Mr. Kirwan’s Reply, &c. 179 
fuch a manner that it was impoflible it could come in conta with. 
the leaft particle of flame or {moke (Mem. Turin. 1769, ps71-)s 
Mr. Cavennisu exprefles his furprife at my afferting, that 
the black powder, which Dr. Prizsriey formed out of an 
amalgam of mercury and lead, was exactly the fame as that 
eut of which he had extracted fixed air; but, I think, I have 
affigned very fufficient reafons for my opinion: how far I was 
right will beft appear by Dr. PrresrLey’s own letter, in the 
hands of the Secretary, of which the following is an extra. _ 
‘<I certainly imagined the two black powders you write 
<< about to be of the fame nature, and therefore did not at- 
“< tempt to extract any air from the latter; but immediately. 
“¢on the receipt of your favour of yefterday, I diflolved an 
‘‘ ounce of lead in mercury, and expelling it by agitation, 
«* put the black powder, which weighed near 12 ounces, into 
«< a coated glafs retort; then applying heat, I got from it about 
<¢ 20 ounce meafures of very pre fixed air, not .*,th of which, 
«¢ remained unabforbed by water.” 
Fourthly, it is impoffible to attribute the fixed air, produced 
by the diftillation of red precipitate and filings of iron, to 
the decompofition of the plumbago contained in the iron: 
for the quantity of fixed air produced in Mr. CAveNnDIsH’s 
own experiment is more than ¢wice the weight of the 
whole quantity of plumbago contained in the quantity of 
iron he ufed, fuppofing the whole of the plumbago to. 
to confift of fixed air, whichis not pretended ; ; and more than 
eight times the weight of the quantity of fixed air which 
plumbago really contains. For Mr. Cavenpisu, employed in 
his experiment 1000 grains of iron and 500 grains of red pre- 
cipitate, and obtained 7800 grain meafures of fixed air, which 
are equal to 30 cubic inches, and weigh 17 grains. Now 100 
Zara grains 
