of Atmospherical Air, Be. 139 
not owing to the presence of pure air; since it takes 
place when air, thoroughly phlogisticated, is kept 
with nitrous air in the same circumstances. | This 
isa strong proof, that the nitrous test cannot be de- 
pended upon. By all other eudiometers it appears, 
that atmospherical air contains about one fourth of 
its bulk of oxygenous gas. And when this and pure 
azotic gas are mixed in such proportion, they form 
a fluid exactly similar to that of the atmosphere. 
Dr. Priestley’s experiments on the composition 
of azotic gas are certainly very curious and interest- 
ing, and deserve great attention.—I therefore regret 
much, that I have not been able to succeed in any 
attempts to repeat them. I speak only of those in 
which he confined rusted iron and inflammable air 
in glass vessels by mercury or water. Those in which 
he kept the oxygenous and hydrogenous gases in 
a wet bladder are not admissible: the azote is, in this 
case, formed by the decomposition of the bladder; 
and is produced though no inflammable air is pre- 
sent.* J kept the red oxyd of iron, which I put into 
a phial with hydrogenous gas, in a glass jar over 
water for several weeks; but no diminution took 
place. A large piece of rusted iron was then suspend- 
ed in a jar with inflammable air confined by water. 
No diminution took place; and when, in six weeks 
afterwards, the gas was examined, it appeared as in- 
* Priestley’s Experiments, vol. 1, p. 179---181. 
