172 _ Mr. Cavennisn’s Anfwer to 
With regard to the calcination of metals it may be proper ‘to 
remark, that this operation is ufually performed over the fire, 
by methods in which they are expofed to the fumes of the 
burning fuel, and which are fo replete with fixed air, that it is 
not extraordinary, that the metallic calx fhould, in a fhort 
time, abforb a confiderable quantity of it; and in particular 
red lead, which is the calx on which moft experiments have: 
been made, is always fo prepared. There is another kind of 
calcination, however, called rufting, which is performed in 
the open air; but this is fo flow an operation, that the ruft | 
may eafily imbibe a fufficient quantity of fixed air, not- 
withftanding the fmall quantity of it ufually contained in the 
atmo{phere. | 
Mr. Kirwan allows that lime-water is not rendered cloudy 
by the mixture of nitrous and common air; but contends that 
this does not prove that fixed air is not generated by the union, 
as he thinks it may be abforbed by the nitrous felenite produced 
by the union of the nitrous acid with the lime. This induced 
me to try how fmall a quantity of fixed air would be perceived 
in this experiment. I accordingly repeated it in the fame man- 
ner as defcribed in my paper, except that I purpofely added a 
little fixed air to the common air, and found that when this 
addition was ...th of the bulk, or 2,th of the weight of the 
common air, ve effect on the lime-water was fuch as could 
not poflibly have been overlooked in my experiments. But as 
thofe who fuppofe fixed air to be generated by the mixture of 
nitrous and common air, may object to this manner of trying 
the experiment, and fay, that the quantity of fixed air ab- 
forbed by the lime-water was really more than .*.th of the 
bulk of the common air, being equal to that quantity over 
‘ and 
