of the gai/lic acid and pruffic acid. It 
‘is; in fhort, from the fame confiderations that our lithic 
acid has been placed in the fame clafs or referred to the fame- 
genus. It has not indeed any four tafte, and is very little fo- 
luble in water; but it reddens turnfole paper when it is rub 
bed on it, with a little water, and it combines rapidly with 
cauftic alkalies, and brings them to the ftate of falts by fatu- 
ration. If it does not decompofe foap or alkaline carbonates, 
it 
