t 39^ 3 
I have found, that the earths of all denomina- 
tions, even the cryftalline and the talcky, which are 
thought to be infoluble in acids, yield a pure dephlo- 
gijiicated air^ when treated in the marnier mentioned in 
my former letters; but that the calcareous earths, and 
fome of the earths of metals, as red lead and the flowers 
of zinc, yield it in the greateft plenty. Upon the 
whole, I think, it may fafely be concluded, that the pureft 
air is that which contains the \Q 2 .iS.pbIogiJion : that air is 
impure (by which I mean that it is unfit for refpiration, 
and for the purpofe of fupporting flame) in proportion as 
it contains more of that principle ; and that there is a re- 
gular gradation from depbldgijiicated air, through com- 
mon air, and pblogijiicated air, down to nitrous air ; the 
lafl fpecies of air containing the moft, and the firft-men- 
tioned the leafl: pblogijion poflible, the common bafis of 
them all being the nitrous acid ; fo that all thefe kinds 
of air differ chiefly in the quantity oi pblogijion they con- 
tain ; though with refpe6t to nitrous air, there feems to 
be a farther difference in the mode of combination. By 
attending to the quantity of pblogijion contained in the 
fubftances ^with which the fpirit of nitre is mixed, any 
of thefe kinds of air may be produced at pleafure, and 
fometimes all the kinds will be produced in the different 
ft ages of the fame procefs. White wood-aflies yield an 
exceedingly pure air ; but the leaft bit of charcoal in the 
aflies depraves the air; and if there be much charcoal 
in them, the whole produce will be ftrongly nitrous. 
The phaenomena of detonation (which has been a very 
puzzling 
